Building Science

The Physics of Thermal Inertia in Low-Slope Roof Design

By Thomas J Taylor

January 13, 2021

Commercial building with a masonry facade

What are the key material properties?

In a previous article the use of thermal inertia to slow down heat flux through a roof assembly was discussed. In buildings where air conditioning costs dominate and heating use is relatively low, higher thermal inertia assemblies can potentially improve energy efficiency. This is particularly the case of buildings such as offices that are only occupied during daylight hours. Thermal inertia could delay the transmission of heat into a building towards the end of the day, increasing thermal comfort and allowing facility managers to reduce cooling during the day.

While the previous article provided an introduction to thermal inertia and its potential benefits, it didn't discuss the origin of the property. Also, the related but different terms, thermal inertia and thermal mass, were used somewhat interchangeably.

This article describes the material properties that contribute to thermal inertia and provides a basis for material selection when increased thermal inertia is a goal.


A building enclosure with high thermal inertia will slow down the transmission of heat. This effect has traditionally been used in the Mediterranean and other temperate climates to improve thermal comfort during the summer. As the building exterior surface heated up during the early afternoon period, high thermal inertia reduced the heat transmission into the interior. This was advantageous in the days before air conditioning. Today, designing with higher thermal inertia, as described in part one of this series, could improve energy efficiency in certain situations:

  • Buildings located in areas where energy costs are dominated by air conditioning.
  • Buildings only used during the day and not on a 24 hour basis. Thermal inertia can push heat loads into the evening, allowing for either temperature settings to be raised or HVAC fans speeds to be reduced.
  • For high thermal inertia within the roof assembly to have a significant effect, low to moderate rise buildings with larger footprints would be best.

Thermal Inertia Effects

There are two key characteristics of thermal inertia; the decrement factor and time delay, both of which can be readily modeled and experimentally verified. A schematic showing these is shown below:

The time delay is calculated as:

Φ = t[Tout(max)] - t[Tin(max)]

Where t[Tout(max)] and t[Tin(max)] are the time of day when the inside and outside surface temperatures reach maximum. The decrement factor, DF, is calculated from:

DF = (Tin(max) – Tin(min)) / (Tout(max) – Tout(min))

  • The time delay and decrement factor could be used by building designers and operators to better optimize thermostat specification, HVAC use, and temperature schedules. As noted earlier, a large time delay could significantly reduce daily AC cooling demands for office buildings.
  • Larger decrement factors can improve occupants' thermal comfort. Temperature swings are reduced, which in turn reduces HVAC cycling.
  • Importantly, high thermal inertia will not reduce the energy reaching the interior, it only delays the transmission. This can make increased thermal inertia part of a strategy to improve energy efficiency in buildings occupied during the day but not necessarily for those occupied on a 24-hour basis.

Roof Configurations and Thermal Properties

Building designers and construction professionals are familiar with the property of thermal resistance, which is a measure of the extent to which a material blocks or resists heat transfer. It is expressed as R-value or U-value, which are numerical descriptions of the extent to which a material can resist heat flow.

However, there is far less familiarity with the time delay of heat transfer, or the extent to which a material slows down or delays heat transfer. As will be seen, a material can have low thermal resistance but still delay heat transfer. This is the crux of the underlying properties "thermal diffusivity", "thermal mass" and "thermal inertia".

Fundamental Thermo-Physical Properties

There are three fundamental thermo-physical properties of materials that are the components of thermal diffusivity, thermal inertia, and thermal mass. These are thermal conductivity, density, and specific heat capacity.

Thermal Conductivity

Building design professionals are very familiar with thermal conductivity, k, measured as watts per meter per Kelvin (W/(m.K)). It is normally used to characterize individual materials, for example polyiso foam, and is a measure of the heat flow through a material when a temperature gradient of 1K (i.e 1°C) is applied.

A related term, the u-factor, is used to describe the thermal conductivity of a system. This could be a window, i.e. a combination of glass, air space, and frame etc.

Thermal conductivity is measured when heat flow has equilibrated and it doesn't include any time delay.


Density is a measure of the mass per unit volume, ρ=m/v. While this is straightforward for most materials, products such as polyiso can be harder to define. There are the facers and a small density gradient within the foam. Later, as values are listed, the facers will be ignored and the foam's density considered as an average.

Specific Heat Capacity

Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 kelvin, or Cp = J/(kg.K). It is worth considering the simple diagram above, showing the measurement of thermal conductivity. At the onset, heat flow through the material is delayed by its specific heat capacity. It takes energy to raise its temperature, an action required before heat can then transmit.

Derived Thermo-Physical Properties

As noted, the fundamental properties described earlier are all measured at equilibrium and do not include any time lag or delay. For that, the following derived properties need to be considered.

Thermal Diffusivity

Thermal diffusivity is a measure of the rate at which a temperature propagates from one point to another point in a material. It's the rate of transfer of heat from a hot side to a cold side and is calculated as:

In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'. In a sense, thermal diffusivity is what is meant when the topic of thermal inertia is discussed casually.

Thermal Inertia

Thermal inertia is the slowness with which the temperature of a material approaches that of its surroundings. It is a product of thermal conductivity, density, and specific heat capacity. From a building enclosure perspective, it could be considered as the rate at which the interior surface can supply heat into the interior, assuming a temperate climate.

It is arguable that thermal inertia is not the best property to use to characterize a building enclosure component in terms of thermal lag.

Thermal Mass

It is often broadly thought that thermal mass is essentially equivalent to gravimetric mass. Conceptually, this gives rise to the view that the more massive a construction the better. However, consider two blocks - one of steel and one of concrete - each having the same gravimetric mass. The two materials have different specific heat capacities and are not equivalent in terms of thermal properties. In addition, steel has a significantly higher thermal conductivity versus concrete.

Thermal mass is a property of a material that enables it to store heat and is the product of density and specific heat capacity:

Importantly, thermal mass doesn't fully describe thermal lag. High thermal mass would change the decrement factor, dampening out heat transmission but is only indirectly linked to thermal delay.

Roof Component Thermal Properties

Two deck types are predominant in North American construction of big-box type architecture; steel and concrete. Two possible idealized roof assemblies are shown below, based on the two roof deck types:

  • Both systems have most roof assembly layers adhered. The steel deck system has a first layer of mechanically attached gypsum board to act as a substrate for the adhered first layer of polyiso.
  • Thermal bridging is either minimal in the steel deck based system or absent in the concrete deck system (ignoring any real world penetrations).
  • The first layer of gypsum board in the steel deck case could be used as a substrate for a vapor retarder.

The membrane, adhesive layers, and steel deck are relatively thin and have no significant insulating properties. The following table shows the fundamental and derived thermo-physical properties of the other components:

thermal inertia chart

  • Polyiso and HD Polyiso specific heat values are estimates.
  • Concrete properties are very dependent on aggregate type and moisture level. Data shown represents average values for dry concrete.

Unsurprisingly, the data shows that concrete has significantly higher thermal mass and inertia versus the other materials compared in the table. As discussed in the first part of this series, insulation is best placed closest to the building exterior, and thermally massive materials placed closest to the interior. The southeast region tends to use concrete decks more than many other regions of the US, largely for improving the strength of the roof deck. This could also mean that, especially for those buildings occupied only during the day, such as offices, schools, and the like, have better opportunity for energy-efficiency benefits with well-designed roof insulation systems.


  • Thermal property data could be used in modeling exercises to better understand how to design and optimize energy-efficient buildings.
  • Clearly, concrete has a far higher thermal mass and inertia than other materials commonly used in roof assemblies.
  • Very lightweight concrete and gypsum board have very similar thermal mass, thermal inertia, and density, but have notably different properties for thermal diffusivity and conductivity.

Thermal property data is fundamental to the materials used in low slope roof assemblies. However, it needs to be converted into values specific for the thickness and weight of actual roof products. That will be a topic for the next part of this series.

Sources Used

  1. Verbeke, S., Thermal inertia in dwellings. Quantifying the relative effects of building thermal mass on energy use and overheating risk in a temperate climate. PhD Thesis, University of Antwerp, 2017.
  2. Balaji, N. C., Mani, M., and Venkatarama, R. B. V., Thermal performance of building walls. 1st IBPSA Italy Conference, Building Simulation, 2013, pp. 151 – 159.

About the Author

Thomas J Taylor, PhD is the Building & Roofing Science Advisor for GAF. Tom has over 20 year’s experience in the building products industry, all working for manufacturing organizations. He received his PhD in chemistry from the University of Salford, England, and holds approximately 35 patents. Tom’s main focus at GAF is roofing system design and building energy use reduction. Under Tom’s guidance GAF has developed TPO with unmatched weathering resistance.

Related Articles

Concrete decks
Building Science

Concrete Roof Decks Have Several Advantages Over Their Steel Counterparts

Concrete decks are one of the more common types of low-slope roof decks for commercial buildings. Steel and wood roof decks are the other most common types. Concrete roof decks make up approximately 13-14% of the new and retrofit low-slope construction market, according to the National Roofing Contractors Association (NRCA) 2015-2016 Market Survey. This article examines the advantages of concrete roof decks, the various types, and some of the precautions that should be taken to ensure success. Advantages of Concrete Decks While there are many advantages to concrete decks, there are two primary reasons to consider them. These are based on desired fire ratings and structural requirements. Concrete can have a wide range of densities depending on the type and amount of aggregate used, the amount of water used in the mixture, and whether air is introduced as a foam via the use of surfactants and the like. Concrete for roofing applications generally fits into one of several categories, depending on the requirements. The properties of each type cover a range depending on the raw materials. The following table shows density ranges followed by typical property values that can be expected. * Values for the concrete structural board are estimates based on the published density. Designers and specifiers should always note that actual property values will depend on aggregate type and source, the volume of water used, and other raw material factors. Always consult with local suppliers to ensure requirements are met. When concrete is used as a structural component, designers typically specify the compressive strength. This is only loosely related to the compressive strength of the coarse aggregate and the American Concrete Institute (ACI) 213R-03 reports that for typical building-slab compressive strengths – up to about 5,000 psi – "there is no reliable correlation between aggregate strength and concrete strength." There are five general types of concrete roof deck, as follows. Structural Concrete Decks These are cast-in-place and become an integral part of the structure. They use normal weight concrete and are designed to carry heavy loads. Foam insulation such as polyiso can be adhered on top, followed by the roof membrane. Also, the structural roof deck can be insulated with poured in-place lightweight insulating concrete with the roof membrane installed on top. Structural Concrete Composite Deck These are based on a steel panel deck system that is overlaid with normal weight or lightweight structural concrete. Loads are carried by the combination of the steel deck and concrete, which act as a single component. Typically, the steel panels are embossed to ensure mechanical coupling to the concrete. Like structural concrete roof decks, composite decks are normally insulated using adhered foam insulation panels or poured-in-place lightweight insulating concrete. Lightweight Insulating Concrete Decks Lightweight insulating concrete (LWIC) can be poured over a variety of structural deck systems that have been designed to carry loads. These can be either of the two structural concrete deck types already described or a structural steel deck. While published properties of LWIC are based on the use of lightweight aggregates or foamed concrete, LWIC can also incorporate foam boards. The following picture shows lightweight insulating concrete being poured over boards that have been set into a thin layer of LWIC. When this is done, the concrete completely encapsulates the foam, which is frequently expanded polystyrene. The picture is of a Siplast system, and the resulting structure is shown here: By arranging boards of different thicknesses, combined with smoothing of the surface, a positive pitch can be achieved to allow for good drainage. Precast Concrete Decks There are a number of types but usually, the precast members are comprised of reinforced, pre-stressed or post tensioned normal weight concrete. They can be T-shaped or hollow form planks. The precast assembly is normally covered with a layer of poured lightweight insulating or structural lightweight concrete, which serves as a leveling layer. Because the precast planks are usually covered with a poured concrete, they are considered as equivalent to poured structural decks for the purposes of this article. As with the previous types, the roof is completed with adhered foam insulation and a membrane. Structural Concrete Boards These are relatively thin (typically ¾ inch) factory formed boards that are screwed down onto a structural steel support (e.g., "C" channels). Conceptually, they can be thought of as being similar to steel deck panels that are installed over steel joists, but offering greater fire protection. Insulation can be mechanically attached or adhered, followed by application of the membrane. Advantages of Poured Concrete Decks Whether they are structural or insulating, poured-in-place concrete decks have a number of advantages. Fire ratings: In accordance with ASTM E119, Standard Methods of Fire Tests of Building Construction and Materials, the American National Standards Institute (ANSI) and Underwriters Laboratories (UL) list minimum concrete thicknesses required for various fire ratings. The following table is summarized from ANSI/UL 263 Design No. J718. As the density decreases so does the thermal conductivity and hence the fire rating is improved. However, as density decreases so does the compressive strength. Wind uplift ratings: As wind rises up and over a building, it creates uplift pressure that exerts an upwards force on roof membranes. By greatly restricting air flow up into a roof assembly, the resistance to that upwards force is increased. Poured concrete decks can be effectively sealed at penetrations and roof edges and greatly reduce the amount of air that can penetrate upwards. This is shown in the following schematic comparing a poured concrete deck with a steel deck. There are two main reasons why concrete decks can potentially improve wind uplift resistance, depending on concrete specification and overall roof assembly design. First, fasteners in structural concrete have a much higher pull-out strength compared to steel. With #14 fasteners for example, it's by a factor of over 2:1. Second, a steel deck system that has an adhered membrane and mechanically attached insulation can be ultimately limited by the fastener pull-out strength. This tends to limit the wind uplift resistance for roofs with steel decks to approximately 1-180 psf in 12x24 wind uplift testing. However, fully adhered structural concrete deck systems have achieved 1-300 psf or higher in similar testing. Even though wind uplift resistance is often discussed in terms of wind speeds, the actual cause of roof wind uplift is slightly more complex. Roof damage caused by wind occurs when the air pressure below the roofing assembly is greater than the air pressure above the building's roof. As wind flows over the building, the pressure directly above the surface of the roof decreases. At the same time, internal air pressure increases due to air infiltration through openings, cracks, etc. The result is a net upward force on the roofing system. While concrete decks, when installed properly, can greatly reduce internal air flow into a roof assembly, if penetrations through a concrete deck are made later, these must be carefully sealed to prevent upwards air movement. Air barrier: The International Energy Conservation Code (IECC) considers concrete in the following forms to be an effective air barrier, provided that all seams and joints are sealed. Cement board having a thickness of not less than 1/2 inch (12 mm). A Portland cement/sand parge, or gypsum plaster having a thickness of not less than 5/8 inch (16 mm). Cast-in-place and precast concrete. As noted previously, poured concrete decks effectively seal around penetrations. However, with any concrete deck, it's important to carefully consider the roof to wall termination and ensure that all gaps are closed up and sealed. Thermal mass: Improvements in the energy efficiency of buildings are commonly achieved through increased thermal insulation. In fact, most model codes use thermal insulation as the only method of improving energy efficiency of building enclosures. However, there are many indications that thermal mass could also offer a means to both improve occupant comfort and lower energy use. Thermal mass affects the dynamic flow of heat into and out of buildings but has not received as much attention as thermal resistance for improving the energy efficiency of building enclosures. Concrete can add significant thermal mass to an assembly. That thermal mass could help to dampen interior temperature swings and thereby improve occupants' comfort. Also, it can dampen and delay heat fluxes caused by the sun, moving peak heat flux from the mid-day to later in the evening. This could help improve energy efficiency especially for buildings only occupied during daytime hours. Ability to Add Slope: A slope can be added to a poured concrete deck during installation. This was discussed in the LWIC section earlier. By achieving a positive pitch with the concrete, it is possible to eliminate the need for tapered foam insulation while enabling drainage. Disadvantages of Poured Concrete Decks Weight: Roof designers need to consider the added weight introduced by concrete decks. It is important to understand the weight of locally sourced concrete versus simply relying on data from national handbooks. Always consult with a structural engineer. An advantage of structural concrete boards is that the added weight is lower than for poured concrete. However, concrete boards then require careful sealing at joints and penetrations to reduce air movement up into the assembly. Water content: Roof decks that are cast-in-place contain a high level of water when poured. This is true for normal weight structural concrete and, importantly, the amount of water is greatly increased with the use of lightweight structural concrete. Normal weight structural concrete uses regular aggregates (i.e., hard rocks) with a low moisture absorption rate, while lightweight concrete uses shales and clays that are expanded with air to make them less dense but they have a higher moisture absorption rate. On a job site, regardless of concrete type, additional water is often added to the mix to reduce the viscosity to ensure concrete easily flows so there are no gaps or voids in the final product. Aggregate for lightweight concrete is often intentionally loaded with water because of the voids. This is necessary so the water needed for concrete curing is not pulled into the aggregate. Typically the roofing industry has required a 28‐day curing period prior to testing the roof deck for "dryness" and suitability for roofing. This recommendation was garnered from the concrete industry's recommendation for the appropriate amount of time for concrete to cure and develop adequate compressive strength; however, the 28-day time frame is not related to the amount of water in the concrete, only to the cure time. The roofing industry now understands that concrete continues to cure and release excess moisture (i.e., dry out) for a significant time. It's important to note that lightweight concrete's higher moisture content (relative to normal weight) means a much longer drying time is needed. When a concrete deck is poured, some of the mix water is used up by the curing process, and some evaporates; but the rate of evaporation is slow, so large quantities of water remain stored within the structure of the concrete for extended periods of time. Moisture retention is exacerbated by construction methods that install concrete over non‐removable non-vented metal forms (or other impermeable substrates). While the concrete itself is generally not damaged by this moisture, the moisture typically migrates into the roofing system where it is absorbed by materials that are more sensitive to moisture. There are several technical advisories that have been issued to raise awareness about the potential for moisture issues associated with concrete roof decks. These include the National Roofing Contractors Association, the Asphalt Roofing Manufacturers Association, the Single-Ply Roofing Institute, the Polyisocyanurate Insulation Manufacturers Association, and the International Institute of Building Enclosure Consultants. Allowing concrete to thoroughly dry is most appropriate; however, it is reasonably impractical to go beyond 28 days. So more realistic ideas must be implemented to accommodate a potentially substrate; those follow. Installation of a vapor retarder on the top surface of the concrete deck. The Midwest Roofing Contractors Association (MRCA) advises that a vapor retarder of less than 0.01 perm is necessary over new concrete roof decks. From a practical standpoint, a vapor retarder of less than 0.01 perm is effectively a vapor barrier—almost no moisture passes through into the roof assembly. For more on the use of vapor retarders see this guide. There are three classes of vapor retarders/barriers: Class I, II, and III. Each has a varying permeability rating. A Class I vapor retarder has a perm rating of 0.01 to 0.1; a Class II vapor retarder has a perm rating of 0.1 to 1.0; and a Class III vapor retarder has a perm rating of 1 to 10. MRCA is advising to use a better-than-Class 1 vapor retarder over new concrete roof decks because of its ability to prevent moisture from entering the roof system. Although industry guidance points towards the use of a vapor retarder over concrete decks, designers must consider the potential issues with installing a roofing system with a double vapor barrier. Moisture that gets in between the roof membrane and the vapor retarder at the roof deck is essentially trapped. Use a venting base sheet in conjunction with vents and venting edge details (at edges, parapets and penetrations) to provide a pathway to allow the moisture in a wet concrete deck to escape slowly over time. Lightweight concrete should always be installed over a vented steel deck to allow downward drying. For more information about moisture in concrete decks see this article by Kirby. Advantages and Disadvantages of Structural Concrete Boards Structural concrete boards improve the fire resistance of roof assemblies but, due to their relatively low thickness, do not offer the same fire resistance as poured concrete decks. Concrete boards are factory cured and therefore do not have the moisture issues associated with poured concrete decks. Concrete boards are similar to steel roof deck panels in that they do not prevent air and moisture from migrating up into the roof assembly. The following schematic shows that such systems can be "enhanced" by sealing around penetrations and applying a self-adhering vapor retarder to the concrete board deck. Adhering the vapor retarder directly to the concrete deck ensures that it is below the dew point so long as a code-required level of insulation is installed above the vapor retarder. A good example of such a product is GAF's SA Vapor Retarder. For most vapor retarders, application direct to a steel deck doesn't meet many fire codes. Concrete boards can provide a good substrate for the application of the vapor retarder while also improving fire performance. Conclusions Concrete decks have a number of advantages over steel decks. These include improved fire ratings and wind uplift ratings. Also, concrete is classified by the IECC as an air barrier provided that seams and joints are sealed. Care must be taken to ensure that moisture in poured concrete decks is prevented from moving upwards into the roof assembly. This is typically achieved by the installation of a vapor retarder over the deck. Structural concrete panels are factory cured and can be a good substrate for self-adhered vapor retarders when there is a risk of moisture migration from within the building up into the roof assembly.

By Authors Thomas J Taylor

September 14, 2021

Roof Penetration
Commercial Roofing

Dealing with Low Slope Roof Penetrations

Low slope roof penetrations can be a source of problems if not done correctly. Pipe, vent, and conduit penetrations through low slope roof assemblies can cause problems for an otherwise tight membrane, insulation, and deck design. With many intermediate layers in roof assemblies, such as a vapor retarder and cover board, there are opportunities for things not to be done correctly somewhere in the assembly. It gets even more complex when we consider that in order to use a vapor retarder, there might be an additional cementitious board above a steel deck. This article provides an overview and is intended to be used as general guidance only. For specifics refer to the GAF Single-Ply Pro Field Guide and, when using the GAF SA Vapor Retarder also review the Guide to Vapor Retarder Design in Low-slope Roof Systems. Let's look at each part of the roof assembly, starting at the bottom: Deck Level: If a separate vapor retarder is being installed, this is where it will be placed; either direct to deck or onto a cementitious board or HD polyiso. A self-adhered vapor retarder installed at the deck significantly reduces the diffusion of moisture and blocks the movement of interior humid air up into the roof assembly. Any penetration needs to be flashed carefully to prevent interior air from bypassing the vapor retarder. Best practice is to field fabricate a collar out of the vapor retarder and wrap it around the pipe as shown here: A target sheet is then placed over the collar and any remaining gaps can be sealed with GAF Flexseal™ Caulk Grade Sealant. The completed flashing is shown below: Important – as shown in the schematic above, sometimes the gap around a deck penetration is larger than can properly support a flashing. When this is the case, be sure to fill the gap with foam, using a foam pack, to act as a support and to block air flow. This also helps further reduce the risk of air infiltration from the building interior up into the roof assembly. Insulation / Coverboards It is not unusual to see gaps around penetrations through insulation and coverboards. These gaps don't just act as thermal bridges but they also allow air to freely move up into the roof assembly. This is especially true for mechanically attached membranes during wind events. Best practice is to fill the gaps with foam as shown here: By filling any gaps between penetrations and insulation and coverboards, the risk of interior humid air reaching the underside of the membrane is minimized. This in turn reduces the risk of condensation issues in cold climates. Important - Always make sure that the two layers of insulation have staggered and offset joints. If coverboard is used, also ensure that its joints are staggered and offset from those of the topmost layer of insulation. In this way, air movement up through the assembly can be minimized. Single-Ply Roof Membrane Penetrations through single-ply roof membranes can be sealed with field fabricated flashings or prefabricated accessories. While field fabrication can be successful, the risks of inconsistency and errors can be reduced by using one of a range of prefabricated accessories designed to help flash in penetrations. The GAF TPO Accessories are a good place to start, allowing many situations to be addressed such as inside and outside corners, penetrations, vents, and skylights. These are discussed in more detail here. Keeping with the example of a pipe penetration, the following shows how to properly install a pre-molded pipe boot: Note that for a mechanically attached membrane, an additional four fasteners should be used around the penetration. A target patch can be required if the four fasteners need to be spaced further away from the penetration to ensure a good anchorage. However, the GAF vent boot is designed with a large 6 inch flange that often eliminates the need for a target patch. Important – as has been stressed before, do a final check that gaps around the penetration are sealed with foam, before doing this final installation. In many cases, condensation issues first occur around a penetration due to the ability of interior air to bypass the insulation layers and reach the underside of the roof membrane. Note that GAF offers custom cylindrical pipe boots which can be custom fit to the penetration to eliminate any air on the inside of the accessory which can help reduce condensation. Important Considerations The purpose of this article is to provide some background information and design considerations for addressing roof penetrations. GAF manufactures and sells roof materials but is not responsible for building design and construction. Design responsibility remains with the architect, engineer, roofing contractor, or owner. This information should not be construed as being all-inclusive, nor should it be considered as a substitute for good application practices. Please consult your design professional for more information.

By Authors Thomas J Taylor

June 17, 2021

Building Science

Vapor Retarder Frequently Asked Questions

Vapor retarders are increasingly being specified for inclusion in low slope roof assemblies. They can help manage humid air migration from the building interior up to the underside of the roof membrane. Also, they can help limit the amount of moisture migrating from a concrete deck up into the roof assembly. In fact, we offer the GAF SA Vapor Retarder, a self-adhering sheet product, to help reduce this risk. If you are designing a new roof and want to reduce possible moisture risks or are replacing a roof assembly where there's evidence of moisture issues, this article may help you to understand more about the use of vapor retarders. In addition to this article, we also have a Guide to Vapor Retarder Design in Low-slope Roof Systems which describes best installation practices. The guide and this article are intended to address the basics of vapor retarders for designers who want to address moisture migration. Later articles will cover more fundamental considerations. The answers to these frequently asked questions may sometimes repeat key information, and the reader can jump to those questions of most interest. But reading all of the answers will help get a better overall understanding of the function and role of vapor retarders. What is a vapor retarder? It is a material that, depending on its exact specification and correct installation, blocks or slows down the transmission of moisture from one side to the other. Vapor retarders can be coatings, boards with taped joints, or membranes. Looking at this schematic, it is clear that in an actual roof assembly any penetrations have to be sealed otherwise the vapor retarder's function is degraded – more on this later. How does a vapor retarder function? A properly installed vapor retarder at the deck level can help slow down or block the movement of moisture from the building's interior migrating up into the roof roof assembly. Blocking or slowing down the movement of moisture can be part of an effort to lower condensation risks within the roof system during cold winter months. Vapor retarders can also have some degree of moisture permeability. There are different classes of vapor retarding material (I, II, and III) and each class of material allows differing amounts of moisture vapor to pass through the materials via diffusion. The ability to limit vapor movement, but allow some moisture vapor to pass through the material, can be important because it can prevent having trapped moisture within the roof system. Roof membranes are moisture impermeable, so if moisture does get into the assembly, a properly specified vapor retarder with some degree of moisture permeability can allow the moisture to slowly escape downwards. How does a vapor retarder differ from an air barrier? To state the obvious, properly installed air barriers block air, and as a result will also block the movement of humid air, thereby retarding or stopping most moisture movement. So, they can appear to be very similar and sometimes identical. But, the use or application of each is normally different. For more on this, check out this article. A good roof membrane, such as GAF EverGuard TPO will not only block air and moisture but also withstand ordinary wear and tear. A good vapor retarder, such as GAF SA Vapor Retarder, is normally used within a roof assembly to reduce moisture movement. It will have limited or even zero permeability as its primary purpose is to reduce the movement of moisture. While it can act as a temporary roof, it is not intended to be wear- and tear-resistant in the same way as a roof membrane. Is a vapor retarder required? Building codes do not require the installation of a vapor retarder in roof assemblies. A determination as to whether to include a vapor retarder must be made by a design professional. Other answers in this article may help frame what could be considered for such a decision, but a design professional must make the ultimate determination based on the specific conditions at a given project. Basically, a vapor retarder can be specified and correctly detailed in order to manage the migration of moisture vapor to prevent wetting and enable drying within a roof assembly. Does vapor retarder use depend on a building's location? The location has a significant impact on the decision to incorporate a vapor retarder into a roof assembly. The building designer should consider the main direction of moisture drive within the building enclosure. Keep in mind that moisture drive is normally from warm (high vapor pressure) to cold (low vapor pressure). If the building is located in the north, moisture drive is the strongest in the winter. The building interior is usually at a warmer temperature than the exterior. Any interior humid air that reaches the external enclosure layers could cause condensation due to the lower external temperatures. In a roof assembly, a vapor retarder located towards the bottom side of the roof assembly can help reduce or throttle back the migration of water vapor from the interior warm side to the exterior cold top of the roof assembly. For a normal building occupancy and where the building is located in a consistently humid climate, the moisture drive is predominantly towards the interior of the building. In this case, exterior hot humid air that is able to penetrate through the building enclosure can form condensation on interior colder surfaces. Roof membranes are inherently vapor retarders so downward or inward vapor drive is blocked. For buildings with high occupant moisture generation, or that are located somewhere with a mixed vapor drive depending on the season, the roof designer should consider the appropriate roof assembly for the application. If moisture drive from the interior up into the roof assembly could lead to condensation within the roof assembly, then a vapor retarder should be considered. Isn't the roof membrane a vapor retarder? Why do I need another one? Roof membranes are generally vapor impermeable, but to be considered as vapor retarders one has to consider their use. In northern buildings where vapor drive is upwards through the roof assembly, the roof membrane is acting as a barrier to the external weather. It can also be used as an air barrier, preventing interior conditioned air from escaping, but it doesn't prevent interior humid air from moving upwards through the roof assembly. If water vapor is able to migrate upwards towards the roof membrane, then there can be a condensation risk depending on factors such as the exterior temperature and the interior humidity level. Where should a vapor retarder be placed within a roof assembly? The simplest answer to this question is as close to the interior conditioned space as is practically possible. However, always check that local fire codes allow for self-adhering membranes applied directly to steel decks. In many cases it is necessary to first install a gypsum or cementitious board over a steel deck which is then used as a substrate for the adhered vapor retarder. Always check with the roof system designer to make sure that a proposed system meets all necessary codes. Alternatively, the vapor retarder could be applied to the topside of the first layer of insulation, but in such a case, the designer would need to confirm that the dew point would be above the vapor retarder. Can I use black poly (e.g. Visqueen) as a vapor retarder? Black poly sheet, technically 6 mil polyethylene, is often used as a vapor retarder in residential crawl spaces. However, its use in roof systems is generally not recommended for several reasons: It does not self-seal around fasteners that penetrate through it. Vapor retarders such as GAF SA Vapor Retarder are designed to meet a self-seal test described in ASTM D1970. Polyethylene is notoriously difficult to adhere to, which makes flashing and sealing around penetrations very difficult and unlikely to be durable. 6 mil polyethylene is essentially impermeable, which means that any leak in the roof covering will let in water that can't escape. Also, if some water has been present when the roof was closed up, from dew or light rain during the previous night, it will not be able to escape. Properly specified vapor retarders have some degree of permeability that will allow for migration of water from within a roof assembly downwards. The only exception to this may be for a building with a very high interior humidity when it might be advisable to have a vapor retarder with essentially no permeability My building is in the north, so do I automatically need a vapor retarder? No, a roof designer needs to evaluate the risk of condensation occurring. The building use, the type of building, and the roof assembly design are important considerations. An evaluation of condensation risk asks questions including: What is the humidity level likely to be in the building? Office buildings can be expected to have lower levels versus buildings with restaurants or indoor pools. If activities within the building could generate high humidity levels, has the HVAC system been designed to reduce the levels with make-up air? What is the building's location and what are the coldest exterior temperatures that could be expected? Will the roof assembly inhibit air flow without the use of a vapor retarder? Some roof assemblies, particularly those that have adhered layers, are more restrictive of air flow than others. Once interior humidity levels have been estimated and outdoor cold temperatures known, then the building designer can calculate where the dew point will be in the roof assembly. If the designer specifies a vapor retarder, it should always be located below the dew point. How should a vapor retarder be tied in, flashed to penetrations, etc? To be successful, penetrations through the vapor retarder need to be flashed and air tight. Also, the edges need to be terminated to the walls. Care has to be taken to ensure that interior air cannot readily move past the vapor retarder and up into the roof assembly or a parapet wall. The GAF Guide to Vapor Retarder Design in Low-slope Roof Systems provides system details to help guide good design. Should a vapor retarder be used with a concrete deck? In new construction, it can be difficult to ascertain when concrete decks are sufficiently dry to allow the roof assembly to be installed. If significant levels of moisture are present in the concrete deck after the roof is closed up, then problems can arise. For more on the topic of moisture in concrete roof decks see this article by my colleague James Kirby. Briefly, as advised by industry groups such as the Midwest Roofing Contractors Association (MRCA), the use of a vapor retarder over a concrete deck will limit moisture passing through to the roof assembly. In re-roofing situations over concrete decks, there is usually less concern about moisture being present within the concrete deck, providing that there have been no leaks. However, if the roof was originally installed with minimal insulation, it could be that the concrete contains significant amounts of moisture due to condensation, depending on the local climate. Also, any precipitation during reroofing could allow a concrete deck to absorb quantities of water. If there is any concern about moisture in an existing concrete deck, a vapor retarder should be considered. In new construction, is there a concern about moisture from a concrete floor or foundation? Yes, there can be, depending on location and other factors. In some big box construction, when the building has been closed up quickly after a floor slab was poured, condensation issues have occurred during the first year of occupancy. This is related to high interior humidity as the concrete dries out for months during and after construction. Concrete floors and foundations can take a long time to dry and as a result interior moisture levels can be high enough that condensation has been known to occur in climate zones 3 and 4. Building designers and architects of such buildings often include a vapor retarder in the roof assembly in order to reduce condensation risks during construction, after the building is closed up, and for up to 12 months later. What about where the vapor retarder meets the edge of the roof? Sealing and termination of vapor retarders around the perimeter is difficult. Building designers should recognize that the goal of a vapor retarder is to block the movement of interior humid air up into a roof assembly and to controllably allow for some vapor permeability so that moisture that does enter the roof assembly can migrate down into the building. The GAF Guide to Vapor Retarder Design in Low-slope Roof Systems provides edge termination details to help guide good design. I often see dew point and vapor retarders being discussed together. Why? It's important to make sure that a vapor retarder is installed below where the dew point in the roof assembly is calculated to be. The vapor retarder will then reduce the likelihood of moisture reaching that position and forming condensation during cold periods. Calculation of the dew point takes into account the expected interior humidity levels and the possible exterior temperatures. More information about the calculation can be found here. What is condensation risk and should I always include a vapor retarder? Moisture is well known to lead to problems with respect to the durability of a building enclosure. The risk of condensation within a roof assembly should be assessed by a design professional. The analysis should consider factors including climate and building use. It is important to recognize that there often is not a definitive yes/no answer to the question as to whether a vapor retarder is needed. The colder the climate, the higher the risk of condensation within the roof assembly. So, buildings located in northerly regions will generally have a higher risk of condensation forming in the building enclosure. The higher the anticipated interior moisture load, the higher the risk. Office buildings occupied during daytime only are likely to have a lower risk versus a building that includes a swimming pool. A building closed up during construction while a concrete slab floor is still drying will likely have a higher risk. The roof assembly design is also a factor. High wind events can cause mechanically attached single-ply membranes to billow which causes air to be drawn up into the assembly, which can increase the risk of condensation. To minimize condensation risk, roof designers should first consider adhering the roof membrane and upper layer of insulation, making it harder for interior air and moisture to be drawn up into the assembly. If a cover board is being used, it should also be adhered. Conclusions Vapor retarders can be used to reduce the movement of vapor within a roof assembly. They need to be positioned as low as practical within the assembly and any penetrations should be sealed. Vapor retarders in the roof assembly may be beneficial in buildings with large temperature differences from interior to exterior throughout the year, and occupancies with higher than normal interior moisture levels, either from use or during construction. Important Considerations The purpose of this article is to provide some background information and design considerations for roofing assemblies using vapor retarders. GAF manufactures and sells roof materials but is not responsible for building design and construction. Design responsibility remains with the architect, engineer, roofing contractor, or owner. This information should not be construed as being all-inclusive, nor should it be considered as a substitute for good application practices. Please consult your design professional for more information.

By Authors Thomas J Taylor

June 08, 2021

Don't miss another GAF RoofViews post!

Subscribe now