RoofViews

Building Science

Polyiso Insulation Explained

By Thomas J Taylor

October 07, 2020

Commercial roofers installing polyiso insulation

Thermal insulation is an important part of commercial roofing assemblies. The aim of this article is to examine the factors influencing the thermal resistance, known as R-value of polyiso. The prediction of long term R-value and the influence of climate, i.e. temperature, have been of significant interest over the past few decades as building energy budgets have increased in importance. Recent discussions as to what R-value the designer should use and the importance of ambient temperature are reviewed and discussed.

  • Polyiso foam is the most common type of insulation today, due to its high R-value per inch, fire resistant properties, and solvent resistance.
  • Polyiso blocks air flow, lowering condensation risks compared to air permeable insulations.
  • Polyiso's greatest value is seen during summer months when air conditioning usage is at its highest. The cost of electricity is four times as expensive as gas on an equivalent Btu basis. This makes the impact of air conditioning greater than winter heating.

Introduction

Preventing water intrusion into the built environment during precipitation has always been regarded as the basic function of the roof. However, reducing heat flow through the building enclosure is a very important secondary function. Maintaining interior thermal comfort has always been an important part of residential construction, but it wasn't until the early 1970s that the use of thermally insulated steel roof decks became commonplace in commercial construction due to the need to lower building energy costs.

Polyisocyanurate thermal insulation, commonly referred to as polyiso, has proven to be popular due to a combination of its cost effectiveness (i.e. cost per insulation unit), efficiency (i.e. insulation value per unit thickness), and fire resistance, as compared to some competitive materials. Polyiso has recently come to represent over 75% of the commercial roof insulation market. However, as polyiso has grown in popularity, so has the interest in understanding a more exacting insulation value of this material. With rising energy costs it is more important to accurately specify heating, ventilation, and air conditioning equipment (HVAC). In addition, once a building is completed, it is important that the owner/occupier be able to better anticipate future energy costs from a budgetary perspective.

How Polyiso Is Made

With any foam insulation material, the process begins with the plastic or polymer precursor materials being in the liquid phase. Gaseous blowing agent(s) is introduced either by some form of injection into the process or through chemical reactions that create the polymer matrix. Initially, the blowing agent(s) is present as an extremely fine dispersion. In the case of polyiso, pentane is used as the blowing agent and during the subsequent development of the matrix, heat is released. The heat causes the dispersed pentane to expand, forming gaseous cells. Growth of these cells ultimately results in cell impingement, the entire process being indicated schematically below:

When the cells impinge, surface tension tends to cause the material between two cells to thin, and material between multiple cells to thicken. This results in so called cell windows and struts, as indicated here:

The characteristics of the windows and struts, such as thickness, size, and number, influence the overall thermal resistance of the foam along with the blowing gas composition as discussed later.

Key characteristics of polyiso are;

  • The cells are 99% closed. This means that moisture doesn't condense within polyiso and that it limits the diffusion of moisture carrying air up through the roof assembly.
  • The cell material, i.e. the polymer, represents less than 5% of the total foam volume. When it's said that shipping polyiso is like shipping air, it's because 95% of the weight is the gas within the cells.
  • The material is a thermoset – in a fire situation it won't melt and then drip down through openings in the roof deck. Also, it is not affected by solvents unlike some other foams such as polystyrene.

How Polyiso Insulates

There are three ways in which heat can travel through a foam material, these being conduction, convection, and radiation, as shown schematically below;

Conduction – closed cell foams, such as polyiso, consist of the polymer cells and the gas in those cells.

  • The cell material, i.e. the polymer, represents less than 5% of the total foam volume and therefore, the thermal conduction of that material accounts for a very minor fraction of the total heat transfer. Furthermore, the path along the polymer from the hot side to the cold side is convoluted. Manufacturers strive for low foam density and polymer conduction can be generally considered to be negligible.
  • The gaseous mixture within the cells represents more than 95% of the total foam volume. That gas phase accounts for essentially all of the thermal conduction through polyiso. The blowing agent used to create the foam will have a certain conductivity, however over time that blowing agent may diffuse out of the foam and air could diffuse in. The diffusion of gas into and out of polyiso is slower than for other polymer foams, such as those based on polystyrene.

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, from a hot surface towards a colder surface. In foams such as polyiso, the cells are too small for any convection to occur. Also, the temperature difference across each individual cell is too small to cause convection.

Radiation – thermal energy radiates from hot surfaces and is absorbed by materials depending on their opacity and thickness. Polyiso doesn't totally block thermal radiation; cell walls are considered to be too thin to absorb thermal radiation, however cell struts are thought to absorb and then re-radiate thermal energy. Manufacturers aim to make small cells, i.e. more cells per unit volume, to be more effective at blocking thermal radiation.

Insulation's Benefit for a Roof

Insulation has two main effects on heat flux into and out of a building.

  • During the summer, flux of the heat from the sun down into a building is reduced. Similarly, reflective membranes also reduce the amount of heat but do so by reflection. During the winter, insulation reduces the heat flux out from the building, thereby lowering heating costs.
  • Insulation delays the flux of heat into or out of a building. Polyiso has "thermal inertia" unlike, for example, reflective membranes which do not have this effect.

    • For buildings such as offices, this delaying effect is a very important benefit of insulation. It means that the maximum air conditioning effort due to heat flux through the roof is required for less hours of the day.

Specification of Thermal Resistance

Polyiso is manufactured to meet the ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. C1289 specifies the thermal resistance at a mean temperature of 75°F for various product thicknesses and requires that the values at 40 and 110°F be made available upon request.

Important features of manufacturer's published R-values are;

  • The values are an average across a temperature range. The test methods need the insulation specimen to have a hot and a cold side which are required to be at least 40°F apart. Most reputable test labs use a difference of 50°F for accuracy. So, a published value at 75°F is actually an average R-value across the range of 50 to 100°F.
  • To take into account the diffusion of gas out of, and air into, foam, the values are based on projected "Long Term Thermal Resistance" or LTTR, obtained by rapidly aging thin slices of the foam.
  • Beginning in 2003, the Polyisocyanurate Insulation Manufacturers Association, PIMA, established a third party certification program to independently validate LTTR values. This is referred to as the PIMA QualityMark™ program. The LTTR values are considered as "labelled R-values" to be used by building design professionals.
  • Label R-values represent a 15-year time-weighted average value. They can help a design professional estimate a building's energy efficiency without having to be concerned about long term loss of performance, this being factored into the value.

Validation of Label R-values

The PIMA QualityMark™ program requires participating manufacturers to meet the following R-values for the various product thicknesses.

Minimum LTTR R-Values Established by the PIMA QualityMark™ Program

The PIMA QualityMark™ program requires each manufacturing facility to submit to an annual verification of LTTR values. During verification, independent third-party representatives visit each facility and select a minimum of five boards for testing. The overall process is administered by FM Global. The results of a 2015 PIMA QualityMark verification testing are summarized below:

Results of a 2015 PIMA QualityMark verification testing

A total of 33 samples were tested for each thickness at a mean temperature of 75°F. These values, obtained from a third party independent process, are reassuring especially given the large number of samples involved (33 samples x 5 specimens = 165 tests). The PIMA Quality Mark program exists to ensure that member manufacturers are held accountable to produce product meeting published label values.

R-Value and Temperature

As noted earlier, the ASTM polyiso specification requires that the value at 75°F must be published and that values at 40 and 110°F be available on request. The mean reference temperatures for the ASHRAE Climate Zones for winter and summer conditions, assuming an indoor design temperature of 68°F are shown here:

ASHRAE Climate Zones Temperature

  • From the data, it's clear that the R-value reported for a mean temperature of 75°F is appropriate for summer, and in many cases, for winter.
  • Building design professionals designing roofs for ASHRAE climate zones 6 and 7 may need to use the R-value reported for a mean temperature of 40°F when evaluating winter heating requirements.

Also, electricity costs are about four times the cost of natural gas on a British Thermal Unit of energy equivalent basis. When specifying insulation, if gas heat is being used then summer air conditioning costs could dominate and the R-value at the 75°F mean temperature could be the most important.

The "R-Value Rule"

While knowledge of a material's R-value is important to the commercial building market for the HVAC specifier and building owners/occupiers, home owners and individual consumers are generally unable to verify claims as to the thermal resistance. In the aftermath of the 1970's energy crisis, fraudulent R-value claims became so widespread the US Congress passed a consumer-protection law in response, the "R-Value Rule". The R-Value Rule "requires home insulation manufacturers, professional installers, new home sellers, and retailers to provide R-value information, based on the results of standard tests."

Polyiso is used as continuous insulation in residential wall systems and roof assemblies in many apartment and high rise condominium buildings. It is unfeasible for polyiso manufacturers to differentiate between products going into residential versus commercial applications. Therefore, in practice the R-Value Rule covers all polyiso meaning that labeled R-values are legally enforceable.

Conclusions

  1. Thermal conductivity of polyiso, in common with most other foams, is dominated by the thermal conduction of the cell gases.
  2. Contrary to popular understanding, R-values are reported as an average across a temperature range and do not represent a value at an exact temperature. For example, the reported R-value at 75°F is normally measured across a range from 50 to 100°F and should be noted as a mean R-value.
  3. Building designers and specifiers are advised to use the labelled R-values shown for a mean temperature of 75°F. For projects in ASHRAE climate zones 6 and 7, values at 40F °could be used depending on the building's geometry and local energy costs.

Want to learn more? See my recent article in Interface.

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.

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The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these: Roof fasteners have a measurable impact on the R-value of roof insulation. High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners. Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches: Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes. Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study. Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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