How Do Polyiso, XPS, and Mineral Fiber Roof Insulation Compare?
The recommended insulation R-values for commercial roofing continue to increase and save energy costs. States adopt these recommendations at different speeds, but the bottom line is that insulation is becoming more important than ever.
The following table shows how ASHRAE (American Society for Heating, Refrigerating, and Air Conditioning Engineers) and the International Energy Conservation Code (IECC) have promoted greater insulation over the past decade. For example, back in 2004 in zone 5, the equivalent of 2.6 inches of polyiso insulation was recommended compared to 6.0 inches of polyiso in 2015.
Minimum Above Deck Roof R-Values
|Climate Zone||ASHRAE 90.1-2004/IECC 2006||ASHRAE 90.1-2007/IECC 2009||ASHRAE 90.1-2009||ASHRAE 90.1-2010||IECC 2012||ASHRAE 90.1-2013||IECC 2015|
There are various types of insulation boards that are used with single–ply and asphaltic membranes.
The choice is generally between polyisocyanurate (polyiso), extruded polystyrene (XPS), or mineral fiber board. So, which is best for your project? A good place to start with any comparison is with the ASTM specifications, with some of the key values shown here:
|Property||Polyiso – ASTM C1289||XPS – ASTM D578||Mineral Fiber Board – ASTM C726|
|R-value (board thickness), minimum||11.4 (2-inch)
|Not specified; provided by individual manufacturers|
|Compressive Strength, psi||>16 psi (Grade 1)
>20 psi (Grade 2)
>25 psi (Grade 3)
|>15 psi (Type X)
>25 psi (Type IV)
|Water Vapor Permeance||<1.5 Perms||<1.5 Perms||Not specified; provided by individual manufacturers|
|Flexural Strength, psi||>40||>40 (Type X)
>50 (Type IV)
|Tensile Strength, psf||500||Not specified||450|
|Water Absorption, max %||1.5||0.3||5|
Partly due to the popularity of polyiso, its published R-values have been subjected to a fair amount of scrutiny. As a result, the values shown in the table represent long-term thermal resistance (LTTR) measurements and are equivalent to a time-weighted thermal design R-value for 15 years. The manufacturers have created an independent testing and verification program to certify that these values are met.
For mineral fiber board, R-values vary between supplier, but are around R 4.0/inch, significantly less than for polyiso. Extruded polystyrene (XPS) is somewhat lower than polyiso.
There are a number of factors that are very important to the installer and building owner, but are not part of the specifications. Some of the key attributes are shown here:
|Property||Polyiso||XPS||Mineral Fiber Board|
|Fire Performance||Very Good||Melts, contains brominated fire retardants||Excellent|
|Solvent Resistance||Very Good||Poor||Very Good|
|Ease of Cutting||Very Good||Very Good||Difficult|
|Weight & Ease of Handling||Very Good||Very Good||Poor|
Over the past several years or more, there’s been a lot of debate about moisture condensation under single-ply membranes. Polyiso and XPS can both be regarded as vapor retarders. However, mineral fiber is very porous and care needs to be taken to avoid having condensation issues. It’s recommended that a system consultant and/or engineer be involved to make sure that moisture vapor migration is properly controlled if mineral fiber insulation is used.
Finally, it’s worth noting the upper temperature limits for some of these products. Mineral fiber board obviously has very high upper use-temperature limits, but polyiso will also remain very stable at any expected rooftop temperatures. However, as noted in the polystyrene ASTM C578 specification, XPS is not intended to be used for sustained end-use temperatures above 165°F. As GAF has demonstrated, membrane temperatures for darker roofs can easily reach 190°F and beyond! Those temperatures could degrade any XPS being used under darker membranes or in close proximity.
Whatever your preferred choice of insulation board, it’s clear that the amount of insulation required by building codes is going to keep increasing. On balance, after reviewing the specifications and pros and cons of each material, polyiso foam is an excellent option.
Learn more about GAF’s EnergyGuard™ Polyiso Roof Insulation.