How Do Polyiso, XPS, and Mineral Fiber Roof Insulation Compare?

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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
1 10 15 20 15 20 25 25
2 15 20 25 20 20 25 25
3 15 20 25 20 20 25 25
4 15 20 25 20 25 30 30
5 15 20 25 20 25 30 30
6 15 20 30 20 30 30 30
7 15 20 35 20 35 35 35
8 15 20 35 20 35 35 35

There are various types of insulation boards that are used with singleply 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)
17.4 (3-inch)
23.6 (4-inch)
10.0 (2-inch)
15.0 (3-inch)
20.0 (4-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)
>7 psi
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)
Not specified
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
Moisture Control Good Good Poor
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.



There are 7 comments

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  1. John Clinton

    I am not sure I understand this statement: “Polyiso and XPS can both be regarded as vapor retarders.” While the foams may have low moisture vapor permeabilty it is difficult to see how they would perform as vapor retarders in the absence of special sealing treatment at the joints between boards and at the perimeter of the roof.

    • Thomas J Taylor, PhD

      John – thanks for the question. The comparison is really to mineral fiber boards, which when used without precautions can allow significant amounts of air to diffuse up through a roof system to the membrane. The foam boards, on their own are relatively impermeable. I’m not implying that as a system they constitute vapor barriers – yes, they would need to be taped etc. However, by using double layers of foam insulation boards with the joints staggered, moisture migration to the membrane can be significantly reduced compared to say mineral fiber boards. As always, if moisture is a concern then a design professional should be involved to make sure that the entire system including perimeter termination etc is specified correctly.

  2. Michael Gallagher

    I’m looking to replace a built-up flat roof that is end-of-life. This for a Miami-area screened-in room attached to a residence. This living area really heats up when the sun hits the existing black roof, so I’m going to install a white TPO cool roof, hoping to make it more livible.

    Now it has crossed my mind that I could make it even more comfortable by adding a layer of iso insulation. The TPO and the iso would sit on an existing cedar plank roof deck.

    What would the iso layer add to the overall goal of improved comfort level? Energy saving is not relevant here since the room is screened, I’m just looking to reduce radiated heat from the roof.

    … and I’ve found a residential roofer ready to do either option.

    • Liza Barth

      Hi Michael,
      The polyiso foam would provide insulation and will further increase your interior comfort level beyond what you’ll get with simply using TPO. Since this is a porch, I would suggest that 1.5 inch thick board would suffice. I can’t advise on the overall installation — you will need to rely on your GAF Factory-Certified Master Elite® Roofing Contractor for that. Thanks for checking in with us.

  3. Shane

    I have a flat roof in SW florida that needs to be re-done. I have an issue with heat in my house because of the flat roof. I am looking at doing a pitch on the flat roof and adding 2″ ISO under my tapered foam with TPO. Would this help reduce the issue with heat in my house during the summer and would bumping it to 4″ be feasible, worth the expense and make a dramatic difference?

    • Thomas J Taylor, PhD

      Adding some pitch is always a good idea – it helps with drainage and prolongs the life of the membrane. Adding 2 inches of polyiso will increase the R-value by about 11 or so. I can’t comment on the feasibility of adding 4 inches of polyiso because it depends on the edges and penetration heights (if any) – best to consult with your local contractor about that. However, adding 4 inches would likely improve the comfort inside your home. If you and the contractor go that route, make sure to use two layers of polyiso, say two 2 inch layers or maybe two 1.5 inch layers, with staggered joints. Good luck with your project.


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