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Community and Q&A

Rigid vs. spray foam for the roof

MarkusT | Posted in Green Building Techniques on

I am finalizing the construction spec on our planned home in Houston (intended to be a ‘pretty good house’) and although this has been talked about here in many forms it’s rarely linked to actual cost numbers.

I originally looked at using 2″ recycled rigid foam above the roof and blown-in insulation below it to achieve the required code values and create an enclosed attic with an unvented roof (as per GBA Cathedral Ceiling Article). The tales of spray foam gone wrong were a strong push in this direction.

However after costing, my contractor has come back and said that there’s a lot of extra work and material in the rigid foam solution (with sheathing both above and below it) and that a closed cell spray foam approach would be approx. $1000 less overall and significantly simpler for him in terms of execution as well as air sealing benefits etc.
The spray foam contractor would be a well established one with a long track record of doing custom and production homes in the Houston area who our contractor has worked with before.

If it were your house and your money, what approach would you take to give you peace of mind in terms of long term performance and reliability (a LOT of roofs in Houston leaked during Harvey…).

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    User-68etc.,
    First of all, can you tell us your name?

    When you are building a house, a $1,000 upcharge isn't much -- but I understand that some budgets are tight.

    Clearly, the option with exterior rigid foam is more effective at addressing thermal bridging through the rafters.

    It would be interesting to know if the R-values of the two options were identical. (In other words, is this an apples-to-apples comparison, or does one option have a higher R-value?)

    It would also be interesting to know if your contractor will use closed-cell spray foam or open-cell spray foam. (I would never use open-cell spray foam on the underside of roof sheathing unless the roof assembly also had rigid foam above the roof sheathing. If the insulation is all being installed on the interior, use closed-cell spray foam.)

    For more information, see these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

  2. brandons | | #2

    I'm a homeowner not a contractor but I'm perplexed at some of your numbers. What is your roof R requirement? Around here recycled polyiso is $16/sheet for 2" thickness . Closed cell spray foam for that same R value would cost twice the price per square ft. Unless cost was substantial (IMHO $1,000 is not) I would stick with the polyiso and even try to do all of the roof in polyiso. I'd imagine the extra polyiso material and screw length would not cost more than the blown in material (assuming you don't need more than 4" above the deck). Also the total R requirement is slightly lower if all of the insulation is above the deck.

    As Martin mentioned above the polyiso roof will perform better. Perhaps your contractor should contact a commercial roofer. Someone with low slope roof experience should have dealt with your situation several times. OR you could shop nailbase panels for the roof and see if he reduces his labor.

    Good luck!
    Brandon

  3. MarkusT | | #3

    Ok, profile should now be fixed to show my name. So Martin, spray foam would be closed cell, based off previous articles here.
    Assume for the purpose of this comparison that R value is roughly equivalent (unlikely to differ by more than 5). So your really comparing air sealing vs thermal bridging qualities and practical issues like observing and repairing leaks in the long term.

    Brandon - with regard to cost, the difference is in the additional OSB layer needed on top of the foam and the additional labor to install it which is substantial. It's an interesting idea to do only rigid on top but I think we would be at 6" to get a minimum of R-30. It's also a fair bit of extra labor doing 3 layers of foam and labor is at a premium here in Houston after Harvey. I can ask how this option compares since it removes need for blown insulation. However I'm reminded of this quote from another GBA article:
    'When Alex Cheimets performed at deep-energy retrofit at his house in Arlington, Massachusetts, he specified 6 inches of rigid foam should be installed above the roof sheathing. As GBA reported, “A box of broken 10-inch screws, a new set of impact drivers, and a week’s worth of frustration later, [the contractors] wondered if it was worth it. In the end, the roof worked out well, but [some of the team members felt that] the extra effort and cost were hard to justify.”

    Any other issues you might see with only external insulation on the roof? There will also be solar panels on this roof.

    I understand the point about $1000 but the reality is there are a dozen of these decisions on a house project and just a little bit more quickly becomes a $10,000 impact on the house so i have to be rigorous about each design decision. However it's also cheap compared to chemical contamination...

    Thanks!

  4. MarkusT | | #4

    Perhaps not - it's Markus.

  5. GBA Editor
    Martin Holladay | | #5

    Markus,
    The other thing you want to check is whether your spray foam installer includes a thermal barrier or ignition barrier on the interior side of the spray foam. In some jurisdictions, local code authorities aren't enforcing the requirements for thermal barriers and ignition barriers -- but homeowners should remember that exposed spray foam is a fire hazard.

    For more information, see this article: Thermal Barriers and Ignition Barriers for Spray Foam.

  6. MarkusT | | #6

    Thanks Martin - looking at your article it looks like blown insulation under the foam would fulfill the role of ignition barrier. This would be the proposed approach (3-4 inches of closed cell and another 4 inches of blown insulation or batts)

    Markus

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