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Open cell foam on roof deck

TNCave | Posted in General Questions on

Hello all,

We’re in the thick of our home construction (zone 4/Tennessee) and I met with the spray foam installer today.  We’re using open cell foam on the roof deck and shooting for minimum R38.  The installer uses Carlisle open cell foam ([email protected]″). 

The installer typically applies 5″ on roof decks in our area.  We’re shooting for a PGH and using the energy star regional guidelines as our performance minimum measures.  Energy Star requires R38 in our region.  We don’t have building codes here..at all.  The foam installer stated that foam installation thickness R-value measurement is doubled for code compliance (Rvalue/in x Thickness x 2).  For example he sprays just over 5″ to meet a nearby local code of R38 (3.7 x 5.1 x 2).

I can do some math, and it seems likely that if we want an actual R38, we will need 10.27″ of open cell foam @R3.7/in.  The installer insists that this is a complete waste of money and he has never installed anywhere near that much foam in a house.  I should mention that spray foam installers are pretty rare in this area and this installer is the #1 recommended by everyone we’ve talked to.

Is this doubled R-value measurement a real thing?  Or do I need to push for the thicker (two pass) installation?

Any thoughts or advice will be greatly appreciated.

Cory

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Replies

  1. user-2310254 | | #1

    Cory,

    Foam contractors always seem to use this pitch, and you are wise not to believe it. I suspect the 5" to 6" value is mostly about market competition and what they can safely spray during one visit. This article (https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing) points out that open cell against roof sheathing can be risky. A flash and batt roof (https://www.greenbuildingadvisor.com/article/flash-and-batt-insulation) would be safer, but you have other options, including a ventilated attic with lots of blown-in insulation (as long as the ducts are located elsewhere).

    I would seek out a contractor who has some experience with energy-efficient construction and see what approach they are comfortable with.

    1. TNCave | | #2

      Thanks Steve,

      Unfortunately we seem to be in a “desert” for energy efficient and green building contractors. The rural TN lack of codes has established large geographies with no market value or support of these practices. I was really surprised how disincentivized we are to build a low quality house with nicer exterior finishes. That’s what the market, and banks, value here.

      All that is to say that I will continue looking for another insulation contractor, but there seem to be very few. We couldn’t find anyone in the area who does high density cellulose and we’re now considering foaming the walls as well.

      Thanks again,

      Cory

  2. user-2310254 | | #3

    Hi Cory,

    How far along are you in the construction process? If it's not too late, I would go with raised heel trussed in the attic and just blow in a ton of cellulose. I'd also go with cellulose in the walls. I've read several articles/threads on GBA that indicate you maximize your efficiency gains by air sealing and putting lots of insulation in the attic.

    If you still want to boost wall performance, consider adding some exterior insulation. EPS or reclaimed would be best.

    1. TNCave | | #4

      Thanks Steve.

      We’re already framed up and in the dry. I recognize that spray foam is not the greenest product and that there are better options. We’ve had to modify our plans in several areas based on limited local knowledge and contractor availability. It’s a definite drawback to living and building in a very rural area without codes. For example we paid quite a bit for energy modeling and an HVAC plan from a third party engineering firm only to discover that no one local would or could build the system as prescribed. It seems that so many practices are dependent on good installations that I’m bending toward what local contractors are comfortable with and familiar with rather than trying to force “new” practices and materials that may be installed sub-standard.

      Regarding exterior foam, I’m completely against the concept in areas prone to high winds. In my region of TN we had two separate wind events this year that damaged buildings and I was amazed at how much rigid foam was carried off and scattered around the region. It was hard to believe. Much of that is still scattered through the woods and certainly a good amount will be carried off into waterways. It really turned me away from exterior foam in our area. EPS especially seemed to break into smaller bits compared to polyiso. Polyiso in large chunks was scattered miles and miles beyond the direct impact areas.

      Anyway I really appreciate the input. We’re doing the best we can out here and GBA has been an amazing resource. I did find one more insulation contractor to try and I’ll be calling them today.

      Thanks again,

      Cory

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