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Spray foam roof insulation questions

user-659915 | Posted in Energy Efficiency and Durability on

In a recently completed home of our design, the contractor – a firm I’d not worked with before – convinced the client to use spray foam insulation for walls and roof instead of the cellulose I’d planned. Part of his justification for accepting lower-than-code R-value of the roof was that the Delta-T of a sprayed roof deck assembly would be lower than if the insulation were at ceiling level. He noted that the air temperature of a vented attic would be some 30° higher than outdoor air. While this true, I suggested that ambient outdoor air should not be the relevant marker for Delta-T through the spray foam, but rather the surface temperature of the roof sheathing itself. With nothing behind it but the foam this would surely not be any lower than the average air temp of a vented attic: in fact probably much higher. Who’s right?

Second question: the home ended up with a reasonable overall HERS rating of 57 but I still feel that as a result of this sleight-of-hand the roof insulation was not what it could or should be. It occurs to me that the spray foam at rafter level could be supplemented at very low cost by rolling out unfaced R30 fiberglass at ceiling level – a variant of flash-and-batt. This would give a much more comfortable total R-value of 55-60 for the overall roof assembly. There’s no HVAC in the attic, no can lights, no storage platform, a tight ceiling hatch, and as the attic is unvented there’d be no wind-washing of the fiberglass. Any red flags for this assembly?

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  1. GBA Editor
    Martin Holladay | | #1

    It took me a while to puzzle out what your were saying in the first paragraph, but I think I understand now -- the contractor's delta-T argument appears to be based on summer performance, not winter performance -- right?

    Anyway, I don't buy it. Below-code R-value is below code. Although spray foam often performs better than fiberglass batts of the same rated R-value, the solution to this discrepancy is to do a better job installing fiberglass, not to do a worse job installing spray foam (by skimping on R-value).

    If you want to install fiberglass batts on the attic floor, I see no reason not to.

  2. homedesign | | #2

    James, I think what you are asking is interesting.
    I have seen the subject brought up before elsewhere.
    I will look for some links later.
    Just Curious...what is the R-value at the roof deck? there crawlspace ?

  3. user-788447 | | #3

    I'm confused. Like Martin says it seems in the first paragraph you are talking about predominantly cooling climate concerns but in the second paragraph you say "a much more comfortable total R-value of 55-60" which I associate with addressing the greater delta T's in a predominantly heating climate. So which climate zone is this house located?

    Can we measure the benefit of adding batts at the ceiling level when you have foam in the roof rafters? If you have foam at the rafters your attic is within the thermal envelope so now you are insulating between to spaces within the thermal envelope; however the attic is still subjected to larger heating and cooling losses than other conditioned spaces so thermally isolating it from the living spaces below theoretically could have a benefit. Is the actual benefit negligible or worth the cost? I don't know how that could be determined.

  4. user-659915 | | #4

    Location is N. Carolina Piedmont, southern edge of 4A. Yes we get very concerned about cooling loads but we have a 3/4 month heating season too. The R-value at the roof deck is about R28, which of course is below prescriptive code. The question was when running a calc., what would you use as the outside temp. - yes, specifically in the summer.

    JC - I'm not too concerned about calculating the exact benefits from adding the extra insulation as the cost would be so low: I just wanted some input on whether there might be unforeseen consequences. If anyone could identify problems it would probably be Martin, so I'm reassured.

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