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Community and Q&A

Polyiso in bonus room

Bonusroom1 | Posted in Green Building Techniques on

I’m starting the process of finishing my upstairs I’m located in piedmont of north Carolina. My house is 5 years old I have soffit and Ridge vents 1 dormer simple roof line 2×6 rafters . I found a deal on Craigslist for polyiso the problem is its foIL faced on both sides . My plan was to cut and cobble 3.5″ pieces between rafters flush to interior of rafters leaving a 2 inch gap from soffit to Ridge vent then install 3 ” polyiso on inside of rafters to adress thermal bridging then 1×3 nailers for sheet rock. Is this a sound plan?

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  1. user-2310254 | | #1

    Eric. Are you zone 3 or zone 4? Either way, you are a long way from R-38 (zone 3) or R-49 (zone 4), which is the 2012 code requirements. Do you have the headroom to create a deeper rafter space (with gussets and additional framing)?

  2. Bonusroom1 | | #2

    I'm in zone 3 we call for r38 the polyiso is rated 6.5 per inch.

  3. user-2310254 | | #3

    I didn't catch the second layer of 3-inch foam or the Piedmont local in my initial read. Sorry. Too much multitasking today.

    You probably will be okay with 6.5 inches of total foam in Zone 3. But the real-world r-value of your foam is likely to be closer 5.7 per inch. And the thermal conductivity of the outer layer may increase when the temperature drops below 50 degrees.

    Is the bonus room ceiling connected to your main attic space? If not connected (or if the opening between the two spaces can be sealed), it might be safer to create an unvented cathedral ceiling. (I've read somewhat conflicting info on using foam-faced polyiso for site-built baffles, so I'd like some clarification on this point as well.)

    But lets see what others in the community think.

  4. Bonusroom1 | | #4

    I've considered a unvented setup but have read it can be difficult with cut and cobble. I have also priced icynene open cell but lost trust when they assured me 5.5 inches would be sufficient. Had estimate for hd r38 and hd r15 in knewalls but I didn't want unconditioned attic behind kneewalls and I was having to scab out rafters with 2x6.

  5. GBA Editor
    Martin Holladay | | #5

    Your approach will work. While the cut-and-cobble approach is risky for unvented ceiling assemblies, it can work if you are able to include a vent channel.

    My only question is whether you can really vent all of the rafter bays if your roof includes a dormer. Usually, a dormer interrupts the continuity of some of your rafter bays.

  6. Bonusroom1 | | #6

    It would interrupt it, Is it possible to make the dormer unvented and vent the rest or should I abandon the the the idea and just cram the polyiso directly against roof deck and caulk or foam around edges

  7. GBA Editor
    Martin Holladay | | #7

    Personally, I don't believe in taking chances with cathedral ceiling details, because I've seen too many cathedral ceilings with mold, rot, or ice dam problems.

    If this were my house, I would use closed-cell spray foam (or rigid foam on the exterior) for any rafter bays that didn't have a continuous vent channel from the soffit to the ridge.

    If the rest of your roof has rafter bays that can be vented from the soffit to the ridge, then your original plan will work for those rafter bays.

  8. Bonusroom1 | | #8

    Thanks for the answers I think I will have the insulation company price closed cell for the whole thing because of the work it will save me.

  9. Bonusroom1 | | #9

    Ah gother several companies quotes they all say I don't need closed cell and 5.5 inches opencell will perform as well as r38. I'm back to original plan as long as foIL facings aren't a problem I'll just take my time and log in a lot of labor.I'm just skeptical of all foam companies in my area.Or are they right?

  10. GBA Editor
    Martin Holladay | | #10

    Q. "I got several companies' quote, and they all say I don't need closed-cell, and 5.5 inches of open-cell will perform as well as R-38. Are they right?"

    A. No. They are wrong and wrong.

    For more information on why you want closed-cell foam, not open-cell foam, see this article: High Humidity in Unvented Conditioned Attics.

    For more information on why 5.5 inches of open-cell spray foam (R-20) won't perform as well as R-38, see this article: It’s OK to Skimp On Insulation, Icynene Says.

  11. Bonusroom1 | | #11

    Yeah I've been reading and studying alot I know it's I know its not right they just all have the same story and they can do closed cell but the cost would be astronomical and it's just not necessary in my climate.

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