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Community and Q&A

Exterior rigid foam and interior mineral wool for hip roof

Phil Ray | Posted in Energy Efficiency and Durability on

I am planning some envelope and HVAC upgrades on a 1958 brick-veneer home in climate zone 5a (Southeast Michigan). Would the GBA community be willing to check/advise my insulation/roof plans?

Right now I’m looking at the 2nd floor which has three bedrooms, a hall, and a bathroom; I have a manual J calculation complete.

My plan is to bring the attic into the conditioned space and insulate to R49. The attic will not be living space or even used for storage. It’s currently unconditioned and contains ducts for air conditioning (heat un the house is via hot water baseboard).

Proposed roof stackup, from the exterior: asphalt shingles, synthetic underlayment, ice and water shield at valleys and eaves, new OSB, 2x4s on the flat to provide vent space, 2 layers of 2″ polyiso with staggered seams, existing plywood roof deck with taped seams, 7.25″ Roxul batts between the 2×6 rafters. OK? Any concerns?

Is it important to fir out the interior side of the rafters to match the Roxul? I assume it could friction fit just fine in the 5.5″ cavities.

BTW, the reason I am not looking at adding ducts within the existing conditioned space is that there just is little space available; ceilings are at 7″6″. I considered ductless heads in each bedroom but they would be oversized (cooling loads calculated at 2,915 BTUh, 1,926, and 1,642 for the three bedrooms).


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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Your planned roof assembly sounds fine to me. For more information, see this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    P.S. I just noticed that you are talking about a hipped roof. That means that ventilation gets tricky; some of your ventilation channels will die into the hips. There are various possible solutions, including using ridge vents on the hips, or leaving the 2x4s a little short so that the ventilation air can move sideways at the top of each ventilation channel. Ultimately, it doesn't really matter much, because this type of roof -- one with exterior rigid foam -- doesn't really need to be vented.

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