Home buyer with cathedral roof problem in Michigan.
I recently entered a home purchase in SE Michigan and now find myself with a cathedral roof problem. There are cathedral ceilings throughout the upper level of the 2 story home; built in 1979. The main floor living room area is open all the way cathedral roof, and also to the basement with an open stairwell (serious stack effect going on). There is evidence of condensation on the walls coming from the cathedral wood panel ceiling which is attached to the underside of the rafters. The seller reported that they had the “roof done about 8 yrs ago” and then added the current ridge vent a few years after (which is only ventilating about half of the rafter bays). The asphalt shingles are shot and at least 100 SF of rotted sheeting is present.
From opening up a soffit vent during the home inspection, it appears that the roof is constructed with 2×6 rafters (measured exactly 5” at soffit) running from the fascia to a center diagonal beam, like fletching on an arrow. It appears to either have blown in fiberglass or R11 batts in the rafter bays. Owens Corning R11 appears to be present in exterior walls behind the T1-11 siding.
From the research I’ve done so far, it sounds like the best approach to upgrading this roof may be to go over the current roof deck with multiple staggered layers of foil faced polyisocyanurate rigid foam since 4” of available space in the rafter bays will not allow for adequate insulation.
I plan on removing the current sheeting to check the rafter bays for moisture damage. After that I’m a bit lost to what should be done. Would there be benefit to adding insulation to the rafter bays since that would essentially become part of the interior when adding rigid foam above? Would staggered rigid foam atop the sheeting act as an effective air barrier once the soffits are closed off with foam above the exterior walls? Or perhaps it would be best to completely fill the rafter bays with 5” of open cell spray foam, trim flat so that it would be up to the sheeting, then add the amount of rigid foam atop the sheeting needed to get at least a combined total of R38 for the entire assembly?
Thank you so much for assistance, this is a bit overwhelming since we are scheduled to gain possession of the home Nov 16th and should probably have the roof replaced before the snow comes in mid Dec. Having a degree in biochemistry, I also wondering about the potential release of chemicals into the interior from the various insulating products?
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