Using rigid foam to upgrade roof insulation in MN
My 1930’s 2-story Tudor style home in Minneapolis needs a insulation upgrade. Problems include a challenging roof design, with 3 knee-wall attics and multiple sloped ceilings in the upstairs (finished off in the 1960’s). Although areas that are accessible are insulated well, the majority of roof area is not. Over the years I’ve sealed/caulked with abandon, installed new storms over the original double hung windows (which are in overall good condition) and had a blower door test recently done which found the house to be within guidelines for air leakage. My thinking is, I believe the roof deck and to a lesser extent the total building envelope is causing a bad convective loop which is accelerating heat loss, causing ice dams and making the house feel cold. Since it’s time for a new roof, I’m proposing using a appropriate thickness of rigid foam panels (such as ISO) over the entire roof deck couples with 1×3 strapping and covered with 1/2” CDX plywood. I reference Fred Lugano’s article “Upgrading a foam insulated roof” in Fine Homebuilding 115 for technique. Then shingle normally over that. This should help with thermal bridging of the roof deck. Since I would be air-sealing the roof totally, in my climate, do I need to ventilate it further or not? Short of tearing out lots of drywall and re-insulating I think this may offer the best solution to my house’s problem. Any thoughts?