Here is your chance!
Thanks to Bo Jespersen, President of The Breathable Home in Manchester, Maine for contributing February's home performance head scratcher.
Bo was stumped after his company was hired to install some spray foam insulation in a second story bedroom side attic.
When Bo arrived, he noticed that the work was exclusively on the north slope of the home, and that the owner had installed three layers of 2" thick rigid foam in-between the rafters, and applied one-part spray foam along the edges (where some stuffed fiberglass was found as well). A small air channel was left between the top of the foam and the bottom of the roof sheathing. The ridge beam was exposed and required ending the rigid foam to the southern slope to make it air tight.
The solution to this problem is to prevent warm moist air from contacting a condensing surface. This is difficult, because it is nearly impossible to create a perfect air barrier – especially one that maintains a seal over time in a climate with large temperature swings and between materials that have different coefficients of thermal expansion.
One solution would be to pull up the roofing material (singles, slate, whatever), and add rigid insulation underneath, overlapped (2-3in), then add lathe and sheathing (the space between the lathe creates an air channel) and then add the roofing material. The insulation will need to be thick enough so that the air under the sheathing won't reach the dew point even during the coldest nights. This option is expensive.
A cheaper option would be to remove the rigid foam, spray the underside of the roof deck with foam, and then place the rigid board back in while the foam is still nice and gooey, essentially as a continuation of the foam. That will get rid of the air space where condensation can occur. It would be nice if the foams had the same thermal expansion coefficients, and there weren’t air bubbles at the interface, but that is probably inevitable. The risk here is that there isn’t enough vapor permeability through the foam and that any water that gets under the roof deck from roof leaks will stay there. This is a risk with all under roof deck foam applications, and demands a good job maintaining the roof once the under-deck insulation is installed.
To just solve the water issue, a ridge vent could be installed and the soffit area could be re-done to create an air channel under the roof deck, but this would mean continued air leakage. Far from ideal.