Baffles and Spray Foam for Low-Slope Vented Roof
I’ve been reading the attic sealing/insulation articles on GBA, and the many posts/questions that deal with this topic, for years in the hopes of finding the optimal solution for my attic, but I haven’t been able to reach a conclusion yet. Here’s my situation.
Split level home with upper and lower attics, both hip roofs, 4/12, no HVAC in attic. Zone 5, north Chicago suburbs, house at present is poorly insulated and sealed and is prone to ice damming. Because of the roof pitch, the vertical gap between the outer wall and the sheathing is 4 inches.
I’m disinclined to go with an unvented assembly, and rigid foam above the sheathing is prohibitive, especially given that my roof is only a few years old. On the other hand, it is impossible to adequately seal and insulate the outer perimeter because of space constraints and accessibility issues. So, is it possible to do a hybrid assembly as follows?
In all rafter bays install full span baffles with 1.5-2″ air channel and blocking as you normally would to prevent wind washing of insulation (though in this case they would serve as a barrier for foam). Rather than use cellulose at the perimeter, use spray foam: 1-2 feet around the perimeter would be foamed from the attic floor to the baffles/blocking. The remainder of the attic would be treated as a standard vented assembly: all penetrations in the attic floor would be sealed, and cellulose installed to code. There would be roof venting and continuous soffit venting to provide for a slightly pressurized attic.
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