Best Spray Foam Insulation for 100 Year Old Home – GreenBuildingAdvisor

We will be finishing the attic in our 1930 house in Atlanta, GA this year (Climate Zone 3A, winters include multiple days in the 20s and summers include multiple days in the 90s). Currently the attic is a nonvented and partially conditioned space: a two year old heat pump dedicated to the attic supplies HVAC to the space and will remain in place after the renovation. Currently the attic has insufficient foam batt insulation above the 2nd floor ceilings and a combination of batt and rigid foam along underside of roof decking in the finished portion, with R value between 5-10 in most places. The ductwork is for both the second floor furnace unit and attic heat pump is all outside of the insulation envelope.

The roof consists of older clay tiles but with ice and water shield installed (i.e. a “lift and relay” was done at some point in the past). Roofers have inspected and confirmed the roof has lots of life left despite the many of the tile appearing to be original to the house. 

After the attic is finished it will remain nonvented and conditioned, and the roof assembly will remain the same. There is not space to add venting so this will be a “hot roof” assembly. I want to greatly improve the insulation as part of the project and would like to air seal it.

Should I use open cell foam, or closed cell foam? I have read and been told by various foam contractors that closed cell carries more risk from a roof leak but open cell carries more risk of moisture passing from living space into sheathing. I am especially concerned about not doing anything that would require reworking the roof system later given the expense and complexity of the clay tile roof system. 

Please help me decide the best way to insulate.