Blown-in cellulose insulation directly against the roof decking?
I have a question about blown-in cellulose insulation for an attic roof. Here in freezing snowy upstate NY, fifteen years ago, we had blown-in cellulose insulation added to the walls of our 103 year old house. Recently a free energy audit, which included IR imaging, showed some settling of the insulation which is to be expected but it also showed several empty areas that the original contractor missed, which was not expected. We now plan to have new insulation blown into the settled and missed areas as well as adding insulation for the attic roof. Here’s the question: the contractor’s plan for the attic calls for placing Tuff R-2 foam board against the rafters with cellulose then densely packed between the foam board and the roof decking, i.e., the cellulose would be packed directly against the roof decking. (We then plan on finishing the attic, installing new electric and drywall and turn the attic into a living space.)
I know that placing fiberglass insulation directly against the roof decking is a no-no as fiberglass insulation requires separation with a spacer to allow for proper ventilation to prevent mold growth, ice damming etc. However, the contractor told us that when cellulose and foam board are installed properly no spacer is needed. He claims the dense pack cellulose with the foam board create a proper barrier so the moisture does not condensate at the bottom of the roof decking. He explained that there is a whole building science field now so they have a much greater understanding now how the different materials respond under different circumstances.
Does having cellulose densely packed directly against the roof decking without a spacer for ventilation make sense? how does doing so prevent moisturefrom gathering and causing mold, ice dams, etc.
My thanks in advance for your time.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part