Roofing Fasteners and Condensation
I am getting ready to order roofing materials for a low-slope and a gable roof. The low-slope roof will have tapered Polyiso, cover board and then TPO. The gable roof will have a top-vented Polyiso nail base panel. In each case, my exterior de-rated Polyiso insulation r-value to total r-value will be greater than the minimum required for my climate zone (4). The insulation on the interior side of the roof deck will be dense-packed cellulose. I will be using #14 roofing fasteners on the low-slope and SIPTP fasteners on the gable roof. Based on the nailing pattern required by the cover board and nail base manufacturers, most of the fasteners will miss the joists and will be penetrating through the deck at least 1″. Should I be concerned about the potential for these cold fasteners to cause condensation to form where they penetrate the underside of the roof sheathing (Advantech on low-slope and Zip on gable roof)?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
The tips of the fasteners will be in the cellulose?
That's not a big deal. Any condensation that forms at the fastener tips gets wicked-up and redistributed by the cellulose. In a zone 4 climate the total amount of moisture accumulation should be WELL within the safe buffering capacity of the cellulose. There isn't any convective air transfer to speak of from the conditioned space through dense packed cellulose most of the moisture transport is diffusion, and since you have some margin on the R-ratio it won't need to buffer very much at all.
Thanks Dana. Yes, the fasteners tips will be in the cellulose. Also the rooms below the gable roof and low-slope roof will be conditioned space and moisture diffusion from the interior into the rafters should be minimized as I will be using a vapor control membrane (Majrex) in the ceiling. Finally, no Aprilaire humidifiers in this house :)