Is my plan to insulate scissor trusses correct?
I’ve made the mistake of not planning my insulation details before building, but because of this GBA (and specifically Martin Holladay) I’ll be much more prepared next time. In the mean time, here’s where I’m at.
I’ve come up with, what I believe to be, an adequate plan to insulate my addition. I’m open to hearing suggestions, pitfalls or critiques. Thanks in advance.
I’m currently building a 30’x33’ addition on my house in Maine (Zone 6a). We used scissor trusses with a 14” energy heel. The top chord is 8/12 and the bottom chord is 4/12. The trusses are spaced 24”oc, and it will be vented from soffit to ridge vent. We are planning on using 1” polyiso to build the rafter vents with a 1 1/2” space for the air flow. I’m planning on using 24” of blown cellulose. The cellulose will be sitting on the air barrier which will be either 1/2” osb taped with 3m all weather flashing, 1” xps foam (taped) or, my least favorite option, 5/8” gypsum taped and mudded. Next, I plan on installing flat 2×4’s to create a small service cavity. I like the idea of the service cavity for the ease of installing our lights and running electrical without having to seal around them since the layer above is providing the air seal. The lights I’m going to use are recessed, but only require 2” of installation depth instead of the usual 7 1/2”+ recessed can lights require. Finally we plan to install 3/4” tongue and groove as the exposed layer.
I’m doing all the work myself. My issue with 5/8” gypsum is that it’s heavy, more expensive than the osb option, and provides next to nothing for insulation value. I’m leaning towards the osb option, but I’m open towards other options with a reasonable explanation.