Poor man’s spray foam for cathedral ceiling
I’m building a Tiny House (8’x12′) on top of a snowmobile trailer to be used as both a grandkid playhouse and guesthouse. Walls and rafters of 2x4s @ 16″ oc and a cathedral ceiling of exposed rafters. I want to add a little insulation to the roof by making a sandwich above the rafters by layering 1/4″ plywood, 2x2s centered over rafters, 1.5″ pink foam between 2x2s, OSB sheathing, tar paper, and finally metal roofing.
The walls will be filled with fiberglass and sheathed with exterior plywood on outside and plywood panelling on inside. For the roof insulation I would undercut the foam sheets and seal tightly with cans of spray foam. Inside will be a small kitchen with fridge, cook top, and sink. It will have an electric heater and small air conditioner, however it will not be used once the real cold winter arrives.
I’m also making the floor as a 2×4 framed sandwhich with plywood both top and bottom. I’d like to insulate that as well if possible. Best way for that?
I’m mostly worried about the roof assembly creating condensation caused rot. I realize I don’t have thermal break through the wood structure. Comments?
Replies
continuous...
I think XPS is a good choice. Have you thought atbout foam insulation made with borates? Might help with insects.
Borates fo no borates, why not go with continuous foam?
Place your rafters, top them with 1/2" ply instead of 1/4" ply to act as your structural sheathing AND as your finished ceiling surface when viewed from inside. Then full sheets of foam, continuous, on top of the plywood. Sure, you can gap the sheets and use canned foam to seal the gaps.
Run horizontal furring strips across the roof (perpendicular to the rafters) on top of the foam, driving your fasteners (screws) through the foam and ply and into the rafters.
Run your metal roof, installing it to the furring strips.
Note that there are many different pre-designed schemes for "hot roofs" over cathedral ceilings. You'd probably be better picking one vs inventing your own.