How to insulate cabin underfloor
Hi!
I have a small recreational A-frame cabin in Baring, Washington. This is in climate zone 4, right near the crest of the Cascade Mountains where it changes to zone 5. So I’m on the cold edge of zone 4.
We have moderate summers and cold snowy winters with snow on the ground some winters for 2-3 months. The underside of the cabin is raised by sonotubes about 4 feet off the ground, and we have large roof overhangs, so snow doesn’t get under the cabin, and air can flow freely under the cabin.
Our heat sources are wood stove and an electric cadet wall heater. No air conditioning.
We only heat the cabin when we are there, which is approximately 2 weekends per month. So during the winter when we aren’t heating the cabin, the temperature inside the cabin sometimes gets down into the teens.
Dog water bowls do occasionally get spilled on the hardwoods and mud and snow does occasionally get tracked in.
My question is about how to insulate the underside of the cabin so we don’t have such cold feet. On top of the 2×10 floor joists is ¾” plywood (not T&G) subfloor, then paper underlayment, then ¾” hardwood floors. My goal is to get a lot of insulation without causing moisture to condense on either the joists or the underside of the plywood subfloor.
1 – It would be very easy to install 2” rigid insulation to the bottom of the floor joists, making it so the joists don’t thermal bridge at all.
2 – But fiberglass insulation between the joists would add a lot more insulation value. I’m just worried that without a good vapor barrier below the fiberglass insulation, the fiberglass would absorb atmospheric water. Should I do the fiberglass insulation in the joist bays plus rigid to the underside of the joists?
3 – If I installed fiberglass insulation, would I want it tight to the underside of the subfloor, or stapled to the bottom of the joists? Would I want the facing paper up or down?
4 – Should I install a vapor barrier? I’ve read that a vapor barrier should be between the living space and the insulation? Or just tape the seams of the rigid insulation?
5 – I’ve seen small spray foam insulation kits I could buy. Would my best bet to spray the underside of the subfloor with spray foam, then install rigid foam to the bottom of the joists?
Lots of options. Thanks for the help!
Jeremy
Replies
Hey, Welcome to the forums!
Given your climate and the specifics of your cabin's setup, I'd recommend a combination approach. Using rigid insulation on the bottom of the joists to prevent thermal bridging, along with fiberglass insulation in the joist bays, could provide both high insulation value and moisture control. Ensure you add a vapor barrier on the warm side of the insulation to prevent moisture issues. If you go with spray foam, it can serve as both insulation and vapor barrier, which might simplify the process. Good luck with your project!
So I'd want the vapor barrier in the joist bays directly below the subfloor, then fiberglass insulation, then rigid foam to the bottom of the joists?
Seems like it will be difficult to get good vapor barrier perimeter sealing.