Tiny house subfloor vapor envelop design help
Hello all, I’m going to be framing out my subfloor for my tiny house on wheels soon. I have insulated the bed of the trailer with 2″ and 3″ inch XPS insulation. Going to be framing a subfloor with 2×4 s and then filling it with half an inch of XPS to eliminate a thermal bridge between the steel, I will be adding rockwool insulation on top. I’m deathly scared about moisture in the floor causing mold and rot. I’ll be using cedar wood for the framing and a sill gasket between the steel and wood. Should I use a vapor barrier between the framing and the plywood subfloor? I’m worried the XPS insulation will be acting as a vapor barrier not allowing moisture to escape. I’m located in climate zone 6A and plan to use both heat and AC. I’ve attached a picture of my trailer, and a photo off Google for what the subfloor framing will look like. Look forward to everyone’s thoughts.
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Normally with a floor in a cold climate you want a vapor barrier against the interior (top) and you want it to be able to dry downward, so you have no vapor barrier on the bottom. All of the vapor drive is going to be from inside out. To protect the insulation from damage you clad the bottom with something vapor open like plywood. So that XPS foam is on the wrong side. It also doesn't seem like it would do much insulating since it's completely bridged by the steel of the trailer frame.
So I'd be thinking a sheet of plywood against the trailer, then a framed floor filled with insulation, then a vapor barrier below the floor of the interior. If you want, that vapor barrier can be a layer of foam.
With a tiny house there is the additional consideration that if it's ever moved in the rain you'll get water hitting the bottom. If it's just for short journey's I'd paint the plywood with exterior paint, latex paint is sufficiently vapor-open to allow it to dry to below.
Yes that's a concern of mine having a vapour barrier on the bottom. I see many people insulate the frame with XPS and still put a vapor barrier below the Interior side of the subfloor. I just don't see how any moisture could escape though. Are you suggesting I should just leave the frame of the trailer uninsulated and just put some plywood to cover the bottom?
You want all of your insulation above the frame of the trailer. Steel has an r-value that's basically zero, insulation that is bridged by the steel isn't going to do anything.
When it comes to insulation there's no substitute for thickness. Even six inches of floor is marginal. If the height of the floor is a concern have the walls rest on the trailer frame and have the floor float. Put a layer of plywood against the frame, the several layers of foam, then a layer of plywood or OSB. If you use tongue and groove for the subfloor it will act like a single piece, and if you run a baseboard around the perimeter it will be trapped. With the floor essentially a single piece of foam you don't have to worry about vapor.
This works better if you can avoid running plumbing or electrical through the floor.
BTB123,
You may find these past discussions useful:
https://www.greenbuildingadvisor.com/question/tiny-house-insulation-detail
https://www.greenbuildingadvisor.com/question/tiny-home-on-wheels-floor-assembly-insulation
https://www.greenbuildingadvisor.com/question/floor-insulation-for-a-tiny-house-on-wheels