I purchased a used (but unbuilt on) 8x20' tumbleweed trailer a few months ago and am trying to get over a few hurdles before diving into wall framing. So far I have insulated the cavity of the trailer with 3" rigid foam and installed a layer of 3/4" advantech sheathing for the subfloor, using sill seal to prevent thermal bridging. I was ready to start putting up the walls but have had second thoughts about whether this is a sufficient amount of insulation for the floor--I live in central Maine and hope to be able to live in it year round. Other sources I've seen went with a 2x6 framed subfloor to allow for 5 1/2 inches of flooring insulation.
So, what would be the best way to proceed?
Option 1: Frame an additional subfloor with 2x4s on top of the existing sheathing. I could insulate with 3 1/2" of rigid foam or possibly wool batting which is what I would like to use for the walls and ceiling. Then install a vapor barrier and another layer of sheathing. My concerns about this are that it seems unnecessary to have 2 layers of heavy sheathing and it raises my floor height 4 1/4". Should I pull up the first layer of sheathing and try to reuse it on top?
Option 2: Leave the subfloor as is and start building. If it proves to be cold I could always put straw bales around the exterior of the trailer. Maybe even heat the underside with a compost pile or wood chips?
Any plans for plumbing in or through the floor? I am assuming your trailer has steel tube "floor joists" every 16 to 24 inches, and your concern is about the relatively little insulation at those points if the Advantech is attached basically directly just through the sill seal.
How do you plan to anchor the wall to the floor frame?
My off the cuff suggestion without seeing pictures or plans would be to consider buying some 2x2s (1.5" by 1.5") for sleepers, put them like a rim on the perimeter and install them 16" oc (offset to miss the existing joists), put in 1.5" worth of foam (extruded polystyrene) in the cavity. Glue the sleepers down with polyurethane based construction adhesive (sticks to advantech) and nails/screws (predrill). Then glue/nail down 5/8 plywood onto that. That is 2 1/8 floor buildup, ideally adding r 7.5 minus bridging. Staggering the sleepers from the joists helps the bridging.
I havent fully thought out the plumbing situation; I'm using a composting toilet but will need greywater drains for shower and sink. The metal joists on the trailer are 24" oc and they are I-beams, allowing the foam to recess into the joist leaving about 1/2" uninsulated between foam boards. I am using Simpson holdowns to attach wall framing to the trailer.
I like your suggestion. This gives me the peace of mind of having enough insulation without taking away from headspace too much.
Marc, what kind of heating do you plan for the structure? The goal is to replace btu's lost and then add in some. As for those hay or straw bales, I would not do it. My experience has shown that the mice move in every winter. That, in and of itself, may not be so bad- but the mrs will see those mice running around all the time and be.... well, let's just say not happy. And the idea of heating the underside of your tiny home with any combustibles is just too scary for me to contemplate.
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