Weekend 1: Subfloor Underlayment and Sealing

So we hauled the trailer over the mountains with much trepidation, and arrived without a problem. The trailer actually tows very well. Everyone who sees her says she’s beautiful! Here was the view welcoming us:

First step, laying the subfloor. Since we have fancy extendo-crossmembers we actually have to frame in 5 boxes, two on either side of the fenders plus the main area. Which brings us to the first conundrum: what type of underlayment do we want? It needs to be strong enough to handle the errant stick, keep out moisture, rodents, and other would-be residents, and protect the floor insulation.

Tumbleweed recommends aluminum flashing nailed to pressure treated lumber stringers laid on their flat side, framing, followed by extruded polystyrene foam with expanding foam insulation to fill in the cracks. Nasty chemicals, the whole lot, and would definitely off-gas! And we discovered that to use aluminum flashing would cost $200. Ouch.

PAD recommends marine grade coated waterproof 1/4″ plywood, followed by wool insulation (treated with only borax). We really like the wool idea (especially since it is grown in Oregon, and sold in Ranier!), but coated marine grade plywood is, again, nasty stuff. In general, adhesives and sealants are the hidden bugaboos for MCS-ers trying to build a house, and marine-grade plywood fails on both counts, although it is thin, lightweight, and waterproof. Traditional construction adhesives all typically use solvents that off-gas, at minimum, a lot of formaldehyde. Now, the pig-headed construction orthodoxy will argue that natural wood off-gasses formaldehyde too, and technically they are correct, so if you see a wood product that says “No VOCs”, they are lying. However, the RATE and AMOUNT is so tiny as to be almost unmeasurable, so as close to natural wood as possible is the way to go.

So about plywood. Interior grade plywood often uses urea-formaldehyde adhesive, which is the worst of the adhesives; ironically the very stuff you would have closest to you, breathing in the goodness. No surprise, as once again, most construction adhesives are derived from fossil fuels. Exterior grade plywood typically uses phenol-formaldehyde, which still off-gasses, but at a much lower rate. You can find interior plywood at greater cost that is urea-formaldehyde-free (meaning they use phenol-formaldehyde), but you cannot find a lo-VOC exterior grade plywood. At least we couldn’t. OSB (oriented strand board) is, along with particleboard, the nastiest of all the plywoods, heaviest, and turns to oatmeal when wet, so no OSB anywhere in our house. It’s only benefit in construction is that it is a little cheaper.

So we went for the least of evils:

Galvanized steel flashing, which cost half as much as the aluminum, a 3/8″ thick sheet of CDX (a type of exterior grade) plywood on top of that, followed by our stringers of 2×6″ pine. To seal the cracks in the flashing, we used a type of adhesive flashing tape… joy! Back to the computer, looked it up, and there are two large groups of flashing tapes, those made with asphalt and those made with butyl rubber. Apparently the latter is not good, but not terrible as it does not off-gas any VOCs unlike asphalt tape. Here’s the article for your interest: http://www.bestmaterials.com/PDF_Files/flashing-tapes-manual.pdf

Main compartment flashed and taped

And for your information, metal flashing is a pain in the butt to get tight enough if you are just screwing it straight into your cross-members, which is probably why Jay used the stringers as he did. We found out the hard way that it leaves lots of gaps around the edges, and a lot of gaps in the middle if it is not perfectly tight, which is fine if you’re using polystyrene board, which no creature is terribly interested in. So we added the 3/8″ plywood the weekend after, as we thought it would be better able to protect the wool insulation, and likely why PAD used a solid wood underlayment. Next week: finish the subfloor!