Quote:
Originally Posted by JayForJohnny
Did you see the one where he says floor framing is unnecessary? Been wanting to ask here what people think of that video. Is there more purpose to framing that it'd be worth still having?
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He's correct that you don't need framing to prevent the foam board from being crushed (after all, they do build entire houses on top of the stuff), but IMHO you
do need it (or rather
might need it) in order to prevent the plywood from warping upward at the seams over time. You can glue the foam to the steel floor and the plywood to the foam, but glue/construction adhesive doesn't really adhere to foam very well (since foam board is great under compression but not that great under tension, so it's easy for the glued part to break off). I've tried glueing XPS foam board with many different products and found nothing that works really well, although Gorilla Glue brand construction adhesive seems to be the best of the worst.
In practice, the warping problem can be dealt with via tongue-in-groove plywood (or OSB or whatever) although this is a bit more expensive. Also, it doesn't seem like warping really even happens all that often to people's floors (why I say framing "might" be necessary).
For my floor I used some pretty crappy (but very cheap) Craigslist plywood salvaged from crates of South American juice concentrate and I wanted to make sure I had a solid mechanical connection between the steel bottom and the plywood, especially at the seams. I welded wood screws upright on the steel floor and then spun pieces of oak dowel down onto the screws through matching holes in the XPS foam board, then screwed the plywood into the dowel pieces. This created a solid subfloor while also minimizing thermal bridging from wood (which is another minor reason not to do framing).
One thing I hate to see is people framing their subfloor with 2x4s on their sides. This is completely unnecessary and makes the thermal bridging problem worse. I saw one floor where the guy used 2x6s on their side 12" on center - the floor was more than 50% wood and less than 50% foam board, which is definitely not what you want from an insulation standpoint, since wood conducts heat 5X more readily than XPS.
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