At a website named homeenvy.com, someone asked if the best method for insulating his now-being-built workshop floor was to use FG batts underneath. The host said FG was bad for many reasons, so: “forget the fiberglass altogether, and leave the underside of your floor frame completely open. Instead, install a 1 or 2-inch thick layer of rigid, extruded polystyrene foam on top of your subfloor, then cover this with another layer of 5/8 or 3/4-inch plywood on top. You’ll get a fantastically warm, durable and moisture-proof installation. Extruded poly foam is dense enough that no strapping is needed under the top layer of plywood. Just put it on the foam, then secure the whole thing with screws driven down into the underlying floor joists.”
Intuitively this sounds SOMEWHAT GOOD, in that the top covereing would be almost entirely isolated from the outside.
I wonder how it would stack up next to, say 2″ rigid foam jam-fit down on ledgers at bottoms of joists, with dense-pac cels the rest of the way up?
Replies
Gee, is this the dunbest thread ever? Not one single response?
Put your yankee impatience aside while everybody gets another cuppa joe in them and brains crank up. I'd like some genius answers here too 'cause up in Atlanta I'm taking a prefab shed and turning it into a shop I can actually afford to heat.What's insulating your walls & ceiling, Jawjuh? And how far south are you? I'm north of Atlanta.
Seems that just keep the structure tight and any heating system is going to work well down there.
In the large towns and cities, where civilization especially prevails, the number of those who own a shelter is a very small fraction of the whole. The rest pay an annual tax for this outside garment of all, become indispensable summer and winter, which would buy a village of Indian wigwams, but now helps to keep them poor as long as they live.
Parolee # 53804
Edited 5/1/2007 3:30 pm ET by rez
bj,I grew up in Columbus, GA, but haven't lived there for many years. I'm retired now,and travel a lot.
"I wonder how it would stack up next to, say 2" rigid foam jam-fit down on ledgers at bottoms of joists, with dense-pac cels the rest of the way up?"
Depending on joist size, the option with the DP cels is way better, even with the joists conducting better than the insulation. Wood rates R-1 per inch of thickness, so a 2x12 would still offer R-10 above your ledgers, and the bays would be more like R-50.
Your way will cost more, but perform better.
Bill
say 2" rigid foam jam-fit down on ledgers at bottoms of joists, with dense-pac cels the rest of the way up?
I'll echo Bill, and say your idea is better than foam above the sub-subfloor.
Even better, though, would be to install, oh, 1" rigid on the bottom of the joists, and then fill the bays with cellulose. Loose blown in would be less spendy that DP, some. But, the real deal would be getting the floor structure isolated so it will bridge less. Otherwise, poultry netting stapled to the joists would hold cellulose more than adequately.
Or, you could just run rigid up the foundation wall, and into the wall-roof inuslation planes, and call it even. For a shop, you probably need to get some circulation under the floor--but how much and how are not real obvious on the buildingscience site <g>. Depending on climate, controlling moisture under the shop floor might be more serious than anything else.
The warmest floors result from an air space directly below the floor on top of air impermiable insulation.
The air in there will be the same as room temp and keep the floor that way too. Of course enough R value is needed but the air tightness comes first. Jammed in foam will eventually leak since wood does shrink over time. Layering the entire deal sounds good too.
I spray foam so thats how I air seal. R20 is fine if your air tight, R30 if your paranoid.
Stu
This is really interesting! I assume that sprayed Icynene shrinks and expands with the structures (joist or rafter or stud bays), and therefore never (within reason) admits air infiltration? That's the claim, which sounds good, though I don't know what proof there is.I like the air space idea. The idea that air separation would give the floor level a temperature close to the temperature in the space above, assuming good insulation below.So, is my floating floor just TOO nuts? Sprayed Icynene between joists. 1 1/2" square blocks, every 16" on each joist, supporting 3/4" ply above, with engineered wood above, or whatever? I know, a logistical nightmare! Placing all those little blocks. Affixing 3/4" ply to blocks!Just thinking out loud here.
Yes the Icynene moves with the building. Comes with a building lifetime warranty!!Now, you want blocks to minimize thermal bridging I suppose? Just run the 2x2 perpendicular to the joists and you have the same deal. An intersection only 16" sq.Go to it, it works!Stu
Yeah, the little blocks are probably overkill.