insulating floor HELP!

spaceman · September 19, 2011, 06:23:19 PM

Hello I am planning the insulation for my 12 x 14 cabin floor. 2x6 PT joists, 16" centers. Do you think I can "sandwich" insulation between PT plywood (bottom), roxul insulation r22, vapour barrier, then subfloor. Does each floor joist space need to be vented? Or should I just use 1/8 hardware cloth instead of PT plywood? The space between ground and cabin is approx 1ft.

spaceman · September 19, 2011, 06:25:52 PM

Ps Winters are cold here!

September 19, 2011, 06:54:00 PM

I would just use the hardware cloth. You can get some shark teeth to hold the insulation up until you get the hardware cloth in place. You really need to lay a vapor barrier ( 6 ml plastic) on the ground if you are closing in the sides w/under pinning to create a crawlspace. I would be afraid that the ply might trap moisture.

spaceman · September 19, 2011, 07:00:48 PM

i was going to leave the sides open for ventilation, and lay plastic on ground. wondering how the r value is affected by having open joists?

ScottA · September 19, 2011, 07:13:12 PM

If the sides are open I think you can skip the plastic. I sealed my floor up. From the top down: 3/4" wood flooring, 15# felt, 3/4" OSB subfloor, fiberglass insulation, 3/4" foamboard with all joints taped. The foamboard seals the bottom. I don't know if that's the best way but that's what I did and we survived -20f last winter. I'll cut it open a couple of years and inspect for moisture. The framing was dry when I closed it up. IMO most moisture issues in dry areas are caused by central heat and a tight house. We use a wood stove which continuously vents fresh air into the house avoiding moisture buildup inside.

September 19, 2011, 08:29:44 PM

One foot between the bottom of the joists and the ground? That's awfully tight. I have 19 to 20" between the joist bottoms and the ground and think that is horribly tight whenever I've had reason to go underneath. Is it already built?

I vented each floor joist bay near the outer ends. The floor joists cantilever over the support beams by a foot or so. The portion of the joist bay outside the beams were sheathed solid with 3/8" plywood. There is a 5 to 6 inch wide vent strip along the inside of the beam for each joist bay. Those are covered with 19 gauge, 1/4" hardware cloth. That stuff is sturdy enough to keep out even large rodents. The balance of the underside of the joists are sheathed in 3/8" plywood.

Q: is the plan to insulate the floor before applying the sub floor sheathing on top? If so, I'd carefully think that out. There is always the danger of water getting through from above and wetting the insulation. I had planned on doing that but was disuaded by the opinions of many on this forum. I was glad I held off till the structure was dried in as we had a very rainy period while the walls and roof were being constructed.

spaceman · September 19, 2011, 09:13:03 PM

Thanks everyone for replying so quickly. Mountain Don, i haven't built the foundation yet, - do you have any photos of your foundation? I'd love to see your venting strategy. Basically you have a 5" vent at the end of each floor joisti Thanks again!

MountainDon · September 19, 2011, 10:01:10 PM

No photos of the underside, but here's a drawing.

The entire build thread is located....

http://countryplans.com/smf/index.php?topic=2335.0

My thinking was that I wanted some ventilation but didn't want the wind whistling through. Hence the joist bays are sheathed outboard of the girders. The hardware cloth venting was installed along the inside of the girders to shield from the wind; theory anyways. The 3/8 ply laps over the hardware cloth. The hardware cloth is bent to an L-shape and nailed in place with roofing nails.

Girders closer than 12 inches to the ground should be PT wood according to the IRC. Joists closer than 18 inches to the ground should be PT as well.

Note the drawing does not show any bracing. The photos of the build also do not show some of the bracing. Pier to girders have 3/4" plywood gussets (approx 16 inch square) well nailed to the inside of the girder/pier and T-plates lag screwed and nailed to the outer face.

There will soon be some retrofit lateral bracing installed in the form of a shear wall. More on that later.