Styrofoam insulation between studs

[wgrim]

Any tips on cutting and installing styrofoam between studs? My ext walls are 2x6. Should I leave air gaps on both sides, or push styrofoam against ext plywood?

[suemarkp]

You don't usually install it that way. Sheet foam is best installed over a wall sheathing or up against a concrete wall. If you want to use it in a wall cavity, I suppose you could cut it and fit it between the studs. But you'll have gaps that should be sealed, probably with a spray foam insulation. I would also push it to the outside sheathing.

Are there pipes or wires in these walls? Why not use fiberglass batts or rolls? Do you need a vapor barrier? What and how are you going to do for that?

[Fischer]

Just did it. Salvaged foam from ISP's and extra foam from low pitched roofs.

What I did, and it took a lot of time, was to rip 4x8 panels down to 15 7/8" wide by 4'. Gang ripped them with a 16" Makita Beam saw. I then measured each stud bay in 4' increments. Say the first bay was 14 1/2" at the top and 14 3/8" at 4' lower. I set my table saw at 2 1/2 degrees off of vertical and ripped a 2" thick panel at 14 1/2" to the narrow point. Then took a surform and slightly tapered to the 14 3/8" end. Then took a 2 x4 block and nudged it in with a hammer. It was snug to say the least. I then staggered the joints between the 2" pieces, so there would be no clear path for air infiltration. A surform and a stiff short saw are invaluable.

Let me repeat- a "LOT" of work.

Now for the benefits.

1. I salvaged everything. Kept it out of the dump, and didn't have to have more made

2. It's an old church and it used to sway in a 30 mph gust. You could stand in the choir loft and rock the church. We had a huge storm here this winter with 70 mph sustained winds. I went into the church during the worst of it, and it din't so much as budge. I had put the foam in the rafters (2 x 6 with a 13/12 pitch) and only had finished 3/4's of the roof and it stiffened it up that much.

3. It went in so tightly that no additional foam has been needed to seal it.

4. If you stand near it, you can feel the heat from your body radiate back to you.

Also used 1 x's on each roof side of the rafters put in with a pin nailer to allow for ventilation. The ridgid foam negated the need for additional baffles. Ended the process with 4 x 8 x 1/2" Rtech foam that had a vapor barrier both sides, put in with cap nails and taped all the joints. The same process on the stud walls.

When cutting on the table saw, pressure the piece to the fence and the table equally and do not let up on it till it drops off the back of the saw. You need to hook up a shop vac to the table saw when you're doing it. Shake out the filter when the suction wanes. An N-95 respirator is advised. If you can get someone to cut while you're swinging the block and hammer it will go 3 to 4 times faster. I didn't have that luxury.

I would have made more by going to work and paying for Icynene sprayed in. I chose to salvage. I don't think the Icynene would have stiffened up the building as much. It cost me time and about $400 for the foam for (approximately 1200 square feet of pitched roof and 1200 sq.ft of stud walls)and the RTech is about $6 a sheet. More insulation, no additional vapor barrier and less air infiltrations were added by the RTech.

When putting in foam this way, you not only have a greater R factor than fiberglass, but the big energy savings is in air infiltration abatement.

It's a lot of work! If you need any more info, email me and I'll be glad to help. I acquired all the foam using craigslist.org.