Cheap backer for Insulation

rhall31 · 03-06-2009, 11:56 AM

Hey guys. I'm in the process of finishing off a bonus room(500 sq.ft.) over my garage and master bedroom. I'm almost done the framing and had an insulation contractor here yesterday to give me an estimate. He said that
I need to consider my kneewall(5 ft. high) an outside wall and that it needs some sort of backer on ths back side, behind the insulation, to stop
any drafts. Makes sense to me, so, anyone have any ideas of a cheap material that I can nail to the back side of these studs?

Ron6519 · 03-08-2009, 04:27 PM

Why wouldn't the vapor barrier in front of the wall do that?
Ron

jaw22 · 03-08-2009, 11:27 PM

You can use 1 or 2 inch pink or blue foamboard nailed to the studs. be sure to use the nails with the washers. Or you could actually use tyvek housewrap there. Be sure to tape and seal all the seams no matter which you use. Use tyvek tape for both or srayfoam for the foamboards.
Also be sure that the spaces under the kneewall where the bottom plate meets the floor/rests on the floor joists are all closed off. Here cut blocks of 2 inch foamboard to fit and seal them in place with sprayfoam.
You should also treat the floor of the bonus room/garage ceiling as an outside wall as well. If its open just dont just put fiberglass in there and cover it thinking it will do well...it wont. You need to completely fill the space with insulation making sure it touches the subfloor, and then airseal it totally. If its covered already and there is space available in the joists see about pumping in more insulation like cellulose and dense pack it thru holes in the garage ceiling.

jaw22 · 03-08-2009, 11:30 PM

Quote:

Originally Posted by Ron6519

Why wouldn't the vapor barrier in front of the wall do that?
Ron

Its called windwash effect i think. If the fiberglass is exposed to cold air currents in the back of the wall it will lower its effective r value rendering the fiberglass ineffective. Thats why it needs to be covered since fiberglass works well only in dead air space.

rhall31 · 03-09-2009, 08:06 AM

Yes. Thats just how the insulator explained it. Imagine how the insulation
on your outside walls would work if there was no plywood and tyvek behind it. It wouldn't. The wind would find its way right through any crevice.

The garage ceiling is insulated so I'm Ok there. But I know I need to fill
the remaing cavity. I was looking at the r-1 and r-3 stuff yesterday.
I'll tape caulk foam away when the time comes. Thanks!