Cold air from behind kneewall
The recent article in FHb about enegry nosebleeds pointed out air infitration into the joist bays from behind knee walls.
This jumped out to me as a prime source fo the cold floors/ceilings in my 1 1/2 story farm house. Don’t know why i didn’t think of it before.
The suggested remedy was to seal the openings between the joists with foamboard sealed in place with spray foam. This wouldn;t work for me because the floor is made up of true 1×10 boards with varying gaps between the boards. So the cold air can still slip through the cracks, so to speak.
I was wondering if I could blow cellulose, back as far as the knee wall and form enough of a barrier to limit cold air movement?
Any other suggestions?
Thanks
Angus
Replies
bump
I'm not picturing the geometry well, which is likely my problem, but, are you only looking at insulating the upper floor out to the knee walls, or the ceiling flat behind the kneewalls?
looking at insulating the first floor ceiling from the knee wall out the edge of the roof.
A.
insulating the first floor ceiling from the knee wall out the edge of the roof
Ok, I think I'm seeing this. And, that surface has roughly-fitted "floor" boards over it, correct?
I'm going to guess that the pitch is around 10/12 12/12, since you said 1.5 story--so there's access, just not large, behind the knee walls, right?Occupational hazard of my occupation not being around (sorry Bubba)
Yup, you got it now.
10/12 slope, but a fair bit of space, I would say 4' between knee wall and roof, and the wall is 3 1/2'.
Almost all of the joist bays are boarded over, with the last 18" open.
A.
Almost all of the joist bays are boarded over, with the last 18" open
So, can you see into any soffit that's out past the first floor top plate?
Might be easier to slip foam sheets in, spray-can (or gun-n-tank foam) the sheets to the joist-rafter-top plate joint, then carry up the roof, and then into the upper story, above roof, wall (insulating that area being yet another kettle of fish to tackle).
If your AHJ requires roof venting, you can set some 1x2 along the rafters as a spacer for the foam sheets, then foam them tight to the rafters.
Why go to the roof plane? Well, for one, it's a little bit easier. Second, any warm air "exfiltrating" through the first floor ceiling (ceilings being rife with air leaks anyway), we're "encouraging that warm air to rise up and collect back where the second floor walls meet the roof. Thirdly, you are adding a "cushion" of warmer air against the bottom of the kneewalls. Lastly, it's the hypotenuse of the triangle, so it's a tad less material in your case than along the joists, then up the knee walls.Occupational hazard of my occupation not being around (sorry Bubba)
If possible I'd pry up the board just this side of the knee wall and fit a dam in there.
In any event you need to insulate the ceiling below the knee area somehow, so blown cells would make sense, blowing in (if no dam installed) enough to get a couple of feet beyond the knee wall, if not all the way to the center.
Not too keen to pull op the floor boards. Those old square nails have really had a chance to get a good bite into the joists over the last 150 years! Also with the low roof not a lot of room to work with a crowbar.
A.
Blowing cells into the floor will work if there is something to dam up the cells out past the kneewall, like Dan said. Otherwise, you'll probably wind up filling most of the floor. Not that there is anythinng wrong with that. You might like the sound damping quality between floors anyway. We've done it intentionally in the past.
Another option would be to insulate the rafters from the top of the kneewalls down to the rafter plates and bring the space behind the kneewalls inside the thermal envelope.
Steve
Edited 10/10/2007 7:52 pm by mmoogie
I did just that in my house, and it's worked fine. Don't worry about getting a bit of "overspray" beyond the kneewall. It won't hurt anything and shouldn't result in a whole lot of extra insulation being used. Of course, if it's convenient, you can stuff some fiberglass between the floor joists back under the kneewall and avoid "overspray" altogether. You should fill the joist spaces a bit higher than the floorboards to account for any settling, but with cellulose, there won't be much.
I wouldn't mess with pulling up the floorboards without being sure they are not functioning as ties to counteract the roof forces. Probably would be OK, but no reason to tempt fate on this one.
Mike Hennessy
Pittsburgh, PA
Always good to hear from someone who has tried it with success. I guess it is the way to go.
Not sure by what you mean with "fill the joist spaces a bit higher than the floorboards"
Do you mean the open space out beyond the floorboards? or put down insulation on top of the floor boards?
A.
If your joists are 8" deep, specify at least 9" of fill. (More, if you want the added R value.) I.e., fill an inch or two above the top of the joists. It won't hurt to cover the floorboards, if they are in unused space. That will keep a cold air path from opening up between the top of the insulation and the bottom of the knee wall if the insulation settles a bit over time. That said, if you push fiberglass "stops" under the kneewall to keep the celulose from going under the floor too far, that should do it as well.
Mike HennessyPittsburgh, PA
Edited 10/11/2007 8:49 am ET by MikeHennessy
Filling the floor joist cavity still won't stop cold air. If you dense pack the cellulose, you may slow the air, but really, the best thing to do is either pull up the floor board next to the knee wall and stick rigid foam dams in each cavity (and then fill the cavity from exterior wall to foam dam with cellulose) or move the insulation up to the roof therefore bringing the knee walls into the conditioned space.
Have a blast,
Dan
Senior editor
I'm begining to think you're right.
I'm really not looking forward to pulling up those boards, like I said they don't pry up so easy. However I think that if I look after this source of cold air will improve the overall insulation of the house quite a bit.
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