Insulate knee walls
Situation: Currently have knee walls with no insulation behind them or the cathedral portion of the ceiling. The rafters are 2x6's. Ventilation is provided from vents installed between the exposed rafter tails. Blown in insulation is currently blocking off the top of the cathedral portion. I live in zone 5.
There was some haphazardly blown in insulation that had fallen from the top which looks like sheep's wool but has fiberglass in it which I pulled out(about 6 contractor sized trash bags). There was ice damming and some mildew on the roof deck. I plan to clean this with a mixture of hydrogen peroxide and vinegar to make peracetic acid.
Cut and remove some of the flooring. Through this access install bat insulation under knee wall and foam to seal.
Install soffit baffles and place batting under floor boards towards the exterior wall. This batting will go up against the soffit baffles and also be foamed to seal.
There are 22" between rafters and knee wall joists so I will cut 2" xps into 21" strips 8' long.
put 1" blocking on roof deck at corner of each rafter bay from knee wall side and attic side to ensure ventilation gap is maintained.
From the attic feed down one strip of xps and then another strip of xps under it. Hope and pray any protruding nails are not a huge obstacle.
Install baffles at the top of the attic, place batting under for reinforcement and foam to seal.
Go into the knee wall and attach a piece of rubber hose onto the end of a great stuff spout, maybe even attach some heavier gauge wire to the rubber hose to make it easier to guide. Guide it up to the top of the cathedral portion and fill the gaps on either side of the xps.
Apply two layers of xps to the knee wall as well and put back blown in insulation until the R-19 under the floor boards is brought up to R-49.
The one knee wall I'm looking at I will leave an access door because there is a stack pipe and bath vent and I'd like to keep access there. The other three I will glue xps to the back of drywall and cover the edges of the xps in caulk to air seal my once opening.
The cathedral portion should be R-49 but the inspector said he'd be okay with me doing this and doesn't expect me to rip out all the dry wall and sister beams to the 2x6's to give enough space to bring it up to R-49.
This would cost me ~$584 to do all 4.
Or I could have it done with foam in an unvented style. Block off the bottom of the cathedral portion and have that filled with the stuff they put in your walls and then have the knee wall portion sprayed. Foam alone would cost me ~$1354 at $1 a board foot and an estimated 64.5 sqft per each knee wall area and 5.25" of foam. This would actually exceed code because unvented attic assemblies in zone 5 only require R-20 but this would be ~R31
What do you all think? XPS or foam or something else?
Posted Fri, 03/28/2014 - 16:37
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