Insulation retrofit
I am remodeling a house with a slab foundation and stucco exterior. The process will be done bit by bit, and am wondering about insulating the exterior walls. I will be going down to bare studs on most of the exterior walls. I am thinking of adding interior offset wall to gain approx. 3″ for cellulose insulation and to decrease the cold bridging effect. Since it is was built in 1926 there is no exterior air/vapor barrier. My question is how do I deal with the air barrier issue, and further increase the r-values of the walls. Do I add rigid insulation on the interior? Would I choose the foil faced, or does this really only work when there is an open space next to the foil face?
I live in Minnesota with cold winters and some hot and humid days during the summer.
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Replies
MN code calls for a sealed air barrier, can be poly or ADA (airtight drywall approach), foam would also work. Foam on interior walls has been used over the years but makes the attachment of drywall a bit tougher, you need longer screws. If you are intent on leaving the stucco exterior I would say a strapped wall, a double wall or a layer of foam under the drywall would be your options to increase R-value and limit thermal bridging.
The slab on grade needs to be addressed with some rigid insulation from the plate line to about frost depth, or you can go down vertically for about 18" and do a horizontal flare of about 30".
Make every effort to upgrade the entire building envelope, a piecemeal approach will lead to poor building performance.
Corey,
A staggered stud interior wall to increase cavity insulation is an excellent strategy, and you can't go wrong with cellulose insulation if it's properly dense packed to 3 pcf. If you want higher R-value, then an interior layer of 1" XPS foam board with taped joints and sealed at penetrations can both serve that function and created an interior air/vapor barrer. There should not be a vapor barrier on the outside in your climate.
You're correct that the foil facing on rigid insulation will not create a radiant barrier unless there is an adjacent air space, and on walls such an air space should be no more than 3/4" thick to avoid convective loops.
Doug is correct that slab edge insulation will be critical to your upgrade, but it's a myth that the total vertical and horizontal length of insulation needs to equal the prescribed frost depth. If you look at the shalow frost-protected foundation design manual (http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf), you will find that less than that has proven adequate and is accepted by the IRC. Any slab edge insulation will be a significant improvement.