A basement for my double wall dream house
I've really struggled to find a solution to all the issues of putting an economical basement under a house built to the "pretty good house" insulation plan (r60 ceiling,r40 walls,r20 basement r10 sub slab). The ceiling is easy, cellulose is cheap and energy heal trusses routine. For the walls two common choices double stud or exterior foam. Double stud wins on cost but moisture issues and durability need attention, I think I've got that figured out with plywood on the outer face of the inner wall as the primary air/moisture control and fiberboard exterior sheathing to maintain drying ability in both directions. But the basement is not so obvious, exterior foam is simply expensive. How to brace the walls with the floor deck yet keep r40 through the rim joist is a great challenge. Ideally both main floor walls should have the same amount of wood in cross grain and parallel between their foundation support and the roof truss to essentially eliminate problems due to unequal shrinkage. Footing insulation and sub slab insulation, it's either plastic foam or unconventional using perlite. I'm sold on perlite, it's a real bargain! My basic plan is a thickened edge slab (also thickened under interior basement walls that support the floor above), sitting on 8" of bagged perlite, under the whole house footprint with metal shear plates sticking up under where the center of the poured concrete walls will be, about 10" in from the outer edge. Once the slab is cured the outer, wood walls will be built and stored in the area that will, later, be back filled. The outer walls are built of basement grade PT lumber using 2x6 16" OC and 3/4 PT plywood, they have a 2x4 top plate flush to the outside of the studs and are the height that the poured concrete wall with it's mud sill will be. 8" poured concrete walls with outer faces 6 1/2" inside the perimeter are next. After the forms are removed r23 Roxul is put in the stud cavities of the wood walls and they are erected sitting up against the concrete wall on the floor slab. A 2 bag (of perlite) high perimeter is placed around the slab and wood wall base. the 2x8 PT mud sill is installed with it's outer edge 3 1/2" away from the 2x4 top plate of the outer wall, the gap is filled with a strip of r15 Roxul. The rim joist wall segment is built in 8', or less, segments. The studs of this wall are homemade I joist the height of the floor truss (14") . The I consists of 3 layers of 1/2" plywood with the center layer 9 1/2" x11 and the other 4 pieces 3 1/2"x11. 8 foot top and bottom plates begin in the center of one end's stud and overlap the other end by 3/4" a 14" x 8 foot piece of plywood is offset similarly on the opposite end on what will be the inner face, the actual "rim board" The beginning and end segments of this wall are laid out so the 24" floor truss layout center lines fall either 4" or 12" from "I" stud center lines. The segments are installed on the mud sills. While this is happening the interior basement bearing walls are built of 6" CMU and topped with 2x6 PT mud sills. Floor "Trimjoist" are set between the interior basement wall and the rim joist "pony wall". Another detail is the sub floor begins 1" inside the inner top plate of the pony wall and aligned with the outer face of the inner stud wall that sits on it providing for a 3 1/2" space between the walls. When it's time to add insulation, after the roof is on, the rim joist pony wall will have, between the I studs, 3 layers of Roxul, 2 of r15 for wood studs and a middle layer of r10 for steel suds.this with 2 or 3" spray foam on the inside makes the 14" rim joist well over r40. Since the studs in the rim joist are not aligned condensation issues could occur but the spray foam also cures that. The spray foam is the only foam left in my house design and it's too good at air sealing to try to replace it. The intent of the rim joist I studs is to transfer all wall and roof loads to the inner wall. The outer wood basement wall assures the r 20 goal and captures any thermal mass benefit the concrete offers. The reasons for using a wood wall and comfort bats is the tremendous difference in costs either high density mineral wool which would need to 5" thick or plastic foam which would only need o be 4" thick. Owing to perlite's low cost I have about r72 of slab perimeter insulation, slab heat loss should be really low.
Tying the floor truss to the wall top is now very conventional, with steel joist angles. What am I missing? Do you for see any problems?
Edit: After a bit more analysis i'm changing the I stud design to 1/4" center and 5/8 outer pieces. that still has very low shear stress as the area in shear is 11 x1/4", if I had a source orf economical 1/8" plywood it would still be plenty strong. Reducing he thickness really helps reduce heat flow.
Posted Wed, 03/13/2013 - 13:54
Edited Thu, 03/14/2013 - 06:18
Other Questions in Energy efficiency and durability