What is the best way to hold a double studded wall on a >R20 foundation with exterior insulation (ie rigid foam)?
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| Posted in Green Building Techniques on
We are aiming toward PassiveHaus standard and plan a double studded wall with dense pack cellulose for the main envelope sitting on top of a foundation preferably with deep exterior insulation (ie 4″ rigid foam). The plan would be for the exterior 2×4 (or SIP) to be the roof loading wall (large snow load in northern NH). How do we best insulate the basement/ foundation to allow structural integrity and load and avoid the wall sitting inside of the foundation insulation?
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Replies
Phil,
We are basically doing this exact detail on a PH we're consulting on (but not designing). We've pulled the insulation to the interior of the concrete stemwall/footing - which picks up all our loads. The interior wall simply bears on the thermally isolated concrete slab. It's not ideal, but seemed the best solution possible.
We don't, however, have a basement. THERM image here:
http://bruteforcecollaborative.com/wordpress/wp-content/uploads/2012/04/1115_ThermDetail_SlabWall_ColorInfrared-500x499.jpg
Might your concerns be addressed by doing something like the Sunrise Home (search here), a Larson Truss, etc? Dense pack real thick walls and skip the foam above grade, installing as much as you want to below. ????
Thanks Mike:
We plan a basement which then would require interior insulation in the basement (not preferred). If pulling the insulation interior into the walkout basement (hillside), what is your preferred approach? We were thinking about spray foaming 2" then mineral wool or cellulose if had to go interior in the basement. Some concern re vapor/dampness on the interior of the spray foam building up in the cellulose (ie where would you place the air and vapor barrier)?
Can ICF's with 4" exterior foam support a load bearing 2x4 wall?
Any experience with cantilevering....?
I am starting to see why so many Passive homes lack a basement (and gable roof)!
Thanks
Phil
Phil,
I'm facing the same issues. My solution is 1. Design the concrete basement so it does not depend on a floor above to support back fill pressure. 2. Use rigid foam (3" XPS) on the insides of the concrete walls. 3 use high compressive strength XPS to insulate under & around basement floor slab. 4. build interior basement stud walls to support the floor framing,sub floor and inner exterior walls (rim board is directly over this stud wall & sub floor ends at rim board). (interior basement walls will be filled with mineral wool later). 5. Use a wide strip of EDPM foam under outer wall's treated plate extending over sub floor under inner exterior wall as capillary break & air seal. 6. Build outer stud walls (in my case must be 2x6 to support snow loads) on sub floor but tip them up onto concrete.
I'm planning a wall service cavity so I'll have plywood on the outer face of my inner exterior walls. I'll also have a ceiling service cavity So after my walls are up I'll install ceiling joist then plywood on top of them before installing trusses. The service cavities make it much easier to have a complete air barrier boundary.