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Community and Q&A

Basement & double stud wall concept

user-1137156 | Posted in Energy Efficiency and Durability on

Rim joist and floor decks are usually full of thermal bridging & moisture hazards.
Why shouldn’t the first floor, it’s walls. the ceiling and roof all be supported by the basement slab floor, even with concrete exterior basement walls? The basement floor would have vertical perimeter insulation of XPS foam sufficiently strong to transfer the back fill pressure on the concrete exterior basement walls & footings to the floor slab. The footings under the concrete wall can be significantly narrower as they only support the concrete wall, no floor or roof loads. The concrete basement walls need to be designed to support back fill pressure without the assistance of a floor on their top. The basement slab needs to be designed to support all but the concrete basement walls. The basement slab is a floating slab sitting on generous insulation. Insulation, either EPS or mineral wool sits between the concrete and an inner structural stud wall. The inner stud wall in the basement is sheathed on it’s exterior face with “ThermoPly. The main floor deck sits on the inner basement stud wall. The outer main floor exterior stud wall sits on a (crush) strip of mineral wool or foam and a capillary break of EDPM foam. . The inner exterior main floor wall is sheathed with plywood and is detailed as the primary air barrier forming a wall service cavity. & sits on the sub floor of the main floor deck Plywood in full sheets is used to tie the inner and outer stud walls together at their tops. After setting the roof trusses a plywood ceiling plane is completed and air sealed. 2x4s are screwed to the bottom truss chords through the plywood to form a ceiling service cavity.. This should eliminate thermal bridges except in the roof trusses, ceiling plywood and around windows and doors.

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  1. GBA Editor
    Martin Holladay | | #1

    Your idea contains many suggestions. Most would cost more than alternative methods.

    Your idea of using a thick slab to bear the entire weight of the house is a method used by many Passivhaus builders, although the slab is usually above grade -- not a basement slab. It's called a raft foundation; here is more information: Foam Under Footings.

    To protect the top of the basement wall from the weather, it sounds like you are proposing a double-stud wall with the interior stud wall used as the structural bearing wall; this method is frequently used by designers of superinsulated homes. If you enter "double-stud wall" into the GBA search box, you will find many discussions of this method.

  2. user-1137156 | | #2

    Other than the service cavities rather than ADA What has a lower cost alternative & gets passive haus performance levels? It may be practical to pour the slab, AKA insulated raft, first, then build the basement walls & use them as the inner 1/2 of the forms for the basements concrete walls & their footings.

  3. GBA Editor
    Martin Holladay | | #3

    If you are the contractor, be my guest: go ahead and innovate. Experiment away!

    If you are the owner and you plan to hire a contractor, the proof is in the pudding. Get some bids.

    I doubt that a contractor will build the way you suggest at a price below other, more conventional methods of construction.

  4. Expert Member

    Jerry, basements have usually been incorporated into buildings for three reasons. They provided cold storage, were needed to get the foundation below the frost level or were excavated when good structural bearing material could not be found near the surface. In the absence of any of these imperatives, why include one in your design?
    Whatever the other merits of your suggestions, you will not see any decrease in the width of the foundation wall. The vertical loads from floors and roofs are simply transferred down to the footing by the concrete foundation, which at around 3500 psi could do so at a quarter of it's usual thickness. They form a negligible portion of the forces at work on a foundation.

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