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Insulated suport for basement walls, how?

Normally a basements walls are restrained against earth pressure by the slab at the bottom and the floor above at the top. Concrete is a miserable insulator! Between the slab and footings high strength XPS (which shouldn't be loaded above 20 PSI to avoid creep issues) appears to be applicable. But around the slab edge there is only a relatively small area (4"slab) a higher strength material is needed. It seems Foamglass doesn't have creep issues and is available in strengths up to 300+ PSI Has anyone used it? There is a similar problem at the top of the walls as well & it seems Foamglass could be useful there as well.

Asked by Jerry Liebler
Posted Dec 26, 2012 9:27 PM ET

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8 Answers

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1.

Jerry, you are right, my comments are diverting you from getting the advice you asked for. I have deleted them.

Answered by Malcolm Taylor
Posted Dec 26, 2012 11:24 PM ET
Edited Dec 27, 2012 4:12 PM ET.

2.

"...which shouldn't be loaded above 20 PSI to avoid creep issues..." XPS can be ordered as 100 psi, and if you believe in the "3x rule", it will take 33 psi. It seems houses are getting along fine w/ less psi foam, though, so maybe the 3x rule is generous, or people are using rather wide footers. You can also tie the slab to the edge beam/footer/foundation wall and insulate around and under the whole thing. Floating slabs are not always necessary, apparently. A structural engineer and a builder I spoke w/ about this said they'd never heard of a floating slab.... Interesting.

Answered by John Klingel
Posted Dec 27, 2012 2:31 AM ET

3.

Malcolm,
Certainly a basement can be designed to not use the floors to brace the top & bottom of the walls BUT such a design, for reasonable back fill height and wall length, requires counter forts or buttresses and will use significantly more footings, concrete and reinforcement.. It is common practice to use the floors for bracing! Using the floors saves a significant amount of material but demands that back fill be delayed till the floor above is completed. The sides parallel to the joists, simply, have blocking that transfers the force to the floor deck. My question is about how to use the basement slab AND the floor deck above to brace the walls and maintain thermal isolation.

Answered by Jerry Liebler
Posted Dec 27, 2012 11:36 AM ET

4.

deleted

Answered by Malcolm Taylor
Posted Dec 27, 2012 1:19 PM ET
Edited Dec 27, 2012 4:14 PM ET.

5.

Malcolm,
This is from the 2006 IRC but the same is in the newer code.
"1805.5 Foundation walls. Concrete and masonry foundation
walls shall be designed in accordance with Chapter 19 or 21,
respectively. Foundation walls that are laterally supported at I the top and bottom and within the parameters of Tables
1805.5(1) through 1805.5(5) are permitted to be designed and
constructed in accordance with Sections 1805.5.1 through
1805.5.5."

I'm simply asking how to insulate the "laterally supported at I the top and bottom"

Answered by Jerry Liebler
Posted Dec 27, 2012 3:42 PM ET

6.

Here is a link to a basement design showing counterforts.
http://www.basementwall.com/basement_wall_sample_project.html

Answered by Jerry Liebler
Posted Dec 27, 2012 3:51 PM ET

7.

I've been thinking about this myself. Why not insulate the interior of the foundation wall and also place insulation on top of the slab, and then cover that with a floor?
Also, why can't vertical steel reinforcement pinning the foundation to the footing replace the lateral restraint the slab normally provides?

Answered by David Argilla
Posted Dec 28, 2012 1:32 AM ET

8.

David,
"Why not insulate the interior of the foundation wall and also place insulation on top of the slab, and then cover that with a floor?"

I'm very sure this would be far more costly than other methods. It also eliminates the thermal mass of the slab from any action to stabilize indoor temperature.

"Also, why can't vertical steel reinforcement pinning the foundation to the footing replace the lateral restraint the slab normally provides?"

This would take a very large amount of re bar as the compressive strength of the concrete is the limiting "weak link. A far better approach, that I've seen on many drawings, is to cast a "key way" into the footing centered under the poured wall, which when poured will create a mating key. Forming the key way may well be more expensive (labor cost) than Foamglass blocks which would also serve as a screed line for the slab.

Answered by Jerry Liebler
Posted Dec 28, 2012 6:34 PM ET

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