Replies

1. 
   jklingel | Dec 27, 2012 01:31am | #1

   "...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.

2. 
   user-1137156 | Dec 27, 2012 10:36am | #2

   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.

3. 
   user-1137156 | Dec 27, 2012 02:42pm | #3

   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"

4. 
   user-1137156 | Dec 27, 2012 02:51pm | #4

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

5. Expert Member
   MALCOLM TAYLOR | Dec 27, 2012 03:12pm | #5

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

6. Expert Member
   MALCOLM TAYLOR | Dec 27, 2012 03:14pm | #6

   deleted

7. 
   Dayton | Dec 28, 2012 12:32am | #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?

8. 
   user-1137156 | Dec 28, 2012 05:34pm | #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.