Subslab insulation and bearing wall footings
I am getting ready to do the foundation work on the new home we are finally getting started with. I have some load bearing walls in the basement that will sit on poured footers. The footers are -2″ down from the bottom of the slab so the insulation can extend over the projection and run into the foam on the ICF wall. However for the interior load bearing walls, I was considering installing these
-2″ as well and continue the foam over top of them, and then the slab. As we all know compressive strength of XPS foam far exceeds loads imposed in residential applications.
Any see any problems with this that I am not seeing or other suggestions?
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That foam easily supports the loads imposed by residential construction is a dangerous generalization and you shouldn't be designing your structure based of such suppositions.
A point load on the slab can easily be in the region of 10,000 lbs. This load is spread by the footing to a larger area of the soil or fill below which has a bearing capacity of around 2500 lbs psf. By placing foam between the footing and the slab experiencing the point load you are replacing concrete with a capacity of 3500 psi with material at a fraction of the strength. It is a very bad idea.
I never said I was putting a point load on the foam or on the slab. I do have 3 point loads that have isolated spread pads with the post sitting directly on the pad, -6". I probably should have clarified my statement with foam can typically support most uniform loads in residential applications.
The question is what people are doing to maintain continous foam under bearing wall locations with uniform loads from floor structure only above.
There are a number of ways of doing it. The simplest is using integral footings - which are essentially a thickened slab where bearing is required, and the foam can till be continuous underneath.
I am sorry f my response seemed abrupt but I have noticed a disturbing trend among GBA posters who appear to have transferred their experimental approach to energy issues to the load bearing parts of their buildings. This is serious stuff with serious consequences and shouldn't be approached in a speculative way by people with little or no construction experience.
I appreciate the response and concern, and do agree with the general assessement of some posts. However I would not be so quick to over assume or generalize the experience level of posters.
The typical way we design is with a thickened slab, as you say this is easy. Trenching, cutting the foam and angling it down into the trench for the thickened slab does not seem like the best/easiest way, which lead to my question. (at least I am assuming that is what you are implying).
Jesse, the fault is mine. I shouldn't post here. Hope you get the advice you need and good luck with your build.
Have you got any detail drawings or sketches?
If i understand you correctly, you are saying there will be insulation between the footings and the slab?
If so, how would the footings tie into the slab?
In case you're interested, here are a couple of links to my blog that explain how I did my own footings and foundation so that the entire thing is wrapped in rigid insulation:
There are several "penisula" footings for load bearing walls. The biggest drawback to the approach I chose was the need to backfill and compact between the footings before the slab could be poured.
Malcolm- I certainly did not imply not posting, your information is valuable.
Lucas- thanks for the links. I have got through your blog in the past, but must have missed that one. yes, the thought was to separate the slab from the continuous bearing footing. Mind you, this is only interior uniform loads. exterior bearing footings the ICF walls sit directly on the footers. I did a quick detail of my thought. If I need to trench it out and take the foam down into the ground under the footing (pouring a monolithic slab/bearing footings) I can do that too. However I thought something like this may be cleaner/easier.
Thanks for the quick detail.
I've never seen anything quite like that before...
Malcolm's concerns about concentrated loading of the foam may not be unjustified and if it were my own home, I would like the footing and slab to be physically tied together somehow.
Do you have access to a creative/interested engineer?
If so, it would be interesting to hear what they would have to say about your proposal.
I keep thinking about how in this day of TJI's and glulams real bearing walls are rare. My grandparents house[ca 1940] had a beam and wall in the basement that held up the bearing wall on the first floor and the bearing wall above that.
My point is, have you calculated the weight on that wall? If foam can transfer the load in your detail [IOW 20 PSF or less] then I cannot see the point of the footing at all. A post point loads a floor and needs a footing. A wall that holds up half the first floor and half the second floor and half the attic needs a footing.
What is this wall holding up?
the wall is 24' long, holding up only a single floor with no additional loads on it other than floor L & D loads. The tributary area on the wall is about 375 sqft.
SO 375 sq ft at 40+10 is 18750 lbs supported by 7 sq ft of wall[3.5/12x24] 2678 lbs/sq ft or 18.6 lbs/sq in
So you are up there.
I still don't see that your sketch would be any different whether the footing was there or not..
To me the question is if you are going to put in a footing why no put in two square footings and a beam so 75 years from now when someone knocks that wall down the house does not sag.
My gut is that a bunch of extra rebar at a right angle to the wall would be sufficient[and more useful than a footing under foam] but as has been mentioned I would consult an engineer. Get a recommendation so you get a guy who is familiar with foam under footings and the like.
our numbers differ slightly from yours, as I was using 15lbs for dead loading and a 2x6 wall for bearing (11 sqft) which puts it just about 13 psi.
I ran it by an engineer we use for a lot of commercial stuff, and he said he thought the formular 250 would be fine and the detail should work, but would feel more "comfortable" doing it in my revised detail. It really is not any harder to do it this way.
I would support the load bearing wall directly on the footing by way of 4" wide x 6" high blocks. The sub-slab foam would then sit between the footing and the slab, butting up to the 6" block. Same way on the exterior footing, isolate the slab from the footing and the foundation wall with rigid foam.
Just remember that you need foam which has an advertised psi that is 3x what psi you are loading on it, or it may creep. Your 13 psi then needs 40 psi foam.
Hi Jesse....I have the same exact condition you are describing and discussing here. My engineer calculates that our bearing wall needs to support 700 lbs/ft...and I was planning to install 40 psi EPS foam between the footer and slab so as to get a continuous layer of insulation....which with a 2x6 bearing wall above can support 2400 lbs/ft. Can you tell me how you ended up handling your situation? And now that we are 4 yrs removed from the discussion....how have things performed?
Much obliged for a prompt answer as I need to make this decision in two days!
It looks like your questions have been posted on two different pages. In case any GBA readers are interested in following this thread, it continues on a different page -- see the link below: