quick question,
architect drafted up plans that show the 2″ foam under the slab in the basement laying on top of the footing. Mason says he always puts foam up to footing flush with the top so the slab sits right on the footing. does it matter?
I see a complete thermal break doing it the way drawn, but will the foam give at all and allow cracks?
Join some of the most experienced and recognized building professionals for two days of presentations, panel discussions, networking, and more.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
i agree with the mason, i've never seen the foam actually placed on top of the footings...and the thermal break is either around/under the perimeter of the slab..
the foam is gonna be placed over flat, well-compacted gravel right?...that's normal practice in my neck of the woods, cracking never a problem..
In doing the research for the insulation of my new shop, which has 2" hi-density foam boards under full slab, not once did I see a drawing that showed foam boards on top of the footers. Call and check with the inspectors, my bet is that they tell you foam is not load bearing and you can't put it there.
Steve.
The foam is load bearing or you would not be using it under the slab. The stone within the footings should be solidly compacted, so it should not move differently from the footing.
The foam forms a useful thermal and capillary break, reducing moisture and chilling around the perimeter of the slab, which reduces the likelihood of mold on the edge of the slab.
According to my building inspector, things that hold up the building are load bearing. Had required inspections of the footers and knee walls before putting up the building. No inspection of the slab or gravel base under it was required.This is in North Carolina. YIMV (Your inspectors may vary).Steve.
Things that bear a load are load bearing. If you put foam under a slab, the slab bears on it. The slab is a load, therefore the foam is load bearing.
None of that has anything to do with what is necessary to inspect.
In my shop the slab and contents (around 30 tons) are bearing on 1053 sq ft of foam. The building and second floor contents (around 15) are bearing on 50.5 sq ft of sill plate. Makes a difference.I guess we disagree on the inspector's perspective. IMHO it would be a real drag to have the inspector flunk the framing inspection and require removal of the foam under a sill plate. Don't know if that would be the case in hvtrimguy's area but that's would happen in this county. If I was hvtrimguy I would sure as heck ask before the plans are done.Steve.
Pay attention to thermal breaks, very important.
I have a 70' long 4" slab sitting on xps, over my footings. Thermal break was my reason. I drive a 4500 lb tractor on it, no cracking.
PAHS Designer/Builder- Bury it!
Tom, seen lots of your posts, always interesting, but I'm sort of in shock here....foam over footings sounds nuts to me....at that point it seems like you just have a floating slab, & no real need for footings at all.....
Thanks, jrnbj. Confusion here somewhere...
You're right, mine's a floating slab, no footing connection at all. But my extremely heavy small house (~350 tons) doesn't sit on a slab. It rests on the footings. There's no connection between either interior or exterior slabs and the walls/footings.
The important point here is to consider interior mass and what's exterior of the insulated shell. A thermal break is what you want to disconnect the two. In my case, footings are part of my interior mass. The slab I mentioned (over the xps) is exterior. The building department didn't want to understand why I insulate under an exterior slab and not my interior one. Not complicated, necessary for my system. We got into it when I asked for a footing depth variance. After my first house like this, they paid little attention.
My insulation umbrella goes down the exterior of the walls, across the footing, and extends under the exterior slab. Slab floats there just fine. I used plastic, rather than welded wire. No need for rebar.
I do have a tractor I don't drive there. At 23,000 lbs it might crack the concrete. Don't remember which xps I used, but it comes with different compression ratings. That's all you have to pay attention to. For instance, that xps goes under a portion of my rooftop retaining walls. Not much weight on it, no issue. That thermal break is compromised with periodic rebar sticking through, so the retaining wall doesn't slide off the xps and kill somebody.
More important than losing a little heat. <G>
PAHS Designer/Builder- Bury it!
"But my extremely heavy small house (~350 tons) doesn't sit on a slab. It rests on the footings. There's no connection between either interior or exterior slabs and the walls/footings."
"My insulation umbrella goes down the exterior of the walls, across the footing, and extends under the exterior slab."yup I'm cornfused....still sounds like your walls are sitting on top of foam.....
yup I'm cornfused....still sounds like your walls are sitting on top of foam.....
Which may be the crux of this whole thread.
Far as I know, footings always extend beyond where the wall sits. Otherwise, they wouldn't be footings. That leaves a few inches, or feet, where a slab could sit on insulation right next to a wall that has no insulation under it.
Here, my 12" cast-in-place walls sit on at least a 24" footing. That leaves 6" of footing on both sides of the wall. That 6", which is not under the wall, is where my xps lives (with a slab on top).
My footings, and probably everybody else's, are not totally topped with insulation. Only the outer portions. The only place I have a wall sitting on insulation is my rooftop retaining wall, which weighs very little. With upwards of 7000 lbs/lin ft (49 psi) of house wall, I don't have that much foam compressive strength.
Looked at an old Amoco bulletin which lists their xps in the neighborhood of 9-12 psi with 10% deflection. No problem for a slab, big problem for my house walls- if I'd tried that.
Hope this cleared the air. PAHS Designer/Builder- Bury it!
There are some good building details here:
http://buildingscience.com/designsthatwork/default.htm
The foam will support the slab, unless the center isn't compacted properly.
If the entire weight of the slab ends up on the edges it will sink or crack or both.
The problem will not be the foam, but the fill.
EPS will do the job, you don't need the high buck blue or pink.
Joe H
i agree with JoeH that there is a far better chance of cracking if the weight of the slab is on the edges (footings)...if the base under the slab gives just a little u could be in trouble...i wouldn't think it to be worth the risk...u can achieve thermal breaks without putting the foam on the footings...
Does the foam also go up the side of the slab?
There should be a break there too, 1" or less will do as long as it isolated the slab from the wall.
Joe H
drawing shows a 4" rip of foam between the slab edge and the inside of the wall.