Slab-on-grade perimeter insulation requirements

[bnickeson]

We are starting to see more jurisdictions go to more recent energy conservation codes which now require rigid insulation to pass between the perimeter edge of the slab-on-grade and the perimeter concrete stem wall. Prior to this, we always just placed a 1/2" expansion joint between the edge of the slab and the stem wall (or CMU as the case may be) and called it good. But the new energy codes require a very awkward detail in our area of the country where 2" of rigid insulation must be placed between the slab and perimeter wall. I am concerned that the natural shrinkage of the concrete slab will cause it to pull away from the rigid insulation and leave a gap between the two. The building I'm working on now has a 130'x200' reinforced slab-on-grade with no sawcut joints so the shrinkage could be significant. Am I correct in worrying about this detail around the perimeter? Does anyone have any experience with this and have any recommendations on a proper perimeter detail? I suppose I could place a 1/2" expansion joint between the insulation and slab edge, but I've never seen that done before.

This energy code requirement is proving to be quite the pain and was clearly not written by anyone with any practical construction experience.

 

[driftLimiter]

Yea we've been struggling with that one for a few years now. I have asked them to punch the insulation to allow for dowels to pass through but its not my favorite. That's a pretty big slab with no joints I think your concern is valid but maybe its better for it to be free around the edge.... if you did have it locked in then you would expect more stress cracking wouldn't you? We have had some architects be able to take the insulation up to the bottom of slab then run it flat in towards the building a certain distance to meet the requirement (I think). I liked this detail more.

 

[bnickeson]

The slab is completely free around the edge with no attachment to the foundation at all, so it is free to shrink uninhibited. It's really just the insulation between the edge of the slab and the perimeter stem wall, with the question being what to do about the shrinkage gap that could potentially form.

The insulation below the slab and on the inside face of the footing are still there as has been required for quite some time, but the insulation between the perimeter wall and slab-on-grade is something relatively new.

 

[dik]

Best to have the insulaton on the outside of the foundation wall, beneath parging if main floor is significantly above grade. Separation of the foundation wall from the SOG, by a significant amount, may be a local requirement, and a thinner insulation space in the order of 1/2" may work.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MotorCity]

My first concern is your jointless slab. I know there are companies out there that have proven methods and products, but these slabs are unicorns in my area. Whether you use rigid insulation or expansion board around the perimeter, the slab could shrink and pull away from the wall. How did you handle this in the past only using expansion board? Also, when the slab shrinkages, friction created by the weight of the slab will develop between the slab and subbase. The entire weight of the slab will be "dragged" across the subbase as it shrinks. Without joints to mask the cracks, how do you propose to eliminate cracks in the slab, or are cracks acceptable? Regarding the insulation detail, place rigid insulation on the outside of the stem wall. Above that, the insulation in the building wall will take over. You can continue using expansion board between the slab and stem wall but all you really need is a bond breaker. We typically use building paper or tar paper.

 

[bnickeson]

I've attached the section I'm discussing since there seems to be some confusion about what it is. We cannot place insulation on the outside face of the wall since grade comes up within a couple inches of the interior slab elevation.

With regards to the large jointless slab, that is necessary because it's a basketball court floor. The flatness requirements generally mean placing joints and grinding the floor down in numerous places or pouring a jointless floor. We're going with the latter and reinforcing it with rebar to keep cracking minimal.

 

[driftLimiter]

I've heard great successes doing jointless slabs in my area. It seems like the builders are ahead of us engineers on this topic. There are admixtures that can nearly eliminate cracking in lightly reinforced slabs. I guess the shrinkage gap is mostly a problem for: surface finish, maybe building envelope? Not really sure if there is another structural consideration beyond the slab being tied into the stem vs not being tied in.

 

[dik]

The insulation on the outside face is far more effective. As it stands, the gradebeam is cold and transfers more cold to the slab beam interface. If the gradebeam were 'warm', then less heat would be transferred and the 2" 'slice' of insulation may be reduced to 1/2". The insulation under the slab is also less effective. Pricier, but even horizontal insulation outside the perimeter of the building allows geothermal heat to assist.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik