Slab-on-grade perimeter insulation per IECC C402.2.5

[bnickeson]

The energy conservation code has - apparently for some time now - had a requirement for insulation at the perimeter of a concrete slab-on-grade. This requirement is pictured below.

The key language in there is "The insulation shall extend downward from the top of the slab". Up until recently, virtually everyone in our area of the country had either not known about this requirement or had ignored it. Building officials didn't seem to know about it either because no one had ever commented on drawings about it until relatively recently. Now a few of them have started taking notice. Basically this requirement is specifying that the perimeter of your slab-on-grade has to have 2" of insulation between it and your exterior wall. Your exterior wall could be masonry, concrete, a small concrete stem wall under a light gauge exterior wall (as is most common in commercial projects around here), etc. Aside from this requirement being utterly ridiculous, it presents some very annoying detailing and construction challenges. I've pictured three typical construction details below...assume that the exterior grade is level with the slab. The left one is our classical way of detailing a perimeter wall, the center one uses the 2" of insulation between the slab and perimeter wall, and the right one tapers the insulation and slab to a tip which is apparently permitted. The two "code-compliant" details on the right are a pain for obvious reasons. They're unsightly if you have exposed concrete floors, the tapered tip of the slab is likely to chip away over time, they don't offer a sealed joint to accommodate slab shrinkage, door thresholds at exterior openings are a nightmare, and architects tell me they're not supposed to leave this insulation exposed due to fire reasons.

So I'm asking a) Is everyone else here detailing their perimeter edges according to this code provision; and b) if you are, how are you detailing it? Over the last couple of years neither us nor our architectural clients have figured out a good way of detailing this without something being a problem. And since this detail is incredibly pointless even from an energy conservation perspective, I'm at my wits end with it and am looking for suggestions. Thanks in advance.

 

[WesternJeb]

Same boat as you with local architects and no one knowing about this code provision, even though we have brought it up a couple of times.

Our local code officials either don't know about this provision, or just don't care. I think I actually heard one of them note that they thought this code provision was silly / thought the cons outweighed the pros and so they never marked it up.

If it gets marked up on the submittals, I would petition the code reviewer with a few reasons why it simply isn't a good idea, and hope they have half a brain, and move on.

 

[dik]

It's better to have the insulation on the outside of the grade beam.

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

-Dik

 

[phamENG]

We went through this in my area about 5 or 10 years ago...and still going through it with some architects who managed to resist it back then.

1) I agree with dik - exterior insulation is better, and a lot of areas are going to a R(interior)-(exterior) rating...so you fill between the studs with insulation, and then you have to run continuous insulation on the exterior. It helps to reduce loss through the thermal bridging of the studs.

2) Nobody actually does number 1 in my area (hot summers and mild, damp winters), so I go for the middle one if it comes up at all. The left one doesn't comply and the right one will just break. Never try to make concrete form an angle like that. It'll just break. It's why we chamfer corners. If we don't think it can manage an exposed 90 degree angle, what makes you think it can do 45!?

Don't worry about the couple inches of insulation around the edge. By the time you add in 1/2" of gyp, 3/4" of base board, and a bit of shoe molding, you might have 1/4 to 1/2 of floor that could be 'walked on' that isn't directly on concrete. And the insulation you or the architect are spec'ing should be capable of supporting the design live load anyway as it is under the slab and in the load path.

 

[dik]

extruded polystyrene, load rated as required.

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

-Dik

 

[bnickeson]

It's better to have the insulation on the outside of the grade beam.

How does that work with something like brick veneer? Seems impossible to have continuous exterior insulation when the brick is bearing on your foundation. Also, I wasn't really given a reason, but apparently architects hate the exterior insulation option.

Don't worry about the couple inches of insulation around the edge. By the time you add in 1/2" of gyp, 3/4" of base board, and a bit of shoe molding, you might have 1/4 to 1/2 of floor that could be 'walked on' that isn't directly on concrete. And the insulation you or the architect are spec'ing should be capable of supporting the design live load anyway as it is under the slab and in the load path.

It's an aesthetics issue. You don't have materials like gyp, base board, and molding to cover it up when your perimeter wall system is a curtain wall or storefront. Without flooring, that insulation is completely exposed. It's what I'm facing on our current project.

I'm completely in WesternJeb's court here, the provision is ridiculous and any jurisdiction with the slightest bit of common sense should ignore it. I've tried to contact ICC about it but I need to pay $200/yr to even ask a question.

 

[phamENG]

Brick veneer actually makes it much much easier. There's usually insulation between the brick and the wall which can be carried down to the foundation. You may want to change up the kind of insulation below the weeps (making sure it's closed cell and not susceptible to absorbing water), but still quite doable. Then, run more insulation on the outside of the brick/block below grade. It's not perfect, but the bridging effect of the footing isn't really something to worry that much about.

I totally disagree that these are ridiculous. As somebody who lives in an older home that was not designed with energy efficiency in mind, and knowing what modern homes built to the energy codes are managing in my area, my power bill hurts. Are there upfront costs? Sure. But they're usually worth it. There's usually someplace to hid the insulation. Here's a system that looks like it's based in Iowa, so not too far from you: Thermomass. I'm sure with a little effort one could find the necessary background to design something similar that could be site built without anything proprietary.

 

[dik]

You detail it so the face of the grade beam and insulation and parging if you have it provide for your normal brick overhang. The insulation may stop a few inches from the brick support. There is a bit of a cold spot, but it's a lot better than having the entire grade beam subject to freezing. Running the insulation under the slab reduces any geothermal heat in the soil beneath the building.

There is also foamed glass that can be used for load bearing conditions with veneer that provides an insulation effect. Foamglas by Owens Corning.

https://www.owenscorning.com/en-us/insulation/foamglas

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

-Dik

 

[dik]

pham... "But they're usually worth it." I suspect these costs will be increasing quite a bit...

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

-Dik

 

[bnickeson]

A couple comments on the latest posts. First, interrupting the exterior insulation where the brick bears on the foundation completely defeats the purpose of the continuous insulation requirement. Allowing the thermal bridging between the brick and foundation is worse than the original detail of the slab butting up to an expansion joint in terms of thermal transfer. I did find that Owens Corning makes an insulated rigid material called Foamglas Perinsul that can replace the lowest course of brick and maintain the thermal isolation continuous insulation requirement. It's only good for stacking brick up to about 20 feet though.

And I obviously wasn't referring to all energy conservation requirements when I said this was a ridiculous provision. I just meant this one in particular. Realistically, how much thermal transfer is there going to be into this slab with the classical way of detailing it? Is your slab going to get, what, two or three degrees colder at the perimeter? It has to be practically negligible.

I see dik just posted the Owens Corning product as well...

 

[dik]

For light loads, like veneer, you can used foamed glass as I noted. For heavier loads like transferring loads through building walls for pipe racks, etc. I usually use a Fabreeka product.

https://sweets.construction.com/Man...reak--Thermal-Insulation-Material-NST577712-P

30 years back I used to use HDPE for a thermal barrier.

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

-Dik

 

[dik]

Not totally ridiculous... but, none are particularly good.

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

-Dik