Building insulation at slab.

[JStructsteel]

Got a building department saying slab insulation does not provide required R value. Current detail has insulation edge of wall, tapered to a point at the slab top.

New detail is something like this. I dont like it at all (focus on the insulation mainly)



What details are you folks using?

 

[jhnblgr]

What’s your floor R-value?

 

[jayrod12]

All of our insulation in 99% of our projects is on the outside face of the gradebeam. But we also swing down to -30 celsius without windchill.

 

[dik]

concur...

 

[Harbringer80]

I don't like that detail either. I would not have a stud wall partially bearing on insulation.

 

[dik]

I would not have a stud wall partially bearing on insulation.

The detail is modified, and the stud wall does not bear on the insulation.

 

[HTURKAK]

Northern Europe typical detail is similar with the following . Insulation should be at exterior face and continuous to avoid cold bridging.
ASCE 32-01 is excellent for this subject. ( Design and Construction of Frost-Protected Shallow Foundations )

 

[XR250]

I agree outside makes more sense but it gets torn up by the weed whacker and looks like crap after a while.

 

[JStructsteel]

Having the insulation to the face of the stud is fine, as long as the slab has a floor covering. They are doing a exposed polished slab.

I have never seen the detail with insulation on the outside.

I think this is all relatively new with changes to the energy code.

 

[JStructsteel]

The detail is modified, and the stud wall does not bear on the insulation.

Dik, what do you mean?

 

[jhnblgr]

Having the insulation to the face of the stud is fine, as long as the slab has a floor covering. They are doing a exposed polished slab.

I have never seen the detail with insulation on the outside.

I think this is all relatively new with changes to the energy code.

Pretty sure there’s nothing new here, it is just one of two options. Inside or outside. Code requires flashing overtop to prevent damage. Best option is to 45 the top so the concrete covers the foam. Polishing concrete close to the edge is difficult so large molding should be used to hide the edge. Concrete cups in the edges giving an uneven finish.

 

[XR250]

Pretty sure there’s nothing new here, it is just one of two options. Inside or outside. Code requires flashing overtop to prevent damage.

Around here, the tract builders cover it with some sort of stucco. Weed Whacker goes right thru it.

 

[JStructsteel]

Pretty sure there’s nothing new here, it is just one of two options. Inside or outside. Code requires flashing overtop to prevent damage. Best option is to 45 the top so the concrete covers the foam. Polishing concrete close to the edge is difficult so large molding should be used to hide the edge. Concrete cups in the edges giving an uneven finish.

If the past, the detail has always been insulation up the inside wall and under the slab. This is the first job that insulation has been spec'd to be vertical to the top side of the slab. Either we (me, architects) have been doing it wrong, or their was a change in the code.

 

[Enable]

I would doubt that it was a local code change so much as people are moving more towards Joe's details. You can find a bunch of Dr. Lstirburek's details for this condition here. If you only insulate the inside vertical face and underneath the slab, then the slab is still coupled to the stem wall, which reduces thermal performance. The bit of insulation at the slab edge is intended to mitigate that issue. However, the preferred approach (generally) is to put a continuous exterior borne insulation plane that begins at the footing (projected, if required) and continues for the entire building envelope. Though one drawback to that approach is that you need to protect the insulation at grade (see here).

 

[phamENG]

I usually just reject these details until they go away. The polished slab is a complication...when they insist I just tell them they have to move it over so the wall isn't bearing on the foam. The floor finish typically covers it. If they complain that it's not strong enough to support the floor, I counter with how they expect it to support the wall. That usually stops them.

If that doesn't work, then it's outside. Yes, cheap tract builders will give you a product that falls apart quickly, but better design, workmanship, and landscape design makes it work well.

 

[JStructsteel]

Thanks all. This is the first time I have had a building department comment on it. The submitted detail was insulation that tapered to the wall/slab interface, but they rejected that.

 

[JStructsteel]

I would doubt that it was a local code change so much as people are moving more towards Joe's details. You can find a bunch of Dr. Lstirburek's details for this condition here. If you only insulate the inside vertical face and underneath the slab, then the slab is still coupled to the stem wall, which reduces thermal performance. The bit of insulation at the slab edge is intended to mitigate that issue. However, the preferred approach (generally) is to put a continuous exterior borne insulation plane that begins at the footing (projected, if required) and continues for the entire building envelope. Though one drawback to that approach is that you need to protect the insulation at grade (see here).

The Dr. does not seem worried about the wall cantilevering over the foundation wall??

 

[Enable]

The Dr. does not seem worried about the wall cantilevering over the foundation wall??

I've seen this condition built. It's a non-issue if you use appropriately rated XPS (HL variety) coupled with some projection into the floor area. If you read the article you'll note that Joe even says while this can be done, it's annoying. And, as mentioned, exterior borne insulation is preferred.

EDIT - Unless you were referring to, say Figure 8, of the article? In which case, same answer about appropriate XPS. We support firetruck loading with it on garage roof decks routinely. There are no concerns with some small amount of wood structure. Also, see phamENG's response below. 2x6s are used for cavity depth to house more insulation, not strength. In a typical home, a 2x4 exterior wall would do just fine structurally.

 

[phamENG]

The tapered detail is garbage, too. There's a reason we chamfer 90 degree concrete edges to make a pair of 135 degree edges. How do you think a sharp 45 degree point of concrete is going to perform?

In most one and two story applications, the little overhang isn't going to hurt anything. Structurally, you can use a stud that's 2" shallower anyway. So whether you check it with a reduced section or a little eccentricity between center of bearing and center of applied load, it's probably okay. Where the real issue will lie is in the connection of the sole plate to the foundation wall. You're usually pushing those in a bit to get edge distance on the exterior if you have a 2x4 wall...but now that interior edge is encroaching and you'll run out of room. And don't say that it's fine, it'll be embedded down below. Most of the shear resistance occurs at the surface - if it doesn't work at the surface, then you're assuming it's cantilevering 8" out of the block below...

 

[JStructsteel]

The tapered detail is garbage, too. There's a reason we chamfer 90 degree concrete edges to make a pair of 135 degree edges. How do you think a sharp 45 degree point of concrete is going to perform?

In most one and two story applications, the little overhang isn't going to hurt anything. Structurally, you can use a stud that's 2" shallower anyway. So whether you check it with a reduced section or a little eccentricity between center of bearing and center of applied load, it's probably okay. Where the real issue will lie is in the connection of the sole plate to the foundation wall. You're usually pushing those in a bit to get edge distance on the exterior if you have a 2x4 wall...but now that interior edge is encroaching and you'll run out of room. And don't say that it's fine, it'll be embedded down below. Most of the shear resistance occurs at the surface - if it doesn't work at the surface, then you're assuming it's cantilevering 8" out of the block below...

Here is the detail I had.
I will have to check the connection to the footing if they take 2" away to have insulation.