Stem Wall Foundation Insulation--is 4" too much? Any Benefit?

[Super400]

I am currently building a 26x26 attached garage in Northern Michigan. It has a 4-block stem wall foundation. I just finished damp proofing the exterior foundation using Duck Coat (rubberized elastomeric polymer). Code requires 2" of insulation on the inside of the foundation before foundation is backfilled. I installed 4" XPS instead of 2". Did I just waste my money or is there a benefit to the increased thickness at this location? To tell you the truth I'm not sure what the purpose of the vertical insulation on the interior of the stem wall actually is? Will it actually keep the garage warmer in the winter?

I would like to also explore adding some more XPS to cover the top block and create a thermal break from the slab and top block around the perimeter but I'm not finding any drawing or example of how this would look on a garage floor. Lots of examples of a slab for a house but not for a garage with a big door and a sloped floor with a drain in the middle. Any thoughts or suggestions would be appreciated.

I attached some pics so everyone can see the project.

Thanks!

 

[PopcornSutton]

Usually there is XPS laid horizontal from the insulation you already have extending 2 feet inside.

 

[mike93lx]

Here's a picture of insulation detail.

Is that top block stepped in for the slab?

 

[hobie18]

Great information

 

[mike93lx]

Extra insulation always offers some benefit, but at a point, it may not pay back. Looking past the added cost that you have already spent, you'll never wish you had less. That part of the wall is where a lot of heat will be lost to cold outside air, so I'd say it was a good move.

 

[billconner]

Technically, that 4" (3 1/2 maybe) top of stem wall is not compliant with IRC but I believe is often allowed.

Beveling too of foam so it ends at top of slab is also common.

 

[mike93lx]

Technically, that 4" (3 1/2 maybe) top of stem wall is not compliant with IRC but I believe is often allowed.

Beveling too of foam so it ends at top of slab is also common.

IRC has a limit?

 

[ConCretin]

I doubt you'll see a payback from the additional layer in your lifetime and several more. The purpose of insulating the face of a frost wall is to resist frost penetration into the base material under the slab. Any increase in heating efficiency by going from 2" to 4" of insulation in this location would be tiny.

The best way I know to create a thermal break in the slab at the overhead doors is to stop the slab at the inside face of the overhead doors and place a separate apron pad that extends over the top of the frost wall. The apron can be placed slightly lower than the main slab and pitch away from the building creating a dam against water intrusion under the door. You can install expansion material or even insulation between the two placements to create a break between the interior slab and exterior apron. This detail works especially well with a radiant heated slab.

Plz forgive my hastily drawn sketch.

 

[theoldwizard1]

To tell you the truth I'm not sure what the purpose of the vertical insulation on the interior of the stem wall actually is? Will it actually keep the garage warmer in the winter?

It will certainly keep the floor warmer ! If you have in-floor radiant heat, less of that heat will escape.

Extra insulation always offers some benefit, but at a point, it may not pay back.

Correct !

Northern MI can get temps well below 0°F, so I would have gone 4" also.

 

[Super400]

Thanks for the info everyone! The top concrete block on the stem wall is a 6” wide block instead of the 8” block. This is because the wall will be framed with 2x6 studs and drywall installed eventually and this way there won’t be exposed portion or “lip” that I would get with and 8” block after framing. Others have done this in my area and it has passed inspections so I’m hoping it meets code. Attached is a pic of another garage in the area built this way. If you look at the bottom you’ll see what I’m referring too.

 

[billconner]

IRC has a limit?

I believe 6", and my comment pertained to Mike's posted detail - which I like but doesn't meet code because it's 4" at top best I can tell.