Insulating the top of the slab?

[zeekh]

I built a garage a few years back. Like an idiot, I did not insulate under the slab . I'm gearing up now to insulate the walls ceiling etc. I've read of people insulating the top of the slab with XPS foam board. I'm sure that would be OK for just people walking on it but what about driving a car on it. Obviously, I put some type of flooring on top of the foam board. Any thoughts on this? Maybe someone has done this. I live in upstate NY so it can get pretty cold in the winter. TIA for any thoughts

 

[Steve W.]

Virtually ANYTHING you put on top is going to compress when you drive on it. You will need to put another slab of concrete on top to make that work.

In other words, ... don't bother.

 

[Firebrick43]

Some people build a floating floor over concrete with 2x4 sleepers with 1.5" thick xps in between and then a wood sub floor. Its superior for comfort and heating but is typically done in wood working shops and such. While it would be fine from a weight standpoint if the subflooring was 2 layers alternating in direction to hold an auto, you wouldn't want to bring one into the garage during a snow storm or rain storm. Leaking oil wouldn't be good for the floor either.

 

[racecougar]

Bingo. Is this a storage garage or one you're going to park daily drivers in?

 

[zeekh]

Kind of in-between. Maybe in winter wife will park her car when it snows. The of the time it's more of a work shop. Planning on a car lift at some point.

 

[Firebrick43]

Kind of in-between. Maybe in winter wife will park her car when it snows. The of the time it's more of a work shop. Planning on a car lift at some point.

In light of that, I would dig down around the perimeter of the building and put 2" xps on the walls to the footer or put wings similar to a Shallow Frost Protected foundation. Both while not performing as well as full underfloor insulation will perform much better than your average structure.

 

[CraigStu]

I wouldn't bother. Grab several of these mats for where you stand most often.

https://www.harborfreight.com/anti-fatigue-roll-mat-61241.html

Or look at L and HD for the square interlocking rubber mat kits. The ones I have seen are a bit thicker than the HF roll mat. One more option is a pair of boots w/ thick soles for the winter.

 

[Mr.N]

I would look more into what Firebrick posted. I've heard of using the whole 4' to go down in MN, but our frost line is much lower than NY.
- foundationhandbook.ornl.gov

 

[zeekh]

One of my fears with insulating the vertical sides of the slab is undermining the compacted soil around the edge of the slab. I don't have a frost wall just an Alsakan slab

 

[Firebrick43]

If you dig, install the insulation, and back fill in a day or two while its dry you are not going to have issues undermining the foundation. If you leave it open for weeks during rain or freezing/thawing, possibly.

If its an alaskan slab does it already have insulation? It should as that is AKA a shallow frost protected foundation. If it doesn't have the insulation its just a monolithic slab, and depending on the size I am surprised they allowed it to be built like that.

 

[ConCretin]

If your monolithic slab is built on relatively free draining granular material, you are fine from a structural standpoint. There is no need for insulation under or around the slab. If you are heating the building however, there is a probably a nominal benefit to insulating the perimeter face of the slab and extending insulation out a few feet horizontally.

The installation of under-slab insulation might have been minimally beneficial but unless you are using in-slab radiant heat, I doubt you'd notice the difference. I can't imagine it's worth the time and money to try to do something on top of the slab.

 

[zeekh]

If you dig, install the insulation, and back fill in a day or two while its dry you are not going to have issues undermining the foundation. If you leave it open for weeks during rain or freezing/thawing, possibly.

If its an alaskan slab does it already have insulation? It should as that is AKA a shallow frost protected foundation. If it doesn't have the insulation its just a monolithic slab, and depending on the size I am surprised they allowed it to be built like that.

No insulation underneath. Form and rebar were inspected before the pour.

 

[The Cobbler]

I would forget about insulating the slab. it's a lot of work after the fact and it won't make that much difference , really .
Just put some mats down where you stand.