Slab insulation install question ( any photo's available)

scott w.

Homeowner preparing a basement garage floor pour for radiant heat. Not sure how to properly install the insulation for the break around the perimeter walls. Never did this before and have to instruct the concrete guy of how to do this properly. Is the foam board around the perimeter walls visible in the slab after the pour? Or is it installed just far enough below the surface of the slab so concrete covers it and the slab meets at the perimeter wall ? Installing foam board around the perimeter and having two inches of foam visible in the floor is not going to be attractive. Have read some prior posts on this topic but still confused. Anybody have photo's of a slab before and after the concrete pour? Staining and sealing the concrete will be done at a later date. Much thanks for any advice on the proper way to do this.

Jamie Hall

First comment -- be sure you also have insulation under the slab. Not just the edges.

Second. If it were mine to do, I'd insulate outside the foundation walls, not inside, and pour the slab (with an expansion joint and seal) right up to the walls.

hot_rod

A few options, none of them great
Hold the foam down 2”
Bevel the top edge of the foam at a 45

You want that edge insulation for heat loss and expansion of the slab as it warms

Will the basement be finished? A 2x4 plate hides the foam if walls are framed

Zman

I like this detail. Putting the insulation on the outside is theoretically a great idea. In practice, it is tough to detail the portion above grade, especially on sloped lots. The insulation ends up getting damaged before the project is finished.

PC7060

Agree with @Zman, Exterior insulation needs to be considered as part of the overall design and appearance, difficult to hide it otherwise. On this addition, we tied the XPS into a beveled upper detail to complement the original foundation characteristic.  And a lot of water shedding materials and flashing was included in the process.  

I’m also a big fan of closed cell foam under slab but would hate to write a check for it at todays prices. 

psb75

Hey, why not use "Glavel®" under your slab? That is a foam-glass "gravel" product. It offers R1.7/in. Presently available in Burlington, VT. Recycled glass.

Jamie Hall

psb75 said:

Hey, why not use "Glavel®" under your slab? That is a foam-glass "gravel" product. It offers R1.7/in. Presently available in Burlington, VT. Recycled glass.

If you can keep it dry, it's fine. If you can't, it's a good structural support -- and worthless as insulation. No better and no worse than any other granular material of similar size range.

Larry Weingarten

Hi, Building Science has done some thinking about this question: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/slab_edge_insulation.pdf#:~:text=Slab is insulated vertically at the edge and,for Further Research: “Understanding Basements,” Building Science Digest-103,
I built my home using insulated concrete forms and SIP panels on top. This is what the connection looked like. The ICFs were wider than the SIPS and I added insulation outside of the ICFs, so needed the bump out that the flashing shows.

Inside, I like @hot_rod 's idea of using a 2x4 or finding a plastic "U" channel to put on top of the foam to make it sturdy, and able to deal with shrinkage and thermal issues in the slab better.

Yours, Larry

hot_rod

Zman said:

I like this detail. Putting the insulation on the outside is theoretically a great idea. In practice, it is tough to detail the portion above grade, especially on sloped lots. The insulation ends up getting damaged before the project is finished.

Have you done a spray foam under slab, like shown in the detail? It seems to be more common here in Utah, compared to 4X8 sheets? They spray the sub-grade, mesh and tube.

psb75

The Scandinavian system has a 48" apron of 2" pink board just below grade, sloped away from the OUTSIDE of the slab around the entire perimeter of the bldg. It is then covered with topsoil. The edge in contact with the foundation wall is at the same depth as the interior layer of sub-slab insulation. That certainly increases the pink board line item in the bldg. materials budget and may affect your landscaping plans. But consider the long-term operating costs of heating the bldg,

scott w.

I want to thank everyone for their comments and am a bit late in the reply as I caught the virus in July and was down for a week. Comments all very helpful. The garage is under the house built in 1928 so outside insulation is really not an option. Also unable to insulate the walls with 2x4's due to space constraints. Gonna have the plaster guy re-stucco the walls for a new look. Have to take out the old floor due to cast iron drain pipe issues. House has a a combo of in floor heat and cast iron radiators and a WM condensing boiler installed. Boiler and in floor heat done on the initial renovation 17 years ago. The destruction and construction for the garage project starts Aug 4 and am excited about having a warm concrete floor in the garage. Really want the floor insulated as best as it can be done. Want to save as much as possible on the heating dollars.

HomerJSmith

Use ridge foam board not the bubble wrap stuff in the underlayment insulation. Why would one spray foam the 6 X 6 X 10 mesh? But...whatda I know.

Zman

HomerJSmith said:

Use ridge foam board not the bubble wrap stuff in the underlayment insulation. Why would one spray foam the 6 X 6 X 10 mesh? But...whatda I know.

Spray foam is nice because it leaves no gaps underneath and it doubles as a moisture barrier. I don't think anyone is thinking that the mesh or tubes belong in the foam, I think there was some poor wording

TAG

Is the floor at grade ?

Ideally having some thermal break is best .... there is no perfect system. On the front edge of a drive I use the traditional tar strip at the garage door and apron and it cuts that heat loss. It's not going to work like a 2" strip of foam. Normally I use 2" foam under the slabs and have 1.5" come up about an inch above the slab .... then I cover the walls be they block or formed concrete w/ PT 2x on the flats. One of the 2x being horizontal along the top of the foam for drywall/ baseboard. The spray foam guys do an 1.5 inches of closed to fill in between all the 2x. Exterior insulation can be done and I have done it .... but it requires various things to make it all work with traditional exterior products. KISS doing inside works and the small insulation loss has never been noticeable to me in the Mid-Atlantic climate.

Are you doing no interior insulation on the old walls ?