Insulating a floor / foundation tie in

[85Ram]

This post isn’t directly garage related but the project contains garage space. I am adding 800 sq-ft onto my home in Connecticut. The addition will be slab on grade with radiate floor heat. My plan is to insulate underneath and on the sides of the floor with 2” foam insulation. In the living space I don’t want the foundation walls to be higher than the floor (like in the garage). The foundation wall is 10” thick and the regular wall is ~6”. If the floor is poured even with the top of the foundation, how do I insulate the sides? Will the floor settle slightly, leaving an uneven floor around the perimeter?

I’ve done some research on the Garage Journal Board but most of the situations are monolithic slabs or garages with a higher foundation stem wall.

Thank you!
Joe

 

[GMCGarage]

Can you do a sketch? IF a slab on grade, why the foundation walls??

If the subgrade is prepared properly, it should not settle, or should all settle together.

 

[85Ram]

Here is a sketch from the architect. The insulation in the sketch isn't on the sides of the slab like it should be.
The foundation is needed to be bellow the 44" frost line.

 

[85Ram]

Forgot to mention, thanks for the quick reply!

 

[wssix99]

+1 that the floor should not move. As long as you have a proper compacted base and aren't putting it to an industrial use, it won't go anywhere.

The only thing that could move it is heaving. (From freeze/thaw cycles.) So... in your situation it will be important to keep the floor heated. (Even if you aren't using the space and want to save some money, you'll want to keep the floor heated when it's below freezing outside.) Heat will eventually soak through your insulation and heat the pocket of soil between your foundation walls under the slab, which extend to your frost line - so you shouldn't get any heaving. (That insulation on the inside of your foundation walls is critical, also.)

 

[wssix99]

BTW - I have a similar situation where my garage floor meets my outdoor sill. There is a gap of insulation between the two. If you want a nice finish there, (after the cure) you can carve out the top 3/4" or 1" of insulation between the two pieces of concrete and put in a gasket: http://slabgasket.com/

 

[nmk_61802]

Bevel the top of the insulation:

http://www.hydronic-heating.com/installation-details.html

 

[85Ram]

Thank you all, very insightful information. nmk61802, in that picture you sent the insulation on the sides doesn't extend below the frost line. Is that okay?
thank you!
Joe

 

[nmk_61802]

Personally, I would not get get hung up on getting insulation below the frost line. As mentioned above, insulation just slows down temperature equalization. If cold enough eventually the temp will hit equilibrium. On my garage (forced air heat) I used 24" sheets to insulate the foundation as it was the most economical use of materials, which left me approx. 6" short of the 30" frost depth in my area.

 

[Copymutt]

Besides beveling you can also run a border covering the gap. I've used mdf board and also cherry flooring. The field could be carpet or tile. Pic is of mdf with transition strip, done in 1996
Jim

 

[85Ram]

Jim, I like the boarder idea. I plan on tiling most of the area but was afraid the transition would cause the tile to crack. The orange isolation mat (Schueler?) is an option but expensive. Might also slow down heat transfer from the heated slab. Boarder would work.
thank you
Joe

 

[Ed Devinney]

Pretty much how my house and garage were built - foundation walls and an insulated slab poured afterwards, with a bevel. You can see the top of the beveled insulation in the garage, where I don't really care.

Unlike yours, our walls were set to the inside of the foundation wall and the interiortrim covers the edge. I do like Copymutt's wide horizontal trim - tile to the bevel and set a horizontal element that ties in nicely with whatever baseboard you use. It only needs to cover about an inch - could you cantilever the tile over the beveled foam?

 

[ConCretin]

You definitely want to insulate the vertical face of the frost wall to prevent frost from penetrating through the wall and freezing the soil under your slab.

To insulate the edge of the slab you could form a slab shelf in the top of wall i.e. step the wall in at the top. This will allow for insulation to be installed under the wall framing, which would cantilever out over it. This is a fairly typical detail that your concrete guy should be familiar with.

 

[wssix99]

Bevel the top of the insulation:

http://www.hydronic-heating.com/installation-details.html

The OP's vertical insulation will be a few inches away from the interior wall due to the extra thickness of the foundation, so this bevel would not be a good idea in this application.

The OP would get cracking along the bevel if any weight were to be put on the floor near the wall.

 

[wssix99]

Jim, I like the boarder idea. I plan on tiling most of the area but was afraid the transition would cause the tile to crack.

If you tile soon after the pour, you will get cracking. I would wait at least two months before tiling so your slab finishes shrinking. (Otherwise the slab will crack your tile as it shrinks and pulls away from the slab.)

If you tile across that gap, you'll also bridge your thermal break with mortar and ceramic. You may want to go with a mat, just to give you a little protection so that boarder tile doesn't wick heat away from your hydronic slab.

Or maybe you could do the border in carpet or something else?