extruded polystyrene question

[mybuickgs]

This is my first experience with Radiant heat and for my contract as well. Thanks for answering my previous questions. Here is another one.

I am building on a grade with will leave a 3 to 4 foot stem wall exposed on a good portion of the garage. Is there any benefit to extend the insulation on the inside stem wall past the point of the slab? And if so how far? See diagram for added explanation. I live in Kansas and have moderate to cold winters.

 

[Racecarl]

I live in SW Nebraska. I extended my vertical insulation down 4' from the top of the slab and the horizontal insulation 4' from the inner wall. The outside grade on my shop is just below my slab level. I think I would err on the side of caution and get the insulation down to at least the frost line (in my case 42"). I don't have anything built on my slab yet but the snow and ice will not melt where the insulation is, so it must be working.

 

[mybuickgs]

Thanks Racecarl.... That seems to make sense. Anyone else out there agree or have a different experience?

 

[sneezer41]

when heating the slab I would insulate under the entire slab and a bit below the insulation for the walls, but I think going much below that is a waste of time. Heat is going to take the shortest path. If you do not insulate under the entire floor then get at least below the frost line on the walls.

 

[tdkkart]

I won't get into the whole story here, but from my experience with my own system I would say that this portion of your insulation is not to be taken lightly.
I did mine just like your picture, down 24" vertically from the top of the slab, and 2" under the slab. Knowing what I do now I would do more.

The goal here is to prevent heat transfer from the heated slab to the outside, or any other cold area. In your case, doing all you can to prevent heat transfer to the stem wall, and from the stem wall outside is going to be key. Since a fair amount of your stem wall will be directly exposed to the elements, keeping the heat out of your stem wall will do you the most good.
What you can spend on this will of course be the limiting factor, but I can tell you that the money will be well spent.

In your picture, I would make the vertical piece at least 24", if not all the way to the frost line for your area, AND I would at least consider doubling the thickness for about 1/2 that depth.
As a minimum, use 2" vertically right to the top of the slab, basically use the foam as your concrete form. Run this piece down to your frost line.
Then, consider running another piece around the inside of the first from the underside of your floor foam down to approx. half the depth of the first piece. Most people I've seen do not do this, but again you are trying to keep your heat INSIDE. Some of the heat from the slab will reach the ground below your insulation, we need to keep it under the slab.

It appears that your stem wall will be no higher than your slab?? As long as none of your stem wall will be exposed to the inside of your building I wouldn't worry about insulating the outside of the stem wall, as long as you've done all you can to keep the slab heat away from the wall.

I did notice that you are in Kansas, depending on you exact location you could back off on the insulation slightly compared to what I need here in Iowa.

 

[Teikas Dad]

Take a look at this:

http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf

That's how I built my foundation, with 1" extruded poly on the entire outside of the foundation and 2' out from the foundation 2' under grade level.

 

[thammel]

As long as the slab is totally surrounded with insulation underneath and on the side, there's really no significant advantage to extending the side insulation down deeper. This would only be advantageous if for some reason you did NOT insulate the bottom of the slab. There is only a trivial benefit to extending it downward if the bottom of the slab is insulated well. I've been doing thermal analyses for 35 years in a high tech energy industry so this is just more than a random opinion. The key here is the heat loss or conductive heat transport calculations. To do this thoroughly, the best approach would be a FEA model where an isothermal boundary is taken at the frost line, say. Then the model would be run both with and without extended side insulation. Given that the slab heat perturbs external temperatures very little due to low heat flow from the slab to the exterior, the side insulation extending downward should have very little impact on net heat losses.

Now, that the technical nerdy answer has been supplied, just be sure to wrap your slab (really, the entire volume that is desired to be heated) well and be sure to snuff out any leaks through gaps.

Tom

 

[sillyhillbilly]

This is my first experience with Radiant heat and for my contract as well. Thanks for answering my previous questions. Here is another one.

I am building on a grade with will leave a 3 to 4 foot stem wall exposed on a good portion of the garage. Is there any benefit to extend the insulation on the inside stem wall past the point of the slab? And if so how far? See diagram for added explanation. I live in Kansas and have moderate to cold winters.

to below your frost line
or the top of the foundation of the wall

 

[Dead Skunk]

What Teikas Dad showed is very similar to Swedish home building standards. What the extra apron of foam does do for an insulated slab is allow ground temperature (about 50 deg. where I live) to come up to the underside of the slab insulation. This will reduce the heat conducted through the slab insulation, especially near the perimeter. Doing the apron rather than insulating a foundation allows them to avoid digging down as far. Probably 6 of one and half-a-dozen of the other.

 

[Teikas Dad]

My foundation is only 2 feet thick at the edges. 1" poly down the entire side and 2 feet out horizontal at the bottom. The booklet states that 1" of extruded polystyrene is the equivalent of 4 feet of dirt. My local building inspector recommended this type of foundation to me. I didn't put any poly under the floor, only on the outside edges.

 

[jklingel]

As long as the slab is totally surrounded with insulation underneath and on the side, there's really no significant advantage to extending the side insulation down deeper.
Tom[/QUOT•• Tom: Good info. What would be your wildest guess about how far to insulate down the foundation wall up here, in Frb, AK? The general consensus is to run 2" of XPS down to the footers (typically on the outside of the foundation wall; slab is often poured over the top of the found. wall), then run 2" XPS horizontally, 2' from the footers. This seems to keep the heat in pretty well, according to the isotherms I have seen on such a set up. Thanks. j