Radiant Slab

Vinny baga doughnuts

I am looking for advice on a below grade radiant slab.
I intend on running radiant in the slab (about 26'x40') the slab buts up to foundation walls that are about 4-5' below grade (Boston climate).

I am going to install reflector board under the slab to reflect the heat up. My question is should the exterior foundation walls underground be insulated also or is this a waste of money effort and time?

Ken_8

If you want to do it right

insulate the entire slab and walls.

You may want to look at ICF's.

I'm using them on our VT house - along with SIPS exterior walls and roof.

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Constantin

Insulate the edges in particular

Ideally, you'd insulate the slab and the exterior foundation walls with 2" of XPS (See buildingscience.com on why this is the case). However, even the folks at the building science corporation acknowledge the difficulty of covering and protecting 2" of insulation above ground.

If you can do it (and it will take some work on the part of the architect and structural engineer to get it right), it seems like the best approach. With the walls and slab inside the conditioned space, they'll act as thermal buffers, evening out any heat gain and heat loss spikes.

I happen to think that reflecting foil is for the attic, maybe. Below ground, IMHO only XPS has a role to play. If money is tight, insulate with 2" around the edges of the foundation (i.e. within 4' of the walls) and 1" elsewhere (or less, some people use nothing under the center of the slab, believing the earth there will also act as a thermal buffer).

I'm planning on using 2" XPS below ground throughout. Above ground, historic considerations require the use of brick on the visible exterior. Thus, I will only have inside insulation in those areas. However, areas under the porch, etc. will also be insulated with 2" XPS as a matter of principle.

Kevin_in_Denver

basement walls

I had similar tricky insulation details in my house. Part of the problem is if you leave out the exterior insulation anywhere, then the exposed part acts as a fin for heat transfer. Now you have to insulate roughly six times as much wall on the inside (because the R value of concrete is 1 per foot)

The cost of the inside insulation isn't the problem. It's the fact that now the wall is thermally decoupled from the house. You lose the thermal mass effect of the wall, which you really notice in the summer.

Other problems are moisture related as investigated by the Building Science guys.

Constantin, did you consider "thin brick" which is applied like tile? It's another fussy detail, but you could have your brick all the way down to grade. Parging or flashing would protect the transition at the top of foundation.

Unknown

reflector board?

No underslab install I am aware of will benefit from reflection from below. Either the slab is in direct contact with the reflective surface, or something relatively opaque is between them (like insulation) which absorbs the conductive or radiant heat and in turn conducts it to the reflective surface.

In both cases, reflectivity is not useful. It doesn't help against direct conductive heat transfer which is the primary method of heat loss in both of the above situations.

Insulating the foundation walls is important. You should also use a thermal break between the slab and the foundation walls.
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Constantin

Hi Kevin,

Yes, I did consider thin brick. I do wonder a bit about the longevity of such systems at the extremes (1/4" thick and all that). It simply wouldn't do if Constantin Jr. manages to bonk out a row of bricks with a errant baseball or somesuch. However, even if we install a 2" thick XPS exterior foamboard as we have done below ground, that would leave 2"-2.5" of brick to support the sill edges.

However the big "killer" WRT to brick veneer was the plan to use the exterior brick as a part of the load-bearing wall. We have cast a 4" brick shelf into the otherwise 12" thick foundation wall. Originally, the weight of the sill above was supposed to be held up in part by the brick. However, the sill came out somewhat "wavy" once the house was plopped down, even with wooden supports holding up the overhanging mud sill.

Our next move was to pull out the Hilti system, order some heavy duty bolts, angle irons, and the like and apply them below the mudsill. As a result, the sills are now perfectly horizontal and supported entirely by the concrete wall, the bolts, and the angle irons. If this stuff is load rated as I think it is, we may now have the opportunity to install 1.5" XPS behind the brick and to use 2.5" brick veneer instead of 4" brick... might be worth investigating.

Kevin_in_Denver

skinnier brick

Constantin,

If that approach can work, don't sweat finding the right brick, just rent a brick saw and make your own 2.5" bricks.

Constantin

Yup!

However, it ought to be easy to find a combination of XPS and brick that fits within 4-4.5" of thickness. The real question is whether the structural engineer will have a heart attack when he hears about the veneer... again.

Even ½" of XPS would do alot to decouple the concrete wall from the brick exterior rather efficiently.