Frost Protected Monolithic Slab

One of the cold spots on a house is where the edge of a slab is exposed to the outdoors. This thermal bridge, is really more like a thermal runway because the outdoor air is a lot colder than the ground in winter, and hotter than the ground in summer.

No surprise that insulating the edges of slabs is one of the items on the Energy Star checklist.

Begin with a piece of ground big enough to dig a hole in, and dig that hole.

A frost protected slab doesn’t need to be dug below the frost line, depending where you live, 12, 14, or 16 inches deep is enough.
A bed of compacted gravel caps the ground under the slab, and a drainage trough should be installed in wet areas.

Lay plastic atop the compacted gravel and then pour the slab, making sure to add rebar as required.

Wrap the edge of the plastic up and install rigid foam insulation along the outside edge of the slab. This foam is about 2 inches thick, which is about R-10.

Lay another board of foam horizontally over the drainage gravel, sloping away from the house.

This piece should extend about four feet from the house to completely protect against frost heave.

Cover the foam with some sort of protective layer, such as a stucco parging or rigid panels suitable for ground contact.

Backfill the hole, and frame the walls on top of the slab.

A piece of flashing above the insulation keeps bugs and water from sneaking in.

Install the OSB, WRB, and siding; plant grass, and then take lunch—confident that this slab will stay warm and dry for a long, long time.

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[Drummer]

Ive done my last 4 houses like this and have never had a problem

[widehotep]

1:38 Note: the 2 x 6 bottom plate extends over the side of the slab by at least the thickness of the outside foam (aka thermal break).

[jmarsh]

for the best install, you should not excavate in the area within 45 degrees of the bottom of a footing or perimeter of a monolithic slab. I think this area is called angle of repose. (according to my engineer this can be slightly more or less depending on soil or aggregate type) . This is the same for typical foundations with 4ft frost wall on footings or deeper ones for a basement. The drain piping should be moved away from the footing if you want it below the footing or raise it so the bottom of the pipe is at same elevation as the bottom of concrete slab footing

[stevepailet]

Gave me a lot more confidence when I saw that it was Steve Baczek who designed this

[Thoracius]

Why do you turn the plastic up? Won't that trap any moisture that gets onto/into your footing from draining away? Seems turning it down would be ideal. I'd also put a layer of dimpled plastic membrane just for peace of mind, to keep the footing ventilated and draining.

[joeshow]

to the author your music in this video is as useful as the sun shining in your eyes when your driving into the sunset.

[habenarohank]

If I add Pex tubing in the slab and heat it for free in the fall with evacuated solar tubes would the ground retain the heat (heat sink) or would the heat loss be absorbed down? From my understanding the R value of the ground will keep the heat under the slab.

[onlyscience]

Must be the only almost code compliant (Gravel should be under the perimeter) R403.3 FPSF/frost-protected shallow foundation on youtube.

[joshuasmith]

Kinda surprised you don't tell people to put foam underneath the entire slab....

[sebastiantevel]

How do water drain around the perimeter if the trench it is covered with a layer of non permeable eps? Will the water pool on top of the EPS foam board before ever reaching the drain?

[Real_Tim_S]

I'm concerned about the detailing on this - one of the long standing complaints I've had watching "perfect building systems" videos is the lack of clarity as to where the conditioned space starts and stops. It's for this reason, I like the REMOTE building method where the entire structure is inside the conditioned space, the vapor barrier is on the outside surface of the structure rather than among the rat's nest of detailing between floors and roofs (etc.), and the insulation is all outside the vapor barrier so that the vapor dew-point transition is always outside the wall unit.

My question with this method follows that problems always come down to how the foundation is coupled to the earth - regional code often requires foundation edge and post in direct contact with the grade. This coupling creates a huge thermal connection usually with very efficient conductors of a high mass. One of the reasons that the insulation has to be extended so far away from the slab here is rather obvious to me: the slab is conducting heat into the grade soils, and IMHO this solution is at best a band-aid for the problem - which is that heat is getting into the soil at all. In areas where frost heave is a problem, again it seems that the actual problem is that water is allowed to build up next to the structure, which this band-aid would probably make worse (although shallower).

It would seem to me that a better approach would be to insulate the majority of the slab from the ground surface, planning the bulk slab to be within the conditioned space. If code requires direct foundation contact with the ground, there is usually a wiggle room for not connecting the slab to the foundation wall but rather using a thermal break material (RPS or equivalent), then thermally breaking the foundation wall from the structure and conditioned space. This would make the temperature of the foundation wall closer to that of the surrounding soil, negating the need for this shelf of RPS.

My biggest gripe is the discontinuity of the vapor barrier - it's shown to just be covered under the edge of the RPS, typically you'd want to have the wall's vapor barrier bond to that wrap so that the joint will shed water, rather than being wicked into the foundation. Once the concrete has hydrated, I'd want to protect it from as much as I could manage to prevent degradation.

There is no drainage of surface water in the above video, only ground water up to the foundation footing. To address the frost heave, I'd want to see a very wide and deep drain, lined with filter fabric and back filled with #5 crushed stone probably a minimum of 2 feet wide and >3 feet deep, with a drain pipe at the bottom, and geotextile on the top, then covered with 4-6" of loose coarse mulch. This would promote rapid drainage around the edge of the structure of liquid water. If icing was a problem, I'd want a method to convert that solid water into liquid again to speed drainage (i.e. heat). Preferably the structure would not be designed in such a way that snow fall for example can land right at the base of the wall Barring that, I'd put a very shallow polyethylene sheet from the edge of the foundation to a drainage trench offset from the wall - sloped like the video's RPS. The idea would be to make any water build up surface-only and very shallow.

[haighyvshaighy]

wouldn't a drainage tile at the sloped end of the insulation board be better? any moisture from above would follow that slope wouldn't it?

[johnjacobjinglehimerschmid]

If you have that foam board insulation continue from below ground to the top of the concrete. How do you protect the foam from the elements and damage?

[governormagoo]

Hi, A couple of quick questions. How deep should the ridgid insulation go? Just to the bottom edge of slab or deeper? Also, should the rigid insulation be attached to the slab with adhesive or something.? Thamks

[stonebody]

Steve I am in Warwick RI and would like to utilize this frost protected slab protocol for my 32/32 two story garage.
Any suggestions on how to achieve this in RI
THANKS
Doug

[bluecollarcrypto]

Why would you block the weeping tile with ridged foam?

[ИринаЗыль-ьс]

Dan,
How do you make insulation for manholes of the drainage system? The grain tubes are insulated with a skirt around the house, but I afraid hatches could be frost bridges...
Regards, Irina

[donatop]

Should you put foam under the slab as well?

[MrRerod]

1:16 cover the foam with what? Something strong enough to withstand a weed eater string

[kanadadayasamak]

you installed that sloping foam wrong, do you wanna know why? 1:08 you dont know