OT: Opinions wanted... Slab for Earth Bermed Home

I've been kicking around this idea for a while.

I live in the norther U.S where it gets very cold. I've got the slab inside a ICF peremeter. My primary concern is settling and shifting of the slab from frost. Is my design O.K. or totally off track ?

Are there any other problems with my design ? Any and all comments appreciated.

For keeping the walls dry, consider a product like "Tremdrain". This is basically a waffled plastic sheet covered with a silt barrier fabric. The idea is to direct infiltrating water down to the footing drains.

For the slab, overexcavate the area by about 1 foot. Put down filter fabric over the undisturbed native soil. Over this, install a 10 foot x 10 foot grid of 4 inch perforated PVC pipe covered with silt separator "sock" fabric. Slope the whole works towards one corner where you will have a gravity drain to daylight or sump pump.

Fill the area under the slab to grade with 3/4 inch washed drain rock. Conveyor trucks work very well for distributing it, if you have them in your area. Compact the material in 6 inch lifts with a plate compactor.

If you do not have a heated slab, you do not need insulation in the basement floor. If you do have a heated slab, or your slab is less than 5 feet below grade, then you need insulation more-or-less as you've drawn it.

Use a 10 mil or better specialty vapor barrier, not 6 mil plastic from Home Depot. You can lay this directly atop the rock, or on top of your insulation. You will want very much to avoid the use of the usual 3 inch sand blotter course between the vapor barrier and the concrete -- it acts as a sponge and it will take 30 years for the trapped water to diffuse through the concrete. Use a mid-range water reducer, super-plasticizer, and/or fly-ash to keep the water/cement ratio in concrete delivered for the slab below 0.40. You are looking for a 4 to 6 inch slump for workability, but keep the amount of water down below 0.40 or the slab will curl without the blotter course. You should personally call the concrete company and explain what you want to do. Get their recommended mix for the job, personally inspect the batch sheet delivered with the truck, and personally supervise the flatwork guys and keep them away from the water hose during the pour.

The flatwork guy will hate you for departing from typical practice, but with the right mix it isn't difficult to place and finish, and the result is a warm, dry basement with a floor that is free of cracks and as hard as steel.

Use a grid of 1/2 inch rebar on 12 to 18 inch centers instead of welded wire mesh. Use plastic rebar chairs to support it. I would not personally dowel the slab to the walls, and I would recommend a purpose-designed 1/2 inch flexible expansion joint between the walls and the slab, all the way around, not the pressure-treated 2x that you have drawn. Isolate the slab from the footing with some of the Trem-drain, both to allow the slab to "float" and to allow any water that leaks through the cold joint at the wall/footing interface to get under the slab and into your sub-slab drainage system, rather than being forced up between the wall and slab. Use "quick strips" or sawn crack-control joints on 10 foot centers to control shrinkage-related cracking in the slab.

I did all these things on my house in Oregon. The concern is more water infiltration than frost or severe climate. But I can heat 6000 square feet (including the basement) through an 800 degree-day month for about $50. And the basement has stayed dry, so far.

The front ICF overall thickness is ~12". The 2x8 wall will be just over 8" thick, leaving a natural "knee" if positioned the way I show it. If I move the wall to the outer edge of the ICF I would have 2 inches of exposed foam in the floor 2" from the lower corner of the front wall and 2 inches of foundation exposed right at the corner.

True, I do loose 4 inches of space at the front of the house. But the intent is to encapsulate the slab (which will be heated) from the outside. Placing it like it is is the only way I could do it and make it all work.

Thanks for the comments.

I would be far more worried about water & swelling soil than I would be about frost - if your house is occupied, it likely will not freeze under the slab. Water, anyhow in my clay soils, loves to do the unexpected & really mess things up.

Since your footing is supposed to be below frost, & you have insulation also to below frost, that middle spot of dirt under the house will not be exposed to cold unless you leave the house unoccupied.

Around here, that really isn't bermed, we have full basements in deep and a bermed house near here has sod on the roof.....

--->Paul

I too am concerned about freezing under the slab. The ICF Panels to a depth below the frost line should help with that. However, charts do show possible frost levels to 6 feet for my region. I am also concerned about loss of heat from the slab to the ground, that's why I added the 2" dense foam under the slab. My plan is to totally insulate the slab from ground contact. I've heard that the slab needs to be poored the same year the site is excavated to help prevent these ground shifting problems.

Water is another concern. Sight excavation and fill will need to include getting the slab above grade to prevent overland water problems. Capilary action is possible, but shouldn't be a problem if the water table isn't high on the site.

The design I'm looking for is one that gives a balance between the warmth of a well insulated basement and a view straight out into the yard on any side. Ideally, my plan would allow me to do most of the labor myself using conventional materials and techniques.

Some of the underground homes you mention can be very energy cheap. There's a company called "Davis Caves" (you may know about) from Southern MN that markets a really neat design which is guaranteed not to leak.

Thanks for the comments.

You"re going in the right direction. My somewhat similar house has passive annual heat storage (link). If you move your insulation to outside the perimeter and put a poly umbrella over it, under the top soil, you"ll have the best of both worlds. The ground beneath will stay dry (assuming reasonable site drainage) and it will never freeze, a little like the shallow frost protected footing idea.

Take a look at the link but forget the architecture. My place looks more like yours. Without dirt overhead you won"t get real annual heat storage, but it"ll work better than any of your neighbors. Even without the underground air tubes.

The insulation umbrella is the trick, simple and cheap. 17� outside this morning, mid-60"s inside with a cold woodstove. We have no heating system. Your climate is considerably colder than ours, but the author built the first one in Montana. They work.

• PAHS

Thanks for the ideas.

If you place a small insulation umbrella starting from your bermed wall and going out 10-15' to a French drain, you get both the benefit of earth tempering plus the lack of worry about frost heave.

This I did for a friend who built a traditional frame house over a basement. Worked like a dream, as long as there's no landscaping to disturb. With insulation (and poly) over the earth mass, it works very well to help heat the house. And no worry about basement leaks with dry dirt on the outside of the walls.

Any way you could tie the slab to your CMU walls with long "L" bars?

Mitch

Thanks for the input.

Being off grid poses additional challenges for you.

I have considered ways to use a fireplace or stove to add head to the PEX system... such as coils around the stove or pipes hidden in a stone/brick fireplace. One concern I have is the possibility of water getting too hot. I haven't read to much about this, but it's a great idea. What type of piping were you considering ? How would you couple it to the boiler/PEX system ?

Best wishes on the new home.