Choosing a Foundation Type
I’m grappling with what to choose for the foundation / basement of our home. We’re self-building a rectangle 1.5 storey home in Climate Zone 6, and we had originally thought that we could do a simple insulated slab on grade (either a raft or monolithic FPSF) foundation, since we didn’t want/need a basement. The idea of a simple foundation with limited excavation and minimising concrete was appealing from both an environmental impact and cost point of view.
It turns out that because our site slopes down about 7% from the highest corner to the lowest corner, we’ll need to bring in engineered fill to build it up for a raft or monolithic slab. We have the additional problem of a very large tree in the middle of the house footprint, which will leave a large hole and disturbed soil once we pull the stump, and preliminary conversations with engineers have warned us against different soil substrates (since it leads to differential settling of the foundation), so it sounds like we’ll need to dig a big hole for the whole house footprint and grade it with engineered fill just to build a raft or monolithic FPSF slab, therefore making it a much more expensive option that we originally thought.
Now I’m looking at concrete frost-walls with an internal slab. Since the bedrock is between 75 – 100 cm below grade, it seems like a good option could be to pin the stem walls to the bedrock, eliminating the need for footings. Based on my research, we’d then fill inside the four concrete stem walls with engineered fill, then lay rigid foam horizontally under the interior slab and vertically on upper part of the inside stem walls (which will connect the horizontal under-slab insultaion to the vertical insulation in the stud walls), a vapour barrier and then pour the internal slab on that. This type of construction should eliminate the need to use engineered fill to grade the site, since we can just build the stem walls higher to match the grade highest point. Does this type of foundation make sense for our site, or am I making it too complicated?
I was also advised informally by an engineer to insulate the outside of the stem walls and add insulation skirting around the base of the stem wall to prevent forst heave. I thought frost heave was a problem in the soil not in the bedrock, so to me this doesn’t make sense since if the stem walls are pinned to bedrock, what are we trying to keep warm?
Finally, I was reading about something called a ‘baby basement’ (from Glatthaar Keller) where a portion of the area underneath the ground-bearing slab is a basement. I also saw an episode on the Build show network where they did something similar and called it a ‘hybrid basement’. I think this could be interesting because then we would only need to fill a portion of the foundation with the (expensive) engineered fill, and also utilises only the deepest area without needing to blast or jackhammer the shallower parts of the bedrock. I’m not sure about the detail of the basement ceiling / house floor though. I was assuming from the Glatthaar drawings that it would be concrete poured at the same time as the rest of the slab, but then how would the continuous insulation layer work, and how would the floor be supported above the basement? Has anyone done this?
…and I will be consulting an engineer – it’s required to have stamped drawings from an engineer for slab foundations in Canada – but I would love to hear experience from the GBA community to help guide my conversation with the engineer.
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