Variation of Slab Free Foundation
I have done a few slabless “floating floors” for clients wherein rigid foam is placed on compacted crushed stone, then two layers of subfloor, then the finished floor. I like the idea to reduce use of concrete and overall carbon footprint, but my concern is that overtime since it is “floating” cracks and gaps in the flooring could open up or the floor may sink slightly. I’m curious if anyone thinks that concern is warranted. I suppose if the crushed stone and soil beneath it is compacted enough it shouldn’t shift and the rigid foam shouldn’t lose its structural integrity but I always like to go overboard on framing, so I was thinking why not just pour footers and stem walls as usual, add crushed stone, vapor retarder, and rigid insulation like a floating floor but instead of adding plywood on insulation, just adding mudsill on foundation wall and spanning joists / subfloor across as usual. That way below would just be an insulated crawl space. I’m sure this has been done but was curious about thoughts on this.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I have thought about doing exactly that on a few projects, including an addition I'm planning at my own house. Architect Elizabeth DiSalvo has done that several times, using open-web joists, so she can also run electrical and mechanical items in the space. She shared her details here: https://youtu.be/iGXoAN4znTI?si=_M9YgeJVwqzjNNim
Thanks for sharing this, Mike. I think I will plan to do that with my house then. I'm planning on using open web joists as well for the whole house for that reason. I also like that open web joists can span further than solid sawn, which is nice for an open concept floor plan.
AGoldstein,
The only downside I can think of is a more complicated sequence of construction as there is a bit of a conflict between running your subfloor so you can frame up the walls, and leaving the floor open to place the services in the cavities. Nothing insurmountable, just something the needs thinking through.
That's a good point. I was thinking some of the MEP could be run after joists are installed but before subfloor is installed and then I could make a floor hatch perhaps in a closet that would provide access to the crawl space for additional work down there.
AGoldstein,
I think codes may come into play. Set ups like Elizabeth DiSalvos appear to be viewed as floor systems because of the absence of any space between the joists and foam below. If it is accessible, it would have to meet the required minimum clearances - and you would essentially end up with a conventional crawlspace.