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PNW Conditioned Crawlspace VS Insulated Slab

nickmokan | Posted in Energy Efficiency and Durability on

We are a home builder in the Seattle area and I’m building a personal home. The architect isn’t a high performance architect, so that leaves me to detail some features- which I totally don’t mind. 
My main question is the foundation floor systeme- I know that IF we;ll build a crawlspace it will be a conditioned, floor insulated with a rat slab, foundation walls insulated and the perimeter of the rim joist sealed with spray foam. Just like a basement.  HRV for air exchange.

The only question I keep asking myself  is WHY do I need a crawlspace? We will be doing ductless mini-splits the house so mechanical isn’t an issue. 

Now IF I do a slab on grade with lets say R25-R30 under slab, R20 foundation wall insulation, will this be a colder floor than the non vented crawlspace approach? I do like the soling feeling of a SOG floor.

I agree with the fact that slabs are faster, more economical, and easier to detail and insulate. I just want want a colder floor assembly.

Open to suggestions and options.

Thank you 

Thank you all!

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  1. Expert Member


    The most adaptable house I can think of is one built on a crawlspace with a trussed roof. That's the big advantage. How much that matters to you is a different question. When I built my own place in the PNW I went with a slab and a stick framed roof. I like not having anything below the floor to worry about. No pests, no chance of water intrusion.

    The slab isn't warm to the touch, but tracks the indoor temperature within a couple of degrees. If feels cold because it's exposed concrete. If I had built a suspended slab over a conditioned crawlspace I'm sure it would feel the same, and if I installed a finished floor over my slab I bet it would feel just like the same flooring over a crawlspace.

    The houses I've built over the years are probably split pretty evenly between slabs and crawlspaces. What determines which is primarily whether the lot slopes or not. One of the nice things about slabs on a flat site is they give you a good relationship to the surrounding outdoor spaces. You lose that when the lot slopes, so often a crawlspace then makes sense.

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