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What kind of foundation?

brentwilson | Posted in General Questions on

I would like some recommendations and pros/cons of various foundation types for my parent’s retirement house which will be built in west-central Idaho (climate: cold/6B). Frost depth is 28″ for construction purposes. My parents don’t really want a basement. I think the two likely candidates for foundation type are slab-on-grade and crawlspace.

I have looked a bit into pricing for the foam SPFS slab forms – they seem pretty pricey. I am considering a T-shaped house with about 1681.75 square feet on the main/ground floor, along with about 439.3 square feet of covered porch/patio space. Legalett pricing with shipping comes in at close to $30k with their in-floor heating solution (not including concrete, stone, and sheets of wire mesh). Without their in-floor heating option, it would be approximately $22k (plus the concrete, stone, and wire mesh). ByggHouse WarmFörm foam would be somewhat cheaper (maybe in the $15′ ballpark without the stone, concrete, or rebar). But theirs doesn’t include an engineering stamp for my specific house. Iso-Slab would likely be a similar price to WarmFörm, and they do offer project-specific engineering for a fee.

Slab Pros:
– Thermal mass (the house will likely be orientated in a pretty ideal direction for taking advantage of passive solar)
– Simple/easy/quick installation
– Allows for easy and effective installation of in-floor radiant heating
– Could go with strong and economical polished/etched/stained concrete as the finished flooring
– Low-level entry points (ideal for an aging-in-place house)

Slab Cons:
– Potentially expensive
– Difficult to make plumbing/electrical repairs/changes
– May need to be kept heated in winter to prevent frost/freezing damage
– Harder floor surface may need rubber standing mats at kitchen sink and other places where a lot of standing takes place

Given the location of the property, I could probably get by with a vented crawl space for much cheaper. But I am not overly excited about that prospect. A conditioned crawl space with a shallow concrete floor might be getting me up close to the $25k to $40k price point of an SPFS slab-on-grade – especially by the time I calculate in framing and decking for the porches in place of the concrete patios.

Crawlspace Pros:
– Easier access to plumbing and electrical under the floor
– Potential mechanical/storage space
– Might be cheaper than slab
– Wood is not as hard as concrete – a more forgiving surface

Crawlspace Cons:
– Potentially shared air between house and crawlspace
– More excavating and more complicated installation (multiple concrete pours, etc.)
– Minimal thermal mass added to house interior
– Subfloor would need flooring installed
– Raised floor (built on top of the foundation stem walls) would likely mean building wood decks instead of using concrete patios on grade
– Need form material for footers and stem walls

Any Advice?
What are your thoughts? What am I missing here? Are there viable ways to build an SPFS slab that are cheaper than the options I mentioned above?

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  1. Andrew_C | | #1

    I leave insulated slab questions to experienced builders.
    A few other thoughts, though -
    1) You probably don't want a vented crawlspace. Crawlspaces should be sealed, insulated, and within the thermal boundary of your house. There are many Q&A's and some articles on this topic at GBA. Think of a crawlspace as an inconveniently short basement.
    2) This is a green building website, so the advice skews in that direction. If you're building a new house, it should be air-sealed and insulated well enough (search for PGH articles on GBA) that in-floor radiant heat won't be a good choice. In a nutshell, if you keep the floor warm to touch for your toes, the house will get so hot you'll be roasted. There are a lot of Q&A's on this topic as well, but that's the summary.
    3) Personal opinion - wood decks are awful and a fire hazard. Patios are much better.

  2. Expert Member
    Dana Dorsett | | #2

    An insulated sealed crawlspace with a heavy vapor barrier on the floor is fine, and uses less concrete than a slab-on grade, and only slightly more (if any) foundation insulation. From at total-verditude perspective, less concrete is better, since it comes with a hefty carbon footprint.

    With the IRC code-min R15 continuous insulation on the crawlspace walls the crawlspace is fairly freeze-up protected, even if the floor joists get insulated (which isn't required by code if the crawlspace walls are insulated to code-min), isolating the crawlspace from the heated conditioned space above.

    While heated floors are nice, they're not necessarily $8K nicer, and certainly not $15K nicer (by most peoples' reckoning.)

    The floor of the crawlspace doesn't need to be at grade, and it's possible to build such that the finished floor level is only 1-2 steps above grade level outdoors, though in snowier parts of zone 6B it might be desirable to build it higher to manage the snow-drift issues.

    1. brentwilson | | #3

      But my understanding with insulated crawlspaces is that often there is a grate or something that allows (either passively or actively) for air from the crawlspace to circulate in the house itself. I'm not a big fan of that idea. I don't necessarily trust the crawlspace air to always be the cleanest air. What are your thoughts on that?

      How valuable is having a slab-on-grade for its thermal mass if we were to go passive solar?

      1. Expert Member
        Dana Dorsett | | #4

        >"But my understanding with insulated crawlspaces is that often there is a grate or something that allows (either passively or actively) for air from the crawlspace to circulate in the house itself. "

        That's not a requirement. The IRC calls out a minimum of 1 cfm per 50 square feet of crawlspace floor area, but it doesn't need to be recirculating conditioned space air (thought that is one allowed option.) It is also allowed be exhaust-only, depressurizing the crawlspace relative to the rooms above, guaranteeing that no crawlspace air gets into the room above. It can also be completely unventilated, provided there is mechanical dehumidification.

        See section R408.3, subsections 2.1 & 2.4:

        >"How valuable is having a slab-on-grade for its thermal mass if we were to go passive solar?"

        Thermal mass in the floor is not nearly as valuable as spending the money on insulation and better optimized windows & shading, as would be pretty obvious in any decent simulation tool.

  3. Robert Opaluch | | #5

    Rather than use a manufacturer's insulated slab product (at a high price point), have you considered building your own forms, and using EPS insulation etc engineered to your design specifications? Most builders of insulated slabs would take that less expensive route.

    Polished slab floors have become more common now for high performance homes. There are materials that can reduce carbon footprint of concrete slabs. Passive solar in Boise would be possible, but I'm not sure how different your climate is from Boise. Certainly solar tempered (daytime heating) would be worthwhile, but wouldn't require a slab floor thermal mass (to store heat energy for overnight or a subsequent cloudy day).

    Why a "T" shaped building? It's easier, cheaper, and more energy efficient to use a rectangular (or close to rectangular) shape for the building footprint. For example, a rectangular foundation with south/north sides less than twice the length of the east/west sides. Small bumpouts for architectural interest don't matter too much for energy efficiency, but do add to construction costs.

    1. brentwilson | | #6

      The climate at my parent's property would be relatively similar to Boise - although a bit further north and probably a little cooler.

      Where would I go for more information or an engineer to help with building my own foam forms? I am sure there is a cheaper way to do these raft slabs vs. the expensive pre-manufactured products. But depending on the price difference, I'm not sure if the hassle of doing it myself would be worth the price difference. ???

      While my parent's property has good southern exposure, there are key views to both the east and west that more of a t-shape would allow the great room area to take some advantage of them. Basically, a vaulted great room area in the middle, with two master suite wings sticking out each side. The southern facing wall-length would be longer than the depth of the house at the deepest point.

  4. Expert Member
    BILL WICHERS | | #7

    I’d go with a crawl space. Crawl spaces have the advantage of allowing you to access, repair, and modify mechanicals if you need to. I’d make the crawl space AT LEAST three feet high, and ideally four feet for easier access. Really cramped crawl spaces are even more miserable places to work in that regular crawl spaces.

    With a sealed crawlspace you don’t reply have any air quality issues. A sealed crawl space is basically a basement with a very low ceiling. Full basements do have advantages though, so I’d price out digging a full basement while you’re getting bids.


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