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Slab Foundation vs Conditioned Crawl Space

I live in the Pacific Northwest (zone 4 marine). My wife and I would like to build a home starting next year. I've been doing a lot of research on foundations and can't seem to find a "definitive" answer to "slab vs condition crawl space".

I've read a number of articles on this site describing the methods to create a conditioned crawl space (very good articles by the way). But I haven't seen an article (maybe I just didn't find it) describing the advantages/disadvantages of a slab vs crawlspace. Part of me thinks just go with an insulated slab, but maybe that's too easy? (Note: we will probably NOT go with in-floor radiant heating regardless of what foundation we choose).

Does anyone have a good answer to the question of slab vs conditioned crawl space?


Asked by Chad Rockwell
Posted Oct 20, 2011 2:55 PM ET


8 Answers

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There is no single answer to your questions.

A crawl space can be damp, unless you plan the floor of the crawl space to be higher than the exterior grade. That's a good solution -- unless you need handicapped accessibility, because you end up with a long ramp. So if you might use a wheelchair some day, choose the slab.

Crawl spaces make it easier to change the plumbing. Who wants to cut the slab up to move pipes?

A crawl space is a good place to put a furnace. Except then you have to worry about the air quality down there.

Lots of people argue these issues all the time. There is no resolution to the arguments. Just choose whatever foundation system you prefer -- it's your house.

Answered by Martin Holladay
Posted Oct 20, 2011 3:40 PM ET


Martin - thanks for the quick reply. What is the typcial cost difference of a condition crawl space vs a slab. Is one way more energy efficient than the other?

Answered by Chad Rockwell
Posted Oct 20, 2011 3:48 PM ET


Is your lot flat or sloping? Other things being equal, on a lot that's dead flat a slab will almost always cost less, while on a sloping lot you may need to spend a bundle on compacted fill, and if the grade is steep enough, on engineered foundation walls to contain it. A friend once calculated the break-even at an average fill requirement of about 24": below that the slab will be cheaper, above that a crawl space is the lower cost option. That was calculated at a specific time and place, so YMMV.

There are other considerations of course. As Martin suggests, a properly-constructed crawl space is a handy spot for mechanicals, especially your air handler and ductwork. If you go with a slab just be sure you have an alternative strategy for keeping all that within the conditioned enclosure:

Answered by James Morgan
Posted Oct 20, 2011 9:53 PM ET


Here's another option to consider: if you choose to build a crawl space, why not have your backhoe dig just a few feet deeper and build a basement?

Answered by Martin Holladay
Posted Oct 21, 2011 4:48 AM ET


I probably wouldn't do a slab unless I was going to install heating tubes into it. Slabs don't have any "give", and are less comfortable to live on than framed floors. I also don't consider "dead flat" to be the criteria to apply, because the site might have topsoil to be removed and fill to be installed before a slab can be poured--I had to dig a crawl space sized hole for my shop and then fill it just to get a slab base. In new construction, it should be quite easy to install drainage outside the foundation to keep the crawl dry. You can easily do remodel and repair work on plumbing and wiring in a crawl space. And, Martin is right, you might want to go ahead and do a basement instead.

Answered by David Meiland
Posted Oct 21, 2011 9:51 AM ET


The property is very flat. I hadn't considered a basement in the past. I've always been put off on the idea of a basement. I was always concerned about moisture/musty smell/leaking. What's the recommendation for building a basement? ICFs or standard with foam or ICFs and foam? Is there an article on your site describing a good approach to building an efficient/dry basement?

Answered by Chad Rockwell
Posted Oct 21, 2011 12:41 PM ET


Any concrete contractor can build a basement. Codes have required footing drains for years; for a belt-and-suspenders approach to keeping your basement dry, include a waterproofing system instead of just dampproofing. You can add foam insulation on the interior or exterior, or use ICFs. This isn't rocket science.

But if you are building your own foundation, or setting up your own forms, and you have never done concrete work, there are a lot of pitfalls -- so you should work with someone with experience if it's your first foundation.

Answered by Martin Holladay
Posted Oct 21, 2011 3:19 PM ET
Edited Oct 21, 2011 3:44 PM ET.


On a property that's very flat, basement drainage via gravity is an issue.

Answered by David Meiland
Posted Oct 21, 2011 3:35 PM ET

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