Crawl space – conditioned – construction
alfie
| Posted in Construction Techniques on
I am planning/designing a new-construction house. There will be a crawl space. A full basement is not feasible because of water table and other conditions.
Having read quite a bit about Building Science, the right way to do the crawl space seems to be:
– conditioned
– with concrete slab
– insulated/sealed like a basement
– provision for venting soil gases
I would appreciate references to details of construction, something like a detailed cross section through the foundation wall. I subscribe to JLC (recent), FHB, and FHB online, and could pull up anything on those web sites. So far I have not found anything that addresses all the above elements.
Replies
I think you already have the answer--look for a basement cross-section that you like--the walls on yours will be shorter.
Thanks - I was thinking along those lines too. One difference - relative to the foundation footing, the slab will be higher for a crawl space. I am trying to understand if the slab just "floats" on a crushed stone base of appropriate depth way above the footing, or should there be a "shelf" created by the foundation wall narrowing, where the slab terminates.
Typically, around here the slab sits directly on the footing. You have the right idea, basically a short basement. Your slab need only be a few inches thick on 6 mil poly vapor barrier on 4" compacted granular fill. I would also do drain tile and a sump due to your high water conditions.
Thanks. When we start digging we might have to go deeper than the standard footing depth for our area, because of soil conditions, and because there is some area that appears to have been filled and re-graded and may therefore not be suitable to put footings on. So then I wonder if I'll end up making the crawl space as deep as the footings, or keep the slab higher to give me just enough crawl space height.
I thought John had some good points. If you are going to have a true conditioned crawlspace, I would also use foam under the slab. You can do a search in the archives to get some ideas. As far as depth, I would go as deep as the footing, although cost will be an issue in terms of the time to dig it out and get rid of the fill, if it cannot be used on site.
Thanks. Yes, it is my intent to put foam under the slab. I have to check again, but off the top of my head, it would be slab, then foam, then vapor barrier, then coarse aggregate, then compacted earth below.
If putting the slab on the footing is the way to go, I might well end up with a basement after all, which would be good. Also, I can use the material for fill right on the site.
Edited 6/13/2008 2:03 pm ET by alfie
are you building in conn., don't you have some issues with footers not below frost line?
Yes, Connecticut. The footings will definitely be "extending below the frost line" as the IRC (which is adopted in Connecticut, with some modifications) calls for.
Check out this website: http://www.crawlspaces.org/
I have a crawl space in a flood zone and I sealed mine.
For a variation on the idea you might look at the way Michael Chandler (ShelterNerd here) likes to do crawl spaces: http://www.chandlerdesignbuild.com/files/tentedCrawlComplete.pdf
I don't know nuthin' but I have great confidence that he does.
-j
a full basement is not feasable because of water table and other conditions.
and yet you ask in another post if you have to put the footings deep do you have to put the crawl space concrete deep or can you put it at just the right height.
are you sure that there is no way to drain by gravity? a french drain around perimeter will not daylight at some point?
it just seems a shame to do all the work for a basement, and not get a basement because of water table.
and you are in conecticut where lot prices are by the square inch?
there must be a way to waterproof exterior of concrete and allow for drainage/removal of water.
Thanks. If there was gravity drainage to daylight, it would be great. Drainage would have to involve pumping, and I was trying to avoid that to keep things simple.
I lived in a house with a high water table. The basement footprint was small..only about 400 sf but the sump pump kicked on every ten minutes, every day of the year for the 20 years I lived there. The basement was dry and finished. I've seen basement with two sump pumps because of the high water. I once saw a basement that filled up to within one foot of the deck! The sumps kept it dry. Bob's next test date: 12/10/07
I'd prefer to have a basement. Given the conditions, I keep thinking that it might be asking for ongoing water and pumping problems. When I read posts such as yours, I think maybe it is not such a problem after all. I am wondering how to determine a reliable answer for my conditions.
Dig a hole. Find out where the actual water table is. Our sump pumps cost us under $100. I had to replace it twice in my 15 or 20 years. If I was worried, I'd do a dual system. It's highly unlikely that both pumps would go bad. You would have to be ready for a power down scenario though. Bob's next test date: 12/10/07
Thanks. The way I understand it, two pumps is the way to go. We do have many power outages, for some reason.
The other factors, which I didn't get into during this discussion, are still a consideration (ledge, uphill septic leach fields which are slightly more than the 50' required by the code, wetlands all around) . So the jury is still out.
Build your house up.The last house we built in MI was on a farmlot that needed an engineered septic system. To facilitate the downward flow, we placed the footing on the existing foundation and poured the basement. We then backfilled the entire basement with the soil that we removed for the pond. Your "basement" might get dug a foot into the existing grade, and four foot of fill put around it. That would create a "daylight" basement which is the best value around. Your water table would probably not affcect your basement at all. Bob's next test date: 12/10/07
Thanks. Build up is kind of what I was thinking of, even if the specifics in my case are a little different from what you chose to do.
You can get some cool looks to the porches too (by raising your finish floor), and for aesthetics consider bricking or stone on the foundation wall for a good look. If you don't mind 4 plus steps this is a good alternative for a daylight basement as others have mentioned. That might also eliminate the need for a dual pump system, although having two wouldn't be the end of the world. I personally would be bothered by a sump kicking on every 10 minutes though. I have won and lost battles regarding venting/not venting crawl spaces with building officials, so always good to check ahead of time with them. There are many things to consider (site, natural drainage, rainfall, and personal preferences) and your on the right track considering these issues now rather than later. If you do end up raising your first floor also consider extra room in the garage for a garage to house stair/landing.
Yes! A conditioned crawl fan. You're on the right track. it never made any sense to me to vent a crawl. All you're doing is introducing moisture into it all summer.
Besides, as I always say, "ever seen a vented basement?"
You also wont have to worry about cold floors and frozen pipes.