I am interested in comments on the attached PDF of a cross-section detail through a crawl space. I started with a standard detail that showed a basement, and modified it to show a conditioned crawl space with slab (a short basement). Is the termination of the slab at the foundation correct?
SEE NEW ATTACHMENT in later post.
Edited 6/17/2008 11:48 am ET by alfie
Replies
When I took drafting class, we were taught to do that detail a little differently--we had the slab edge resting right on the footing with an expansion strip between it and the wall. Maybe things have changed.
There are a couple of things I didn't see that you may think about (or maybe are included, but just not in your drawing): Re-bar in the bottom third of the footings (2 parallel bars running the length of the footings. (Where to place the re-bar is often argued about, but we were taught to put them near the bottom to help the concrete in tension (since concrete has little strength in tension).)
On the outside of the footing there should be drain tile and gravel, separated from the fill with landscape fabric (or perf. pipe contained in a special sleeve of fabric to prevent clogging with silt). We were told to also show drain pipe inside footing, but my feeling was that why provide a path for water to the inside (they were connected to the outside footing drains.) Lastly, I don't remember if your drawing had these things, but there should be in this order, under the slab from the undisturbed soil up--compacted gravel (or pea gravel, which is rounded and from what I've been told doesn't need compaction), rigid insulation, vapor barrier, then slab. Vapor barrier under the slab is very important. We put wire mesh (6x6 welded wire fabric, if I recall correctly) near the bottom of the slab too, to prevent cracking.
Foundation details are very important--house rests on it (goes without saying) and it is very hard to fix things after it's done!
Edited 6/17/2008 7:55 am ET by Danno
Wall insulation works better on the outside of the foundation ..
Thank you for taking the time to write your thoughtful response. I took a .jpg from the Building Science Corp. web site and tweaked it a little as an illustration of the conditions around my question. For some reason on this particular drawing they didn't mention the vapor barrier. Also, the steel isn't shown, but we can agree that it should be provided consistent with normal practice. Based on what I have read on building science, from the the ground up it should be compacted earth, stone layer (no fines), vapor barrier, rigid insulation and then slab.
Most people I talk to, and details that I have seen drawn, show the slab resting on the footing, just as you described. However, the premise for my inquiry is that there is a high water table, surrounding areas are wet, and there is an engineered septic system. So the idea is to avoid footing drains, but to have a crawl space. If there were to be footing drains, they would end up at approx. 6' below grade. This would put them lower than the level of the wet areas, and the only way to remove water from those drains would be pumping up. The idea is to keep things simple and build upwards.
So question to all: What are the implications of building the slab the way I depicted it in my first post in this thread?
Why not do something like this:
I could go for something like that. Thanks.
I wasn't sure as I wrote my reply whether the vapor barrier should go below or above the rigid insulation. Building Sciences should know, so whatever they say, I'll go with.
I don't know of any problem with slab edges not resting on footing. Might eventually settle unevenly, but if the gravel is compacted and so on, don't really see that happening. Seems like I've seen it done your way many times, so it can't be very bad! There's a guy named Brownbagg here that doesn concrete for a living, maybe he'll respond. I'm not sure how a person goes about notifying someone of a thread other than posting directly to them and then the rest of the forum can't see the post.
Maybe I'll try posting to him and see what happens. --Edit: I tried, he isn't recieving emails from Breaktimers, so....
Edited 6/17/2008 10:16 pm ET by Danno
Thanks. I am attaching an updated detail. I'll check again but I believe the vapor barrier goes below the rigid insulation, otherwise the insulation would always be wet.
I appreciate your reaching out to Brownbagg. I am hoping somebody who does concrete will see this and respond.
A qualified "Yes" to your question about the slab termination detail.
If the frost proof depth of the footing lies below that elevation of a 30" crawl space then the drawing is accurate. Live where frost depth is 48" then with a 30" crawl you are going to have 18" difference between the bottom of the footing and the slab.
All depends on location.
Thanks. Please see updated detail, now showing frost depth of 42". Note, the top of slab is 18" below grade, rather than 30".
In the real world you are going to have compacted fill down to the footing level inside the foundation at least around the perimeter. Tough to form and pour the footing without having room to build it. So the picture should really show either a sloped fill line starting at the bottom of the footer and sloping up to the level of the gravel beneath the slab or a gravel filled cut inside the foundation wall. Minor detail though.
They can't get your Goat if you don't tell them where it is hidden.
Thanks, that's a helpful point.
Have you thought about doing a frost protected shallow foundation system.Basically that uses foam insultion under the ground on the exterior to increase the frost depth so that the footings can be shallower..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Hadn't thought about it but I will now. First place to check will be the building code. If it does not say you can do that, I'll have quite a time getting an OK from the BI. Thanks.
Why is the slab recommended if it's only a 30" crawl? I've been under some houses with "rat slabs"... maybe 2" of concrete, pretty roughly detailed, and it was a pain in the knees, or whatever other part of the body was crawling on it.Unless it's a stand up crawl, we use 10 mil plastic material with taped seams. Some builders run the plastic after the walls are built, then form the piers on top. It eliminates the sketchy detailing of wrapping and taping the piers. Problem with that is the wear it can take during construction.Consider having your rim joists foamed, much better sealing than fiberglass... foam companies will also do the vapor barrior and spray the walls.Conditioned crawls are a good way to go.
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Thanks. I'll think about that.
"U.S.Code AcceptanceProvisions for FPSF are included in the 2000 and 2003 edition of the International Residential Code (IRC).The American Society of Civil Engineers developed a standard, "Design and Construction of Frost-Protected Shallow Foundations", ASCE/SEI 32-01. This standard is referenced by the 2000 (by amendment) and 2003 International Residential Code (IRC). ASCE/SEI 32-01 is also adopted by the 2003 International Building Code (IBC) which allows FPSF to be used on multifamily buildings, houses four stories and above, very large houses, malls, schools, and offices.In areas of heavy termite infestation, local codes may restrict the use of rigid foam insulation on the outside of the foundation, or may require termite resistant insulation."http://tinyurl.com/6a9fr9
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thank you - I'll investigate this further.