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I’ve read most everything posted about vapor barriers & crawlspaces. I have decided that I want a good vapor barrier under my new house which will have a crawlspace. We have a temperate climate here in Arkansas, so our footings are only 18″-24″ deep by 24″ wide. I think GeneL said something about “capillary action” causing your footing to absorb groundwater. What I’m considering is putting a 6 mil barrier down in the trenchs before the footings are poured, letting it extend inside the footing (crawlspace) about 6 feet. The crawlspace barrier would then lap over this, and I’ll put crushed rock around the edges to hold it in place.
Will this create any problems?
Replies
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I've seen questions raised about how long viz-queen will last under concrete.
There's a product that's made for such an application, Mirafi Miradri 860. I don't have any first hand experience - I've just seen it in a catalogue, and there's limited info available at their site: http://miradri.com but it might give you a local contact. If not, track down your local masonry supplier.
The illustrtion in my catalogue shows it as being applied on top of the footing and under the foundation.
Also, one has to wonder how likely wicking is in such a situation. Check with local builders or home inspectors. (Sometimes builders don't know, sometimes they don't hear about the problems that arise down the road; home inspectors see them as part of what they do. The only time I've seen wicking damage to sill plates is when they've wrapped the vapor barrier up to and fastened it to the sill. Even then, it takes a fair amount of moisture penetration in the foundation to cause problems.)
Bob
*Rusty, If a lot of water is unlikely to come in direct contact with the footings and your concern is normal ground moisture being sucked out of the soil by capillary action, then placing the footings on a compacted bed of stone or gravel will stop the capillary action. This seems to always work for us.Bob
*Rusty: I'm not sure how low Visqueen (polyethylene sheeting) will last, but it will last much longer than it does above ground, in the UV. The gravel bed will serve as a capillary break, but the sheeting will be better at keeping the moisture in the concrete and giving you a better, stronger cure (seeing as how no one hangs around to keep it wet for the first week like they should).I've spec'd some really substanial vapor barriers when fast-food joints were built on top of my clients' toxic waste sites. Multiple layers including mylar and HDPE. I'll see if I can dig it up, but any civil, environmental or landfill engineer might know.I like your idea of lapping the footing vapor barrier under the crawlspace vapor barrier. I'd use the (red) vapor barrier tape that usually goes just behind the drywall. Very sticky and vapor tight. -David
*Hi there,I would agree with Bob. The inclusion of plastic under a footing would be a band aid solution at best. It's always better to solve the problem at it's source.Put the footings over well compacted granular material and ensure that your drainage tile is adequate to keep the moisture level BELOW the footing level.Gabe
*I thought Gene's post referred to breaking the capillary action between the footing and the foundation walls by placing a membrane (EPDM, etc) on top of the footing, and pouring the walls on top of the membrane. Thus, creating a break between the footing and the foundation wall.
*I think you're right, but if you can keep moisture away from the footings, wouldn't that be a step better? Also how are you going to get adhesion between the concrete block and the footing if you have a membrane between? Or is that a concern? What are the mechanics of the footing/block interface?
*Hi there,Only a fool would put a mambrane between the wall and footing.We specifically install rebar and keys in our footings to ensure a solid interlocking of the two components.Gabe
*"Only a fool would put a mambrane between the wall and footing."Well, I don't know if it works, but in my concrete contractors suppliers catalogue, 2 manufacturers Miridri (http://miradri.com/) and Tremco (http://www.tremcosealants.com/) show membrane products being used between the footings and foundation walls. In both, they show the membrane going on top of a "structutal" slab (Mirdri) aka a "mud" slab (Tremco) the top of which is even with the top of the footing, and covered with a "topping" slap (mriadri) -no name given (Tremco) which rests on the top of the footing.Actually, the Tremco is a "1-component, bitumen-modified mositure curing polyurethane. Use as an inter-layer membrane ...." The Miradri is a "self-adhereing sheet membrane consisting of 56 mils of rubberized asphalt laminated to 4 mils of polyethelene...."Bob
*Hi Bob,There's a world of difference between a commercial structural application and a common residential concrete wall application.I've got catalogues that show products that you've never seen or much less used, but that doesn't mean that they are any better than what would be commonly used on any jobsite on any given day.Try not to confuse the issue and don't bother to try to impress anyone with name dropping of products you don't understand or use.Gabe
*Gabe,"don't bother to try to impress anyone with name dropping of products you don't understand or use."I thought it was obvious that I wasn't trying to impress ("Well, I don't know if it works, but ...")All I was trying to do was pass on some information that _might_ be of interest to Crusty. I freely confess there's a whole lot more in this world than that of which I know and/or have experience with.I quoted your "only a fool would ..." statement because it seemed to me to be exaggerated. Arguably, "only a fool would use a commercial structural application in a residential setting" but even then ....Bob
*harsh, Gabe, harsh....u go stand in the corner and think about the abuse you just heaped on Bob....unless of course its a slow nite and then by all means have some fun....wanna play tag -team abuse... say me and Bob against you and Fusco?
*Sorry Mike,We don't shoot it out with unarmed dudes!Gabe
*is that unarmed dudes as in ...in the battle of wits ,...some people are unarmed...hah, hah, hahand if so... wud u be referrin to Joe.. er me?
*Hi Mike,Neither Joe nor I need help in a shootout. I don't do "battles" as a rule, just the odd skermish.The interesting part of this exercise, is the fact that, on one hand, our industry has enough common elements to form a common understanding of the building process while on the other hand regional influences introduce enough differences to make it and keep it fresh and alive.(I think that I've learnt more about new products, in this forum, in one year than attending trade shows for the past ten years)If you notice, most of my posting tend to take a direct and simple approach to the question. I subscribe to the "KISS" theory.Others, tend to complicate the process.Although 90% of my work is commercial in nature, in replying to the questions, I try to focus on the residential aspects and the DIYs.Chat laterGabe
*Gentlemen. As we all know, the junction of the footing and the foundation wall is a "cold" one. If one wants a capillary break between the footing and the foundation wall but is worried that polyethylene, for example, will prevent the foundation wall from locking into the footing, use an elastomeric compound to coat the top of the footing. It is my understanding that the keyway in the footing is to provide some stability for the wall if the wall is not braced during backfilling....few residential foundations are.GeneL.
*Hi Gene,And we all know its not worth the effort or money!Gabe
*i've never had a problem with capillary action of the foundation.. and i think its cause we build in such high water tables and poorly draining soils, that i long ago adopted the practise of over digging my holes by 12 inches , laying a pad of 3/4 inch crushed stone about two feet bigger than the foundation , and setting our walls on top of the pad...then we run a 4 inch pipe system just inside the wall, in the 12 inch of stone and either lead it to daylight or lead it to a sump...we don't have wet basements...as long as the owners watch their sump pump maintenance during the wet season... and i guess the 3/4 stone is acting as the capillary break too....the 4 inch pipe system also serves as a radon mitigating system if it's needed down the line... if you buy into the radon threat
*Mike. My last personal residence wa sconstructed that way. We filled trenches with crushed stone--no fines--and poured the footings on the stone. This, gave me a capillary break,allowed me to use less concrete, and not go down to the frost line. That was 11 years ago and no frost heaving from the shallow footings.GeneL.
*Gene..we did a commercial bldg. that way.. the site had been filled over a swampy area..we got a narrow trench thru the fill, and the unsuitable material.. dumped ( 4-5 inch stone).. into the trench to a uniform 4 feet below finish grade and then poured our footings on top of the filled trench...at the low end i think we saved about 8 feet of concrete wall...
*Gene, Mike... standard practice around here is to dig your footings to the desired depth and pour on top of "undisturbed soil" -- they only use crushed stone if you get into some bad soil. Assuming I have good, solid ground when the footings are dug, does it make sense to take out another 6-8", backfill and then compact? Seems like either a good bit of work, or a good bit of expense. We're in a "temperate" zone here in Arkansas and frost heave is not a problem.
*C rusty, with high water tables and bad drainage soils, it makes sense to me...whatever u develop as ur strategy to handle water, roof runoff, groundwater, moisture in yur crawlspace / basement.....look at the topography of yur site, plan where any water will come from , be colected , and drained or pumped away....that will tell u if stone under yur footing / slab is something u want
*MikeyJust caught up with this thread. I've got a reno coming up turning a poorly built/owner built seasonal into a retirement home. Gotta jack it up, put in a foundation and add on. The original was built with one end on bed rock and the other on fill at the edge of a cedar swamp. Your gravel base idea interests me.. .but youri "we got a narrow trench thru the fill, and the unsuitable material." is a bit confusing. . . sounds like you went down to solid material anyway??? How did you savei "about 8 feet of concrete wall..."????? I'm lookin for a way to get a good footing without going down god knows how far in spots...any thoughts?????-pm
*paddy... peat is peat.. the thing that we did was dig thru the filled material (which was good material)..but underlaying that was the peat... we dug thru that and got to a fine sand bed.. that is the level that we started filling from... and the narrow trench we backfilled with 3-4 inch stone tailings as soon as we dug it so it wouldn't fall back in.......i would think that in your situation.. i would dig a couple test pits to see if you can get thru the peat at the cedar swamp end.. if you can.. then i would bring that level up and continue it as a cushion over the bedrock portions so the bearing would be uniform without any hard spotsb but hey, whadda i know?
*This rubble-trench method was first described by Frank Lloyd Wright, BTW. He loved cost-effective techniques. Works well for above-frost foundations, as has been noted.
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I've read most everything posted about vapor barriers & crawlspaces. I have decided that I want a good vapor barrier under my new house which will have a crawlspace. We have a temperate climate here in Arkansas, so our footings are only 18"-24" deep by 24" wide. I think GeneL said something about "capillary action" causing your footing to absorb groundwater. What I'm considering is putting a 6 mil barrier down in the trenchs before the footings are poured, letting it extend inside the footing (crawlspace) about 6 feet. The crawlspace barrier would then lap over this, and I'll put crushed rock around the edges to hold it in place.
Will this create any problems?