How to Stop Capillary Action Beneath Footings
churcharch
| Posted in Green Building Techniques on
Hi,
I’m going to monopour my foundation walls using form a drain and ICF foundation walls. What product can I use to stop capillary action below my footing? I’m thinking of using a 10-15 mil vapor barrier beneath the footings prior to setting the forms up. Any thoughts would be greatly appreciated.
Thanks
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Replies
Martin Holladay recommends capillary breaks over the footing, not under the footing. Check out this article: Capillary Breaks Above Footings.
Best defense is preventing water from reaching the foundation in the first place. Install washed drain rock below your footing and install a well-designed subdrain system below the bottom of the footing to capture any free water that comes in contact with the foundation. This is best done prior to any formwork - wrap perforated drain piping (holes face down) in a filter fabric 'burrito' to prevent fines contaminating the drain rock/clogging the drain pipe.
If you're working in a wet climate, the drain will serve to keep the worksite much drier while under construction - I've never regretted spending more time/money on drainage.
I'd install 10-15mil poly and tape the seams as belt and suspenders. Once the foundation is in, the ability to rectify mistakes is difficult and expensive.
On the other hand, if you can't install a quality drain system that prevents water level from reaching the footing/wall, then I'd agree with Martin Holliday's advice about using a membrane that adheres to green concrete. But it's a poor second choice (albeit all-too-common in residential construction).
Granular,
It' an interesting approach, but I see a couple of complications trying to do that here.
Under our building code any footings that are not placed directly on undisturbed soils require the approval of a geo-technical engineer, and I'm doubtful they would okay building on drain-rock.
One of the useful roles of footings is as a dam against water moving into the area under the house. I'm not sure providing a path for water to move under the footing is a good idea from the perspective of structural integrity, or water management.
Malcolm,
I believe IRC states compacted drainable rock can serve as a footing/foundation itself. It's not much different conceptually than a raft slab or thickened edge monopour with continuous rock underneath except for some higher potential loading with the basement walls.
Water is another matter but we know concrete doesn't typically take care of that on its own without the aid of other waterproofing measures, say drain tile with a sump.
Jason,
Under our code both raft slabs and thickened-edge footing move the foundation from the prescriptive solutions allowed in Part Nine, to Part Four which requires an engineer.
I think the big difference conceptually between having a slab on drainable rock, and the same thing under the footing for a stem-wall is their relative heights. A slab will be above the surrounding grade where the rock only has to act as a capillary break. The footings are at level where water can be expected to flow and accumulate.
On a wet site I'd much rather try and keep the water on the exterior of a foundation than allow it to move through a continuous layer under the house and have to rely on a sump.
Depending on the concrete plant and what type of mixed they have. The mix I used from the plant was called a hydra-mix and when dry/cured, the concrete does not wick moisture like normal concrete would. It does work. I've seen water sitting on top for hours and the slab did not absorb it.
I paid $130 per yard for the mix with a 4,500 psi cured rating. I used this on my footings, driveway apron and garage/house slabs.
I would abandon the mono-pour and cast the footing first. The footing can then have a fluid applied membrane on top to act as a capillary break between the footing and foundation wall to stop the "rising damp". When you form the footing you can ensure there is a key-way to provide lateral strength at the footing/wall cold joint.
Then with the proper placement of drain rock and damp proofing membranes around your foundation and under your slab (if you are having a slab) there should be no way for water to wick from the soils into your structure.
I don't know how good my advise is because you are doing a monopour. I did footings first and used the blue delta footing barrier. If I have to give any advice about it I would say it has to be applied right after footing is poured and same time the key is being made so basically as you are pumping. If you wait even just a little bit it's harder to make the key and harder for it to adhere.