Under-slab vapor barrier under footing?
Greetings. First I would like to say that I am awed by the knowledge and expertise on this forum. I have purchased a steel building and am going through the process of a slab/footing-piers. I believe the foundation design is sound( had to go back to mfg. as local concrete guys were speccing different and woefully inadequate foundations)
My question is about extending the vapor barrier under and around the footing to grade. It would seem to make sense and be correct but many internet pics show it stopping at the slab before the foundation. The contractors I have been querying about the job don’t think a barrier is needed- they don’t normally do them etc. I would like to limit moisture as much as feasible.
Thank you for your time, Ken Cohenour
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Replies
Ken,
You definitely want the capillary break, and polyethylene is one way to do it.
Whether or not the capillary break belongs under the footing or between the footing and whatever is placed above the footing depends on (a) the type of foundation -- whether it is a thickened-edge slab, a footing and stemwall, or some other type of foundation; and (b) the height of the drainage pipe that drains the crushed stone layer.
If I am visualizing your foundation correctly, then your solution -- to extend the polyethylene under the footing -- is a good way to go.
There is lots more information on GBA and elsewhere on this topic. Here are some links:
Is a capillary break between the footing and foundation wall really necessary?
Capillary break in deep footing way below slab level?
Capillary break over footing vs. vapor barrier under footing
Video: Installing a capillary break on top of concrete footings
Delta footing barrier
Capillary Moisture Transport and Foundation Implications
Thank you Martin. I altered my search words to define it as a monolithic slab and found what I was looking for. Thank you for the links
Hello,
I know this is an old thread but this is the same question I have and my search led me to your post.
My small monolithic slab is about 10 ft x 4 ft.
Ken, if you see this post, what did you decide to do? Did you extend your vapor barrier under the footing and up to grade?
Thanks,
Hoang
I would be very hesitant to place the capillary break under the footing. Reason being,you want the concrete to have intimate contact with the soil. The poly will leave minuscule voids under and around the concrete. They can become pathways for water migration which could possibly lead to erosion. That is one reason I'm not a fan of the bag type footing forms. You can say I'm over reacting,I'll give you that,however,I'd rather be safe than sorry. Put it on top of the footing. Better yet,use an integral water proofer in the concrete mix and call it a day.
Fred,
Isn't that exactly how we pour monolith slabs and those with integrated footings?
Yes Malcolm,however,I feel the best approach to this concern is to use an integral water proofing add mix directly in the concrete.