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- To: seaint(--nospam--at)seaint.org
- Subject: Vapor Barriers
- From: Jim Kestner <jkestner(--nospam--at)somervilleinc.com>
- Date: Wed, 14 Oct 1998 15:16:52 -0500
Thanks for all your responses. No one except Brian Smith really addressed the constructibility issue regarding if it rains and the top 3" of sand gets saturated what does the contractor and engineer do about it? It won't drain since the vapor barrier is usually taped at the joints and turned up and taped to the perimeter foundation walls. About the only way it will "dry out" is by evaporation and will 3" below the surface still be wet. Anyone else have a better answer? Someone ask what is wrong with having the vapor barrier directly under the slab? The top surface of the slab will dry out eventually while the bottom will stay wet. This will cause the slab to curl (just like a wet piece of paper sitting on a table...the top dries and the bottom stays wet). The 3" of sand acts as a blotter so that the slab cures and dries more evenly. Plastic shrinkage cracking and bleeding is also much more common with a vapor barrier directly under the slab. We typically only use a vapor barrier under slabs that have finish material on the slab that could be damaged by excessive moisture. We might use 4 mil polyethene in residential, 6 mil in commercial and something heavier for special conditions. If the water table rises to the slab elevation or above, it is not intended to prevent water from coming thru the joints. If that happens, you need a structural mat to resist hydrostatic uplift and some waterproof membrane. Although some of you referred to the vapor barrier as a capillary break, I believe it is the course self draining granular subbase below the vapor barrier that is intended to be the capillary break (ask your friendly Geotechnical Engineer) and the vapor barrier is just intended just stop upward vapor transmission. That is why I think it may be OK to punch holes in the vapor barrier (although I have only done this once or twice many, many years ago....in California) for the reasons that William Riddle stated. I believe there are 3 items that cause problems with finish floor materials: upward water flow (capillary action), upward movement of vapors, and condensation (sweating). Each of these problems can be solved by: course granular subbase, vapor barrier and a good HVAC system. Thanks again for making the discussion worthwhile and interesting. Jim Kestner, P.E. Green Bay, Wi Go Packers!
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- From: Dennis S. Wish PE
- RE: Vapor Barriers
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