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 Question:  What is the best plastic to use for a  vapor barrier?

Answer by Lee Hinsley: I highly recommend only Engineered Vapor Retarders/Barriers for any application where one wishes to reduce or control vapors.  If one goes with an engineered film that has been designed for this purpose, and passes some level of the ASTM E 1745, preferably Class A, then one can be well assured they have a quality product that will last when in contact with soil.  Today as in the past, huge numbers of structures are built with no vapor retarder, or almost as bad, using Construction/Ag Grade Polyethylene sheeting.  I strongly recommend using a vapor retarder under all building structures, and just as strongly recommend that only engineered films are used in this critical application.  The big problem with Construction grade film (aka Visqueen) is that it will normally degrade when in contact with soil, and this process is usually significantly underway within 2 years.  Anyone who has been around construction for any length of time has dug up this kind of product.  Ask yourself how often this sheeting is in one piece?  Does it still have the strength it had when buried?  Not likely in either case.  In addition, the Construction grade sheeting is not even guaranteed to be pinhole free when brand new.  It often has high levels of something called Slip agents.  These make it easier for the manufacturing plants to make this lowest grade of polyethylene quickly and inexpensively.  The slip keeps the tapes normally used on this type of sheeting from permanently sealing at the seams.  Vapor Barriers and Vapor Retarders are often used interchangeably, but the preferred term in the industry today is vapor retarder.

Answer: Thank you for sending us your question.  First I would like to commend you for knowing that the plastic sheeting sold in most hardware stores is not ideal for this application.  My top recommendation would go to an engineered plastic sheeting that passes at least some level of the ASTM E-1745.  These materials are tested such that they will not degrade when in contact with soil, plus pass a number of other important criteria, like puncture strength and water vapor transmission rates.

Most customers prefer a white liner in crawlspaces, because it brightens the space and makes it look really nice.  Our top pick is the VaporBlock 20Plus, a 20 mil white/gold product that blocks virtually all of the radon, methane, VOC's and does an excellent job with water vapor.

If you want to stay with a material that is white on one side, we also have  Dura Skrim in 8, 12, and 20 mil versions, plus our Crawlspace 1800, 18 mil reinforced crawlspace liner material.  Any of these do a nice job in crawlspaces.  The heavier materials are usually used for the floors, and the lighter materials for the walls and columns.

If one is just concerned with water vapor transmission, then the VaporBlock line of products is pretty hard to beat in today's marketplace.  The 10 and 15 mil versions are blue in color, and pass Class A,B,C of the ASTM E-1745.  We also have 2 versions of the 6 mil available, a 6 mil blue, and a 6 mil VaporBlock Plus, both pass ASTM E-1745 Class C.  The VB6 Plus is currently being closed out as the plant has decided to discontinue this product.

The tapes used are also quite important to the finished crawlspace.  We have 4" wide seaming tape made specifically for this application, called Vapor Tape, Rolls are either 4" x 180' or 4" x 210'.  For sealing along the top and around protrusions, we supply a double-sided Butyl Rubber, Reinforced Tape we feel is the best in the industry.  Rolls are 1.5" x 100'.