High-Performance Vapor Barriers

On more than one occasion, I have had to slither across the damp dirt in a crawl space. I would emerge from the space encrusted with dirt. Sometimes, the dirt would smell of chemicals. The air in the crawl space was heavy with a dank, musty smell.

Three years ago, a friend of mine was considering leasing a house. He asked me to inspect it, as the lease had an "option to buy" clause. The house had been vacant for approximately one year. As I crossed the threshold into the home, I knew instantly that this modern house had been built over a crawl space. I asked him about this. He said that there was a lower level storage area. The house had been built on a hillside. We went around the back of the house. Yes, there was a storage area alright. The entire house was constructed over a giant walk-in crawl space. I guess you would call it a walk-space in this instance. In any event, the musty odor was overpowering. There was gravel spread throughout the entire area, however, there was no vapor retarder beneath the gravel. Needless to say, my friend decided against leasing the house.

Water Vapor Retarders

When water evaporates, it is attracted to warmth. This happens because warm air has the ability to hold more and more water vapor. Just visit New Orleans or Houston in July and you will know what I am talking about. This same thing happens in your house on a smaller scale. Do you live in an older home, say one built before World War II? If this house has a basement, I'll bet that there is a good chance that the basement always feels slightly damp. Why? Because vapor retarders were not used back then. In fact, the widespread use of vapor retarders didn't begin until the mid 1960's.

Unless you live in a desert climate where the soil is extremely dry, water is constantly leaving the soil and evaporating into the air. It does this in your house as well. Water from the soil will saturate concrete foundation walls and basement floors. This water evaporates when it is exposed to the air in your basement. If you have a crawl space, the water leaves the soil and looks for warm air. The warm air acts like a giant magnet. The water vapor will penetrate wood flooring, insulation, carpeting, etc. It will do just about anything to get into the warm air of your living space. When it gets there, it brings along any musty odor as well.

Stopping the Flow

Virtually every builder I know uses polyethylene sheeting as a vapor retarder. You have probably seen this material. It is usually either clear or black. It comes in rolls usually 100 feet long. Often it is available in different widths ranging from 8 feet to 20 feet. It is also available in different thicknesses: 2 mil, 4 mil or 6 mil. Here in Cincinnati, everyone calls vapor retarders 4 or 6 mil 'poly'. I used this material on all of my jobs. I used to think it was very effective. I recently found out that this material has its limitations. It is not nearly as good as I thought it was.

Apparently, many of these products are made from low density polyethylene resins. The material used to manufacture them is often scrap or reprocessed resins. Fillers are sometimes added. Pinholes can exist in the material. What's more, this material can actually degrade or break down when it comes into contact with certain alkaline soils.

The Right Stuff

As it turns out, there are high performance vapor retarders. These materials are made from polyethylene. However, it's very different from the previously described polyethylene.

The high performance materials are made using virgin high density polyethylene. When they are made, most of them are cross laminated. That means that two separate sheets are fused together with a hot melt resin at a 90 degree angle. The process is very similar to the way standard plywood is made. The net result is that there are no pin holes and that these materials are often 50 times better at resisting water vapor transmission!

They are very resistant to tearing and punctures. Special pressure sensitive tapes are made which allow you to seal overlapping joints. These vapor retarders materials are unaffected by most soils.

Where are They Used?

There are a variety of places you can use these high performance vapor retarders. They make an excellent cover over dirt in crawl spaces. You can and should use these same materials beneath concrete slabs in basements & garages, exterior wall vapor retarders and as an exterior vapor retarder around foundation walls. You simply use them anywhere you wish to stop water vapor transmission.

Are They Worth It?

Often people shy away from high quality materials because of their high cost. I can understand this when you are dealing with cost differences of hundreds or thousands of dollars. For example, you may want granite countertops which can cost thousands of dollars. But, because of budget restraints, you must settle for a less expensive top.

Vapor retarders are often a one shot deal. That is, you only put them in once. For this reason, you must not let price affect your decision. You probably think I'm setting you up for a major price increase. Surprise! Yes, the high quality vapor retarders do cost more than the poor performance polyethylene products. Often they cost 5 times more. But wait, let's see what this means in an ordinary job.

Let's say you need to install a vapor retarder in a crawl space. The crawl space measures 14 feet by 20 feet. So, you buy an 8 by 100 foot roll of low density polyethylene. Here in Cincinnati, this would cost you $16.00 plus tax. This stuff sells for about $.02 to .03 a square foot.

The high quality vapor retarder, on the other hand, sells for about $.09 a square foot. This would cost you $72.00 plus tax. So you spend $56.00 dollars extra for a high quality product. Big deal! Not only that, you have over 75 percent of the roll left over for another project!

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Author's Notes:

I received this email from Michael P, Cincinnati, OH. Here's how he helped with his crawl space problem.

"I live in Turpin Hills and wanted to thank you for your advice for sealing a crawl space. This is our third winter in our home and the basement is always much colder than previous basements from other homes. I did as you suggested and put heavy duty, thick plastic on the gravel and put doors on the opening of the crawl space. It's been a few weeks and I can see water droplets forming under the plastic.  I may put a second layer of plastic for good measure. The basement is warmer and so is the room above the crawl space. ;I don't smell the mustiness either.  Thanks again for the tip."