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Sustainable Building Materials
And Vapor Barriers

Question:

I was trying to do research on material for a house that would have green or sustainable materials. Is there such a thing a green vapor barrier? Why or why not.

Charlie

Reply:

The short answer, I think, is no.

The purpose of a vapor barrier is to prevent the movement of gaseous water molecules through walls. When water vapor leaves the building it carries a lot of energy with it.

There is a well-known chemical action known as diffusion whereby molecules in a higher concentration automatically migrate to a region of lesser concentration. Simply put, if you drop some sugar in your coffee and don't stir it, the sugar normally will dissolve all by itself, eventually. My understanding is the process of diffusion is greatest for vapors.

"Vapors just wanna spread out", a new hit song by 1980's pop singer Cindi Lauper.

It just happens that most natural materials are quite porous. Wood has very high vapor permeability (a measure of how easily water vapor can penetrate a material) and the other common household building materials like paper and gypsum have virtually no ability to block diffusion or movement of water vapor.

Synthetic materials, especially plastics, have completely different chemical structures, and can easily be made water-tight and resistant to vapor transmission. I can't recall the book where I saw the info, but I can remember seeing some vapor permeability numbers a few years ago, and the permeability ratings of simple plastics like polyethylene (the usual vapor barrier material) are almost zero, while the ratings for the usual list of building materials are very high.

Which leads me to a thought: A thin sheet of metal could be used for a vapor barrier if plastic was not desirable. But sealing the edges of the material could present some challenges. I've never heard of anybody doing this. Glass has very low permeability, but glass is a very ornery building material. Just try driving a nail through a sheet of glass.

Of all the plastics used today, polyethylene is one of the most versatile. Many people, environmentalists in particular, seem to have a hatred of plastics. Having a half-dozen college courses in Plastics Technology under my belt, I feel I can speak with a small degree of confidence when I say that plastics in general have far more benefits than potential side effects. And the polyolefin family of plastics, which include polyethylene and polypropylene, are one of the best. Polyolefins are very highly chemically inert, which is why they have become the most popular form of packaging. Gasoline cans, for instance, are made from polyethylene, because nothing attacks polyolefin molecules. In fact, this trait is a problem for the plastics industry because getting paint (for printed packaging) and labels to stick to polyethylene is darned near impossible. What they do, I'm told, is literally expose the packaging to a hot flame for a split second, to alter the chemical structure at the surface, so paint and glue will stick.

That's a pretty good material... it's chemically inert by nature, and to make it chemically active you need to give it special treatment. I can't think of any other materials that are so lucky. All the metals will corrode in aqueous liquids, except gold perhaps.

Is polyethylene 100% safe? I can't say that. Would you be exposing yourself to any significant risk by using a polyethylene vapor barrier in a house? I can't say "no" with 100% certainty. But I've never heard of any evidence to suggest that polyethylene is even absorbed by the human body, let alone shown to cause any health problem.

But when you look at comparative risks, like the risk of injury and death from even a short automobile trip, polyethylene vapor barriers look pretty darn tame. When you look at environmental damage caused by human activity, using a polyethylene vapor barrier is hardly the most egregious attack. Building a house in the country is by far a greater environmental assault, because of the eternity of car trips generated by traveling between house and city for work, for children's events and for the necessities of human consumption.

The best thing an environmentally-minded person can do is live in the central part of a city, or at least an older suburb, take mass transit to work every day, and carefully remodel an existing house with high-quality materials that will last a long time. If this subject interests you, I urge you to read the book "Suburban Nation" by Andres Duany, et al.

I'm moving back to the city as soon as I'm done remodeling this old farm house.

Bruce W. Maki, Editor.