I was working in my shop this morning and uncovered a partial roll of Tyvek. Just for grins I cut a piece about two feet square and bungeed it over an empty five gallon bucket. I left enough sag in it to make a basin of sorts. I faced the writing side into the bucket, and poured a half gallom of water in it. After seven hours of sitting I poured the water off into another bucket, and then back into the milk jug it was originaly poured from. Near as I can tell I got 100% back in the jug. I found no water in the bucket the Tyvek was over.
DuPont claims Tyvek is a permeable one way membrane. It will let moisture out of a wall from the inside, but will not let moisture or air in from the outside. While my test was decidely unscientific, my thought is that Tyvek a vapor barrier on the outside of a wall.
Comments one and all. Let’s beat this thing up agian!
Dave
Replies
Well then now you have repeat the test except with the lettering on top this time. Tell us what you find>
~Peter
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I thought it is only supposed to be permeable to water vapor, not liquid.
A simple test for vapor permeability, the ability to let water vapor, not droplets, out, you could do something similar.
I'm thinking a five gallon bucket would work. Into this squirt a small amount of water, one ounce seems about right as vapor move small quantities over great times, and foil tape Tyvec over the top. Make sure everything is sealed. Then set the whole thing in a sunny location that does not get any rain. Some source of warmth is needed. without heat the water will remain liquid much longer and you want it to be vapor for a fair test. You might be able to substitute a light bulb. The bucket should get slightly warm to the touch.
For the most accurate results you would weigh the assembled bucket with water on an accurate scale. Wait a week of so and weigh it again. Any weight loss could be assumed to be a result of water turning to vapor and escaping into the atmosphere through the Tyvec.
If you don't have a triple beam, you might ask a local drug dealer, A rough idea might be gathered by keeping the sealed bucket warm for a week or two and looking inside. If the inside is dry, or drier the water can be assumed to have escaped through the Tyvec.
Actually your first test was, IMHO, a victory for the Tyvec. As I understand it this material is meant to keep liquid water and the majority of air movement out while allowing any small amounts of water that might make it into wall, no vapor barrier is perfect, to turn to vapor after absorbing interior heat, and get out of the wall by migrating out through he Tyvec or tar paper.
Even easier, place some water in the bottom of the bucket, tyvek sealed to the top (with lettering facing out), and place in sun. If condensation or droplets appear on underside of tyvek, it is not a permeable membrane (ok, well, the rate may matter, but wet is wet) or insufficiently permeable.
I don't think that that would really be a fair test. Permeable membranes, I'm most familiar with Goretex used in outdoors clothing but the principle is the same, are a balancing act. A lot depends on how much moisture gets into the wall. In theory a perfect vapor barrier and construction would allow no water in so none would need to get out. In that case you could poly the wall and walk away happy.
Of course no construction or vapor barrier is perfect. so water vapor has to be able to get out while keeping liquid water out. Some old construction used used no barrier. This makes for a well ventilated but inefficient, energy wise, wall. Others used tar paper. It keeps bulk water out but allows some ventilation.
Tyvec, et al, tries to keep more water out and limit air infiltration, supposedly higher energy efficiency, while allowing an adequate path for water vapor to escape. As long as water can get out quickly enough to avoid collecting, soaking the insulation and causing rot the wall will do fine. If it can do it while increasing energy efficiency they have quite a product. That is the question.
As I understand it it doesn't matter which side points out. The water repellency and permeability is intrinsic to the material not limited to one side.
Just because there may be some condensation it does not mean that some water, hopefully enough to make the wall system work properly, is not escaping. A longer wait would allow more water to escape. As I understand it not much has to escape. Just more than gets in and this process happens over hundreds of square feet of area.
A few points from prior research:
Permeability is typically listed in "Perms"- the number of grains, 7000 grains per pound, a square foot of the rated material will allow to pass per hour of time given standard conditions.
A few points of reference:
1/4" exterior ply, glued joints, has a rating of 0.7 perms
Two coats of vapor barrier paint 0.3 to 0.5
4 mil poly 0.08
6 mil poly 0.06
Aluminum foil 0.35mil 0.05 perms
water is roughly a pound a pint, 16 oz.
So a 100 square foot wall with a 4 mil poly vapor barrier and moisture on the inside, say a bathroom, that wants to get out.
100 sq' x 0.08 perms = 8 grains per hour entering the wall.
8 grains / 7000 gn/ lb x 16 oz = .018 oz /hr entering = aprox .5 cc per hour.
This goes on each hour until the exhaust fan clears the moisture from the bathroom after that Hollywood shower. If the infiltration barrier can match that rate the wall would likely dry in a reasonable time. This doesn't take into account the inevitable construction flaws or holes in the vapor barrier. I'm guessing that if Tyvec has a perm rate around one it would breath enough for the wall to stay dry, the sheathing unrotted and the house energy efficient, comfortable and long lasting.
But then again I may have totally misunderstood the concepts and gotten everything bass ackwards.
Sounds like a reasonable test. I'll give it a try, but instead of a constant amount of heat, I think real world temperature swings would be more real. If I can get it set up today, I'll run it for a week, and see what happens. I don't know any local drug dealers, so weigths may have to come from a baby scales. I also have a product called Barricade (sp) that is a polyprolene I think. Might run them side by side.
Dave
Agreed. I was giving a rough a dirty test. One could calculate the amount of moisture that the Tyvek should pass from its permeance, and then put the proper amount in the bucket for the given area of Tyvek covering.
If you wanted to be real scientific, I guess you could put differing amounts of water in the bottom, put them all out in the sun, and see which one condenses first. But, the permeance of the Tyvek might be such that the vapor pressure cannot reach 100%, so you would never see condensation. In that case, I would say it passes with flying colors.