My understanding for house wrap is as a water barrier into the structure while allowing air flow to pass and moisture from the sheathing side out to evaporate, to some extent.
After running to pick up a roll at the best pricing of $80 a roll I was wondering how Tyvek was able to justify it being any better or important enough to become code. For instance why isn’t plain visquen plastic with overlapping slits anymore or less effective than actual wrap?
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Replies
Poly sheeting is much less vapor permeable than housewrap. It would trap a lot of vapor in the walls, assuming there is vapor trying to migrate out. Housewraps themselves vary quite a bit in their performance characteristics. Worth learning about it you are a builder, and easy to find lots of info online here and elsewhere.
I realize you don't want to seal the structure with plastic nor can you by code. But if the plastic was to be slit it really performs the same function. Truthfully you could take the idea as far as to say 1' plastic, 1" nothing, 1' plastic, and so on, some in some out.It's more about what really makes it so much more special that it's use is legislatively mandated and in turn more expensive. It seems the wax coated cardboard we're pulling off a 100yr. old farm house has been just as effective.
" It seems the wax coated cardboard we're pulling off a 100yr. old farm house has been just as effective."
You will find a lot of support for that point of view. Discussions on building paper vs house wrap tend to elicit equal support for both choices.
Barrier to liqid water from outside while allowing water VAPOR from inside to pass thru.
"while allowing air flow to pass"
NO - it stops infiltration if installed right.
Plastic stops both liquid water AND vapor equally from both sides, trapping moisture inside, where it condenses into liquid water, causing damage.
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"The unique nonwoven structure of DuPont™ Tyvek¯ HomeWrap¯ blocks ***excess*** water from entering into walls."
"The breathable structure of this house wrap allows moisture vapor to pass through to ***help promote*** drying in wall systems which ***can aid*** in ***preventing*** the growth of mold and mildew."
"The unique non-woven fiber structure of DuPont™ Tyvek¯ HomeWrap¯ ***stops air movement*** through the walls ***helping*** to provide a more energy efficient home.""DUPONT EXPRESSLY DISCLAIMS ANY WARRANTY, EXPRESSED OR IMPLIED, INCLUDING THE WARRANTY OF MERCHANTABILITY OR FITNESS FOR PARTICULAR PURPOSE OF THIS PRODUCT FOR THE PROJECT."Not the strongest of verb usage for the sake of mandating code, unlike "smoke detectors can, will, and do save lives".
All those statement agree with what I said.in spite of the fact they were obviously reviewed by a lawyer before publishing
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Not quite correct. Originally housewrap was provided as an air barrier -- it DOES NOT allow air to flow through -- while allowing water vapor to pass through, and, incidentally, serving as a secondary water screen. Housewrap IS NOT a "vapor barrier" -- quite the opposite.
Plastic prevents airflow but also prevents water vapor from passing through.
How significant these differences are depends on your location and the construction of your home. In a "conventional" (stud wall with fiberglass insulation) home in northern climates (where temps dip below zero for extended periods) keeping the wind out while allowing water vapor from the inside to escape is of enormous help in terms of both energy efficiency and the well-being of the structure.
In other climates, and with other forms of construction YMMV.
The selling of housewrap as a secondary rain barrier started after it was first sold for the above purposes (apparently abetted by the advent of vinyl siding which is a relatively poor rain barrier). Regular (white, thin) Tyvek is not really intended for this purpose, though it does a passable job. There are other products that are a bit more reliable as rain barriers.