# tyvek question



## roughsawn (Mar 2, 2011)

Does anyone here know if tyvek can be used for roof underlayment w/ metal roofing. Right now I have a giant billboard tarp for underlayment but I'm now afraid that it might melt in the summer sun. Also can tyvek be used for floor underlayment. My roof decking and sub floor are both made of rough sawn 1x of varying widths. I will have six inch red maple shiplap for finish flooring over the tyvek. also, can tyvek be used on inside bof studs to cover insulation in mi9chigan. thanks


----------



## md2lgyk (Jan 6, 2009)

Why would you use Tyvek for these things? Especially the roof. 30 lb roofing felt is a lot less expensive. Same for underneath flooring - I use roofing felt there too. And for covering insulation, 6-mil poly sheeting is far less expensive than Tyvek.


----------



## roughsawn (Mar 2, 2011)

*free tyvek*

tyvek was free
tyvek breathes, poly does not
the reason I want to use tyvek on the inside walls as well, is to cover insulation because I am using square-edged rough-sawn boards for wall panelling.I just want to know if the tyvek is suitable for this, as it breathes, but will keep out fiberglass particles.


----------



## md2lgyk (Jan 6, 2009)

OK, I certainly understand free. But you don't want your vapor barrier to breathe - that's counter to its purpose. If the wall insulation is unfaced, you need something impermeable like poly. If it's faced, you don't need anything else.


----------



## Gary in WA (Mar 11, 2009)

You could use it to cover the insulation on the attic side of a knee wall, or on the joist bottoms of the crawl space. Not in the other areas mentioned. 

Gary


----------



## roughsawn (Mar 2, 2011)

Not to be a pain, but why not? It seems that a double vapor retarder would benefit if any thing. please elaborate. Trust me, I put a lot of blood and sweat into this house, I want to do it right. I'm not worried about moisture vapors getting in.They alwasy find a way. I want them to be able to escape w/out drafts and actual water being able to get in, thus the use of tyvek vs poly. The idea of tyvek on the inside walls vs poly was to cover unfaced insulation and seal the gaps between square-edge board panelling. Plus I have enough tyvek to do all this and then some to spare. I've seen tyvek used around here (northwest michigan) for . underlayment w/ metal roofs. Maybe I am misunderstanding the mechanics of moisture vapor. anyways we heat w/ wood, so the air stays pretty dry in the winter. Could you please elaborate? Thanks


----------



## Gary in WA (Mar 11, 2009)

Tyvek has a perm rating of *58*. Check the ratings of different classes of vapor retarders;

http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-water-resistive-barriers




Gary


----------



## md2lgyk (Jan 6, 2009)

roughsawn said:


> Not to be a pain, but why not? Trust me, I put a lot of blood and sweat into this house, I want to do it right.


I do understand the sweat and blood thing (and you left out broken bones). My wife and I (both in our late 50s) built our log home entirely by ourselves. We did much research to be sure we did everything right. If you want to do the same, why won't you listen? You seem bound and determined to use an inappropriate material despite our advice.


----------



## roughsawn (Mar 2, 2011)

I'm listening. I was just looking for a more detailed explanation, so I can better understand the reasoning. I'm not bound or determined to do anything but build a solid house. Otherwise the tyvek would already be up. From what I understand there are two schools on the issue, the old timers that say a house should breathe, and the sealed envelope type. I was just hoping to gain a better understanding of one vs the other, rather than just a don't do it. thanks


----------



## roughsawn (Mar 2, 2011)

Anton TenWolde: “The calculations show that even with very low air pressures across the assembly, and even with a very good air barrier, sufficient moisture can bypass a poly vapor retarder, degrading its performance. In practice it doesn’t matter what the permeance of the vapor retarder is, because the air leakage will go around it for moisture transfer. I came to the conclusion that the idea that we need a vapor barrier to keep our walls dry doesn’t hold a lot of water, so to speak.”
John Straube: “The whole reason we’re talking about vapor barriers is not because vapor diffusion control is so important, but because people believe it is so important. The question comes up, have we seen diffusion-related building failures? And the answer is, very few — maybe in rooms with a swimming pool. Assuming that the vapor came from the inside, you would have to have a very high load before you would see a problem. I think that solar-driven vapor is much more important. The moisture is coming from the other side of the assembly.”
Joseph Lstiburek: “In North Carolina, for whatever reason, they build their walls with fiberglass insulation and with poly on the inside. Depending on the cladding — brick and stucco being the worst — the walls rot like crazy.”
André Desjarlais: “We can’t assume that the building envelope is perfect. We have to assume some level of failure: some rain will get into the wall, and there will be imperfections in the air barrier.”
Achilles Karagiozis. “It’s all related—the vapor control strategy, airtightness, and whether or not there is a ventilation cavity behind the exterior cladding. If you have a ventilation cavity behind the cladding, it doesn’t matter what kind of vapor retarder strategy you use.”
Bill Rose: “In the South, no vapor barrier. In the North, as long as you have insulated sheathing that meets the dew-point test, also no vapor barrier.”
Anton TenWolde: “When you put enough foam sheathing on the wall you get away from the cliff rapidly, and there’s no reason to worry about vapor barriers any more.”


----------



## JCarsten (Jul 6, 2010)

roughsawn said:


> From what I understand there are two schools on the issue, the old timers that say a house should breathe, and the sealed envelope type. I was just hoping to gain a better understanding of one vs the other, rather than just a don't do it. thanks


The old timers school of thought never took into consideration energy efficiency. The house may breathe, but your heat/ cool air would go right out the door/window/wall/ etc. The new theory is seal up everything, but then it can't dry out if it gets wet. Wall assembly, type of insulation, vapor barrier all need to work together. There is never a perfect system. It will always be penetrated by nails, screws, electrical boxes, plumbing pipes, etc. And the codes for how to construct energy efficient homes is ever changing.

Jason Myrlie
www.jcarstenhomes.com


----------



## Tom Struble (Dec 29, 2008)

you seem to be confusing vapor retarder and air barriers


----------

