I have been installing Tyvek or equal on homes for 20+ years. Now i hear from a friend/competitor, who just went out for more training for his licence, that Tyvek is bad and we need to go back (to the dark ages) felt. It has something to do with trapping moisture behind the wrap and rotting out the walls. He also learned not to put foam at the rim joist, as this also allows rot. Both of these practices combined with 6mill poly/black death/caulking/air tight electrical boxes (@ $5.00 each) to create as airtight as an building envelope as possible and mechaniclly ventilate it, to gain a healtly living enviroment. Another educational fact he picked up was to install ice & water over the ENTIRE outside of the house, then add 2” of foam, then strips, then siding. I can’t wait to try that one out!
What have others heard about the pedulum swinging away from super air tight?
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I've been really interested in this topic, because it changes so often. I was reading another post here last night that talked about the slow evolution away from a single set of standards that was designed for a certain climate. There is finally starting to be a recognition that you need to build completely different wall structures for heating-only, mixed, and cooling-only climates. It seems that some builders have been doing this for a while, but the codes haven't been keeping up. I hate to think what their inspections have been like.
The best resource I have found is the Building Science Consortium ( http://www.buildingscience.ca/bsc/ ). Check out the buildingscience.com section of that site for some great articles or the building science press section to buy their books. The have builder's guides to different climates that used to be published by Taunton, but it looks like they may not be anymore. I bought the one for my climate, and found it very useful.
There is something that doesn't make sense about what your friend told you. He said he learned that Tyvek is trapping moisture behind it and causing rot, but also told you to put Ice and Water Shield (IWS) on the whole house. I gaurentee that will trap more moisture than the Tyvek did. The real issue is understanding how moisture moves in your climate, and where the dewpoint will be in your wall constuction. I think the goal with that IWS and exterior foam method is to reach the dewpoint in the foam, or at least somehwere outside that IWS, so the condensation doesn't happen inside your wall contsruction and rot out your studs.
-Rich
-Rich
I was thinking the same thing as rfarnham when he mentioned that the ice shield will trap more moisture than the Tyvek. Unless you are in a hot, humid climate with no other vapor barrier, I would think that would be a mistake. Tyvek supposedly allows water vapor in and out--I don't think water & ice shield does, but I could be wrong--just seems awfully thick to allow water vapor through. You can go to http://www.buildingscience.com for more about insulation and vapor barriers.
The I&W wrap with 2" foam makes sense to some degree if in a warm humid climate, except that I fear too many guys would be in the eternal hurry up mode and rely on the I&W instead of proper flashings and lap directions, so water would still be getting trapped.I'm thinking the guy who went to that class was not listening with both ears. I have too often sat in a class and then in the rehash on the way home or the luinch break discussion hear guys repeating what was not said or even saying exactly opposite of what was being taught.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
True about people often not hearing the whole lesson in a class.
Then there's the guy I used to work with who was sent to a class on computer modeling of transportation networks so we could predict traffic loads and decide where improvements needed to be made and so on. He was supposed to go and take notes and bring the info back to share with me. He didn't take any notes and when I asked him what the seminar was about he said, "Well, it involved lots of math--algorithms." That's all he could tell me. I already knew it involved algorithms. That's like telling someone that building a concrete retaining wall involves cement. (Thinking of another thread here!)
If you have such a tight inside wall, then there is something else wrong to be traping moisture in the walls. Tyvek can breathe to the point of allowing moisture to escape. I'd like to know the source of his info.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
I'll chat some more with my friend and get more clues to his education. I think i'll just keep on w/ Tyvek, foam and all
http://www.umass.edu/bmatwt/publications/articles/leaky_housewraps.html
http://forums.jlconline.com/forums/printthread.php?t=10926&page=1&pp=15
The system with 2" foam over the ice and water shield everywhere is sound because the foam moves the wood studs and sheathing to the inside of where the dewpoint falls, meaning any condensation theoretically occurs outside of the waterproof layer of I&W.
The other thing sounds wrong. Tyvek and 15# felt both breathe enough to dry out small amounts of water. There are good reasons to prefer felt to Tyvek on a house, and they have to do with cost and longevity in the presence of tannins. Like Piffin said, no housewrap can compensate for improper flashing details that let in the rain.
Foam at the rim joist is only a condensation issue if the foam is pieced in the joist bays. Holding the rim in from the wall line and packing it out with foam before sheathing the wall is how I learned it in the 80's, and that works just fine, so long as you span across the rim with your sheathing sheets. This was done on 2x6 walls, BTW.
Bill
In many cases, the placement of foam outside of the framing will indeed prevent the dew point being reached within the wall framing and any insulation there. I am assuming we are talking about a cold climate here; as pointed out, for a hot and humid climate (US gulf coast, fro example), everything is very different. However, just where the dew point is reached in a wall assembly can and should be calculated for a particular wall assembly that is contemplated.
For steady state heat transfer, the rate of heat transfer across each layer of the wall will be constant. Thus the temperature change across each layer will be proportional to the R value of that layer; the absolute difference across the layer will be that layer's fraction of the total wall R times the total wall temperature difference. In the example cited, a 2" layer of foam outside the sheathing was proposed. That would provide about R10 if we are talking XPS. If inside that is an R11 batt of FG in a 2x4 stud wall, and assuming that with sheathing and siding the total wall R value is R22, then the temperature drop across just the FG batt will be about 11/22 or half the total. For, say, 65 F inside the drywall and 5 F outside temperature, the drop across the FG would be about half that 60 difference, which would make the temperature on the inside of the sheathing 65-30=35 F. That is 10 degrees below the dew point of air at 70 F and 40% RH. For such a wall, having a vapor retarder under the drywall AND tight sealing to prevent convective flow of inside air into the wall cavity still would be essential. Given that, the diffusion of water vapor into the wall cavity and ultimately to the outside would be slow, and the dew point would be much lower than mid-40s. This of course assumes that there is no vapor retarder on the cold side of the wall to trap the water vapor inside. If one wants to keep the temperature inside of the sheathing above 45 F, then more of the total R value must be outside of the sheathing (or less inside it, but that increases heat loss). If there is a wall cavity, it makes most sense to fill it with insulation and keep the warm-side moisture out of it.
You are correct. The specific wall should be calculated for the climate it lives in.In the real world, I seriously doubt that the R-11 batts, combined with R-3.5 studs, plates, and headers, will outperform the continuous layer of 2" XPS outside the sheathing.Bill
Oh, I fully agree. My point was just that temperature profiles within exterior walls can be calculated, rather than guessed at. There are some useful sketches on the http://www.buildingscience.com site. Actually, FG batt insulation is getting hard to recommend for use in a heating climate, in light of what now is known about how its performance gets progressively worse as the outside temperature plummets. Someone in these threads has called the stuff an air filter in the wall. One could use a FG batt to add R value on top of foam used to do air sealing, as long as the temperature drop across it is not severe. For the case of the wall in question, would it be cheaper to apply 2" XPS (R-10) outside or have closed cell foam sprayed on the inside (R-21 at aged 6.0/inch, for whole-wall R of around 18)?
two inches of foil backed polyiso foam on exterior - about $.80 per sq. ft.
Local quotes for any type of sprayed in foam are much higher.