Vapor Barriers: Need one or not?

[Music] it looks like I'm coming to you from CIA headquarters but in fact we're recording at our underground building science earlier all right guys hard topic today we're talking about vapor barriers and I was trying to figure out how I could shoot this at a job site but I'm just not using any vapors on jobs these days so I thought we'd record this here at the office where I can have access to the computer and show you some images both in the past and from a few places around the web on this important topic specifically what's happening is I'm seeing vapor barriers being used incorrectly on jobs all the time and I'm getting builders asking me when do I use them when do I not now when I say vapor barrier most builders are looking at this image right here this is some rockwool unphased bats that's how they're coming and then the Builder whether it was the inspector or the Builder said hey let's cover those with polyethylene six mil poly is typically what's being used out there that I'm seeing and when I was a builder in the 90s we were told on the warm side and the winter is where we're gonna put that that polyethylene vapor barrier poly vapor barrier and so that's where you're seeing it right here now let's go to a couple places in the web that I uses some trusted sources find home building comm you know this is a great image from their web site talking about vapor barriers if you look here we've got a house here's the cold outside that's you know maybe under ten degrees outside here's the inside of the house and the heaters on or maybe sixty eight degrees outside and this blue right here you're seeing that's some vapor that's been diffused on the inside air inside our houses were breathing we're cooking we're showering we're boiling pasta and so the humidity typically in our houses in the wintertime is much much higher than it is outside cold air can't hold a lot of humidity so if you look at this diagram here they've got a sheet of drywall in that water vapor is diffusing through the drywall that's the process where basically those small water molecules that are in the air floating we can't see them are able to actually go from places of high humidity to humidity from warm to cold and so it's diffusing through the drywall you can see it's even wetting somewhere in the insulation there and on the back of the sheathing that's cold it's condensing and if you look to this diagram you think oh my gosh without a vapor barrier this walls gonna get drenched there's just gonna be liquid water all over the place I don't want to get into this article in particular but I do want to jump over to Wikipedia I was curious to see what they had to say on the matter check out this Wikipedia page right here we're talking about vapor barriers specifically here and it defines a vapor barrier it says a vapor barrier is something that's point one perms or less basically plastic sheeting and then interesting where they show where you need it based on the geography here really the only place where you need an exterior vapor barrier they're saying is in Florida and maybe a few parts of Texas and then where you need an interior vapor barrier is basically from there on north everywhere north of this kind of line right here on Wikipedia is saying we need a vapor barrier on the inside very interesting now let's move over to a friend of Mines website this is Allison bales who runs a blog called energy Vanguard fantastic blog you should subscribe to his feed for sure but he wrote an article a couple years ago called you don't need a vapor barrier probably and this is a great article I'm gonna summarize it briefly he basically says that the act of diffusion that water vapor going through the sheet rock and potentially condensing on the inside that act of diffusion is a pretty slow process and it's not gonna move a lot of water in fact through the whole heating season this article says that you're gonna diffuse at most a third of a quart of water through a four by eight sheet of drywall now on the other hand most the time when walls are getting wet it's not diffusion but it's actually air movement which is depositing that water look at this right here again 4-byte sheet if we had a small hole in the drywall and our interiors being heated and we've got forty percent relative humidity air leaking into that cavity could bring with it as much as thirty quarts of water now that's a big deal that's also a big deal for me in the hot humid south because air could be leaking from the outside in and doing the same thing depositing on the back of the sheetrock where I've got a cold condensing surface so basically Allison says on this article there's very few places if any in the u.s. that you really need a vapor barrier the big deal that we need to think about is air barriers now let's go over to my friend Martin Holliday who writes for green building advisor great source of information on building science and his article here I'll link to this below do I need a vapor retarder read this great article but the summary at the bottom here if we go all the way to the bottom the very last line has what's the short version here's a couple rules most buildings don't need polyethylene anywhere except underneath a concrete slab or in your crawlspace floor this goes really for the whole US and the main reason you might use an interior vapor barrier or a vapor retarder rather is to keep your building inspector happy now instead of using polyethylene martin says what we should consider using is a vapor retarding paint that means a paint on the outside of the gypsum sheetrock which is gonna slow that diffusion down or a smart retarder for instance here's a couple brand names right here quoted but the point of this article is that there's multiple different building zones in America and we really need to be cautious about putting that polyethylene up now here's a direct example from from me down here in Texas I did a remodel in this house that was less than ten years old and this has happened to me on multiple occasions where we found on a very young house polyethylene sheeting on the inside and it's a little hard to tell in this photo maybe but there's a little bit of some moisture it looks like depositing in the backside of that and then look who we found on the inside of that sheetrock in several locations not good not good at all if you're in climate zone one two or three you don't want any kind of vapor retarder on the inside of the house whatsoever on the outside of the house you might have a low perm house rap or something that's gonna stop both water and vapor on the outside but on the inside of the walls you want nothing now there this is the climate zone from climate zones from the do-e so this is basically a map of all the different zones in America and if you're in climate zone four and above it actually you might actually use some amount of retarding of that moisture from getting into those walls but it's really only if you get into the very coldest zone in America which is climate zone seven up there where you might actually use a vapor retarder pretty much every other climate zone you don't need one on the inside and certainly you don't want to be using a plastic sheet okay last thing I want to mention going back to the web here go to building science comm and they have a bookstore and there's different climate zones as I mentioned and there's different ways to build and there's different zones the same house that you're gonna build in Minnesota is not going to work if you build that house down in Florida and so you actually want to buy the climate zone guide for your specific area this is your Bible from a building science perspective and there's of course there's a million different assemblies some people are using exterior insulation others are not you really want to get this book for your climate zone and figure out what's right for you but the long and short of this video is if you're in America you don't need a plastic sheet on the inside of your walls you don't need to worry about it the chances of a perm failure or a diffusion Ferrier failure are very very low most of the time if you have a problem it's because of air leakage through that cavity or bulk water that got in from the outside guys super geeky topic thanks for putting up with me doing a slightly different video from the office here comment below if you've got any comments on this if you like this format - I'd love to hear that otherwise follow me on Twitter Instagram or ever see you next time on the build show [Music]

Channel: Matt Risinger

Views: 753,742

Rating: 4.8899117 out of 5

Keywords: building science, vapor barriers, when to use a vapor barrier, fine home building, build show, buildshow, build show with matt risinger, jordan smith builds, build show austin texas, filmmaking, matt risinger presentation

Id: q8fOMBlfh3A

Length: 8min 56sec (536 seconds)

Published: Fri Jan 26 2018