A technical guide to understanding the role weather barriers and rainscreens play in a wall, choosing the right products and installation tips for optimum performance.
They're officially known as water-resistive barriers or WRBs, sometimes weather-resistive barriers, but in practical terms they are more commonly called housewraps and weather barriers. With so many options I plan to randomly switch between terms just to alleviate the monotony and keep us all entertained.
A few other linguistic options to mention - cladding is the same as siding, furring strips are the same as strapping. Those and your weather barrier all installed properly are what defines a ventilated rainscreen.
There are many different potential wall assemblies, and many products with different characteristics that can be used in different cases. For the sake of simplicity and not getting off on a bunch of tangents, in this article we will stick with the bread and butter wall assembly we all know - a stud wall with exterior sheathing, housewrap, strapping and siding. Other options for wall assemblies can be found in our世界杯葡萄牙vs加纳亚盘赔率.
就像帆布或尼龙帐篷一样，水会从防水屏障中流失，但积水会从屏障中流失，尽管速度很慢。由于水分可以通过wrb蒸发，任何在壁面组件中积累的水分都有机会逃逸。
There in a nutshell is the purpose of a weather barrier - to provide a drainage plane that protects sensitive materials from direct exposure to precipitation at the same time that it allows walls to dry out.
It's worth a mention at this point that weather barriers are oftenintended to act as an air barrieras well, but the most common installation techniques (staples, nails and hastily installed tape) mean it is a rare case when they actually do a good job at air sealing. Manufacturers will rightly claim their products are a suitable choice for an air barrier, and they most certainly are. The failure of weather barriers to perform well in that capacity lies entirely with how they are installed.
随意地在你的家庭保暖膜上撒点东西，或者在家庭保暖膜上撒点订书钉，会产生很多小孔，空气可以在这些小孔中进出。此外，当暴露在夏季和冬季之间的极端温度变化，以及风和气压差造成的松散天气屏障的无休止移动时，我们经常使用的溶媒胶带可能会随着时间的推移变得脆弱，不再粘附。
You'll have better luck using weather barriers as air barriers if you place staples where they will be covered by strapping, or put tape over them. Don't over-staple it for no reason, staples only have to hold it up until the strapping is on. And if you can wait for a calm day with no wind you will really only need to staple it at the top (where it will be overlapped anyway) and let it hang. Better still would be forgoing staples altogether and using nails with plastic washers.
Be meticulous when taping joints and try to use brands of tape that are more suitable for extreme temperature changes. Siga,Delta Multi-Bandand Air Stop are three examples of tapes with much better adhesion.
Peel-and-stick membranes:
一种常用的非粘性房屋包装的替代品是“剥离-粘贴”膜，如果你想给它们取一个更有趣的词，那就用这个——自我粘附的空气和防水屏障。
They're still vapour permeable, but membranes that stick to the sheathing are much less susceptible to damage caused by wind pressure.
Since a conventional housewrap is usually held on by staples, the back and forth ballooning of those loose weather barriers can over time enlarge staple and nail holes and reduce their ability to prevent air leakage. With no stapling needed and being fully adhered to the sheathing, wind and air pressure isn't a concern for air barrier durability with peel-and-stick membranes.
根据你贴膜的基材不同，制造商可能会建议先在表面涂上底漆，以获得更好、更持久的粘附力。在混合物中加入底漆也会影响墙面的透气性;稍后再详细说明。
Self-adhering membranes are not all the same:
Like so many other building products, it is important that you understand the properties of different membranes before choosing one, don't just pick one randomly off the shelf based on price or brand recognition. Some are vapour permeable and will allow walls to dry, some will not. They both have suitable applications but shouldn't be randomly interchanged.
For the standard wall assembly we are talking about here, make sure that whatever membrane you choose as your weather barrier is vapour permeable, and the more permeable the better. Look for the vapour permeance rating (how much moisture can pass though it); it should be on the product itself but also on the technical data sheets of company websites.
1 Perm and 60 Ng*(U.S. and Canadian ratings respectively) are the benchmark permeability rates for what defines a type II residential vapour barrier. Products rated below those numbers are vapour barriers, those rated between 1 and 10 perms are considered vapour retarders. The higher the perm rating or Ng, the quicker moisture can evaporate.
Ng代表毫微克，即十亿分之一克水。That 60 Ng number refers to the amount of water that will pass through a square meter of a material in a second [60 ng/(Pa.s.m2)].
1 U.S. perm is actually57 Ng, that 3 billionths of a gram difference between 57 and 60 is what you call 'close enough for jazz'. And forget the 60 Ng for now, we''ll go with perms only.
Categorizations of building materials based on their permeability rates:
| Vapour impermeable | 0.1 perms or less |
| Vapour semi-impermeable | 1.0 perm or less and greater than 0.1 perms |
| Vapour semi-permeable | 10 perms or less and greater than 1.0 perms |
| Vapour permeable | Greater than 10 perms |
Understanding vapour permeability:
All we are talking about so far are the stand alone housewraps everyone is used to seeing, which will range from about 6 to 60 perms. To go further down this rabbit hole, you will find that your sheathing will impact vapour permeability of the wall assembly as well.[Read more about material permeability here].
Conclusions about the vapour permeability of sheathing seem to differ heavily from different sources, We will run with the pack on this one for now, using the numbers most manufacturers cite.
7/16" OSB (oriented strand board) has a rating of about 2 perms; compare that to half-inch plywood which is about 10 perms. And those rates will change with thickness - 3/4 inch OSB is rated about 40 Ng (2/3rds of a perm) which actually qualifies it as a vapour barrier, and can be used as such on the interior.[Here's an example of how osb can be used as a vapor barrier]
所以你需要将墙体作为一个整体来看待，不要认为你选择的天气屏障的烫发等级会定义墙体外部的渗透性作为一个整体。
Starting off with such a low-perm sheathing as OSB means even a high-perm weather barrier may have little effect, so the added cost of plywood is something that may be worth considering investing in for your overall wall durability, as it is 5 times more permeable to vapour than OSB.
Also important to remember is that if you are putting a rigid board insulation on the exterior, that as well will have a vapour permeability rating that should be considered. As an example, 2 inches of Roxul ComfortBoard is rated between 30 and 40 perms, compared to 2 inches of EPS foam which is about 1 perm, and 2 inches of XPS foam is only about .5 perms. Meaning, by putting 2 inches of foam on the outside of the house you have effectively just installed an exterior vapour barrier.
Permeability is tested in two manners -'dry cup' and 'wet cup'. Each method simulates the vapour drive in a different direction, so the resulting numbers vary, and manufacturers may not always tell you which test method was used.
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To use Vent SA from Cosella-Dörken as an example (which to our knowledge has the highest perm rating of peel-and-stick membranes), their 'wet cup' test resulted in a rating of 31 perms, dry cup testing was 50 perms. Not all manufacturers will list both numbers or even tell you which result they are posting, assume they are putting their best foot forward and giving you the more favourable looking number.
A rainscreen allows walls to dry:
“通风防雨罩”这个术语有时会被误解，并有不同的定义。它不是一个单独的产品或材料，而是一个设计特征。天气屏障、绑带和外部包层以及顶部和底部的通风开口共同创造了一个“雨幕”。防雨屏的目的是保护墙壁免受暴雨的影响，同时让任何渗入的水分排出，并让空气通过，使其干燥。
Installing siding directly against your weather barrier will greatly reduce the ability of a wall to dry. This isn't very common anymore but it does still happen. Always install strapping to separate the siding from the weather barrier. And a common practice to avoid is installing strapping horizontally. That prevents both the drainage and air convection that is essential for walls to stay dry. Water drains down and warm air rises - don't stop that from happening.
如果你选择的壁板需要水平的带子(比如木板和木条)，在它的后面包括某种间隔，这样水仍然可以排出去，空气仍然可以上升。没有了后面的空间，水平的带子会导致少量的积水，可以渗透建筑材料。
That gap in a rain screen doesn't need to be significant, just enough for water to drain and air to rise. If the weather barrier is tightly attached, as in the case of a peel-and-stick membrane or is part of a rigid board product, building scientists say as little as 1/16th gap provides a capillary break, that will let liquid water drain out.
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Building walls that can dry © Ecohome
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If you have a loose or bunched up weather barrier that might block that space, you may be wise to install a thicker spacer to ensure that space is continuous. Care during installation is what will determine how well your rainscreen blocks and releases moisture.
Do you need house wrap if you're installing rigid foam on the exterior, that's sealed at all joints?
Hi Marie,
你不一定要在刚性泡沫板的顶部安装WRB，但很多建筑商无论如何都会选择这样做，因为这是一个很好的方法，可以确保由于暴雨而渗透到壁板的任何水不会最终在泡沫板的接缝中，并确保墙壁可以正确排水;它也可以是一个很好的保护措施，以确保水从窗户的闪光不会进入泡沫后面。
Some builders also use the WRB as an air barrier, so before you omit it from your wall assembly you should make sure that you have a proper air barrier.
And how thick is the foam you are installing? Keep in mind thatrigid foam insulation can be a vapor barrier – XPS as of about 1 inch, EPS as of about 2 inches. And Polyiso insulation with foil is a vapour barrier. Despite the fact that we regularly see it being installed on the exterior of houses, that isn’t a great thing since it acts as a vapour barrier. So the thicker you can make it the better; I would propose at least 2 inches, but 3 or 4 would be even better. And with or without a WRB, be sure to have a drainage plane where water can drain; at this link is avideo on how to install siding so walls can dry. And here is another video that may be of value to you abouthow to install a peel and stick exterior air barrier.
I used 7/16 OSB sheathing and will be installing tongue and groove pine siding. I will also install a rainscreen with strapping. My question is, does the type of house wrap matter? I have either Prime Wrap which is a perforated fabric or Tyvek which is not perforated. From your article, because the OSB has such a low perm, I don't really have to worry about how many perms the house wrap has. Thanks!
嗨，Kathy，一旦你开始将不同的材料堆叠在一起，你就真的没有关于渗透率的清晰和简明的信息了。我认为两者都不会错，但我们会说，你的墙干得越多越好。每种材料都应该有一个规格页，上面列出了它们的渗透性，如果你想确定哪种材料更能渗透水分。最重要的是正确安装壁板，见上面评论中的链接。
I have a question involving radiant barrier house wrap on top of ZipR. I am in process of finalizing exterior wall designs and I want to place a radiant barrier with a Perm Rating of 6 on top of the Zip R. The Zip R would be the water barrier and have 1"of rigid foam on the inside for a thermal break and R Value of 6 attached to a 2x6 wall which will have Roxul mineral Wool insulation, R value of 23 for something near a final R vaule of 29. I want to put a Radiant Barrier wrap on the outside that has a Perm Rating of 6. I was wondering if anybody has done this and if that Perm Rating would allow any moisture that got past the Rain Screen and the Radiant Barrier and onto the Zip to dry fast enough to avoid any issues.
Hi Mike - I'm building a new house that is 70% stucco, 20% Cedar siding, and 10% stone. The climate is low humidity and low rainfall/20" rainfall yearly. The plan is to use TYVEK over the OSB. I keep hearing that xxxxxxx would be a superior product to use? Can that be so??
We have blocked out the name because we often get manufacturers on here trying to start discussions on their own products Doug, and honestly it doesn't stick out as either as being either better or worse than all the other competitors. Who do you keep hearing that from? And where are you building?
Hi Mike
I’ve heard commentary about Barricade from our local Boise builders. I was planning to use Tyvek but - everyone has their own opinion I realize that. Actual test data is needed. I just want a good house wrap for my situation. My house is located in Star, Idaho. 83669 Thanks Mike
Doug W.
Hey Doug. There is nothing wrong with tyvek, the product itself works great, but typically people put thousands of holes in it with staples and nails so installation is always where it's faults come from. I would say be very sparing with the staples if you can, and when possible only put them at the very top where it will be overlapped and taped, Strapping will hold it tight to the wall in the center. That works if you come by soon enough with the strapping so you don't leave it flapping in the wind.
Our preference is the peel and stick membranes as they do a much better job at air sealing a house. As for choosing one, check the perm ratings on company spec sheets online and you can see how they stack up against each other for permeability, then it's down to price and what you have access to.
And I'll also point you to our page abouthow to install siding so walls can dry, that will help your WRB do the job it is intended to do.
I have a home built in the late 1960s in MA. The prior owner covered the house with vinyl, which was not done with care. Water gets behind the vinyl. The cedar wood clapboard is rotted in many places. Also, behind the cedar is missing and or torn tar paper. There is a heavy mildew smell outside in places. Inside there is a dry musty smell as well. We are considering removing the vinyl and replacing the siding with new cedar wood along with a rain guard. However, my main concern is wrapping the house in something to air tight to prevent the house from breathing and causing trapped dry musty and mildew smells inside the house. My house does not have central air, which makes an air exchange unit not possible.
What do you recommend for my house? Any help is greatly apprecaited.
Sounds like the exterior cladding was built wrong all around, sorry to say. If there was tar paper against sheathing and then shingles directly on the tar paper, it will not be able to dry. See this page here –How to install siding so walls can dry.
I think you are best to remove it and the vinyl, then do a proper weather barrier with an air space and strapping as explained in that page I just linked. I can’t guarantee the musty smell will go away instantly, but I would think it would dissipate over time if the walls are able to dry out.
As for air exchangers, there is one product that may work for you, it is a great solution for installing ventilation without having to run ducts all around your house, see here -ductless air exchanger for renovations
And I wouldn't hesitate to do these repairs for fear of trapping air, your house will continue to rot if you don't. And even by making the house air tight, if you build it so walls can dry I can't imagine your air quality getting worse even without an air exchanger, even the most airtight house will still have some air leaks and change the indoor air.
I am building a 3 season summer cabin in Maine, rat slab, no attic. The inside will be tongue and groove pine, the insulation Cel Pak, the exterior Hardie Shingle and Hardie Panel and batten.
如果我们使用5/8英寸的CDX胶合板，你会推荐什么样的房屋包装，即透气性等级?
赛乐包里面有什么蒸汽屏障吗?
Thanks.
Our Canadian home is 20 years old and is subject to extreme winds and driving snow and rain. I want to replace the vinyl siding but want to also insulate the outside. I was thinking of putting Rockwool insulation on the outside of the wall and then new foam backed vinyl siding. Do I need to also put a vapour barrier between the rockwool and siding? Your help and advice will be appreciated
Rockwool is a smart idea but I would skip the foam backed vinyl. For cold climate houses, foam is okay on the outside when it is thick enough, but adding just an inch could do more harm than good as it will act as second vapour barrier, and an exterior one, which is not good. Rockwool lets moisture pass through, whereas foam will stop or at least slow it, to the point where you can have condensation form. If foam is thick enough (as in being about 2/3rds of the wall insulation) then the interior surface becomes the vapour barrier, but it will be warm. So, either go withRockwool comfortboard, or if you want to use foam then we would suggset you make sure it is thicker that the existing wall insulation. ThisThermalwall foam panel如果你想使用泡沫塑料，它内部有一个金属轨道来固定它，外部刚性隔热板的另一个问题是找到一个合适的方法来固定它们。
在加拿大马尼托巴建造一个独立的车库，在那里，Painted LP Smartside外部板是成品，所以它直接连接到2X6框架。所有接头都采用螺柱，墙体采用R20隔热，内部采用蒸汽屏障，并与干墙连接。在内部的蒸汽屏障和油漆完成的Smartside外部之间，是否也需要天气屏障/房屋包装?我们曾考虑在Smartside和2X6框架之间做家居包装，但不确定是否需要。评论?
I don't know the product, but from what you say I would be concerned about how the wall is supposed to dry. Is the intention for these to replace sheating and prevent racking?
当你考虑到耐久性时，典型的壁板安装包括壁板和框架墙护套之间的空间，允许墙中的任何水分逃逸。如果你把这条墙板直接连接到螺柱墙上，那么墙板的透气性就变得很重要了，而这在我看来并不透气性。If you look at our page abouthow to install siding so walls can dry你会看到后面的空气空间的壁板，以便湿度的墙可以espape。也许可以看看产品规格或联系该公司，询问该墙应该如何干燥，并让我们知道。我真的看不出来。简而言之，我想要有护套或者至少是交叉支撑，一个防风雨的屏障，垂直的带子然后是壁板，这样空气就可以从后面进入，这样就不会积聚湿气。