The difference between air barriers and vapor barriers
The job of a vapor barrier is to prevent vapor diffusion, and the job of an air barrier is to stop air leakage through differences in air pressure. A wall system should have one vapor barrier, but can have many air barriers. A vapor barrier can act as a very effective air barrier, but an air barrier does not (and should not) always stop vapor from difusing.
A wool sweater for example, is agood choice of natural insulationand will keep you warm when there is no air movement, but will allow the wind to howl right through it. A wool sweater with a raincoat will keep you warm but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to difuse through it.
所以把风衣想象成一个空气屏障，雨衣是一个蒸汽屏障。That is about as far as I can stretch the human to house analogy, hope it helps.
Since warm air expands, there is more space between its molecules compared to cold air. Water vapor is found in that space. When warm air cools as it passes through your walls, it contracts and squeezes out the moisture, leaving you with condensation.
In order to prevent condensation from forming, a vapor barrier should be placed on the warm side of your insulation to stop warm, moist air from condensing on a cold surface inside your wall.
在寒冷的气候，如加拿大，一年中的大多数时间，蒸汽屏障应该在内部的绝缘层。In hot climates like the southern U.S. for example,vapor barrier should be installed on the outside of the insulation, especially where there's air-conditioning involved to prevent condensation and mold.
In both cases, the vapor barrier is tasked with preventing warm, humid air from shedding its moisture as it meets a cool surface, no matter which direction it is travelling.
The most important thing to realize is that there is no fixed rule regarding vapor barriers. Building practices should always be determined by the climate in which you are building.
How water vapor travels:
有两种主要的湿气会穿过你的墙壁，你应该关注——空气泄漏和蒸汽扩散。这是两个完全不同的东西，有两个完全不同的解决方案。
Vapor diffusionis the process of moisture passing through breathable building materials,like drywalland insulation. Vapour barriers are there to prevent that from happening.
Air leakageis due to air pressure differences between indoors and out, which forces air through any holes in your air barrier.
Where the problem arises:
Thedewpointin a wall is the point where the drop in temperature causes air to contract, and water vapor turns to liquid. Since the warmer the air is the more moisture it can hold, where the dewpoint will be in your wall is determined by the difference in temperature from indoor to out, and the amount of moisture in the air (RH -Relative Humidity).
The job of both air barriers and vapor barriers is to prevent moisture from forming at that critical point, they just do it in completely different ways.
Vapor barriers
The rule for vapor barrier installation in cold climates is to have it on the interior with at least 2/3rds of your insulation on the outside of the vapour barrier.Air barriers on the other hand can come in the form of house wrap (WRBs), tightly sealed sheathing, insulation that slows airflow, and well-sealed gypsum board (drywall).
To explain this further, Gypsum board (drywall) is vapor permeable, but stops air flow. This means water vapor can diffuse through it, but air cannot pass through it. So if you were to have a home with no windows and no vapor barrier but simply a sealed gypsum board box all around, you would have an airtight seal with no moisture carried through by air transport.
这里的关键因素是，通过石膏板盒子的水蒸气分子的数量与通过的水分相比微不足道，如果你在盒子上开一个小洞，就会产生气压差。
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The need for a proper air seals in homes is grossly underestimated, and too much faith and focus is put on the vapor barrier. According to the U.S. Department of Energy, "air movement accounts for more than 98% of all water vapor movement in building cavities.”
If you think of how a polyethylene vapor barrier is installed, it will be cut, stapled and taped, then have nails and screws put through it to install strapping and drywall, along with breaches due to electrical wires and boxes. In most cases, the vapor barrier will be perforated thousands of times during the building process.
But a perforated vapor barrier would actually not be a problem if you have a tight air seal. Like that gypsum board box, the amount of water vapor that can pass through a ripped and torn vapor barrier is insignificant as long as the air seal is intact.
Can a house be too airtight? No it cannot.
Unfortunately, air barriers are really not given the attention they should be in regards to the building envelope. In large residential developments, air barriers are often not even on the radar. Crews come and go, and in the interest of mass production, some standard practices can be detrimental to the performance of the final wall system.
A proper air barrier is one of the most important elements of a successful building enclosure, and one of the most overlooked. Given the amount of heat loss due to air transmission and the potential moisture damage from air leaks, air barriers should be getting a lot more attention than they are.
Hey Mike - good article - I see you are showing a picture of enermax - in your opinion - what is the difference or effectivness of foil vs. 6mm as an vapour barrier?
The difference in performance between foil and 6mm poly as a vapor barrier is little enough not to be a concern. It's just that with Enermax the foil vapour barrier is only one component. As a product it also adds insulation and prevents thermal bridging. Of course there is nothing to say you can't create a comparable or even better wall system using 6mm poly with any number of other components.
I would mention though, that often too much focus is put on the 'vapor' barrier and not enough on the 'air' barrier. Well installed and sealed both can be effective air barriers, which is quite a bit more important than the vapor barrier. That can sound a bit confusing, since both products can perform both roles. We have a piece on that as well, have a look and tell us what you think.
Regards,
Mike
目前的房子有石膏板天花板和防蒸汽隔板，防蒸汽隔板上方有吹入的隔热材料。我的女婿正在安装罐式灯，拆除一些不承重墙，这对蒸汽屏障造成了很多损害。他打算在现有的天花板石膏板上再加一层石膏板。他是否应该在两层石膏板之间安装一层干净的防潮层?这意味着他在天花板的暖侧有两层蒸汽屏障。
Hi Syl,
With only drywall between them and no source of moisture it wouldn’t be catastrophic, but what he needs is an air barrier not a vapor barrier, so he could always source a membrane that lets moisture through to be certain. And instead of using just 5/8 strapping, if you use 2x3s instead that would enable you to run wires and use shallow box potlights instead perhaps, and not compromise the air barrier.
Thank you for this great article. I am looking for a air barrier to lay down in an attic floor above mixed insulation to allow water vapour to exit but seal all air gaps in the ceiling. Can you recommend a product that is easy to install (roll-out and tape) and available in Montreal?
The shower area do not have vapor barrier in place. But because of the temperature difference between the shower and the adjacent rooms there is formation of dew which will develop the growth of mold in due course of time. Can someone explain more about this issues.
Yap. The shower is the " inside" and the adjacent rooms are " the outside" in this instance.
Great post, really made me think about air barriers and vapour barriers. Thanks for your writing; I'm looking forward to more related stuff from you.
Never too tight, what about the Biopshpere that nearly aphixated its human lab rats? <1990s Arizona?
Indoor air polution is killing more Americans then all other poisons now. Oops except for tabacco and Booze.
Hi Peter, correct, a home can never too tight. A ventilation system should be designed by an HVAC professional to meet the needs of the building and its occupants. Building materials often do contain formaldehyde and other toxins, we strongly recommendselecting materials that protect indoor air quality.
当我可以通过漏风免费获得“被动通风”时，为什么要花钱买空气密封+一个由工程师为我的特殊房子定制的暖通空调系统呢?
I'm partially joking, but also sincere.
暖通空调系统将室外空气引入和排出。如果暖通空调系统没有热交换器，为什么通过暖通空调通风比空气泄漏更好?
首先，乔纳森，在大多数地区，你需要HRV来遵守建筑规范。指望空气泄漏来提供新鲜空气无疑意味着你得到的新鲜空气比你需要的多得多，你将为此付出高昂的取暖费，或者你将得不到你需要的那么多，很可能会有糟糕的空气质量和潮湿问题。
So the question 'why pay for a proper ventilation system' is answered if for no other reason than 'not paying for a proper ventilation system' will cost way more.
大家好，我们住在马尼托巴省温尼伯市2018年建成的新家。2019年我们在这里的第一个冬天，我们遇到了潮湿的问题，导致地下室地板进水。混凝土墙后面的聚和绝缘(batt r20)有大量的冰。现在我们准备重新做地下室的绝缘。从春天开始，我们去掉了保温层，让混凝土墙晾干。我的问题是，我们是否应该在螺柱上使用一个空气屏障，然后重新绝缘，然后在绝缘上添加6mil聚?我读到的问题是，隔热层紧靠着混凝土地基，因此不允许任何空气流动。聚也安装相当蹩脚太，没有使用指责密封从进入冷墙的温暖空气。防止冬天再次结冰的最佳行动计划是什么?地下室现在没人用，我们有冬季用的hrv系统。 thank you!
Hi Lauran,
The poly is trapping the moisture inside the wall, so in winter what you are likely seeing is that moisture freezing. If you sandwhich a stud wall and insulation against wet concrete with a poly barrier you will just repeat the whole process. Here are two pages to check out that will help, if you still have questions then check in our discussions, and if you don't find all the answers you need they by all means post a question. Hope that helps.
Why basements are moldy and how to prevent it
Best Practices for durable and efficient basement construction
Thank you for your reply. I have read the links and comments. I'm still confused as to what to do. We have a 2inch gap before the stud wall, i read some people saying add the vapor barriers right against the concrete wall, to trap the moisture there. Is this a good practice? We have to install a vapor barrier on the inside over top of the Batts as that is to code. Would spray foam stop this issue? We do not want a repeat of last winter. But also not looking to spend a ton of money.
It's really a matter of physics in that the concrete is wet, it's in the ground, so even if it has the black tar spray on the outside, moisture will continue to be absorbed by the footing. So the concrete is the source of moisture and always will be, not the interior air. If the interior air WAS the problem the vapour barrier would make sense, but it isn't. it's what is fundamentally flawed about building code. You are in effect being instructed to build a wall in a way that is guaranteed to fail.
Sometimes building inspectors realize that and as long as they see a vapour barrier someplace they are fine with it, so if they aprove it against the concrete where it can do good instead of harm, all the better. Putting studs and insulation against concrete then covering it with poly will lead to the same problem, full stop. If the building inspector insists on it then I'd propose a 'smart membrane' like MemBrain from Certainteed, since they look like poly but they allow the moisture to escape. it's basically a way to get the inspector to sign off on a wall, albeit a pretty expensive one. It's one of our biggest peeves. Building code shouldban保利从地下室来，不是坚持。
感谢这篇有用的文章。我想完成我在安大略省未完工的地下室。我住在一幢联排别墅里，两边都有邻居。混凝土党墙的防潮、保温和空气屏障的一般做法是什么?我计划使用2英寸xps硬质泡沫加电池绝缘的墙壁，与土壤接触/未加热的另一边。但我觉得这对党墙来说太过分了，因为双方都很激烈。
I could not find anything in the building code on this. Is it acceptable to use thinner xps rigid foam (just to prevent wood frame and concrete contact) for those walls, and does it need to be insulated to the same R value as the walls in contact with soil?
Hi Aaron,
From a heat or moisture point of view there is no need to insulate the party wall since it will be essentially the same conditions on the other side, give or take a few degrees perhaps. Sometimes people in adjacent units get odours from another unit passing through, is that a problem? That could perhaps be a reason to consider sealing it for air leakage.
现在，如果你在谈论完成的党墙，你可能会考虑换出EPS只是一个6百万聚。它会便宜很多，甚至节省你额外的一英寸空间，甚至更好的工作。尽管它是一面墙体，而且没有暴露在另一边的地面上，但墙体的基脚可能没有受到地面水分的保护，所以墙体可能仍然会无限期地保持潮湿，只要添加一层聚醚就可以保护木材不受潮湿的影响。
If you find there is a lot of sound transmission you might want to stick Rockwool insulation in the stud bays of the party wall, and honestly its also a good idea for the exterior walls as well since Rockwool (mineral wool) isn't affected by moisture and there is a lot of that in basements!
And for your exterior wall, the idea of using 2" of EPS foam against the concrete first and then the stud wall is a good one,at 2 inches EPS begins to act as a vapor barrier, read more here.
Another little tip - The wood-framed walls aren't structural, so a way to further protect them from rotting is to seat them on small foam blocks so they aren't in contact with the concrete floor. That also alows a couple of inches of water to flow underneath without touching stud walls or insulation in case there is ever any minor flooding. You can see what I mean in the main image of this page onhow to prevent mold when renovating basements.
Hi Aaron, A sill gasket wouldn't give you much space and would be compressed pretty easily. I'd stick with the foam for consistency since it's really just a spacer, so just grab a sheet of EPS foam at building supply store.
你好，谢谢你的文章。
I have a question which is a little different as it's based on a campervan but still affected by moisture. A lot of people use vapour barriers on the inside of their insulation, but a van has drain holes so air can still get behind the vapour barrier and the metal skin. Would it be best not to have a barrier at all and just allow air to circulate over the insulation allowing any moisture rich air to replaced with drier air?
many thanks
嗨，谢谢你的这篇文章。我们目前正在为我们的屋顶/天花板重新做绝缘。以前有霉菌在屋顶上，我们想适当地绝缘，以避免这种情况再次发生。我们安装了面绝缘和从谷歌告诉我，牛皮面作为一个蒸汽屏障。我们计划安装舌槽作为最后一层。冬天很冷(下雪并不少见)，夏天的平均气温是85-95。我们现在海拔3000英尺。我们是否应该增加一个空气层，如果是这样，你的建议是什么?特定的MemBrain目前无法使用(我看到你在另一条评论中推荐的)。Appreciate your help and wisdom :)
Great article once again. I had gotten some advise about insulating my basement walls in an old house with rigid and batt filled walls, and my basement is now probably the warmest area in my house. We have just discovered that we are having some condensation issue in 3 outside corners in our main floor, so have decided to gut all the outside walls on the main floor. If I'm understanding this article correctly, would it be good to fill the wall cavities with batt insulation and then some rigid foil faced foam insulation, strap that and then drywall? I think I'm understanding that I wouldt do a vapor barrier as the foil faced rigid would serve that function as long as I sealed all the seams and edges properly. The next question is what would be the benefit of strapping the drywall away from the rigid foam as opposed to putting the drywall directly against the foam? Is this just creating another air barrier? We live in Winnipeg, Manitoba, so cold winters is my main concern. Thanks for the great articles and advise again.
嘿，很棒的文章。
Hi,
伟大的文章。我想封住我的阁楼。我住在温哥华。这所房子建于90年代初。它在天花板上安装了塑料防潮板，所以它位于石膏板和电板之间。根据我的观察，塑料片从外墙顶板到内墙顶板是连续的。我需要沿着顶板喷泡沫吗?或者这个塑料屏障作为空气屏障足够了吗?它看起来像塑料片运行的顶部板和可能是钉在某处墙上的螺柱。
Also, the bathroom exhaust fan is covered with this same type of plastic with batt insulation on top. Do I need to build an ridgid foam insulation box over it? Or is this sufficient?
Any advice would be greatly appreciated! Thanks
Hello,
我来自阿尔伯塔，有冷凝问题在我的地下室(未完成)。去年我的三面墙都结了水气地下室也被建筑商重新隔热了边缘搁栅上还喷了泡沫。今年我在一面墙上有明显的凝结，真的不知道是什么问题。水从底部密封的底部渗出，流到地板上。请告知导致这个问题的原因和可能的补救办法来解决它。去年没发现墙上有裂缝。
另一个问题，冬天湿度在45-50%会导致这个问题吗?聚乙烯安装在棉絮表面。
Please advise
Hi Brad, did you see any of our pages onhow to insulate and finish basements properly?You may find answers in those pages.
It could be a number of things - you may need exterior drainage and grade improvements, exterior or interior perimeter drains, eavestroughs, dehumidifier, I would suggest you put'basement' into the search barand see if any guide pages seem to match your situation, or the discussion Q & A pages. This page is about air barriers so its best to keep them separate.
If you don't find what you need in the basement pages you can post a question, include the wall assembly - do you have exposed concrete or a stud wall, if its an insulated stud wall is it right against the concrete, is there a vapor barrier (and on which side - spoiler alert, it should be against the concrete to keep the stud wall dry, on the outside it keeps it wet). Is there an exterior waterproofing membrane (not likely), do you run a dehumidifier, give us as much details as possible.
The basement is unfinished. There is damp proofing course on exterior of concrete foundation wall. Haven't seen but this is requirement by City and it is ainspection item by City Inspector. There is a 2x4 studwall with gap of about 2" between stud wall and concrete foundation wall. Batt insulation is placed in stud wall and have a clearance betwen concrete wall and stud wall. There is a poly vapor barrier on exterior face of insulation i.e interior face of stud wall. I have HRV. Unfortunately, it was broken when I checked before a week and damper in HRV was not working. Humidity at that time in basement was measured and was 49%. Let me know if you require any further info.