vapor barrier

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    Christine Williamson Cronin (Edited )

    Condensation always occurs on a cold surface. If you're "moving" through the wall in section from the warm side to the cold side, condensation will occur on the first cold surface. In a standard cavity insulated wall (2 x 6 framed wall with batt insulation) this will be the inside surface of the exterior sheathing. This is typically a wintertime issue. 

    To control condensation we have only four possible options: (1) warm the condensing surface, (2) prevent moisture (usually moisture-laden air) from reaching the condensing surface, (3) remove moisture from the environment by dehumidifying, and (4) permit the condensation but make it so that there's enough drying to where it's not a problem.

    In building design we usually pursue these options in combination. In cold climates the standard approach is to add an interior vapor barrier or vapor retarder to attempt to keep the warm, moisture-laden air inside (option 2) and to provide exterior drying through the sheathing and WRB (option 4). Neither of these options is perfect, though and we will routinely get condensation on the sheathing anyway! It's just not enough water to be a problem most of the time. In other words, our design approach is to reduce the problem, not eliminate it.

    The most effective and reliable approach to controlling condensation in walls is to modify the standard wall to include exterior insulation (option 1). This keeps the exterior sheathing warm enough to where the condensation cannot occur in the first place. (No cold surface = no condensation). But continuous exterior insulation is more expensive, which is why it's less common than using vapor barriers.

    This is much more information that you will need for the AREs, though. For the purpose of the AREs, just think of the problem occurring on the cold side of the insulation, and the vapor barrier (or vapor retarder) being located on the warm side. Since the purpose of the vapor barrier (or retarder) is to prevent the moisture from reaching the cold surface, you'll install it in the most practical place to that end. It might also be helpful to think of the warm side as also always being the wet side. 

     

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    Brett Bowers

    Hey Christine, do you have any book recommendations on high-performance building enclosures? This wouldn't be for the purpose of the ARE; I'm already licensed and simply looking to improve my understanding of building science.

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    Christine Williamson Cronin

    Sure! There are three that I think are particularly good for architects:

    High Performance Enclosures by John Straube. This is for commercial and residential construction. Dr. Straube also has a textbook (Building Science for Building Enclosures), but I don't recommend that one for casual readers. It's fantastic, of course, and more comprehensive, but it's much more dense and, well, textbook-like. By contrast High Performance Enclosures is technical but accessible enough that you actually want to read it.

    Pretty Good House by Kolbert, Mottram, Maines, and Bailey. I love this book. It covers building science principles in general and then presents a whole bunch of case study houses and includes data and drawing details. It's definitely good for architects, but it'd be appropriate for some of our more technical-minded clients, too. I particularly love how forthright the authors are about how they dealt with the kinds of constraints we encounter all the time working on real projects with real budgets.

    Moisture Control for Residential Buildings by Joe Lstiburek. Full disclosure: Joe is my father! And he has, very conveniently, written down a lot of what he teaches me and published it. What I love about this one is that it covers both enclosure AND mechanical system design. Not that architects are actually designing the mechanical systems (and that's not the level he gets to anyway -- he's not teaching about load calculations and duct sizing) but indoor air quality is a really big deal, and it's nice to know what to ask for... and what to avoid. This book also has lots of drawing details.

    Straube's book is the most general and Pretty Good House, being case studies, is the most specific. Those two are also more focussed on cold climates. Straube's book is the only one that explicitly deals with commercial construction. The other two are definitely residential in focus, but obviously the principles are applicable to all construction types.

    Hope this helps!

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    Christine Williamson Cronin

    Brett - I tried to respond with some recommendations, but it won't post until NCARB reviews it first. Let's see if I can show you rather than tell you...

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    Brett Bowers

    That's perfect! Thanks! Two of those had been on my radar, but I had heard mixed reviews. I see cold climates mentioned in the subtitle of High Performance Enclosures - is it worth picking up as someone who practices primarily in hot humid climates? I do already have BSC's Builder's Guide, for what it's worth.

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    Michael Ermann

    It’s not just for cold climates…the subtitle is misleading in that it addresses every US climate and is an exceptional book.

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    Michael Ermann

    I’m at the AIA conference in San Francisco right now and was just talking with three building scientists…they were, by coincidence just hours ago, admiring that book.

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    Brett Bowers

    Appreciate it, Michael. I'll pick it up.

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    Frank Dong

    Christine, thank you for your section and explanation.

    When you say "risk of condensation on the interior face of exterior sheathing", is that because the section doesn't have a vapor barrier?

    For ARE purposes, don't we always want the vapor barrier on the warm side? Maybe I am getting confused because you show "water & air control" on the exterior face in your section. I am assuming that's the vapor barrier.

    We would want the vapor barrier would be on the interior (warm side) face so wouldn't the condensation happen on the interior face on the warmer interior?

     

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    Christine Williamson Cronin

    Frank -

    Close. There's a couple things that will help you understand this. First, nothing we ever do in architecture is "perfect". Sometimes we can be reasonably sure that a material we intend to be continuous actually will be... and other times we need to recognize that a material we wish to be continuous will not be. In other words: it's not just that we're not always perfect in architecture, it's that not all materials perform -- or fail to perform --  in the same way.

    Let's take the standard cavity insulated wall that I included in the last response. The interior drywall appears continuous in section... but we know that we run all kinds of services in the wall cavity (plumbing, electrical, etc.) and there will be tons of penetrations through it. So it's helpful if we assume that the drywall will slow how much interior air and interior moisture gets into the rest of the wall cavity (it's better than nothing!)... but it won't stop ALL the air and all the moisture. Same goes for an interior vapor barrier/retarder (which is not shown in that section, but if it's included, it will be located behind the drywall, between it and the studs). We know that we're supposed to seal all of the penetrations through the vapor barrier, but we also know that we'll miss some of them, and we won't be perfect at sealing the ones we do. So because of this we have to regard that vapor barrier as helpful, but not perfect.

    Which brings me to your first question: is the risk of condensation on the interior side of the exterior sheathing because there isn't a vapor barrier? The answer is: sort of. There is a risk of condensation on that surface because it is cold, and that risk exists whether there is a vapor barrier or not. Including a vapor barrier will certainly reduce the risk of condensation (by keeping interior moisture inside and out of the wall cavity), but it won't stop all of the air and all of the moisture, so some risk still remains. That's why it's important that the wall can dry to the exterior through the sheathing and through the water and air control membrane (bright pink in the section).

    Let's now talk about that water and air control membrane shown on the section. I said not everything in architecture that we intend to be continuous actually will be... but in this case, we can be much more optimistic about its continuity than we were about our interior drywall and our interior vapor barrier/retarder. Why? Because we don't have nearly as many services penetrating the exterior sheathing (and water and air control membrane), and those services that do penetrate them are usually installed before the cladding is in place so they're exposed for long enough to let us seal them properly.

    The purpose of the water and air control membrane is primarily to protect the rest of the wall behind from getting excessively wet, and secondarily it's to keep the air inside, inside. This second part helps with comfort, energy efficiency, indoor air quality, acoustics, and pests. Notice that we pick one membrane that serves multiple functions (controls water AND air). This membrane is called the WRB (water resistive barrier) by the building code, and it's required in all framed walls.

    So, if you have a framed wall, you're going to have a water control membrane in the location I've shown it regardless of what other things you do. That membrane is usually also used to control air (be the "air barrier").... And sometimes we can use that same membrane to control vapor. BUT NOT ALWAYS!

    It helps here to think of vapor permeance (how much water vapor a material will allow to pass through it) as a physical property of ALL materials, like mass, density, or even color. All materials are permeable or impermeable to water vapor in varying degrees. Most of the time we pick a WRB that's permeable (or at least somewhat permeable) to water vapor because we want the wall to dry to the exterior when we get condensation on the sheathing (exactly what we've been talking about above). If we pick a WRB that happens to be impermeable to water vapor (a vapor barrier) and we use it on a framed wall in a cold climate and we get condensation on the sheathing, we can cause the wall to rot. This can happen even if we include an interior vapor barrier. Why? Because that interior vapor barrier will likely not be perfect enough to reduce condensation enough to compensate for the lack of drying.

    So why might we want to control vapor on the exterior of a wall? If we're in a sunny, rainy climate and we're using a cladding that can hold a lot of water (like brick). The cladding will get wet after a rain and then the sun will come out and drive that water vapor inside the wall. It can be so much water that it overwhelm's the wall's capacity to dry to the interior and the wall can rot.

    This is very good stuff to know for professional practice... but it's way, way more than what's required for the AREs. My best advice to you is to not get bogged down with the ins and outs of this as you're studying. What I would recommend is that you spend some time understanding the materials included in the standard cavity insulated wall that I included in my first response. See if you can name the parts yourself when you're driving around and pass a construction site, and ask at your firm what materials you usually use for each. This is the most common wall in North America and you can use it as a baseline to compare every other kind of wall you see.

    Then - for the AREs - just remember the old rule about vapor barriers/retarders being on the warm side of the insulation. 

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    Lita DeBoer

    Michael Ermann I purchased High Performance Enclosures based on your recommendation I read in another thread and I really like this book. It's been very helpful.  I tell others in study groups about it.  Thanks for sharing.

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    Frank Dong

    Christine, thank you so much for taking the time and so into such detail to answer my questions! It has been very helpful!

     

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    Christine Williamson Cronin

    You’re so welcome, Frank! Good luck on the AREs:)

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    Ben Thouthip (Edited )

    It should be wrapped like a building envelope. Think of moisture like a smaller particle. It can pass through none dense surface to interior. 

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