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Building Science

Rain Control in Energy Efficient Buildings – Building Science Podcast

This week, Dr. Joe Lstiburek talks about rain control in well-insulated buildings; specifically, how brick is affected by water, how it used to be installed, how it's installed today, how it should be installed, and why. Part 1 of a 2-part series on water management.

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_This podcast series is excerpted from a two-day class called “Building Science Fundamentals” taught by Dr. Joe Lstiburek and Dr. John Straube, both of Building Science Corporation._ _For information on attending a live class, go to_
_This week, Dr. Joe talks about rain control in well-insulated buildings; specifically, how brick is affected by water, how it used to be installed, how it’s installed today, how it should be installed, and why._ _______________________________

Rain Control in Energy-Efficient Buildings

Rain control is really, really, really, really important now that we are going to be doing really, really, really good buildings. Really, really, really good buildings are going to have lots and lots of insulation, therefore they will have really, really, really, really low drying potentials. Therefore rain control is a really big deal. All of the stuff I’m going to talk about from a rain control perspective has been known for a long time; it’s just that we didn’t have to know it. In other words, we knew stuff, wrote it in books. It’s been known for a long time. It fell into disuse simply because the potentials for drying were so great, we didn’t really have to be good at it—we only had to be adequate. Now we have to be very good. In order to demonstrate the water holdout capacity of brick, I used an ordinary garden hose. I’ve opened up the back of the wall to see that water passes through the brick in approximately 25 to 30 seconds, so brick is transparent to water. Now, it wasn’t always this way; the performance of brick assemblies has noticeably changed for the worse from a water control perspective. It doesn’t matter now, pretty much because we assume that brick leaks, so we put a water control layer behind it, a drainage plane. What I would like to talk a little bit about is the mechanism of the water penetrating through the brick Water doesn’t actually pass through the brick. Water doesn’t actually pass through the mortar. Water passes through the connection between the mortar and the brick, and that’s an important thing to appreciate or understand. Water passing through the connection between the mortar and the brick didn’t typically occur until the last half of the last century, meaning that we were pretty good with brick and mortar until relatively recently, and that’s because we stopped letting old people do brick. Dip your brick before you set it down Let me explain how brick used to be done. First, you have to understand a little bit about how brick is made. Brick is made in a brick kiln. It is in the kiln for a while at several thousand degrees. So when the brick comes out of the brick kiln where it has been at several thousand degrees, it is really, really dry. It is kiln dried. Its moisture content is unbelievably low, like zero. We take it out of the kiln, we wrap it in plastic, and we ship it out to the job site. What is the likelihood that the brick is going to be dry? One hundred percent. The old timers, 100 years ago, before they would put the brick on the mortar bed, they would dip the brick in a pan of water so that, when they put the brick on the mortar bed, it wouldn’t desiccate the mortar, wouldn’t suck the water out of it. If it sucked the water out of it, there wouldn’t be a bond between the mortar and the brick. That lack of a bond that we get today allows water to pass through. It doesn’t affect the brick structurally; remember, brick is a big deal from a compression perspective. We didn’t really care much about the tensile strength, so we weren’t really caring about that bond. We now call it a setting bed. What that means is that we have an awful lot more water passing through the first layer of brick. A little bit of water can really add up How much water passes through the first layer of brick? Well, around 1% or a little less than 1%. Now that doesn’t seem like a lot, but it is if that water enters directly into the assembly. It’s easily managed if we direct the water down a drainage layer, a drainage plane, to a flashing system that expels it at the bottom of the wall. The typical way of handling that penetrating water through the cladding is by layering tar paper in shingle fashion. We call this the “drainage plane” — we want to drain the ‘rain on the plane.’ You don’t need much space for drainage I like the term “drainage plane” because it describes what the layer does — it drains water on a plane. Sometimes we call it a WRB, which, depending on which ASHRAE or ASTM committee you’re talking to, might mean “weather-resistant barrier” or might mean “water-resistant barrier.” Well, I’m not interested in the barrier; I want drainage to occur. That’s why I like the term drainage plane. I don’t like barriers. I want drainage. For drainage to occur, you actually need a gap between the cladding and the drainage plane. If you smush the brick so that it is in direct contact with the drainage plane, drainage can’t occur because the water will be held there by capillary forces. You need some type of a gap. A traditional gap — and I use the term “traditional” for a reason, because it is based on tradition, go figure — is around 25 mm or 1 in. All you need is about 3/16 in. for drainage; you don’t need an inch. Why and where do we get the inch from? Well, the inch comes from the typical thickness of a mason’s knuckles and fingers. It’s a historical artifact. If attorneys laid brick, it’d be a 1/4 in. Now does that imply that I don’t need an inch for drainage? I’m not implying it at all, I’m saying it. I don’t need a 1-in. gap for drainage. In fact, I don’t need much of a gap at all; 3/16 in. will give me drainage. Mortar droppings are part of the problem So what’s the big deal about mortar droppings? Mortar droppings don’t affect drainage very much. But what mortar droppings do affect is ventilation, and we have to understand that sometimes we may want to ventilate a cladding as well as drain the cladding. So it’s getting important to the design of building enclosures to actually decide whether we want to drain and ventilate or just to drain. If you want to ventilate, you’re going to need an air inlet, an air outlet, and a clear pathway connecting the inlet and the outlet. That means no mortar droppings. So mortar droppings dramatically affect ventilating a cladding, but they don’t dramatically affect drainage. Why ventilate a cladding? What happens if a wall cladding is able to store a lot of water? It would be a reservoir. We would call such a cladding a “reservoir cladding.” We could probably describe brick as a reservoir cladding, and this reservoir would be charged during a rain event. So it rains on the brick, the brick gets wet, there is a lot of water stored on the surfaces of the brick. It is not drained because it is stored. It is like a sponge: You fill up the sponge with water, and then, once the sponge is filled, only then if you add more water does water drain out. In other words, if you add water to a brick wall, water doesn’t come out of the bottom; it keeps building up until you can’t store any more water, and then the water drains out. The reservoir is charged. Think of this as a water source. Next, the sun comes out and beats down on the brick. What happens to the temperature of the water in the brick when the sun beats down on it? It gets higher. Now there is hot water on the outside of the wall. So moisture flow is going to go from where? Warm to cold. Think of the water as splitting—some of it is going to evaporate to the outside, and some of it is going to evaporate to the inside. The concentration of water in the brick is greater than the concentration of water in the outside air but also the water in the air behind the brick. Moisture is going to go from a higher concentration to a lower concentration. Remember, we will follow the thermal gradient and the concentration gradient. A whole bunch of moisture is going to end up on the back of the brick What we want is a moving stream of air to intercept that inwardly driven moisture and flush it out the top of the wall. So we want to back-ventilate reservoir claddings to uncouple or disconnect the reservoir cladding from the rest of the assembly. Everybody got a handle on the language here? Uncouple, disconnect, back-ventilate, reservoir claddings. This is just the language that we are going to be using to describe these phenomena. Goose is not a vapor barrier If there are mortar droppings behind the brick, we don’t get effective back ventilation, and the reservoir cladding isn’t really disconnected from the wall. Let’s say that the WRB (whatever that is) is Tyvek. Tyvek is a fantastic plastic WRB. It’s very good at handling liquid-phase water. It’s extremely vapor open, so Tyvek would not be a vapor barrier. In fact it’s around 50 perms. Commercial Tyvek is around 20 perms; the residential is around 50 perms. So we would say that water would blow through the Tyvek in the vapor form like shit through a goose. Let’s say that there’s plywood sheathing on the wall. Is plywood a vapor barrier? No, it’s not. As the moisture is driven in through the Tyvek, it will increase the moisture content of the plywood. The plywood will breathe, and then the moisture will end up where? In the wall cavity. So the more vapor-open the sheathing-building paper complex, the more likely that moisture will blow through that assembly. What if instead of plywood I had Dens-Glass Gold? Let’s say that this is a commercial building. Dens-Glass Gold is probably the single most common commercial sheathing. It’s around 30 perms, so it breathes. So moisture is going to blow through the Tyvek and the Dens-Glass Gold like a hot knife through butter. Is the fiberglass insulation going to stop it? Fiberglass is very, very vapor open. It’s going to be around 150 perms for three inches, so it really breathes. So where is the water going to end up? Well, if there is polyethylene in the wall, it is going to condense on the poly and run down and corrode the metal track or rot the bottom plate. If there isn’t poly in the wall, if it’s gypsum board, it’s going to blow through the gypsum board until it hits the vinyl wallpaper. So a really, really bad scenario would be metal studs with gypsum board on the inside with vinyl wallpaper, with Dens-Glass Gold on the outside, Tyvek, a brick veneer and mortar droppings. We would call that a hotel. Bad decisions lead to mold and rot So it rains on the brick, the brick gets wet, the sun blows the whole darn thing through, and you are going to end up with an awful lot of mold at the interface of the vinyl and the gypsum board. Now I don’t really care because it’s trapped between the vinyl and the gypsum board. It’s in the wall, but I’m not breathing it, so it’s not an issue. But the moisture is going to pick up and increase until I’m beginning to actually grow mold on the backside of the gypsum board. Now it’s in the cavity, it’s no longer at the interface between the vinyl and the gypsum board. It’s actually in the cavity, on the cavity side where it intersects the fiberglass. Again, I don’t have much of a problem if it stays there. Well, if I have a dropped-ceiling return plenum which is at a negative pressure and it is now connected to my exterior wall, if you pop your ceiling tile, what’s the likelihood that that interior gypsum board is airtight? Zero. You are going to shovel in a little bit of mineral wool? Yeah, give me a break, a lousy filter. So I’m going to actually suck the mold out of the wall and inject it into the breathing zone of the occupied space. Now, that would be a problem. That would be like having mold on your surface and you would go with your nose and suck it in. That’s a health effect. The point here is that we usually have a whole series of things happening If there weren’t mortar droppings in the wall cavity and the cladding were back-ventilated, there wouldn’t be any inwardly driven moisture, and I could probably (and the operative word here is probably) get away with vinyl wallpaper in places like Chicago. On the other hand, even with no mortar droppings and back-ventilated cladding, I could never get away with the vinyl wallpaper in a place like Atlanta — the operative phrase is “get away with.” If I didn’t have the vinyl wallpaper at all, I could probably not have to worry about too much of an issue with mortar droppings with my brick veneer from a ventilation perspective as long as I had my flashings done right. The liquid water would drain away. I could live with the reservoir because the moisture driven in would actually blow all the way through the assembly and life would be pretty reasonable. So if you have a reservoir cladding, you need drainage. We don’t need much of a gap for drainage, but we need a lot for back ventilation. How much do we need for back ventilation? The answer is: It depends. It depends on what the reservoir is. You could get away with as low as 3 or 4 mm, or 6 mm, or sometimes 9 mm; sometimes you are going to need 25 mm. When you aren’t sure, you basically ask for as much as you can get. It’s a pretty reasonable way of doing things in life: You err on the side of caution.


  1. GreenVerifier | | #1

    Thank you
    Please post more of his works! Good Info...

    Gary Smith
    NAHB Green Building Verifier

  2. homedesign | | #2

    Ventilated Drainage Space
    My questions are for a hot and humid climate.
    Your podcast is describing the effects of a reservoir cladding during sunny conditions and just after a rain event.
    What about all the other times... times when it is not sunny (cloudy and or night)?
    Times when the moisture content and temperature outside are greater than inside?

    Can a well ventilated drainage space sometimes feed more warm and humid air to the wall assembly than a not-so well ventilated space?

    Is ventilation always a drying process?
    Is it always a good thing?
    Are there any downsides to an extra well ventilated drainage space?

  3. Joe Lstiburek | | #3

    On balance, vented cladding is a good thing
    The short answers are - yes, sometimes a well ventilated space feeds more moisture behind cladding than a not-so ventilated space. Having said that, on balance, much more moisture is removed than is deposited. It isn't even close. So ventilating a cladding is always a good thing.

    Please note the following subtlety - subtle but yet significant -
    ventilating a cladding is not the same thing as ventilating the wall assembly - so even though during limited times the ventilating air is moving moisture into the airspace behind the cladding it is not moving moisture into the wall assembly.

    The biggest downside to a ventilated drainage space is dealing with
    insects and small animals. Insect and critter screening is necessary.

  4. AndrewInChelseaQC | | #4

    Name change?
    I always look forward to anything written by the folks from Building Science.

    So with some hesitation, and not to be impertinent, and recognizing the origin of the acronym; but I think you should consider a new name for your podcast as it calls into question the quality of the content. : )



  5. Daniel Morrison | | #5

    One of our few places for humor
    We keep humor out of most of the text in because people searching for information are usually looking to answer some questions. They may even be in trouble.

    Because Dr. Joe and Dr. John are light-hearted in their presentation, and when their text includes phrases like "'ll blow through the Tyvek like shit through a goose." we feel that these podcasts and blogs present one of the few opportunities for humor.

    If enough people really object to it, we'll change it to something more sterile.


  6. homedesign | | #6

    BS and the Humor are Good
    Dr Joe,
    I enjoy your BS and your humor.
    What do you think about this REMOTE Wall for a Cold climate?

    How could it be adapted it for a Mixed/Humid climate?
    How about grafting the roof detail from the H For H Westford House on to the top of Thorsten's wall?
    Running the Structural Sheathing, the Barrier Membrane and Rigid Insulation right up to the underside of the lower roof deck?

  7. Paul Cressy | | #7

    Liquid applied moisture barrier
    We are doing a lrge multi-family project in New Orleans and are getting mixed information about the use of a moisture barrier. The product is from Grace Construction Products. I understand the need for this in colder climates, on the exterior of the walls, but we do not have the grave temperature gaps from outside to inside. Any thoughts?

  8. GBA Editor
    Martin Holladay | | #8

    What type of moisture barrier?
    Are you talking about plastic housewraps (a type of water-resistant barrier, or WRB)? If so, they really don't have anything to do with temperature gaps from the outside to inside; their purpose is to stop wind-driven rain. They are just as important in warm climates as in cold climates.

  9. Lissette Eugenia Amado Rubio | | #9

    I would like to know if you give conferences on sustainabçe building in other countries like Brazil. Is it possible to have your e-mail address to inform you more details? many thanks!

  10. user-797100 | | #10

    Does concrete+rigida foam+stucco wall need ventilation
    Does concrete block+rigid foam+stucco wall need ventilation behind stucco?
    Appreciate your expert opinion.

  11. Dave | | #11

    Metal clad SIPS exterior treatment
    We are soon to build a SIPS house in humid, wet florida. Do I need to have an airspace between the steel cladding and any type of siding.? Is is acceptable to use properly applied EFIS directly to the metal cladding?

  12. Greg | | #12

    air gap behind insulated siding oregon rain screen code
    Oregon just went to a state wide rain screen code Jan. 1, 2010 and I spoke with the building inspector and he told me that all siding has to abide by this code including insulated siding. Do you think that other states will start adopting this new language that Oregon revised to define the 2006 703.1 WRB revision? This code really does not define what a "Means of Drainage" is? This is why Oregon actually defined it to mean at least an 1/8" gap for drainage, but that will not even make it a capillary break? you need at least 3/16" as Joe states.



  13. Anonymous | | #13

    Brick + Foam + Stucco already installed. What
    So what do we do if there's no ventilation gap between brick and sheathing, and foam and stucco already installed over the brick? Is vapor barrior on the inside over 2 x4 wall a problem? Location Toronto.

  14. Joe Lstiburek | | #14

    Many Responses
    Response to John Brooks.

    The REMOTE Wall is anther name for PERSIST (circa 1990) which is another version of the PERFECT WALL which was developed by the National Research Council of Canada in the 1960's and they stole it from the Norwegians who figured it out in the 1950's. There is not problem with building this type of wall in a Mixed Humid climate and yes there is no problem with grafting the roof detail from the Westford House onto the top of this wall.

    Response to Paul Cressy
    The reason for a liquid applied moisture barrier in New Orleans has nothing to do with temperature - it has everything to do with rain.
    It is an excellent idea for New Orleans for rain reasons alone.

    Response to Lissette Rubio
    I am happy to go anywhere anytime to speak about anything.

    Response to Harry Zhou
    No, the stucco in this application does not need a ventilation gap.
    It is a mass wall.

    Response to Dave
    Yes, you should have an airspace behind the cladding. I have doubts about applying anything but a water managed EIFS over the structure.

    Response to Greg
    I do not think other states will follow Oregon in this regard.

    Response to Anonymous
    Installing a vapor barrier on the inside of this assembly is not recommended or necessary. If it is already installed I would probably leave it in place as the risk, although not insignificant, is not high enough to warrant intervention.

  15. Greg Albracht | | #15

    Rain screen code in Oregon
    Thanks Joe for responding to me. You are the Rain Screen Guru in my opinion. Thank you for all of the great articles and knowledge you have you taught to people. You inspired me to come up with my Furring Master product. I am curious why you think that other states will not adopt this same type of code?

    Thanks again,


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