Rainscreen Cavity Venting

Archive of Older Topics Archive - Specifications Discussions #5
1

A few years ago I had a client that believed placing “area vents” in the top of brick veneer walls, @ 10 feet, in addition to weep vents in the bottom at 2’ was a good idea. I can not find him now but my notes indicated it was to “reduce condensate rusting of ties”. Although I did not think much of the condensation idea, I did think more cross ventilating a rainscreen cavity would offer some benefit.

Since they do not look that hot I had to recheck this whole idea for another project and found that cross ventilating actually reduced pressure equilization.

I remember some supplier that had a product designed for the upper head joint ventilation, but now I can not seem to find him either. Any thoughts on this?

2

I believe that this is a recommendationi of the BIA (Brick Institute of America not the Bureau of Indian Affairs). See BIA Technical Notes 27 and 28B.

3

Take a look at our (BIA) Technical Note 27 on brick rain screen walls for more information (www.gobrick.com). It takes an in-depth look at the theory behind the system and provides recommendations on design and detailing. In general, venting of the cavity is a good idea to help dry out the wall system. You can get more sophisticated and design pressure-equalized or pressure-moderated wall systems. These types of wall systems include air barriers, vents and compartmentation to work properly. There is also information on the proper ratio of vent area to cavity volume for pressure equalization or moderation to work. In your case the 10’ recommendation is too far apart. Generally we recommend the spacing of vents at the top of the wall to match the spacing of weep vents at the bottom of the cavity which is 24" (2’).

The vents you may remember were the Goodco vents which are plastic weep vents. They are now being manufactured by Williams Products Co. There are other companies who make similar things - CellVent by Durowal, MortarNet WeepVents by MortarNet, etc.

4

Has the theory ever been corroborated by actual in-service history and/or testing?

I am very reluctant to adopt this idea - not having vents has never caused any problems in any project with brick veneer that I have worked on. Is this a solution looking for a problem? What is the advantage? What is the danger in NOT adding a second line of venting? What are the potential problems the system is taking on in adding this second line of vents (I have heard that this actually caused more water to enter the system, making it worse)?

5

The BIA Technical Note referenced by Brian Trimble was published in 1994. NINETEEN NINETY FOUR. That’s 18 years ago, and the information is 18 years OLD. The vents at top and bottom are recommended in this technical paper to achieve pressure equalization:

“The following recommendations should provide sufficient venting to achieve pressure equalization for the rain screen wall. Vents are installed at the top and bottom of each compartment. Open head joints should be spaced
at a maximum of 24 in. (600 mm) o.c. horizontally in the exterior brick wythe. If clear, round openings are used, they should be at least 3/8 in. (10 mm) inside diameter, and the spacing should be reduced to 16 in. (400 mm) o.c. horizontally. Open head joints may be positioned at flashing locations in the exterior brick wythe and serve as
weep holes. A minimum of two vents at both the top and bottom should be provided for each individual
compartment. The suggested minimum cavity width is 2 in. (50 mm). If the cavity space width is greater than 2 in. (50 mm), open head joints should be used as vents. Vents should not be positioned at corners. Water drainage
provisions in the wall assembly should be evaluated carefully to avoid placing vents at high flow areas such as sills and heads of openings in the wall system. It is imperative that the cavity have no blockage due to mortar bridging or mortar droppings that can collect at the bottom of the cavity closures in each compartment, thus blocking the vent openings.”

If, like me, you have been following the information trail over the last 20 years about pressure equalized rainscreens you will know that, currently, building scientists have concluded that attempts to pressure equalize cladding systems is folly, and completely unnecessary. In fact, the godfather of PE did not intend this to be something that extended to cladding systems - not what it was conceived and designed for.

We have better information available to us now about how exterior enclosures work, and best strategies for various systems. We also have actual TESTS now, from AAMA (508 and 509) that we should all be requiring for cladding systems to ascertain their abilities to keep water out of the building (and off the drainage plane).

So, I say to BIA - please update this tech paper. Get your “PE” system tested in accordance with AAMA 508 and/or 509 and share the results. Do not continue advocating a bunch of extra vents in the system without knowing how the system will actually perform.

I say to others considering adding a bunch of extra vents in accordance with this out-of-date BIA tech paper: Careful! You may be giving the Owner a LEAKIER building over simply going with what has worked very well for hundreds of years. And, do you really NEED a “PE” brick veneer assembly? Does your building sit in an area that gets 60 inches of rain per year? If not, you do not need “PE” cladding systems. And finally - are you sure that you are even getting a “PE” system in following these outdated BIA “recommendations?” Because a true PE system works only as well as the sum of its parts. Read the bit about “compartmentalization” in the tech paper very carefully… This is NOT an easy thing to do for cladding systems (and again, not what the idea behind PE was designed for).

6

Brians response comment about venting of the cavity is a good idea to help dry out the wall system is the primary objective of a pressure-equalized system.

Anons comment that he/she is reluctant to adopt this idea suggests that it is a design choice that has to be adopted on all masonry cavity systems. As specifiers, one our professional responsibilities is to manage risk. At times in our careers, many of us have been confronted with masonry cavity systems that are subject to higher moisture risk than the normal masonry project. Case in point, swimming pools/natatoriums and similar projects with high interior moisture content need to have special attention to moisture drive issues. The adoption of PE considerations is just one of many design remedies available for consideration in high risk conditions. As to the comment that too many vents actually causes more water to enter the system, suggests that the system wasnt properly designed in the first place.

Several years ago, I specified a scientific lab project for the University of Maryland on the Chesapeake Bay housing about ten large vents growing and studying various seedweeds, frogs, and other wetland issues. The structure, which covered several acres, had a masonry interior wythe with rigid insulation and air infilitration retarder in the cavity and a rainscreen veneer of face brick. The primary risk management issue was: what was the potential for future cracks occurring in the interior masonry wall of extreme height and length? The clients demand that we leave the interior masonry walls exposed and protected by a washable vapor retarding coating left us with the decision that the risk was high that somewhere, sometime [Murphys Law] the interior moisture would make its way into the cavity. We designed the cavity with stainless steel wire ties and weep holes [plastic honeycomb type] at 24" o.c. and top vents [same as weeps] at 48" o.c. The biggest problem was getting the masons to actually install the top vents.

I do recall seeing a photo of a seriously deteriorated brick wall as a result of excessive moisture drive through a masonry wall and, it I recall correctly, it came from the BIA files [Brian, is it possibly on the BIA website?].

The rainscreen-drainage plane issues have a much larger impact on systems other than masonry construction such as EIFS, metal panels system, etc. HUD now requires a drainage plane behind a residential grade vinyl siding.

7

That is correct - BIA Technical Paper #27 recommendations result in an improperly designed PE brick veneer cladding assembly. Well known and understood rule of PE system weeps/vents is that they must first address all other forces that bring water into the assembly - gravity, capillarity, kinetic energy, and convection/air currents. Popping weeps/vents at the top of the cavity in a brick veneer assembly does not address these forces - unless the weeps/vents are designed specifically to do this, and most vents on market ARE NOT. In fact, there is no way I can think of to address convection/air current by incorporating two lines of weeps/vents between through wall flashings in the veneer.

Your example of moisture moving from the interior of the natatorium into the brick veneer cavity space has absolutely nothing to do with PE exterior wall assembly design. This is predominately an air movement problem, and to a much LESSER degree a vapor diffusion issue.

8

Primary Objective = moisture free cavity
Cross Ventilation = elimination of moisture

Moisture is the one element most detrimental to masonry. Even in the event that air blown water gains entry into the top vents, a good cross ventilation will quickly reduce it to acceptable levels.

9

The primary objective is not to eliminate moisture from the cavity. No. The objective is to achieve a balance between wetting, drying, and safe moisture storage levels (of exterior wall components).

Cross ventilation does not, and cannot, in and of itself guarantee elimination of moisture. This statement is also untrue.

A pressure equalized brick veneer wall assembly would be prohibitively expensive to build. The term “pressure equalized” is incorrectly used in BIA Technical Paper #27. Designers should focus instead on “ventilated” assemblies. Ventilated cladding assemblies. Below is a link to what is perhaps the best source of information on the subject:

http://www.buildingscience.com/documents/reports/rr-0907-ventilated-wall-claddings-review-performance-modeling

A few highlights from this study/report:

“Vent locations and details are important and should be understood by designers.”

“Ventilation rates are dependent on the cladding and venting configuration (size and type of openings) and STRONGLY influenced by weather events (wind and solar radiation). Brick veneer walls had lower ventilation rates than vinyl siding walls.”

*Of 4 wall types studied, the brick veneer wall assembly contained vents top and bottom spaced every 2 bricks (400mm).

“The larger the cavity, the greater the ventilation flow for similar driving pressures. The vent openings are a critical detail, and should be made as large and unobstructed as possible without allowing rain penetration or bird/animal/insect ingress. Brick vent bug-screen inserts are especially problematic, and by removing the inserts the ventilation rate can be increased by a factor of 10 for similar driving
pressures. Alternately, larger or additional vent openings (between every brick) may be an option to
improve ventilation rates and thus drying potential.”

“The impact of a rainwater leak that penetrates the wall assembly can also be modeled, and it was shown that continual leaks (as a fraction of the driving rain load), can lead to elevated moisture contents even in ventilated rainscreen wall assemblies. Selection of an appropriate leak size is up to the user, and will vary depending on climate and exposure. Further research should be performed to determine the validity of modeling small leaks with a one-dimensional hygrothermal program, and the impacts of three-dimensional re-distribution of moisture.”

So what’s the takeaway from all of this? Adding top vents to brick veneer assemblies may improve drying potential - but this is highly dependent on the type of opening, the amount of rain leakage through these added openings, the cavity depth, the orientation of the building, and the solar and wind conditions.

The conclusions in this paper are not consistent with the out dated recommendations in BIA Technical Paper #27. There isn’t enough guidance in the BIA paper to lead a designer to a technically appropriate and correct brick cladding ventilation design, and will certainly NOT result in a pressure equalized assembly (and this is unnecessary).

More testing and calculations are required - specific to each building - to remove risk from the design professional and ensuring that adding top vents will be a net positive for the building with regard to moisture control.

10

Wow, I’m gone for a few days and people are disparaging what I wrote (all anonomously, hmmm). Please mark your comments as opinions, not as facts, as you certainly don’t have all the information you think you do.

This is an area that has some debatable information and no one has the full answer at this time until our analyses can become more sophisticated.

First, I have to defend our (BIA’s) Technical Note 27 (remember I’m an engineer working for the BIA for almost 20 years). While TN 27 was written in 1994, a lot of the information in that TN about theory is still correct. In fact, our Technical Note on the stuctural design of arches was written in 1967 and that method is still appropriate to use. Let’s see… that’s FORTY FIVE YEARS ago. Oh my! So, yes, we need to update TN 27, but the jist of the information in the TN is still fine. Many of the sections in the TN such as Definition and Principles, Vapor and Air Barriers, Moisture Migration, Axial and Lateral Loads, and Thermal Insulation are all still accurate. Nothing out of date here, even though it was written 18 years ago.

The current discussion among many people is the issue over the term “pressure-equalized”. Can a cavity ever become pressure-equalized becasue of the highly variable nature of wind? I have heard people calling this pressure-moderation instead since true pressure equalization may never happen. I think that is fine, we don’t have all the answers since this is a highly complex issue. The TN does not result in improperly designed walls as written. If anything, the walls just convert back to drainage walls which work just fine thank you. Drainge walls have been working well for over 100 YEARS, so I guess that says something.

As to putting “holes” in the wall making these walls to be improperly designed, I think the two (or one) anons are just wrong. Putting in vents in a masonry wall helps deal with moisture issues not create more. Vents that are recommended are not holes that direct water into a wall, they are meant to allow air into or out of the wall. Williams Products Company manufactures a product called their Williams/Goodco PVC brick vents. This is a great type of vent that keeps out some wind-driven rain and allows water to run out when used at a flashing location. So the holes are not as random and useless as the anons make them out to be. Maybe the spacing should be 16" o.c. to produce better air flow. Cavity walls have already been designed to address forces such as gravity, cappillarity, kintec energy and convection/air currents. I believe that vented cavity walls actually help dry out walls faster and may avoid increased problems with efflorescence. Hmm, maybe not such a bad thing after all.

So here are a couple of questions that I have:
How does wind know to flow directly upwards that we need to have vents at the top and weeps at the bottom lined up? “there is no way I can think of to address convection/air current by incorporating two lines of weeps/vents between through wall flashings in the veneer.” Gee, from my OLD physics textbook it states that warm air rises (I think that is still true), so natural convection currents will allow the warm moist air to rise and exit through the vents at the top of the wall, thus helping the wall dry out.

How has the technical information changed over time? “If, like me, you have been following the information trail over the last 20 years…” So let’s look at the June 2010 Construction Specifier article “More Than a Pretty Face: Masonry Rain Screen Walls”; “Ventilated Wall Claddings: Review, Field Performance, and Hygrothermal Modeling” by Straube, 2009 (“In summary, while some of the past research shows conflicting results, the consensus in recent years is that cladding ventilation can improve the drying potential of wood-frame walls when exposed to initial or periodic wetting events. Measured ventilation flow rates show good agreement with the presented theory, and can be predicted using CFD models. Therefore the ventilation theory could potentially be applied a hygrothermal model to predict field performance.”); and “Rain Control Theory”, Building Science Digest, BSI, 2010 (“A pressure-moderated wall system promotes the moderation of the pressure difference across the cladding. The proper choice of venting, i.e., size, number, and location, and the division of the cavity into stiff, airtight compartments are necessary requirements. Although such walls have heretofore been described as pressure equalized rainscreens (PER) in Canada, instantaneous pressure equalization rarely occurs in reality, and the screen deals with more than rain. Hence the more realistic term, pressure-moderated screened or pressure-moderated drained wall, is preferred.”). So it appears that the literature today is pretty similar to the information from years ago with slight changes in terminology (i.e pressure-equalized to pressure-moderated).

Better to do your own research than rely on some people’s opinion. That’s what I would do, but that’s just my opinion…

Brian Trimble, PE, CDT
Regional VP, Engineering Services
Brick Industry Association

11

Love it:

“Williams Products Company manufactures a product called their Williams/Goodco PVC brick vents. This is a great type of vent that keeps out some wind-driven rain and allows water to run out when used at a flashing location.”

That’s right, Brian, AT A FLASHING LOCATION. Where in TN 27 does it say anything about the recommended top vent requiring flashing? The top vent wouldn’t be a top vent if it occurred at the flashing - this is known as the “bottom vent.”

You’ve helped make my point, thank you.

[I would appreciate any additional comments for this discussion to have the person’s name and company on the posting before approving guest postings. Colin Gilboy]

12

Let me make this a little clearer, an open head joint or vent-type product like I mentioned can be used as both a weep and as a vent. A weep is placed at the bottom of the cavity above the flashing. A vent is placed beneath a shelf angle or similar item that just… vents. No flashing wanted to required at this point. Please see our (outdated) Technical Note 27 for more information.

13

Thank you Colin and, to Anonymous, I tend to discount most of the information posted here anonymously, regardless of the subject matter. If you really believe that your information is factual, then have the courage of your convictions and stand behind them.

14

Anon, you’re an irritant. Please leave. You twist information like a politician. You’re not getting any votes here.

Brian and BIA are very clear about placement of weeps and vents as well as the need for proper compartmentation to achieve any semblance of pressure equalization. In fact, BIA typically presents cavity walls as a rainscreen, not a PER, and identify vented rainscreens as a way to dry out the wall since we know that every wall allows water intrusion regardless of what material you use as cladding. BIA has simply offered various ways to manage the water and to deal with it in case of intrusion. They offer numerous options for design professionals to choose from. If you don’t like the options, use something else. Personally, I prefer brick to most of the other options. When done right it works beautifully lifetimes on end.

Even the Canadians, who are mostly to blame for introducing the PER concept to North America, acknowldege that achieving a true PER is almost impossible in the field regardless of building veneer.

The fact is that a vented brick rainscreen allows the wall dry out, just as Ron Beard noted.

If you would listen instead of spin, perhaps you’d learn something. Instead you’ve just managed to prove how little you know.

15

I think Doug comments about anonymous posters speaks for many who regularly participate in this forum.

He certainly speaks for me.

16

What’s with all the hatred against anon posters?

It is not hard to understand why people do it. They believe they have something to contribute but they are not absolutely confident that they are right. They WANT to be proven wrong if they are wrong, but they do not want to risk damaging their reputation. If we are required to be absolutely factual at all times, I suspect there should be very few contributions to any conversation.

Others anon posters are embarrassed by their lack of understanding of a topic and thus feel more comfortable asking without exposing themselves as ignorant. If they cannot comment anonymously, they will simply hold back their comments. Do we benefit from discouraging these people from asking questions? Anonymous posts provide us with the opportunity to clarify issues. This comment string is MORE informative because of the responses that were required to address those anonymous posts-posts that would not have been made without the opportunity to speak anonymously.

The only time anonymous posts are unacceptable is when they are used as a means to stealthily attack other commentators. That is just cowardice, and I have no problem seeing those kinds of comments blocked. (To be honest, I have no use for ANY comments that attack other commentators, anonymous or not.)

17

Its not hatred, its a lack of tolerance for anyone that professes particular knowledge of something, especially if its someone who is refuting a standard or providing technical information that needs to be trusted.

No credentials are given, or even if they are there is no way to verify them. Sort of like submitting a resume, “Hire me, I know a lot of things and I graduated from a major university”. Right, you are not going to do that and no one in their right mind is going to give any credibility to an anon professing detailed information about a system or the adequacy of a standard.

Asking of ‘embarrassing’ questions is not a problem for someone to post Anon, they are asking for assistance, not giving it. You don’t need credentials to shout ‘help’.

There are some here who due to their work environment make it impossible for them to appropriately reply. I am sorry for that, but that’s the way it goes. I am truly sorry it limits their ability to participate, and maybe one day their employer will become more enlightened and less despotic.

But there you have it.

William

18

Those are good points, William.

I have to point out however, that if the person had made the exact same comments without doing so anonymously, it merely would have made the comments seem MORE credible. It likely would not have changed the content of their comments. By posting “anon”, at least we can be more prepared to question their credibility–which, I would argue, is what they wanted us to do.

It seems we only want people to comment if they are the ultimate expert on the subject and can prove it with their credentials. I suppose that would be the ideal. But why would we not go directly to those experts for answers if that is what we are seeking? Open forums like this should not be seen as depositories for accredited expert advice. Open forums provide an opportunity to hear many points of view–some expert, some less so. It is our obligation and responsability to assess the credibility of each commentator. Anyone who takes anonymous posts at face value is asking for problems. But sometimes, the less-informed can provide valuable new perspectives.

19

Having re-read the source material that Anon referenced (it’s something Joe Lstiburek has cited in the past and it is posted on Dr. Joe’s website as linked from Anon’s post), I take exception with the statements and interpretations he/she made. The only possible point to concede to Anon is that BIA TN27 occasionally misuses the term PER since it uses the terms rain screens and PER interchangably. Obviously not all rain screens are PER. Back in 1987, this was possibly a common error. Is it worth tossing out the baby with the bathwater? I don’t think so. TN27 is still a good reference. Besides, it’s up to design professionals to use reference materials correctly.

Staying anonymous for fear of repercussions from an employer is certainly understandable. I dare say, however, that many of us have taken the risk of being considered unknowledgeable by posting questions and seeking solutions on this and many other forums. I don’t believe anyone has ever been rude to those people. To ask questions, seek knowledge, and offer solutions is certainly why we’re here. To offer ideas and suggestions is fine. To pontificate erroneously and to ridicule a fantastic resource person like Brian Trimble is inexcusable.

Just my opinion.

20

Well said, Ken.