Rainscreens, Wildfire Hazard and Other Unintended Consequences
GBA Editor
| Posted in General Questions on
This is to move the discussion out of another thread which has already driven the OP away due to repeated hijacking.
Do rainscreen cladding systems increase the risk from wildfires, and what are the options for areas of both high annual rainfall and wildfire during dry seasons (if any such climates exist)?
Do rainscreen cladding systems create other unintended consequences?
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First, let me reiterate the current building science recommendations for rainscreens.
There are four WRE (weather resistant envelope) strategies: sealed face (caulked joints), concealed drainage plane (housewrap or other WRB behind cladding), drainscreen (capillary break and gravity drainage space between cladding and WRB), and rainscreen (vented drainage space to allow for convective and evaporative drying).
The HUD PATH Best Practices Guide states: "Consider a drained cavity weather-resistant envelope (WRE) system for most non-severe climates and building exposures, or select alternative WRE approach based on climate, site condition and target performance level."
In other words, either use the default strategy or evaluate the local climate severity, site exposure, overhang ratio and moisture index (likelihood of wetting vs ability to dry).
"Leading building scientists recommend rainscreen systems be used in areas prone to wind-driven rain and/or areas that have an average annual rainfall of 40 inches or more".
- Benjamin Obdyke, manufacturer of a variety of drainscreen products.
"In areas like the Southwest that receive low rainfall (less than 20 inches annually), a housewrap or building paper should offer sufficient water resistance protection, according to most building experts. In areas that experience moderate amounts of rainfall (20 to 40 inches annually), protection against rain penetration should include an enhanced housewrap. And for wet and/or humid climates, coastal areas and hilltop exposures receiving high (40 to 60 inches annually) or extreme (60 inches or more annually) rainfall, a ventilated rainscreen assembly is recommended; a rainscreen system is also advised for areas that receive high winds in addition to rain. Rainscreen systems are recognized by leading building trade associations for their effectiveness in controlling rain water intrusion into wall assemblies in areas of high and extreme rainfall."
- “To Build a Better Home,” published by the APA-Engineered Wood Association, 2002
And Building Science Corp. supports the fact that the overwhelming majority of the North American continent is in the low to moderate rain exposure zones.
While the increasingly popular rainscreen approach may increase building and cladding moisture resistance, it also adds unnecessary and potentially problematic complexity, including WRB and flashing continuity, extra materials and labor cost, an exterior fire chase in wildfire country, and the aesthetic challenges of integrating windows with trim and siding. It also decouples the cladding from the thermal envelope and thereby has the unintended consequence of reducing the solar radiant drive which helps drying to the interior within the structural and thermal cavities.
Here are excerpts from the National Fire Protection Association (NFPA) standards on Wildfire Protection:
NFPA® 1144
Standard for Reducing Structure Ignition Hazards from Wildland Fire
2008 Edition
5.2 Roof Design and Materials.
5.2.2 Vents shall be screened with a corrosion-resistant, noncombustible wire mesh with
the mesh opening not to exceed nominal ¼ in. (6.3 mm) in size.
5.2.3 Eaves shall be boxed in with 5/8 in. (15.5 mm) nominal sheathing or noncombustible materials or meet the requirements of 5.5.2.
5.2.4 Where the roof profile allows space between the roof covering and the roof decking, the spaces shall be constructed to prevent the intrusion of flames and embers, be fire-stopped with approved materials, or have additional assembly components of noncombustible materials to prevent ignition.
5.2.5 Attic or foundation ventilation louvers or ventilation openings in vertical walls shall be covered with nominal ¼ in. (6.3 mm) mesh corrosion-resistant metal screen or other noncombustible and approved material that offers equivalent protection.
5.2.6 No attic ventilation openings or ventilation louvers shall be permitted in soffits, in eave overhangs, between rafters at eaves, or in other overhanging areas on those exposures facing hazardous vegetation, as determined by the AHJ.
5.5 Exterior Vertical Walls.
5.5.1 Exterior vertical walls shall meet the requirements for heavy timber construction, ignition-resistive material, fire-retardant-treated wood, or be a minimum 20-minute firerated assembly where walls are potentially exposed to a wildland fire, unless the AHJ determines that the wildland fire risk and structure assessment requires greater protection.
5.5.2 All exterior walls shall be protected with 2 in. (50 mm) nominal solid blocking between exposed rafters at all roof overhangs, under the exterior wall covering on all sides exposed to native vegetation, as determined by the AHJ.
5.5.3 When appendages and projections are attached to exterior fire-resistive walls, they shall be constructed to maintain the fire-resistive integrity of the wall.
And here are excerpts from the CA Building Code regarding Wildfire-Urban Interface:
California Building Code CHAPTER 7A [SFM]
MATERIALS AND CONSTRUCTION METHODS FOR EXTERIOR WILDFIRE EXPOSURE
704A.3.1 General. Exterior walls shall be approved noncombustible or ignition-resistant material, heavy timber, or log wall construction or shall provide protection from the intrusion of flames and embers in accordance with standard SFM 12-7A-1.
704A.3.1.1 Exterior wall coverings. Exterior wall coverings shall extend from the top of the foundation to the roof, and terminate at 2-inch (50.8 mm) nominal solid wood blocking between rafters at all roof overhangs, or in the case of enclosed eaves, terminate at the enclosure.
704A.3.2.1 Exterior wall vents. Unless otherwise prohibited by other provisions of this code, vent openings in exterior walls shall resist the intrusion of flame and embers into the structure or vents shall be screened with a corrosion-resistant, noncombustible wire mesh with 1/4 inch (6 mm) openings or its equivalent.
And here is a link to a PPT "debate" between Steve Quarles and Anton Ten Wolde regarding the competition between venting for moisture management versus fire protection details:
many designers, architects and builders don't view a rainscreen as adding unnecessary and potentially problematic complexity.
the building science map shows half of north america (and a majority of the population/built landscape) in the moderate to extreme categories, which is pretty close to the driving rain index used by the BIA. that would put a significant portion of north america as 'prone to wind-driven rain' - where obdyke and others recommend rainscreen assemblies.
extra materials and labor costs are minimal - it doesn't cost significantly more to install a rainscreen. it doesn't take significantly more time for a competent builder.
we've never run into issues w/ fenestration details on rainscreens. yes, it can take more thinking up front, but since when was a challenge a bad thing?
by design, the rainscreen has fewer moisture issues, including significantly less solar driven moisture. wouldn't this mean there is less need for the structural cavity to dry out?
"Because of its potential benefits in both winter and summer in a cold climate, cladding ventilation is a good choice for mitigating condensation due to sundriven moisture." - kan liyen & joseph pinon
other benefits - belt and suspenders approach, no sealants to replace or sealant 'bleeding', less fasteners through air barrier/WRB, crisper/modern look/detailing, reduced heating load in summer. additionally, rainscreens are required in some municipalities.
there are ways to apply fire barrier or fire stop to rainscreen assemblies, mostly commercial and MFH applications. i'm not really a fan of wood exteriors (rainscreen or not) in areas prone to wildfire regardless. also, as someone who's experienced north county wildfires, landscape management is a huge factor in protecting one's home. i don't see rainscreens posing quite the increased threat as roof vents, due to pressure equalization of the rainscreen.
Dear Mike,
Thanks for your contribution, also. Can you recommend good resources I can look up on approaches to landscape management which reduce the risk of wildfire damage to property?
Or is it all just Common Sense?
Dear Robert,
Thank you for your informative and comprehensive reply. This is exactly the sort of factual information that I sought on the other thread.
The discussion on the other thread made it clear that I am planning a house in a region which is currently soggy (>40 inches annual rainfall) but where the long-term trend is slowly but steadily towards a drier climate. To prevent a later (disruptive, expensive) re-work of the whole approach to siding, I am seeking advice on what approaches might be suitable for both climates.
It would only be honest of me to explain here that comments in the header to this thread about the other one are not relevant. The discussions which prompted the OP to depart the other thread were not about rainscreen siding details and these rainscreen discussions started after the OP had, unfortunately, indicated their intention to depart. Perhaps the OP would still be with GBA had the antagonists moved their discussion to a new thread in a timely manner. I guess we're all learning the need for a mechanism to allow this to happen easily. Robert's decision to move this discussion to a new thread is a good example of what should be happening more often and earlier in off-topic threads.
Reason for edit : honest clarification
TImmy, there is plenty of general information on the web on how to best landscape a fire-break around a home - much of it seems to come out of Australia and much of it should be "common sense" if you are familiar with living in a forested area.
This has been something I have put some thought into for my own home since I live in the bush where summer conditions can get extremely dry. Drought-resistant ground covers (white clover, vetch and some type of short-growing fescue mostly) surround the house - grassy lawns work too but are too water-intensive for my liking.
I've also taken care to make sure there are no trees too near the house - some trees are riskier than others - and that there is no canopy "bridge" from the bush through the fire-break to the house.
I should also mention that even though it may be good practice to include a fire-break around your home, there isn't much that can stop a wild-fire if the wind is really against you.
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Thanks Lucas,
Constructive as always.
Just as many designers, architects and builders don't view many "green" materials and methods as problematic because they don't have a sufficient understanding of building science and hygrothermal dynamics.
To make his argument, Mike misuses the data in the Building Science map by saying, "the building science map shows half of north america (and a majority of the population/built landscape) in the moderate to extreme categories." This is the logical equivalent of stating that the majority of Americans are in the top three of four income categories, or that 9 of the 10 numbers from 1 to 10 are greater than 1.
What the BSC map actually tells us, as I already noted, is that the overwhelming majority of the North American continent is in the low to moderate rain exposure zones (the bottom two of four categories).
Mike goes on to use his distorted interpretation of the map to state that these three of four zones are "where obdyke and others recommend rainscreen assemblies." In fact, an unbiased reading of the quotes I posted indicate that it is only in the most extreme exposure zone that a rainscreen is recommended. I included the Obdyke quote because it's rare to see a company that manufactures rainscreen products not overselling them but instead using accepted building science consensus to limit their market.
To argue that "it doesn't take significantly more time for a competent builder" is moot if there is no legitimate need for the extra materials, labor and cost. And even more so if the extra cost creates unintended consequences.
Mike claims that "by design, the rainscreen has fewer moisture issues, including significantly less solar driven moisture" and asks "wouldn't this mean there is less need for the structural cavity to dry out?"
By design, a rainscreen can reduce potential moisture issues in the cladding and possibly in the sheathing if it cannot dry to the inside due to impermeable insulations like spray foam. But that situation is just another example of the unnecessary complexity required by other methods and materials that interfere with the hygrothermal flux of a building envelope. And most of the moisture movement in a cold climate house is from the inside out, so the need for drying potential of a structural cavity is mostly determined by interior moisture management and the hygric buffering capacity of the cavity insulation.
Drying to the inside becomes necessary when we use moisture-impermeable exterior foam insulation which prevents or limits the dominant drying to the exterior, but the exterior insulation limits inward heat flux and hence inward drying potential. And by decoupling the cladding from the structural envelope, we reduce solar driven inward drying potential even further.
Mike quotes others to support his argument. "Because of its potential benefits in both winter and summer in a cold climate, cladding ventilation is a good choice for mitigating condensation due to sundriven moisture." - kan liyen & joseph pinon. But condensation is not an issue with sundriven moisture in a cold climate, but rather with exfiltration of moist air or diffusion of water vapor from a warm to a colder environment, so these "authorities" also suffer from the all-too-common ignorance about moisture dynamics.
And Mike states the common misperception that a rainscreen is a pressure equalizing technique. True pressure-equalized rainscreens are a sophisticated system developed by the metal cladding industry for high-rise buildings with extreme rain and wind exposures. These systems require laterally and vertically isolated cladding compartments that are fully open to atmospheric pressure and allow bulk drainage. This system is not used in low-rise residential construction.
What a ventilated residential rainscreen does is to create a capillary break between cladding and sheathing to control inward solar radiant moisture drive (which is important only for reservoir claddings), a gravity drainage space for bulk drainage of any rain that gets past the cladding, and a convective path for evaporative drying.
Those can be helpful in preventing vapor pressure drive moisture from separating paint films from cladding materials, but modern water-based exterior paints are much more vapor permeable than the old oil-based paints and water-based stains are hardly vulnerable to moisture movement.
What the building science consensus indicates is that WRE strategies have to be consistent with the degree of moisture exposure, the climatic drying potential, and the quality of the moisture protection elements of both the site and the structure, including - most importantly - overhangs and gutters.
Examination of the previous thread will show that this statement is simply not true. Lucas Dupuis, the original poster, had already been driven from the scene when I made my first post to the thread.
Timmy, TIMMY
Robert's assertion that I make virtually no substantive contribution to this forum is to some extent true. Given that I know very little about building science and techniques, particularly in comparison with the real experts who give so generously of their experience and knowledge, this could hardly be otherwise. I am limited mainly to asking questions and critically examining the replies I receive. As a newbie who has stuck the course, unlike poor Lucas Dupuis, I hope to continue asking and examining.
Yet, every time that "Timmy" used Dupuis' thread for his own ulterior motives, an email was sent to Dupuis' address to further harass him.
Hi John,
I'm not sure what you're trying to convey with your image. Feel free to e-mail to let me know. I've tagged your post as 'helpful' because I'm sure it is trying to be...
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Hi Robert,
I asked a question in my previous post and I'm frankly not interested in anything other than clearing up what the has been going on. If you do know anything about it, I'd appreciate your help.
Wait,
Steve El is now Timmy O' Daniels?
Thanks for mentioning that, Robert, I was unaware that it had happened.
That is completely out of order. If someone thinks such harassment is either funny or helpful in some way then they are deluded. I certainly sent no such e-mail and I condemn whoever did. I will gladly co-operate with any actions taken to trace the perpetrator(s).
Do you know if there was any harassment occasioned by posts other than mine?
I am not Steve El but I've no idea how I can prove that. The GBA moderators probably could, I guess. Frankly I'm more concerned with who has been harassing Lucas Dupuis - apparently on my behalf.
"Timmy",
If you have an off-topic concern, I suggest you start your own thread - perhaps titled "I am not Steve El and I didn't send those emails".
"Poor Lucas Dupuis" would like be left out of this. Thanks for your thoughts on rainscreens Robert. You make several good points and I respect your opinion. However, If you are always going to revert to the idea that questions like mine can be answered by suggesting that I revert to more "traditional" designs I don't think it will be productive for anyone on this site. Traditional design and building practices have their place, However, to suggest that these are the only "common sense" solutions is, IMHO at least, narrow minded. Modern building materials challenge designers and builders alike. Believe me, I would rather, in almost every case, use a durable, proven, natural material over a synthetic "miracle material". However, it's not always an option. I think the reason that the GBA site was created in the first place was to share information so that the interested building community at large can build better, more durable, more energy efficient homes without making the mistakes of the past, caused by blind application of new technology.
thank you
-Lucas Dupuis
are you actually insisting that the majority of north american population and built landscape are outside of the moderate – extreme regions of the BSI map?!? yes, the low exposure region constitutes a significant area of the BSI map, but consists of less than 20% of the total populations of canada and the US. or maybe you believe that the 77,000 people living in nunavut and northwest territory have built a hundred million houses to balance out the built landscape of the atlantic seaboard? too funny.
my reading of obdyke and others is that rainscreens are ideal for two regions:
1. areas receiving 40+ inches, or the high and extreme categories of the BSI map.
2. areas prone to wind-driven rain.
i read ‘prone to wind-driven rain’ as moderate exposure or higher from the PATH best practices manual’s wind-driven rain map. this is how i derived the moderate to severe exposures, which also matches the brick industry assoc. map for wind driven rain. additionally, building height and topography can also affect propensity to wind-driven rain. hardly a ‘distorted interpretation,’ as you so ineloquently lied.
the cost, time and labor of a rainscreen is not significantly more than a drained cavity wall. the point isn’t moot – your overly biased opinion is that a rainscreen is excessive and unnecessary. your [misguided] opinion - you are entitled to it. a rainscreen only creates unintended consequences if the designer and builder are not competent.
i never said anything about spray foam, again a red herring. but thanks for NOT answering my question.
i love how people with peer-reviewed publications are ‘authorities’ (the contempt shines through).
I didn’t state the common misperception that a rainscreen is a pressure equalizing technique - i understand the difference between the two systems. i just don’t see the pressure differential of the rainscreen (which isn't overly large in a one or two story house), as being more than a roof vent. also, many rainscreens incorporate open gaps – which would be a form of a pressure equalized rainscreen. thanks for putting words in my mouth (again). also, you are incorrect in stating that pressure equalized rainscreens are “not used in low-rise residential construction” – they’re fairly common in urban low-rise construction these days.
i’ve never seen any building science ‘consensus’ stating that rainscreen assemblies shouldn’t be used outside of extreme exposure – and even the PATH manual states that rainscreens receive the highest performance rating across all exposure categories.
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Mike,
The primary difference between you and me is that I have contempt for deliberately espoused ignorance and you show contempt for science-based knowledge. I'll leave it to GBA readers to judge which one is, itself, contemptible.
It's hardly a "red herring" to point to the broader ignorance among the "green" building community about the myriad potential unintended consequences of material and method choices. This merely puts the rainscreen issue in a broader context. Nor is it irrelevant to put the "green" building discussion in the broader context of modern society's irrational reliance on increasingly sophisticated technology when that entire history is replete with examples of unintended consequences, some of which have been catastrophic to the continuity of life on earth – human and other.
Since it's my topic, it would seem that wherever I choose to direct it is "on topic".
Which necessarily adds to the cost of a project, making today's single-family home unaffordable to even middle class Americans. The increasing specialization of professional society results in the narrowing of knowledge and expertise to the point at which another specialty is necessary just to coordinate all the specialists. And the cost of ignorance is one of those "externalities" that are not factored into the initial financial cost of a project, but becomes manifest in the long term.
Since I never said that, let alone "insisted" that, there's no need to argue it. What I said was that you used a "red herring" argument by lumping three of the four exposure zones, when the BSC map clearly recommends a rainscreen only for two of them. And the moderate zone, in which BSC recommends only a drainage plane or drainage space (WRB) includes the highly populated (and growing) Midwest and the fastest growing single-family area of the country, the Southwest.
In the PATH wind-driven rain map, the upper two of three zones comprise only part of the two highest zones of the BSC map. So this clearly constitutes an even smaller area of the US where both rain and wind exposure are an issue.
When we're considering low-rise residential single-family homes (one or two stories), building height is not a significant variable. But, as I pointed out and you fail to note, the PATH methodology also considers site exposure (landscaping and neighboring buildings) and roof overhang to wall height ratio (which Canadian studies have demonstrated is the most significant design factor for durable structures).
Another straw man argument. Of course there's not much difference between a drainscreen and a rainscreen. But the baseline for legitimate comparison is a concealed WRB approach, which is the industry standard.
While the competence of the tradespeople and their supervision is always a determining factor for any WRE system, every technology and methodology has inherent consequences that are independent of the quality of application. And it's this fundamental physical truth of the universe that you and most others fail to appreciate.
Which was a major omission, because the function and value of a drainscreen or rainscreen is largely dependent on the heat and vapor flows through the entire assembly. No part of an assembly can be considered in isolation. And that's another critical oversight by much of the building community.
Actually, you did before: "due to pressure equalization of the rainscreen", and you did again: "many rainscreens incorporate open gaps – which would be a form of a pressure equalized rainscreen."
Not according to PATH:
Rainscreen – This type of WRE is uncommon in the U.S. but has been used to some extent in Canada to address severe climate conditions… At a minimum, this approach involves use of an air barrier behind the cladding to resist wind pressures… Also, the cavity between the cladding and water/air barrier must be compartmentalized by use of air-tight blocking or furring at corners of the building as a minimum practice. This feature prevents pressure differences on different surfaces of the building from “communicating” through a continuous cavity behind the cladding, which can cause unintended pressure differences across the cladding that drive rain water through the cladding into the drainage cavity.
Nor did you ever hear me say that – yet another "red herring" or straw man on your part.
What I've been saying (and PATH and BSC as well) is that a rainscreen should be used where it is necessary – in high and extreme exposures – but not necessarily otherwise, given the cost and unintended consequences.
I don't think I ever said that traditional materials and methods are the only common sense solution, but that because they were often based on common sense they are worth considering and are often better than more modern high-tech options. Even Paul Fisette of the UMass Building Technology Department, after extensive testing of all modern housewraps, concluded that he would still prefer #15 felt. And the IRC specifies #15 felt as the standard for WRBs against which all others must be measured.
We are challenged by ever more restrictive and prescriptive codes, but we in the building trades also make options feasible by choosing them. Traditional and even natural building materials and methods are options in many cases and are becoming more broadly accepted even by local, state and national code writing bodies. We can all do our part to encourage that.
at least you are now backtracking to reality based discussion. in the beginning, you only stated that rainscreens were recommended for areas of extreme exposure. glad to see you coming back around to 'science-based recommendations'.
the PATH report, published in 2006, was correct, PE Rainscreens weren't as common. since then, they'e become much more common, especially in urban areas. i can point you to myriad projects incorporating this, but i realize your ignorance of basic facts would cause you to overlook it. and again, you stated they weren't used at all on low-rise construction - which is laughably incorrect.
not discussing spray foam was not an omission (again, stop putting words in my mouth. it's ridiculously asinine). i don't like the stuff, and never use it.
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please point out my contempt for 'science-based knowledge' - i read 'prone to wind-driven rain' as being in the moderate exposure categories. add in the fact that PATH gives rainscreens the highest performance rating across all categories is the reason i don't have an issue with their use in most locations.
now you, on the other hand, apparently think you're a building science 'authority' - yet i find a lot of people find your ridiculous demeanor offputting and pathetic - authoritarian, maybe. you might be taken more seriously if your tone was more... teacher-like.
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Mike,
For what it's worth I've been following and trying to understand your points. It would seem that so much effort would some how begin to make sence. I know that sometimes I have difficulty putting my thoughts into words that work.
The last statement that you made # 29 was clearly a "last straw" effort though. You blew it ~ If you have points related to differances with building tech stuff , as silly as some may be, thats OK, but the personal stuff has no place here. With all of your claimed experience you should know this.
Saying things with the intent to hurt serves no good.
I guess we all have bad days.
Not that I believe it will make a bit of difference, but since you ask and are willing to have me repeat this for everyone else:
It would be clear to anyone who actually read what I posted that I've consistently stated the same thing: that rainscreens are recommended for areas of extreme exposure, but are not necessary anywhere else. And that exposure levels are determined by a combination of climate severity, overhang ratio and site exposure. So, according to the PATH formula, even a high climatic exposure with good overhangs and some site shielding results in a moderate to low exposure which necessitates only a drained cavity or a concealed barrier approach.
But you, being unable to honestly argue that point, instead turned my statement upside down into:
The consensus is in the positive, not the negative: that rainscreens should be used in extreme exposure zones with inadequate site shielding and inadequate overhangs.
So the remaining question becomes, is a rainscreen justified in less than extreme climate zones where it's not - according to building science - necessary.
My answer has always been NO, both because of cost and complexity and because of the often ignored unintended consequences.
And, as I made clear in post #27, your argument has been replete with red herrings, straw men, and deliberate distortions of my statements. Those are all perfect examples of intellectual dishonesty, made more pernicious by projecting onto my arguments the logical failings of your own.
robert,
you haven't been consistent...
Actually, I have if you read my comments in context.
I'll grant that the terminology is not consistent among the various authorities, and this can lead to confusion if the principles are not fully understood.
The PATH formula, for instance, combines three variables to create a four-level exposure rating of negligible, low, moderate or high. The three variables are climatic severity (wind-driven rain - low, moderate, severe), overhang ratio (W:H of 0 to 0.5), and site exposure level (no shielding, partial shielding, full shielding). And their exposure level chart makes clear that a good overhang and some site shielding can compensate for extreme climate exposure and reduce the need for complex cladding systems.
The BSC map shows four annual rainfall zones: low (60"), and recommends rainscreens in the two most severe zones, without taking into account the other relevant factors.
And the NAHB ToolBase standard recommends a simple face barrier for up to 30" of annual rainfall, a secondary drainage plane (WRB) for up to 50" of rainfall, and a rainscreen for areas with more than 60" of annual rain.
And, to make it more confusing, PATH states "Consider a drained cavity weather-resistant envelope (WRE) system for most non-severe climates and building exposures", suggesting that a fully vented rainscreen is necessary only in severe climates and building exposures.
So, if there's confusion about this, it's in the inconsistent use of terms and exposure levels among the various authoritative sources. But the message is the same: the most complex cladding systems are necessary only in the most severe exposure situations, and each building and site must be evaluated to determine the appropriate WRE strategy.
Here's a brief history of rainscreen technology and a suggestion for more accurate terminology (which the building science community is starting to adopt).
The rainscreen idea seems to have been used for centuries in Norway:
"They utilized drained and back-ventilated claddings with both closed and open joints. On buildings with timber claddings, closed joints were adopted, and openings at both the top and bottom of the cladding allowed for drainage and evaporation of any penetrating rainwater. The Norwegians titled this approach the "open-jointed barn technique," since originally it was used in conjunction with the construction of barns."
- Anderson, J.M. & Gill, J.R. (1988). Rainscreen Cladding: A Guide to Design Principles and Practice.
1962 - "Curtain Walls" by Birkeland published by the Norwegian Building Research Institute. Discussed principles behind the "rain barrier" technique. Suggested idea of pressure equalization.
1963 - "Rain Penetration and Its Control" (publication CBD40) by G.K. Garden published by the National Research Council of Canada. First usage of terms "open rainscreen" and "rainscreen principle": It is not conceivable that a building designer can prevent the exterior surface of a wall from getting wet nor that he can guarantee that no openings will develop to permit the passage of water. It has, however, been shown that through-wall penetration of rain can be prevented by incorporating an air chamber into the joint or wall where the air pressure is always equal to that on the outside. In essence the outer layer is then an open rainscreen that prevents wetting of the actual wall or air barrier of the building.
It was in the 1980's that metal cladding industry associations began adopting guidelines for compartmentalized pressure-equalized rainscreens, which is a much more sophisticated and highly-engineered form of the original fully-vented rainscreen and which is intended for large commercial applications.
For instance, in Understanding ‘The Rainscreen Principle’ by the Metal Construction Association (December 14, 2006), they differentiate between
- Drained / Back-ventilated (D/BV)
- Pressure-Equalized / Compartmented (PER)
They state: "There should be no confusion between the rainscreen types, yet confusion seems to be widespread in the industry. Too often we are faced with specifications that co-mingle the elements of both Rainscreen types and even introduce elements of other cladding designs. It must be incumbent upon us as professional representatives to educate the design market and to clear-up these misunderstandings."
And "Successful rainscreen design depends on proper understanding of both of the design principles and of the way in which the technology originated and developed … confusion may lead to the development of hybrid versions, which may or may not achieve the design objectives.”
But my concern is the same as MCA's: that widespread confusion over terminology and technique, as well as over the physical principles involved, not only results in potentially devastating building mistakes but also an inability for designers and builders to effectively communicate with one another.
Because of the confusion addressed by MCA (and to which, I believe, they have contributed), I have been trying to encourage the use of more accurately-descriptive terms which others have invented to better differentiate between the various functions of cladding barrier systems.
Given that the original historical design of the rainscreen was a simple but fully-vented space between cladding and structure, I believe it's best to define the term "rainscreen" in that way. Otherwise, we are faced with a cornucopia of terms which muddy the water, including:
- drained/back-ventilated rainscreen
- fully-vented rainscreen
- pressure-equalized rainscreen
- pressure-modulating rainscreen
- compartmentalized rainscreen
Because designers and builders have developed a partially-vented version, which can serve most but not all the functions of a vented rainscreen, and because material manufacturers have mimicked this idea with textured and corrugated housewraps which allow bulk water drainage, capillary reduction and some back-drying potential, it is valuable to agree on a different term to describe this system. The best one that's been offered, and one which notes both its similarity to and distinction from a rainscreen is "drainscreen".
Below that level in function is the redundant barrier or secondary drainage plane (WRB sandwiched between cladding and structure), and finally the sealed cladding approach.
Thus, with this simple and understandable terminology, we can easily differentiate (and talk with each other about) the four primary approaches to keeping the weather out of our walls:
1. rainscreen
2. drainscreen
3. redundant barrier
4. sealed cladding
Below is the HUD PATH depiction of the four WRE types:
A true pressure-equalized rainscreen (PER) is a highly compartmentalized and extremely rigid panelized system developed for high-rise buildings that are routinely exposed to very high wind speeds. They have to be carefully engineered with a specific vent area to cavity volume ratio and be compartmentalized in both vertical and horizontal dimensions with great care that air pressure cannot communicate around building corners.
These systems are not used on single-family residential buildings, as they are both unnecessary and far too complex.
According to NAHB Toolbase.org: "Pressure-equalized rain screens (PER), an advanced version of the simple rain screen, carefully integrate porous exterior claddings, compartmentalized air spaces, generous ventilation, and watertight, airtight support walls. PER systems employ barriers to compartmentalize the air cavity, thereby allowing rapid air pressure equalization and minimal moisture intrusion. "
Here is a cross-section of a typical PER:
Is contempt compatible with ahimsa? Either way, rumors of my prior participation in this thread are greatly exaggerated.
Steve El
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