Building Science 1: Rainscreens and Weather Barriers
We tend to think that in order to keep water out of buildings, we need to seal the outer boundary of the building as if we were slipping a condom over it. That might work if it were possible. Size issues aside, nobody yet has been able to figure out how to make the outer surface of a building permanently and completely waterproof. The submarine industry seems to have this covered in exchange for habitability and windows, but folks who live in buildings tend to like a prettier, more open environment.
Why is it so hard to seal buildings at the outer boundary? Because just about everything leaks somehow. Building science teaches us that brick and mortar are permeable to water. Panel joints are rarely sealed 100%. Window gaskets and weatherstrips can allow water into the system. The wind makes things worse by driving water into every crevasse. Capillary action can suck water uphill. Internal building pressures can draw the water through the wall like a soda straw. With these types of challenges, keeping a building dry might seem impossible.
The sustainable solution for modern buildings has been a layered approach. The outer layer, the rainscreen, deflects most of the water. The inner layer, the weather barrier, finishes the job.
The outer layer, the part of the building you see from the street is rarely the primary water seal. That surface is the rainscreen. It sheds a large amount of the water and deflects some of the wind gusts in the way an umbrella does, but not all of it. Why not all? Return to paragraph 2, entitled “Everything leaks”. When finished re-reading that, continue below.
The weather barrier, which is behind the rainscreen layer, performs the job of keeping all the remainder of the wind and the last drop of water from penetrating the building. The rainscreen and weather barrier are separated by a layer of air. The air gap is critical, do don’t discount it. In non-window parts of the wall, the weather barrier can be easy to see when the building is being built. It might be a white paper-like material with the ubiquitous “Tyvek” label all over it, or a different brand of “house wrap”. Behind some brick or panel walls, one can sometimes see the inner wall coated with a black, gray or yellow paint-like weather barrier. These layers tend to be not pretty. They are all about function.
Pressure equalization is a management of the wind pressures that is built into well designed multi-layer walls. In this scenario, airspace is openly connected, or “pressure equalized” to the exterior to allow unimpeded drainage and ventilation that can evaporate moisture in the airspace.
The weather barrier (inner layer) is so complete that neither the force of the wind nor the internal building pressures can push air through it. If there is no air movement through the weather barrier, the soda straw effect can’t take place. The building doesn’t suck, in more ways than one. Any water that is able to reach the weather barrier simply runs down in the airspace to the bottom and then back to the outside.
IMPORTANT: When using open, pressure equalized airspaces, be very careful to insulate/isolate the area around window and door penetrations from the freely moving airspace wind, or chilled window frames, excessive air infiltration, unexpected window condensation and even frost on windows can occur. Stuff the airspace around the penetration with insulation and cap it with a weather barrier barrier. Expandable foam can also be used because it insulates and blocks the wind.
Rainscreen, weather barrier and pressure equalization in window systems
In window systems, the layers can occur within the window system itself. Often the design is that the outer weatherstrips act as the rainscreen and the inner weatherstrips act as the weather barrier. They have an impermeable portion which is the glass itself and the solid material that comprises the framing, but between these things are gaps that in some ways resemble plumbing pipes. Some gaps are unintentional and some are designed drainage passageways that conduct water from any part of the system directly to the exterior, or to a collection area where accumulated water can then exit.
In a window system, the rainscreen layer is still the exterior face that you see from the street, but the weather barrier is comprised of internal/interior caulk seals and gaskets that try their best to keep air from penetrating the system. Generally, if no air penetrates the weather barrier, neither does water.
Building science / building envelope consultants like myself tend to obsess about the areas in the building where one “system” meets another. When the wall system and the window system come together, very good things or very bad things can happen. This connection can be understood, detailed and implemented correctly and the building can enjoy a sustainably leak-free existence. Or the opposite can be true, in which case occupants may need galoshes on rainy days.
Integrating different systems involves having a clear understanding of where the rainscreen layer and weather barrier layers exist in each system, then connecting them in the most reasonable, workable way possible.
One of the biggest aspects of reasonableness is the issue of workability. White collar folk who dabble in this area really need to spend time in the field before taking themselves too seriously. Designing intricate details with miniscule tolerances is cynically called “building a watch” by field folk who endure every kind of obstacle in the performance of their work. Even building science rarely touches upon workability.
For a crash course in workability, spend 8 hours on a ladder in high wind, dust, extreme cold and very loud noises, wearing a hardhat, safety goggles, ear plugs, visibility vest, heavy shoes and work gloves, loaded with tools and materials, with your drawings blowing in the wind. Then try to build a large birdhouse on top of a swaying 15 foot pole. Oh yeah, once you get set up and started, go back down and move your ladder to a different position to let the other guy through. Repeat until educated. When released from the hospital, go back and design a wall system.
Redundancy and improved performance in wall systems
In a multi-layer wall system such as masonry with a weather barrier backup, there are better and worse ways to do the job. For instance, the masonry is usually tied back to the backup wall for structural support. Those connections can conduct water from the masonry right through the weather barrier. Make sure they are sealed too!
At the bottom of the masonry wall, the space behind the masonry (the airspace or wall cavity) must be clear to allow drainage. Use drainage mats and well trained masons to keep mortar drippings from clogging the drainage area.
Building Science Rainscreen quiz
Now here’s a quiz: Check which is preferable:
The rainscreen should repel the following percentage of water: a) 100% b) 95% c) 90% d) 50% e)25%
My answer: a) 100%
You: Hey, didn’t you say it was impossible to make a rainscreen waterproof?
Me: Yep. But that shouldn’t stop you from trying. That’s called redundancy. The more the rainscreen can defend the weather barrier, the longer the weather barrier will last. And the longer the building will last. Just make sure the water freely drains from the bottom.
Redundancy in windows
The same concept can be employed in windows. By adding certain redundant seals, you can prolong system life by reducing exposure of primary seals to water. Examples: exterior silicone “cap beads” between glass and frame, or over exterior receptor gaskets, or at mullion seams.
Adding an interior perimeter caulk joint between window system and the surround is a very important redundant seal. In this case, if the exterior caulk fails, the building still doesn’t leak because the interior caulk stops the water, as long as the area beneath the window controls that water. If the interior caulk is the same material as the exterior caulk, it will last much longer because it has been defended from the weather. Detailed right, the interior caulk joint can prolong building life in a big way. Hopefully, the building will get a new exterior caulk job before the interior joint fails.
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