The Grand Canyon, Arizona, 277 miles long, 18 miles wide, 6,000 feet deep, carved through the rock of the Colorado Plateau. By water. — photo courtesy Grand Canyon National Park and ace-clipart.com.
When you design and construct buildings, you are challenging the number-one enemy of the built environment.
Think you’ll prevail? Think again, my friend. Water always wins.
You’re going to build an apartment building? Water wears down mountains. It dug out the Grand Canyon. It sank the Titanic.
You might keep water out of your roof or walls for today or tomorrow. Water will bide its time. It’s been around several billion years already, just on this planet. Eventually, it’ll find a way in.
Maybe you’ll try to keep it out with an impermeable air and water barrier — the way the French tried to keep the invading German Army out with the Maginot Line during World War II.
“You shall not pass!” you are, in effect, saying to water.
Prudence Ferreira, CPHC
Principal, Integral Impact Inc [i3]
It won’t work, says LEED AP Prudence Ferreira, Certified Passive House Consultant and founder and principal of Integral Impact Inc., San Francisco. i3 specializes in using building science and systems-thinking to help design and build ultra-durable, sustainable, energy-efficient and low-impact commercial and residential buildings.
Integral Impact Inc has a long list of such projects throughout California — sustainable buildings that perform at Passive House and Net-Zero levels of energy-efficiency because of their air- and water-tightness.
And yet, it’s a fact, Ms. Ferreira says. No construction is perfect. Water will eventually get in.
Water in liquid or vapor form won’t penetrate an impermeable air and water barrier. But it can bypass the barrier through vulnerable roof-wall or wall-foundation connections.
It can get in through rough openings where window installation may have torn the barrier fabric or peel and stick flashing.
It might bypass the barrier via a leaky pipe.
In cold weather, positive pressure pushes warm moist air from living spaces into the walls through plumbing and electrical penetrations. When that air hits a cold-enough surface, the vapor it carries condenses, and you have water in the walls.
Water has countless ways to get in walls, and it’s been getting in since there were walls. Like the Germans with the Maginot Line, water will flank the air and water barrier.
If the air and water barrier is seamless, continuous and durable like it should be, then likely just a little moisture will get in. If it’s vapor-permeable, that little bit of water can evaporate out again, and no harm done.
But if the air and water barrier is impermeable, the moisture stays. That’s how rot and conditions for mold begin.
It’s the same principle as when you wear impermeable vinyl or plastic rain gear. Sure, you keep the rain off, but you end up soaked anyway, from your own sweat. The human body itself, after all, is more than 60 percent water. Brain — 75 percent water.
Water wins again.
“I advise our construction teams to use materials with the greatest vapor permeability possible,” Ms. Ferreira says.
Ms. Ferreira, who teaches the Certified Passive House Consultant (NaCPHC) training and advanced Passive House technical workshops for Passive House Institute US, assumes walls will get wet at some point, one way or another, even with the seamless, continuous, durable air and water barriers she includes in her design recommendations.
She just wants the walls to be able to dry out again. “If the assembly can dry, there’s no rot. There’s no mold,” she says. Simple.
That can only happen if the air and water barrier, among other building envelope components, is vapor-permeable.
But if a vapor-permeable barrier lets water vapor out, won’t it also let vapor in?
In a 1997 study titled Preventing Indoor Air Quality Problems in Educational Facilities: Guidelines for Hot, Humid Climates by CH2MHill in cooperation with Disney Development Company, researchers determined that diffusion results in a negligible amount of water ingress compared to water vapor carried in through air leaks.
That means stopping air leaks, rather than diffusion, is key to preventing water-condensation from developing in walls. And for that, you need a seamless, continuous, durable air and water barrier. And make it vapor-permeable.
Because as Ms. Ferreira puts it, “If you keep the vapor out, you’ll keep the moisture in.”
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