We ask a very interesting question. We know of course that people, pets and plants all breathe, but how can a wall breathe? The answer lies in a long word that is hard to say, let alone define, called “hygroscopicity.” Look at the word that makes it up: Hygro means water, and when a substance is “hygroscopic,” it is said that that substance allows water to pass through. The trick in whether water will pass through lies in how large the water molecule is and what form it takes.

The answer to our question is, a wall is “breathable” when it allows moisture, in the form of vapor, to pass through. Now most people think of breathing as involving the movement of air, and it does, but air can also carry water in the form of vapor, small water molecules dispersed in air. You are familiar with that when you feel hot and sticky on a humid summer day, or sitting in a steam sauna. That is because a great deal of water is dispersed invisibly in the air as water vapor.

When we say a wall is breathable, we are not talking about its ability to pass air through. No one, not even proponents of breathable, hygroscopic, walls wants that. Why? Because if your wall allows air to pass through, it is not well insulated, and you lose heat in the winter, and warm outside air can infiltrate into your air-conditioned house in the summer.

The whole green building movement is first and foremost based upon the notion that a modern, energy-efficient house needs to be “tight” in order to be “green” and energy-efficient. Their motto is, “build tight, ventilate right.”

We agree with this laudable goal in theory, but every builder will at some point admit that in the real world, this system is prone to failure. This is because when water enters a wall, it often cannot get out because multiple plastic barriers that are designed to keep moisture out trap moisture within when it does get inside. In fact, even if a wall is built to “dry to the outside” with plastic house wrap on the exterior that is supposed to allow water vapor to pass through, that house wrap can get overwhelmed if the amount of moisture in the wall becomes too great, an all too often occurrence.

The problem is, no matter how tight you make the wall, moisture will eventually get in. Mold experts say every building, without exception, will experience a water event at some point in its life. Moisture enters walls either by infiltration of rainwater leaking through caulking or flashing (the layers of material around windows and doors) or rainwater leaking through failures in your external waterproofing system under your siding. Other causes include roof leaks, including ice dams, causing rainwater to drip onto attic insulation and down into walls.

From the inside, we have a whole host of ways moisture can enter a wall, including moisture-laden warm indoor air finding its way through gaps around electric outlets, windows and doors. Even if you tape everything real tight when you build the wall, how long will that taping job last? And what will happen when you pound a nail into a wall to hang a picture? You pierce the plastic vapor barrier on the inside of your sheetrock. All these, and many other ways, create a small path for warm air to seep into the wall cavity. That air carries moisture.

The problem is, once moisture gets into a wall, it can condense and make the insulation and other materials wet. How does this happen? Because in the winter, your wall has an air temperature range between 68-70 degrees F. just inside the wall, to possibly zero degrees F. or less outside the wall. Somewhere in between the interior and exterior lies the dew point, the point at which moisture will condense.

How long does mold need to grow on wet surfaces before it becomes unhealthy? Most experts say all it takes is 24-48 hours for mold to get to spore formation, which is considered the point of no return.

Studies show that up to 50% of modern walls with fiberglass insulation and plastic vapor barriers have water problems, meaning they have mold growing within them. That is a huge, and costly, problem and shows the failure of modern walls to keep us healthy. Unfortunately the paradigm of building tight may work on paper, but it is flawed when implemented in the field. This is because you never achieve the 100% air-tight and vapor-tight wall you designed in the first place. Plus, the tighter you make the wall, the greater the pressure exists for vapor pressure to equalize on both sides, driving more moisture into the wall. So you can exacerbate the problem. Ultimately you are left with the reality that when a “tight” wall fails, it fails in a big way.

We say instead, design a wall to be thermally insulating, meaning keep warm indoor air from leaking to the outside, but allow any moisture vapor or rainwater that seeps into the wall to move out. The wall can then dry out within the 48 hours needed for mold to grow.

Our motto is, make walls “waterproof but vapor permeable.” We accomplish this with our envelope materials and the natural, “breathable” finishes we use.

Old walls built in the 1920s and 1930s had no vapor barriers. They also had no significant insulation, except possibly horsehair or crumpled up newspaper, but often nothing at all. They have always been leaky to air flow and heat loss, but they dried out within 48 hours if they ever become wet. Ask any remodeler who opens walls in old houses and they’ll tell you they never see mold in older walls unless there is a constant source of moisture dripping in. These remodelers only started seeing mold in homes built in the past few decades when vapor barriers started to be used to make the houses tighter. The problem is even worse since the late 1990s, when builders began making houses supertight.

Read more about this topic and how to avoid it in our book!