How Sunlight Can Cause Problems for Brick and Stucco Siding

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Usually when we talk about moisture problems, we mean the stuff caused by ice, rain, sleet, and snow—the kinds of weather it makes sense to protect your home from. But if you have masonry, stucco, or even fiber cement or concrete on the outside of your home, your siding could be collecting moisture from the inside out.

It’s called inward solar vapor drive, a condition that occurs when condensation collects from cool air-conditioned interiors and sticks to the inside of your home’s house wrap. Meanwhile, it rains outside, and then clears up, drying surfaces—and pushing water deeper into the brick in your siding. Don’t worry—solar vapor drive isn’t all that common, but when it happens it could cause a lot of problems. Rampant moisture is poison for interior walls, and it could cause a whole slew of issues like mold, rot, insulation damage, leaks—you name it. Let’s take a look at inward solar vapor drive, why it happens, and what you can do to prevent it in your home.

Row of new suburban brick homes

Reservoir Cladding: How Walls are Protected in Masonry

If you’ve lived in your brick house for a while, you may have noticed that moisture tends to stick to it. Maybe you’ve had some problems with algae, or have noticed that plants tend to take root there in the spring. Brick, concrete, and cement products and stone all absorb water when it rains or snows. They’re what’s known as “reservoir cladding,” which is just a fancy way of saying that they hold water.

That means builders have to be careful when they put in your siding. They can’t lay it flush against your home’s sheathing—that could potentially expose the walls to moisture, and lead to all those problems we talked about before. Instead, your installer creates a drainage plane and an air barrier behind the brick wall. It’s too complicated to go into all the mechanics here, but basically that system behind your walls works to absorb water and dries it out using the airflow inside your home’s walls.

The problem comes when reservoir cladding is combined with a couple of other factors that we’ll go into in depth below. If water isn’t being properly vaporized behind the wall, it can collect, and then suddenly before you know it, you can have inches of water sitting in your walls.

Other Factors that Affect Moisture in Brick Walls

The biggest player in the whole inward solar vapor drive effect is house wrap. In most cases, house wrap is good for a home. It can protect the interiors from the elements and keep heat from escaping when it’s cold outside. But it’s not perfect. Most house wrap is permeable, meaning it’s made with small holes to let moisture escape. But a plastic vapor barrier like polyethylene sometimes doesn’t “breathe” enough for a brick home.

To get the whole picture, though, we have to talk about how that polyethylene surface interacts with air conditioned interiors. You’ll have to think back to the summer, when temperatures were soaring, and you turned your AC up high just to get the slimmest relief. You’ve probably noticed that sometimes that AC unit causes condensation—well, the cooler air from your home can do that when it comes across a vapor barrier that’s not porous enough. Water collects against the back of the house wrap, and guess where the water then stays? Yep, you got it: inside your walls.

measuring rigid foam insulation

What Can Builders Do to Prevent Inward Solar Vapor Drive?

But not all is lost. Builders can make sure you never have to deal with the consequences of solar vapor drive. The first thing, of course, is to avoid a polyethylene or vinyl vapor barrier at all costs. There are plenty of alternatives to prevent moisture infiltration, so ask your house builder to investigate one of these instead.

A builder who knows what they’re doing will also check the tightness and airflow behind the wall. The more airtight a wall is, the less chance it has of drying out, so brick homes should be specially designed with a little extra room in the interiors to let moisture dry.

Another thing your builder might do is forego the fiberboard sheathing behind your new addition’s brick-veneered walls for a different material altogether: foam. Polyisocyanurate foam, also known as rigid foam sheathing, is most often used as protective insulation that’s nailed on top of OSB sheathing for protection. But it can also be used for the walls themselves, which is what brick home builders in the future may do to protect wall cavities from excess moisture.

See? Solar vapor drive isn’t that big a deal. As long as your contractor takes care to use the right materials and techniques to install your new brick exteriors, you won’t find yourself hitting your head against that same wall in a few years.