Brick veneer over a wood frame is a popular siding material and good construction practice says that there should be a one-inch air space between the brick and the wood framing. However, the purpose of this air space is often misunderstood. This article will explain what the air space is for, and why it doesn't always function like it should.
Brick and mortar are somewhat porous, so when they get wet from rain, moisture will move through them from the outside toward the inside of the wall. This process is called diffusion. An example is when you dip a paper napkin into water, and the moisture slowly moves up the napkin. The same thing happens with a brick wall. If the brick and the wood are in contact with each other, then the moisture will eventually reach the wood. If this happens on a regular basis the wood can rot. The one-inch air space acts as a diffusion break to keep the wood dry.
Moisture can also get through brick veneer because of the wind. It isn't unusual for a brick wall to develop small cracks due to shifting of the building. There can also be small gaps between the brick and the windows or doors as the caulking deteriorates over the years. When wind blows through these openings it can force rain into the air space. However, the only way that this can occur is if the rain exerts a higher pressure on the outside of the brick than the pressure that exists within the air space. There must be a pressure difference for liquid water to be forced through a crack.
An easy way to make sure that the pressures on both sides of the brick are the same is to purposefully make some holes through the brick veneer. These are usually at the bottom of the wall and are called "weep holes." Additionally, the air space is usually open at the top, into a soffit. As long as these holes exist, and are open, the air pressure will try to equalize on both sides of the brick. However, for the pressure in the air space to equal the pressure exerted by the wind, the sheathing must be relatively airtight. This can be done by caulking between the sheets of sheathing or by using a house wrap. With an airtight sheathing, when the wind blows on the side of the building, the air space will see the same pressure as the surface of the brick, and the wind won't be able to blow rain through the cracks. This principle is used a great deal in commercial construction and is called a "vented rain screen."
Common Pitfalls
For an air space to function correctly as a vented rain screen as well as prevent the moisture that diffuses through the brick from reaching the wood framing, it is important that the air space be completely empty. Unfortunately, it isn't unusual for mortar droppings to fill up the lower portion of the air space. When this happens, moisture can diffuse through the wall, then through the droppings, and reach the wood framing members. The droppings can also clog the weep holes, making it difficult for the air pressure on each side of the wall to equalize. This can be solved if the masons take extra care to prevent any mortar from falling into the air space. Sometimes, it is recommended that a 2" air space be used rather than the usual 1" space. That way, if some mortar does fall into the space it will be less likely to clog the weep holes and create a diffusion bridge between the brick and the wood. Of course, if any moisture does happen to get into the air space, the weep holes allow it an easy way to get to the outdoors. The bottom of the wall also needs proper flashing, so that any moisture that gets into the bottom of the air space will be directed outward rather than inward.
Another area of concern involves the metal ties that connect the brick to the wood. In order to be long lasting, the ties need to be sturdy and heavily plated for corrosion resistance. They also need to be installed so that they slope down toward the brick. If they slope down toward the wood, they can direct water from the back of the brick toward the wood, resulting in eventual rot.
If all these factors are taken into consideration, the result will be long-lasting, moisture-resistant, brick veneer walls.
