Breaking Down Masonry Cavity Walls

This week I want to explain a little something I should have learned in architecture school, but unfortunately didn’t; and no I didn’t miss out because I was too focused on honing my rubber band shooting skills (two year champ, baby!). This lack of knowledge is simply due to the ignoramuses who thought construction methods were not an imperative in architectural training (apparently we have contractors and engineers for that).

So how did I learn this stuff? It’s called being an intern. Believe me, it is no fun when your boss asks you the correct method of weeping moisture from a wall and you have to wait for him to explain it.

I digress. I want to share with you some things I learned on the job. They might be basic, self explanatory or even slightly incorrect, but I am not an expert yet. However, my wife tells me one of the best ways to learn is to teach a lesson, hence this new post series. I will begin with something easy, one of the most basic tools an architect uses: a wall

For an architect, one of the most efficient types of walls we use is a cavity wall. This is an exterior wall comprised of multiple layers and an air space separating the outermost from the innermost components. Cavity walls are most efficient for insulation and moisture.

The cavity is partially filled with insulation which, combined with the air layer insulates the building very effectively.

Let me explain its qualities. The cavity in the wall separates the cladding from the structural wall and is usually partially filled with rigid foam insulation, giving the wall a much higher R-value (“R” for thermal Resistance).

What most people do not realize is that in a cavity wall, moisture is intended to be absorbed or shed by the exterior cladding. Moisture that gets into the wall (an believe me, it will find a way) is forced down in the cavity to the base of the wall where it is redirected back out of the wall by sloped flashing and weep holes and allow it to be absorbed into the air outside the wall. In a sense, the wall breathes.

Sloped flashing at the bottom of the wall redirects moisture down and out of the wall.

Now, my text books tell me a cavity wall is made of masonry i.e. brick cladding and concrete masonry units (C.M.U.’s). However, I have seen and designed other walls that function in the exact same way and are made of different materials. As long as there is an air space separating two layers in the wall and it functions in the way described above, it can be considered a cavity wall.

For all you nerds, I found this website that can tell you everything you may ever want to know about cavity walls and more. Be warned. It’s quite exhaustive detail.

Thanks for reading!


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