Wind-Driven Rain, Air Pressure, and Windows

This video is an excerpt from the Building Science Fundamentals class taught by Dr. Joseph Lstibureck, P. Eng., of buildingscience.com and founder of Building Science Corporation, back in the day. This segment focuses on drainage, air pressure, and how these factors relate to windows, including both their construction and installation.

TRANSCRIPT:

Cup analogy for wind-driven rain

All right, so I've got my cup. I turn it horizontally, and if a raindrop drops into the cup and it's sloped, the raindrop will drip out—right? You already got that.

If it's windy, the wind will blow into the cup and fill up the cup with wind. It pressurizes the cup. Once there's wind in the cup, no more wind can enter. You can't blow into a full cup. No more wind can enter the cup because it's already filled with wind.

So you're not going to have wind-driven rain carry into the cup, because the cup is already full of wind.

This might require a bourbon tonight. Let me give it to you again: if the wind enters the cup, it fills up the cup with wind. No more wind can enter, because the wind has already filled the freaking cup up. Right? Right.

Role of the baffle

I put a baffle over the cup. Now, air will enter the cup and pressurize it, but the cup pressure is taken by the back. There's no pressure taken by the baffle, because the baffle has a hole in it.

If there's rainwater on the surface of the baffle, it's not going to get wicked in, because there's no pressure to drive the water into the cup.

Why is this important? Because this is the entire basis of two-stage joints in windows.

The whole secret with a window is that the seal between the two panes of glass—or three panes—can never get wet. If you want it to last 30 years, it's got to stay freaking dry.

Windows are like gutters

So let’s put the damn window in a gutter! Well, that’s really stupid, right? But the window extrusion is a freaking gutter. So what do I do? I want to lift the window off the bottom of the gutter on setting blocks, and I have an inner seal and an outer seal.

Which seal is going to last longer? The inner seal—because it doesn't see ultraviolet light and heat. The outer seal is going to go to crap in a couple of years. What do you mean your 30-year sealant is going to be toast in 10 years?

Why air pressure matters

But we don’t care in this assembly if we provide enough holes in the frame at the bottom to pressurize the extrusion. The wind pressure is taken by the back seal. There's no pressure taken on the front seal. So the front seal can go to crap and it won’t leak—because there’s no pressure across it.

Now, we learned in the 1950s that the inner seal needs to be 10 times tighter than the outer seal. Well, it’s very difficult to make the inner seal perfectly tight. It’s real easy to make the outer seal deliberately leaky.

So what we try to do is make more holes in the window on the outside than we really need, so the inner seal is always 10 times tighter than the outer seal—even if the inner seal isn’t freaking perfect.

When leaks improve performance

And as the outer seal goes to crap, the window performs better, because I have more equal pressure—equalization.

Now, a lot of people who don’t know this actually make windows. I can’t tell you the fun I had in Chicago in the ’80s, where I’d drive around and drill holes in windows that were leaking to stop them from leaking. It freaked people out: “We don’t know about this guy, but he’s fixing our leaks by putting holes in things.”

What I’m doing is getting rid of the driving force. Did I invent this? Hell no. This goes back to the ’40s and ’50s.

The next piece is: when you put your window in the rough opening, you need to do the same thing. You need a perfect four-sided air seal on the inside—but then you want air to get in between the outer seal and inner seal to pressurize it.

So we have a perfect seal on the inside—that’s airtight. We seal the head of the window and the jambs. But we want air to get in at the bottom to pressurize the space, to get pressure equalization and drainage. Woohoo!

So you want the outer seal to be leaky, and the inner seal not to be leaky.