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Philibuster: Letting the Air Out – Let’s talk about exfiltration

June 3, 2020  By Phil Lewin

I recently participated in an online discussion that reminded me of an experience I had many years ago that is still a relevant window service issue today. In our industry there is a lot of attention paid to air infiltration. Of course, no one wants to be standing next to a closed window and be standing in a cold breeze blowing right through it. That cold air will ultimately result in the thermostat turning on the home’s heat source, costing money and wasting energy.

Instead, one wants to be standing next to a closed window in the dead of winter and feeling warm and toasty. So, you may be surprised when I tell you that warm and toasty feeling may be a symptom of a problem not unlike the cold breeze from air infiltration!

Many years ago, I was asked to go to a farmhouse in the dead of winter where one of the dealers I worked with at the time had installed a single window. The homeowner was unhappy with the window’s performance and the dealer could not see anything wrong with either the product or the installation.

After a trip to the middle of nowhere, I arrived at the farm house in the middle of an open field. The word “desolation” comes to mind. The owner was pleased to see me and explained that this was the sixth window, each from a different company, that she had installed and not one of them was acceptable. She did show me one window that she liked but was unable to find again. It was warm standing in front of this window.


I was then directed to my window. Truthfully, I could immediately tell that it was not as warm to stand near it. I had brought a laboratory-quality smoke gun with me. These look like a glass tube with a rubber bubble at one end. Squeeze the bubble and out of the tube will come a smoky substance that hangs in the air unless the air itself is moving. They’re great for locating drafts. Using the smoke gun, I checked the complete perimeter of the window. Then I checked the casing. I checked the hardware. There was no air leak to be found. The only air movement was a slight drop. That is normal because as air near cold glass cools, it becomes more dense and gravity pulls it down. The homeowner was not impressed, reminding me that something had to be wrong and once again mentioning the other window.

I admit I really wasn’t thrilled to be asked to use my smoke gun on a product from another company, but the homeowner insisted that I do it. She suggested that the dropping air on my window was the problem and would not be a factor on her one prized window possession. With the smoke applied, at first glance, I didn’t see any air infiltration. Then, I looked down through the window. What I saw looked like a miniature Niagara Falls of smoke coming out the sill of the window! In fact, the window was taking all my smoke and shoving it outside. So, as the air adjacent to the window was leaving the building faster than Elvis, it was obvious what was happening. Air next to this window never had a chance to cool off before it was exfiltrated from the home. In reality, standing next to the window was a matter of standing in a warm draft of heated air. Yes, nice and toasty, but a source of significant heat loss and energy wastage.

Now I had a new problem as I had to explain to the homeowner that her most prized window was not the star that she believed. People can be so resistant when confronted with unpleasant truths.

Here is what I told the homeowner: for the air pressure in your home to remain basically constant, for any air that is added, an equal amount of air has to leave. I pointed out that even in her old home, she had air return grates built into her floors. Hot air enters a room and the colder air is expected to leave, back into the system where the remaining heat would be recycled. Her home was not circulating air efficiently and heat was being lost through the windows.

It all came down to air flow. The moral of the story is, when trying to diagnose an issue, one has to look at the entire building envelope and not just a single component in order to get an accurate picture.

Phil Lewin is sales and marketing manager for GEM windows. He’s been annoying people in the window industry since 1984.

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