Condensation for Dummies - Drexel Building Supply

Condensation for Dummies

On Wednesday Jan 7th, many of us from the 3 stores attended a seminar at the HBA office in FDL. The presenter
was Joe Nagan from “Homebuilding Technology Services.” Joe talked about not only condensation on windows, but how
to prevent condensation from occurring in your wall cavities and attics. There was way too much info for me to try to cover here, but I will try to sum up the key points.

Window Condensation
-Probably the biggest concern that we run into at this time of the year. People often times are uneducated about this subject and assume that their windows are bad. But, can we blame the homeowners for this? It is our job to make a
point of explaining condensation to our customers when they are in here, and purchasing the windows. If we explain
condensation on the front end, they will be expecting to see some condensation, and more understanding of why it
occurs. Fact is, it is not a problem with the window, or a bad window brand, rather the result of air & humidity behaving
the way that nature intended them to.
‐ Condensation occurs on any surface that drops below the dew point temperature in a room. Whether it is on a
window, or a soda can that you take out of the fridge, if that surface is 35 degrees, and the dew point temperature at
that given time is 42 degrees, the humidity in the room will show up on these surfaces. Often times on a cold day, the
glass, which only has an R‐4 insulating value at best, will drop below the dew point temperature, causing the humidity in
the room to show up in the form of condensation on the glass. Studies show that they bottom 3” of the glass are often
times 7‐12 degrees colder than the rest of the glass, which is why condensation occurs at the bottom only.

‐ How can we work to decrease the amount of condensation on the glass? There are a few ways:
1.) Raise the exterior temperature. (out of our control)
2.) Decrease the interior temperature, in turn, decreasing the dew point. (An unpopular solution)
3.) Decrease the interior humidity level, decreasing the dew point. We can achieve this by running bathroom
fans when showering, running the stove fan when cooking, using an air‐air exchanger, etc.
4.) Warm up the interior of the glass. There are a couple ways that we can do this:
a.) keep blinds and curtains open, to allow air flow over the glass
b.) removing screens on casement windows will allow more air flow over the glass also.
c.) Turning on a ceiling fan and circulating more air helps keep the glass warmer as well.
d.) Leaving furnace set at a constant temp will actually help keep your glass warmer as well. The furnace
will run on more consistent intervals, causing warm air to warm the glass more often.

‐Every‐day Examples for homeowners:
a.) Take a cold soda out of the fridge and set it on your counter. What happens to that soda? There is
condensation on it, right? Why? We know that the can isn’t leaking, and we know that the can isn’t making water. So
what is the deal? The condensation that you are seeing is a result of the can being below the dew point in the room, so
the humidity shows up in this location. As your soda can warms up, it will dry off. This is an everyday occurrence that we
have come to expect. Well you know what? The exact same thing is happening on the glass of your window.
b.) The windshield in your car is often times covered up with condensation and frost. How do you get rid of it?
You turn on your defroster. What is your defroster doing? It is warming up the glass. That is exactly what you want to do
in your home. Open the blinds, take your screen off, get some air‐flow on the glass to warm it up. This will help rid the

‐Tools: At the Seminar Joe used a few tools to help understand all of this. They included a small digital thermometer
that also displayed the relative humidity, A psychrometric chart that showed the dew point for a given temperature and
humidity level, and he also used a laser, point‐and‐shoot thermometer for testing surface temps of the window to
determine when it is below, or close to, the dew point temp. CBS/BBS/KBS will all have these tools by Feb 1st, 2009.

Condensation in Your Home
Joe also spent a good deal of time speaking about controlling & preventing the condensation in your walls and
attic. This could be a somewhat controversial subject, and we have not seen many issues with this in the area, so I will just touch on it. Joe basically said you should keep the interior of your house as tight as possible, and then control the humidity with mechanically regulated methods. It is very critical to have a poly vapor barrier on your walls and ceilings, and then seal the seams, corners, even caulk around the outlets and light boxes. You want to prevent any humidity from making it
through any gaps and getting into your wall cavity. Once it is into the wall cavity, or attic, it will reach the point of the wall that is at the dew point temperature, and then turn to moisture. Moisture in your walls is bad news because it will often times cause mold and mildew.
The theory is to keep all of the moisture inside your home, and then send it outside mechanically by use of
bathroom fans, stove fans, air to air exchanger’s, etc.

To learn more about condensation, or if you have questions, contact the author of this article Steve Herriges. Steve, our assistant store leader at Campbellsport Building Supply, can be reached at or at 1-88-715-9663.

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