Condensation on plexiglass?
Marc Delisle| Posted inGeneral Questionson
Hi,
In fall-winter-spring, I have lots of condensation on my double-pane windows. After reading multiple sources on the Internet stating that condensation does not form on acrylic, I fastened a plexiglass sheet on a window to test. I lowered the double-cell cellular shades for the night.
This morning, I realized that the test failed – there was fogging on the bottom part (but no droplets). I guess I read the wrong sources or I did something wrong.
Which one is it?
Thanks for your help!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Acrylic isn't some magical substance capable of defying the laws of physics. If the surface is below the dew point temperature of the proximate air, condensation will form.
The internet disinformation likely arose from a perception of those who had installed plastic interior storm windows. A storm window (any material) lowers the heat loss through the windows, which results in a higher temperature on the interior surface of the interior storm. That makes it more likely that the surface will be above the dew point temperature of the conditioned space air (resulting in no condensation), but there is no way to guarantee that it will be above the dew point in any given situation.
Good morning Marc,
听起来不像你做错了什么how you conducted your test since condensation occurs on any material when the surface temperature drops below dew point temperature. Condensation doesn't much care what the material is; if it's cold enough condensation is inevitable.
It really sounds like you need to be looking beyond your windows and looking at your home environment if you're getting "lots" of condensation on your windows, especially this time of year.
Cellular shades are often a significant contributor to window condensation, but I suspect you already knew that since you specifically offered that little detail in your original question, so no further comments there.
The simplest response, based on your description, is that you have way too much moisture inside your home. This is especially true since it's very early in the upcoming condensation season to be seeing (interior) wet windows even with high levels of moisture in your home. So unless you live close to shouting distance from the North Pole, or you are living inside a tropical rain forest, I am wondering how you could be seeing interior window condensation in early fall.
Any other details that you might offer, such as climate zone, if you have exhaust fans, and so on might be very helpful in determining what's happening in your home.
Greg - Thanks for your answer. I live in Ottawa, Canada. Temperature got down to 5 degrees Celsius. There is quite a bit of humidity, around 60% relative. I also think my shades are part of the problem, because they are very close to the window and fit the frame maybe too well.
D Dorsertt - you are right, maybe I should have known better.
What are my options here?
Marc: the fix is to eliminate the sources of moisture. Do you run vent fans after shower? Have lots of house plants? Dry laundry indoors? Damp crawl space? Vent a dryer indoors?
Hi Stephen,
Actually, I run vent fans; no house plants; no drying laundry indoors, no damp crawl space, no venting dryer inside.
However, I open my windows these days. Temperature is about 18 Celcius by day with relative humidity around 60%.
What should I do, close my windows and start the furnace to bring down the humidity?
Thanks for your help!
Marc,
If you have 60% indoor relative humidity (RH), that would be quite high for winter (when you are heating your house), but on the margins of OK for summer (when you might be either running an air conditioner or opening your windows). In a normal house, there is no need to use cellular shades when the indoor RH is 60%, because the weather is hot. (Unless you are using the cellular shades to prevent solar heat gain. But under those circumstances, you shouldn't get condensation.)
If you have 60% indoor RH during the winter, you should read this article:Preventing Water Entry Into a Home.
Hi Martin,
Thank you for your response. It's not winter yet, but Ottawa, in Canada, is already feeling the chill - it was 4 degrees Celcius last night, and high is going to be 18 today. So I used the shades to keep the cold out at night.
Outside relative humidity is 80% right now. The thing is, I run fans, but the air that gets sucked into the house is also very humid. I feel that I would need to start the furnace to bring the humidity down (I don't have a dehumidifier).
Is it really an issue if indoor humidity level is around 60%?
I read your great article, thank you! So my house is from 1968, so I probably don't have a perfect thermal envelope. However, there are no signs of excessive moisture, and I don't do any the things you mentioned if your article (drying clothes inside, house plants, etc.)
It's interesting that you say that cold air is dry, because our winters get very cold, and the humidity outside is always around 80% or more in winter. UNLESS we get a minus 35 celcius...
Thanks for your help!
The outside relative humidity is a meaningless number without the outdoor temperature. The outdoor dew point is what is relevant, since one can convert to that to the relative humidity at indoor temperatures.
At 20C, 60% RH the dew point is 12C. If the outdoor dew point is above 12C, ventilation would be adding humidity to the indoor air. When the outdoor dew point is below 12C, it would be lowering the humidity of the indoor air. The outdoor dew point can never be above the outdoor air temperature, which is fairly cool in Ottawa in winter.
The mean outdoor air temperature in Ottawa in January is about -10C, and even at a rime-icing fog (100% outdoor RH), raising the temperature of that air to an indoor temp of 20C would have an indoor RH of 11%. That's pretty dry! The average January dew point in Ottawa is actually about -12C or -13C, and thus not a full saturation. Air with a dew point o -12C will be bout 84%RH when the temperature is -10C. Typical season temperature and dew point averages for Ottawa are graphed here:
https://weatherspark.com/averages/28316/Ottawa-Ontario-Canada(The dew point graph is near the bottom of the page.)
The goal in winter should be to keep the indoor RH between 30-35% @ 20C to limit the amount of moisture accumulation in the susceptible wood on the cool side of the walls & roof etc. That is easily achieved by adjusting the ventilation rates, diluting the more humid indoor air with the much drier outdoor air.
And if you still get condensation, adjust the shades to get more airflow behind them.