"Why are my new windows sweating?"
Column #881 01/28/12
On The Level
Q. We had new windows installed this fall in the house and have noticed condensation forming on the inside panes from time to time. This didn’t happen with the old windows. The house is about 50 years old and the old windows were single pane glass with storm windows but very leaky which is why we replaced them. Is there something wrong with these new windows? Should we contact the contractor who installed them and have someone look at them?
A.Your new windows are doing a job your old windows couldn’t. In fact, your old windows were so leaky that they were apparently continuously venting your house so there was no need for anything to condense because whatever moisture was in the house was leaving the house about as fast as it was going into the inside air and didn't have time to accumulate to a level where condensation was an issue.
What you need to do is identify what is presenting the moisture that is condensing on the cool window panes. What is happening is that the interior relative humidity levels are getting to the point where the cool surface of the glass is below the dew point of the interior household air and water in the air condenses out on the glass surface. Much like a glass of ice water will sweat on the outer surface of the glass. The temperature of the water in the glass is drastically reduced by the ice cubes and indoor air heated to 70ºF will be able to hold moisture that will condense on a surface as cold as the glass.
If you are seeing moisture condense on the interior panes of insulating glass windows-- windows that have two panes of glass separated by a space between 3/8ths to 1/2 half inch and some higher end windows have gasses like Argon in between the panes for added insulation-- then you have elevated humidity levels in the house. You’ll want to control those humidity levels for a variety of very goods reasons. Too dry and things dry out so much that good furniture and things crack or cause health annoyances in people such as dry skin and nose bleeds. Not to mention those annoying static electricity snaps when you touch a door knob or another person.
Too moist and the next thing you know you are battling mold and mildew problems. The optimum winter indoor humidity range is thought to be in the 35 to 50 percent relative humidity range at about 70ºF indoor temperatures. Don’t think because the Weather Channel says the relative humidity is 60 percent that it translates to inside the house. It’s 40ºF outside as I write this and the relative humidity is 60 percent, which means the dew point-- the temperature at which the moisture in the air will condense-- is 29ºF. Bring that air into the house and heat it up to 70ºF and the dew point drops off the charts it’s so dry.
There are a number of likely sources in a modern house that can be contributory to uncontrolled higher humidity levels. A malfunctioning heating system-- gas or oil-- can pump moisture into the air along with Carbon Monoxide so rule that one out ASAP. I had a lady contact me saying her windows were sweating but the people next door--in her duplex-- didn’t have that problem. I asked what type of heat she had and it was gas. I asked her if she was having headaches and she was. I told her to get a service tech over there now and she did and they found that a squirrel’s final act was to get into her furnace flue and die. Could have killed her, too.
Unvented gas appliances such as stoves or gas logs can do it. Clothes dryer vents that are not vented to the outside are a big source as is bathing. Wet basements and crawl spaces can add to the humidity loads. I see the results of that up in the attic where the moist air will condense on the underside of roof sheathing, especially on the north facing side. One of my least favorite heating system add-ons is the whole house humidifier. Much safer these days with the addition of a UVC sanitizer light to keep the system free of nasty microbiotics such a viruses and bacteria but I find these moisture sources poorly maintained, leaking or set so far off that they keep pumping moisture into the air stream regardless of what the house relative humidity is or should be.
Check all of the above and you should locate what and where your elevated moisture source is coming from and then you can control it. You can track your home's humidity with a hygrometer-- a fancy name for a humidity meter-- available at hardware and home improvement stores or Radio Shack. They’re fun the play with and they don’t cost much.
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