I finally figured out a way to keep our windows from condensing on the inside during our cold winters. Last winter we had to sop up condensation (or melt it with a hair dryer and then sop it up) to prevent it from pooling on our wooden window sills and causing rot. We have wood heat (no forced air), a relatively tight house and live in a fairly cold area.
Condensation is caused by the inner surface of the window reaching a temperature below the dew point of the air near the window. These are new windows but there isn't much a double pane window can do when it's -20F outside. "Normal" suggestions are to reduce the humidity inside (last year we got it down to 25% and we still had condensation every night), increase temperature of the house interior or add a storm window. Increasing the temp may help if you're just barely below the dew point but we are in too cold a climate for that option to work.
My solution was two-fold. First part is to install winterizing window film on the inside of the window (tape attached to outside of window trim). I didn't want to do the window film but it ended up not looking as tacky as I thought it would. This film gives another "pane" of glass to keep the room warmer. It also traps the air between the film and the window so your showers, cooking and breathing won't keep supplying new humidity to the window to condense.
When I tried just the film, it definitely helped. I didn't keep records but I think the outside temp could get about 10 degrees colder before the condensation would begin. And then once it began the amount was much less (since there was only a small amount of humidity trapped near the window to condense. The problem was that you can't sop up that condensation because the film is in the way So if you barely condense at the coldest part of your winter, this should work for you.
The second part of the solution is to dry out the air that is trapped between the film and the window. To do this I bought a pound of silica gel beads from Hobby Lobby. I put 3 tablespoons of beads in a mesh wedding favor bag and set them on the window sill and immediately put on the window film. The silica gel changes color when it gets half full of water. The color changed about two days after I installed them. I left them in place all winter and had no condensation on all our windows except one. That window may have had more gaps between the trim and the drywall which let more humidity from the room into the trapped area. I made a slit in the film, removed and dried out the silica, then put back in double the amount of silica and taped over the slit with packaging tape. After that it never condensed for the rest of the winter. This was with several night around -22F.
I thought winter was over a week ago and we removed the film and silica from three windows. Then winter returned and were able to remember how good we had it. Lots of condensation. This time I measured it and we had condensation any time the outside low was under about 15F. Inside temps were 63 and humidity was 38%. I'm glad we didn't remove film from all the windows!
These pictures were taken on the same morning after a 5F night. The first window still has the film and silica baggie, the second shows condensation on an unprotected window.
The silica is reusable by drying it out in the oven.
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