Preventing condensation on windows in winter

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.

Bump for that time of year again.  It got down to 5F last night and there was condensation all over.  So I put the film and silica in place and we "should" be good to go for another winter.  Last winter it worked flawlessly again.

Some year if I get a chance, I'll make plexiglass or lexan panes that will fit into the window opening with a closed cell foam border so they fit tightly.  Then with the silica behind them it should work the same but then not need the plastic film.  And we'd have a window sill again.  

wonder how hard it would be to use strip magnets with plexiglass to make the double glazing.

That's a good idea Graham!  I'm assuming one strip on the plexi and one on the window frame?  Would you leave the magnets up all year?  I wonder if they make wood grain strip magnets.....

You'd need a strip that goes completely around on the window frame and on the plexiglass so that you'd get an air tight seal.  Maybe paint the ones on the window frame to match the wood grain?  Or apply a wood veneer if it's not too thick.

Wow, that sounds like a great idea, and I might try it!

What a friend of mine did, was pin up woolen blankets right against the window in winter, that hung right flat to the glass. When you pulled them away from the glass in the morning, for an instant the glass was clear, but then it fogged up and moisture dripped down to the sill before your eyes. So the wool curtains very close to the glass seemed to prevent the warmer moister air from reaching the glass.

What I've basically found is that no matter what the material is, just separating the window glass from the interior of the room helps.  But when it get really cold out (below zero F), even that isn't enough to keep condensation from coming out of the air gap you've created and having it condense on the window.  If you pull back a curtain every day then you're recharging that air gap with new moist room air to condense that night.  

There's a whole range of climates where the is all you'd need.  But where I live, I found the dessicant really makes a tremendous difference.

Graham, I love the magnet idea, I'll pursue that instead of the foam idea when I get around to upgrading my system.  I could put the magnets against the closest piece of wood to the glass (part that cranks open).  Or on the window trim.  I'll have to noodle on which would be better...

Although one concern with the strip magnets is that they alternate from positive to negative as you go down the strip.  So if you put two strips together, they want to jump into alignment.  I guess I could stick them to the window and mate them up with their friend, then press the plexiglass on...

You could also put storm windows up on the outside to avoid looking at your magnetic strips or whatever attachment system you come up with. Same thing, have some weatherstripping around the edge so it seals really well. At my parents' place, before they replaced their old single pane windows, they had little wooden swivel tab things on the outside of the house. Just hoist the storm window into place and turn the tabs to hold it tight. Do it on a dry cold day to avoid trapping moisture.

If you put your window film up when the air is dry, you could probably avoid the silica, too. When we want to get our house's humidity down, we crank the woodstove and get it good and hot inside. Then we vent all that hot humid air and let in the cold dry air from outside.

Yes Jan, I think it would work on the outside of the window as well.  I'll have to look at mine but they're new windows and I'm not sure if there's a good ledge to attach them to on the outside.  Plus it's a two story on one side but I can deal with that.

I do put up the film when it's dry but it seems like I either can't get it dry enough or the humidity is constantly slowly sneaking through gaps in the window trim all winter.  I believe I'd need the humidity at the window surface to be around 10-15% RH to not condense when the glass surface is under 20 degrees F.

I don't think my parents had ledges at all, just the swivel latch things. And yeah, now you mention it, I think they got lazy about putting the second story ones up pretty quick :D

BTW, because the magnets aren't that strong the perspex has to rest on the window ledge. The magnets are there for the air seal.

I didn't know about the steel strip.  That eliminates the issue of two magnetic strips not matching their polarities.  Wonderful stuff!

And I bought a magnetic glazing kit last week and did one window last night.  Made a few mistakes but will correct that in the next attempt.  Not only should one measure twice and cut once, one should at least measure once!

My daughter had a few sash windows retrofitted with thermally broken double glazing units.  I guess a lot of the cost was in the labour and there is a top pane, and a bottom pane, but it cost NZ$2000 per sash window.  This solution is less than 10% of that cost but it's a DIY solution so you need the time, and perhaps more care than I took.

Here in Florida we have the same issue with temp change and high humidity.     I run a couple de-humidifiers to keep the humidity at 60 percent, which stops the issue and stops mold forming in it's tracks.

A tip on the silica jell,    you can toss them in the freezer to remove the moisture from them, just like ice cubes freeze dry so will the silica gel packs.


I recall in Japan they would put much charcoal in bags in the house to regulate the humidity of the home.


Thanks for sharing your tips.

Interesting tip on silica gel. But I'm finding a problem finding anyone who has them!

My second morning with one acrylic pane in place

The company that sold me the magnetic glazing system suggested I get the cheaper version which just used the plastic trim, and it gets nailed into place.  It would have been about NZD50 cheaper, and he said most people never removed the glazing once it was on.  But my experience with something I tried a few years ago was that you can't stop moisture getting into the gap, even with silica gel bags, and it's best to have something you can easily remove to swap out the silica.  

Though it certainly reduces visibility, small-cell bubble wrap readily adheres to a damp window and significantly reduces condensation.   While impossible to look through, it still lets in a lot of light.  Improves insulation at windows as well.  

Hi Graham, I got silica gel from a hobby store called Hobby Lobby.  I think the intended use is for drying flowers.  

Graham - Trademe listing for silica gel

Thanks Phil .. but too late, I've already ordered some from overseas

G'day all,

I've been experimenting with fairly inexpensive USB Cooling Fans in my windows to prevent frost and dry up humidity.

We had a low of -19°C (-2°F?) last night.

One large window, under a running baseboard heater (set to 15°C), blinds left all the way up, had a little bit of dew

One large window, under a running baseboard heater (set to 10°C), blinds brought all the way down and set to "deflect" heat away from window, was very frosted (bottom 4-6 inches)

One large window, under a non-running baseboard heater, with two USB fans (one fan on each window pane) had no frost and no dew

I have loads of USB power banks, so I am using those to power the fans.  Purchasing these fans along with power banks may not be the most inexpensive proposition.

Somebody more resourceful than I could probably McGyver something similar for much cheaper, maybe re-purposing CPU fans from expired computers?

At the moment, I'm using twist ties for anchoring the fans to aluminum foil boxes, to raise the fans up from the window sill to window pane-bottom level.

Cheers and best regards !

Mike Haasl wrote:

I do put up the film when it's dry but it seems like I either can't get it dry enough or the humidity is constantly slowly sneaking through gaps in the window trim all winter.  I believe I'd need the humidity at the window surface to be around 10-15% RH to not condense when the glass surface is under 20 degrees F.

I'm not convinced moisture is sneaking through gaps - you've got wooden windows, and wood absorbs and releases moisture depending on the humidity, so if you have wet fall weather, the wood window frames themselves could easily be the source of the humidity. The silica gel is a cheap, reusable fix. The only other idea I've heard of is boxing out the "storm window" a minimum of 6" to solve the condensation issue, and I've not personally tested that idea, so who knows. We consider -6 centigrade unusually cold, so I have *no* desire for -6 F or anywhere near there!

I'm sure some moisture could come out of the wood trim, but I'm thinking it's much more likely that humid room air is sneaking through the miter joints and any gaps between the wood trim and the drywall.  At least in my house...

We have "ama-do" storm shutters here. When I close them I get no condensation. If I leave them open, it's a soup.

Closing them keeps the house nice and warm too.