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Condensation problem inside storm windows

Last year we bought a 70-year-old house which has old-style single-pane wood casing windows with aluminum storm windows and vinyl lining the base between window and storm. We noticed that moisture condenses and ice forms on the storm windows primarily upstairs in the winter. Sometimes there is a little moisture inside the window too. We tried adding rope caulking to prevent air leakage between the windows, but that didn't solve the problem. I note there is some old caulking at the base of the storm sills which does leave small gaps for a couple of slivers of weep holes on each window. Should this caulking be removed? Should caulking be added on the inside of the storm frames where they meet the wood to prevent air leaking in there? Would reglazing the cracking windows do the trick? (We just had them reglazed this summer.) Could our bathroom upstairs with little ventilation for showers be the problem? Or all of the above? What should we try to do next?

Re: Condensation problem inside storm windows


Really, what the problem is is that you have two systems, and neither one works well.

You have the wood single glazed windows which are leaking air out of your house (and that's what's causing the condensation and frost to form on your storm window when it's cold).

And, you have your aluminum storm window, which doesn't have a proper seal around it so that the two windows together don't behave like a double glazed sealed unit.

What you have is outdoor air leaking around the aluminum window, thereby keeping both sides of it about as cold as the outside air, and you have air leakage through your wood casing window which is the source of the frost and condensation on the aluminum window.

Ideally, the best option would be to install new windows, but that may not be economically feasible.

I think the vinyl and caulking you mentioned in your post are attempts by the previous homeowner(s) to seal the gap around the aluminum storm window, thereby preventing cold outdoor air from flowing around it, and keeping the inside surface of glass warmer, thereby preventing condensation and frost on it.

About the only thing I can suggest is to use a caulk that would ensure a good seal around the aluminum storm window, but still be EASY to remove in the spring when you want to remove the storm windows. Such a caulk is called Kop-R-Lastic:

Simply Google Kop-R-Lastic and the name of any major city close to you, and you should find places in that city that sell the stuff. If not, you can mail order it. It's worth the hassle.

Also, if you have wide gaps between the aluminum storm window and the wood framing it fits into, you want to fill those gaps with something called "foam backer rod" before caulking.

The foam backer rod prevents your filling the gap completely with caulk, thereby preventing the caulk from adhering to the back of the gap. Caulk works best when it bonds only to the opposite sides of a gap, not also to the back of the gap. Foam backer rod comes in 1/4 inch diameter (smallest) to 1 inch diameter in 1/8 inch increments. 1/4 inch foam backer rod will push into a 1/8 inch gap. If your gaps are smaller than that, then don't bother with the foam backer rod, or just use some cotton yarn if you want.

Kop-R-Lastic costs about $7 to $8 per tube, but you can do quite a few windows depending on the size of your gap. It's easily toolable with a finger dipped in a 50/50 solution of liquid dish washing detergent mixed in water. It'll take a few months to fully cure, but once it's fully cured, and you want to remove the caulk (so you can remove the aluminum storm windows), you jus get one end of it started and it'll pull out of the gap like a rubber rope.

I am only concerned that if you caulk with Kop-R-Lastic when you put your storm windows up, there may not be enough time for it to cure completely before the cold weather comes, and then it'll remain dormant and not cure any more. You may have uncured caulk to clean up in the spring.

However, even if you left the uncured caulk on the wood and aluminum to cure over the course of the summer, you could still pull that caulk off easily from both in the fall.

I'd try your WORST window (or two) this fall using Kop-R-Lastic and see if you get any better performance out of the storm windows. If not, the only real fix is to replace your windows.

Cured Kop-R-Lastic is dissolved by mineral spirits. Paint thinner is typically mineral spirits, but you're better off to look for a jug that calls itself "Mineral Spirits" instead of one that calls itself "Paint Thinner". That's cuz mineral spirits will evaporate completely without leaving a residue. If a company sells something called "Paint Thinner", that difference gives the company free license to put anything in it that they think might help paint flow and self level better, and what they put in might not evaporate completely. You could end up with some sort of oil residue remaining behind on your wood that might be a fight to clean off. They wouldn't do that with mineral spirits cuz mineral spirits is understood to be a light fraction of petroleum distillates, and all light petroleum distillates evaporate completely without leaving a residue; it would be false advertising.

If improving the seal around the aluminum storm doesn't do any good, your next step would be to save your pennies and replace the upstairs windows. It'll be the upstairs windows that sweat and frost up the most because warm air rises inside your house, and the warmest air will typically also be the most humid air.

Also, if you find that you can stuff foam backer rod all the way around the aluminum storm window, then don't even bother caulking. The foam backer rod will prevent air circulation around the storm. You can buy foam backer rod at most construction material wholesalers (who will sell to you if you pay cash), and I think most home centers even carry it now.

Re: Condensation problem inside storm windows

What Nestor said plus;

You're looking at new windows. Those aluminum frames are horrible when installed properly so don't go blaming yourself. Someone before you took the cheap way out and put in crappy windows. You can either live with it or buy new ones as the funds come along.

I'd recommend solid vinyl, double pane, low E, argon filled, tilt in, with burglar locks. Decent ones can be ordered at big box stores and installed by yourself, but generally if left to the pro's it is cost efficient to have them installed.

Re: Condensation problem inside storm windows

One remark about "Argon gas filled": the gas finds ways to escape in the first month after installtion, so I consider it a sales tool rather than an efficiency tool.

Re: Condensation problem inside storm windows

Thought I should interject here:

Where I live, the standard replacement window is a PVC casement or awning window, triple glazed, low-e on at least one lite and sometimes on two lites, and argon filled in both airspaces.

It's true that argon does leak out of the windows, but the escape rate is closer to about 5 percent per year. If it all leaked out in one month, not one would sell them and no one would buy them.

This web page says that the "Swiggle Seal" and "Super Spacer" are considerably better at retaining argon than other window spacers:


Re: Condensation problem inside storm windows

Add weatherstripping to your wooden windows. That will stop warm moist inside air from escaping and condensing/freezing on the storm panes. Maybe keep the indoor humidity below 60% in the winter, if you run a humidifier during the cold months, as I do. I had a lot of frost on my storms when I was keeping the humidity at 65%. It magically vanishes when I keep it at only 55%. Go figure.


Re: Condensation problem inside storm windows

Were it me, the first thing I'd do is pull out the vinyl on the sills. Why? Because you're probably going to find rot. When you see a 'fix' like this, it's usually a cover-up for something that needed repairs. Rarely is it a forethought done by someone conscientious enough to mitigate a problem before damage occurs. Second, I'd ditch the storms and put in decent replacement windows after repairing any rotten sills. If the budget doesn't allow you to do them all at once, go at it in stages doing all the fronts, then one side, and so on. Or you can first do the ones that need it now, ie southern-facing in the summer, northern-facing in the winter. As long as you do a whole side at once it won't look too bad from the street. The only time I don't recommend replacement windows for double-hung wood is if there is a historic value that matters more. There's no point in patching someone else's patches, just do it right and be finished with it.

Re: Condensation problem inside storm windows

People that keep recommending new windows must be made of money. We have 42 windows on out 3200 sqft home. I figure we'd be in it for $25k + any damage to the window frame and interior plaster wall or exterior stucco.

I won;t recover $25k in energy bills in my lifetime.

Consider replacing the old storm windows with newer ones. Larson **** storm windows will run you around $130 each depending on size.

As for moisture. What are you humidity levels? what is the outside temperature. Even replacement windows will have condensation and frost. Worse yet, even if your windows don't show moisture, you might have moisture forming inside you wall if you had replacement windows.

On our last house, the mid range vinyl replacemnt windows we installed had moisture on them at 40%RH and outside temps under 20F.

In our new house, I expect moisture will form a lot earlier and e'll have to live with a dryer house. Also, it's sealed fairly well and may not need a lot of supplemtal humidification. Right now it's 95F with a 80F dew point (55%RH). Inside its 77F with a 55F dewpoint (38% RH). I'd say it's sealed up pretty well given the conditions. Plus my AC isn't oversized and runs nice and long in hot weather.

Re: Condensation problem inside storm windows

Thanks Nestor and others for this very helpful advice and food for thought. I think we have a two-fold problem: I do see a smaller gap on the sides of the storm window frames (which are essentially permanently installed on our aluminum sided house) so either no caulking was applied when they were installed, or it has long since dried up. I also know we have higher humidity in our small upstairs (2 BRs plus bath) because we do not have a fan in our bathroom and our steam heat doesn't dry the house out that much. We live in the Boston area where winters are fairly chilly outside too for a good contrast of indoor and outdoor weather. I may try a multi-pronged approach: caulk or foam fill at the storm frames (I'm happy to get more advice about products for consideration if others have opinions), apply some weather stripping on wood windows (not sure what is best for that either), remove caulking at the base of the storm at the sill to ensure weep hole exits, and put in the rope caulking inside our windows at the start of the fall rather than later. If this fails, we'll consider replacement windows down the road or in stages (we have 5 BR windows and 1 bath window). Advice on replacements for 8 over 8 and 6 over 6 paned windows would be welcome as well. Thanks all! Your advice and time giving it is MUCH appreciated.
--hlhood (AKA damp and chilly in Mass.)

Re: Condensation problem inside storm windows

Not having an exhaust vent for the bathroom is a big problem. :eek: Install one large enough to do the job and you'll be helping yourself a lot. :)

Re: Condensation problem inside storm windows

You only want to caulk the storms at the top and sides, leaving the bottom unsealed for drainage.


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