Help for dry­ing up your wet win­ter win­dows

The Labradorian - - Puzzles - Steve Maxwell House Works by Canada’s Hand­i­est Man’ Steve Maxwell fea­tures DIY tips, how-to videos and tool prod­uct re­views.

Cold weather al­ways brings the same puz­zling ques­tions to many Cana­di­ans from coast to coast. “Why is there con­den­sa­tion on the in­side of my win­dows, and what can I do to make it go away?”

Wet win­dows are one of the more con­fus­ing home man­age­ment is­sues be­cause the cause isn’t im­me­di­ately ob­vi­ous, nor is the so­lu­tion. Wet win­dows not only lead to ugly mould growth, but they’re ac­tu­ally an in­di­ca­tion of poor in­door air qual­ity, too. It’s some­thing you need to deal with.

Win­dows form con­den­sa­tion when in­door air near the win­dow glass cools to the point where it can’t hold all of its mois­ture any more. The abil­ity of air to hold wa­ter is rel­a­tive to tem­per­a­ture (that’s why they call air­borne mois­ture lev­els “rel­a­tive” hu­mid­ity). When in­door air in your home cools as it comes close to cold win­dow glass dur­ing win­ter, the abil­ity of that air to hold mois­ture de­creases. If cool­ing hap­pens enough, rel­a­tive hu­mid­ity of the air right next to the glass rises to 100 per cent and wa­ter droplets form and grow on the glass as the air loses its grip on some of its mois­ture. When droplets of con­densed wa­ter get big enough to run down the glass, wa­ter pools on the win­dow sill and causes a mess.

While it’s true that the ul­ti­mate so­lu­tion to the wet win­dow prob­lem is to sim­ply lower the hu­mid­ity in your home, that’s eas­ier said than done in win­ter. A drier home also brings health draw­backs for some peo­ple, too.

So if higher-than-ideal hu­mid­ity lev­els are re­spon­si­ble for wet win­dows, is a de­hu­mid­i­fier the so­lu­tion? No. Be­sides mak­ing noise and us­ing a fair amount of elec­tric­ity, de­hu­mid­i­fiers can’t re­duce in­door hu­mid­ity lev­els enough to solve the wet-win­ter-win­dow prob­lem. They also don’t freshen the air, they just take out some of the mois­ture.

In­creas­ing house­hold ven­ti­la­tion is the best way to re­duce win­dow con­den­sa­tion dur­ing win­ter be­cause out­side air gets quite dry as it comes in­side and warms up. This is why the leaky old houses that used to be so com­mon years ago never had run­ning win­dow con­den­sa­tion. Nat­u­ral ven­ti­la­tion kept in­door hu­mid­ity lev­els low au­to­mat­i­cally.

The chal­lenge for us mod­ern peo­ple is that we want clear win­dows and fresh in­door air, but we also want to re­tain the heat en­ergy we in­vested in the air in our house. Al­low­ing us to have our cake and eat it too is why heat re­cov­ery ven­ti­la­tors were in­vented back in the 1970s. This Cana­dian in­ven­tion uses fans to send stale air out of the house and bring fresh air in, all while re­tain­ing most of the heat en­ergy from the out­go­ing air stream.

There isn’t a win­dow con­den­sa­tion prob­lem any­where in the coun­try that can’t be solved by the in­stal­la­tion of a prop­erly func­tion­ing HRV. The chal­lenge is cost. You’ll pay about $2,000 to have an HRV in­stalled in your home, but dry­ing out win­dows might not come to that. Some peo­ple have suc­cess run­ning bath­room and kitchen ex­haust fans more of­ten than usual. This does re­sult in more heat loss from the house, but it can also solve the wet win­dow prob­lem with­out the need for an HRV.

One of the chal­lenges in all this is find­ing a level of ven­ti­la­tion and in­door hu­mid­ity that’s dry enough to keep win­dow con­den­sa­tion at non-dam­ag­ing lev­els while also be­ing hu­mid enough for com­fort. Un­for­tu­nately, most win­dows to­day re­quire drier-than-com­fort­able hu­mid­ity lev­els to re­main dam­age-free dur­ing the cold­est weather. My rule of thumb is to ven­ti­late enough to pre­vent run­ning con­den­sa­tion, but not nec­es­sar­ily to keep win­dows 100 per cent dry. This is enough to keep win­dows in good shape and your in­door air qual­ity fresh and health­ful.


Wet win­ter­time win­dows like these in­di­cate in­door hu­mid­ity lev­els are too high, and in­door air qual­ity isn’t what it should be. In­creas­ing ven­ti­la­tion solves both these prob­lems.

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