Help for dry­ing up your wet win­ter win­dows

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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. e 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 de­hu­midi er the so­lu­tion? No. Be­sides mak­ing noise and us­ing a fair amount of elec­tric­ity, de­hu­midi ers can’t re­duce in­door hu­mid­ity lev­els enough to solve the wet-win­ter-win­dow prob­lem. ey 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. is 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.

e 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 ven­tila- tors were in­vented back in the 1970s. is 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.

ere isn’t a win­dow con­den­sa­tion prob­lem any­where in the coun­try that can’t be solved by the in­stal­la­tion of a prop­erly func­tion­ing HRV. e 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. is 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 nd­ing 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. is 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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