Frost is in the air!

With winter coming some owners of historic places might be witnessing the formation of frost on their single pane wood windows and storms, most notably on the second storey. The reasons for this will vary and subsequently so will the solutions. Generally speaking, a little frost now and again should not harm the window frame but a more persistent formation will saturate the surrounding material and in the long run, potentially cause significant damage. Should this be the case, ignoring the situation is far from the recommended solution.

Frosty window. Note the closed storm window vent cover. During the winter it should be open.

So what is one to do?  The first step, as with any intervention on a historic place, is to develop an understanding of the problem.  What is causing frost to form on the window? The answer: warm moist air comes in contact with the cold glass and condenses, which then freezes on the surface. So how does one mitigate this? Should you let the house freeze so that the interior temperature matches the outside, or should you turn the house into an oven to eliminate any and all moisture in the air? Obviously, neither scenario is plausible.

So what is one to do – replace the windows?  Speaking from a heritage conservation perspective, replacing authentic historic windows would be the equivalent to someone shaving their head bald because they found a grey hair. Historic windows have, in my experience, proven themselves to be longer lasting than any modern window and can continue to serve their function with proper care and maintenance, while modern windows wear out and routinely need to be replaced.

As air moisture is the general cause of frosted windows, controlling it would appear to be the most appropriate approach. However, there is more than one way to control moisture during the winter. One can control moisture levels with a dehumidifier, or by preventing it from reaching the cold glass with weather stripping techniques. As well, one could better manage the presence of moisture with exhaust fans and insulation. Any combination of these efforts will help to reduce the frost on windows but it should also be noted that there are pre-existing systems that should be taken advantage of in regards to this problem.

Storm windows traditionally have three holes at their bases with a flap cover. During the winter, these should be open to let moisture and condensation escape. When bugs arrive with spring, close the flap. Double hung windows generally have locks where the sashes meet, use them to tighten the window and reduce air leakage.  Should you decide to proceed with a form of weather stripping, concentrate on the interior side of the window and allow the storm window to breathe. This will create a micro climate (similar to the ventilation in one’s roof), which should help prevent frost and therefore better sustain and protect the window.

Hopefully, the preceding information will be useful for those who might be dealing with frosty windows.  There are many articles on the Internet, which tackle this issue and propose similar, different and additional solutions ranging from “free to expensive”.  As a Heritage Conservation Adviser, my advice and that of the Standards and Guidelines for the Conservation of Historic Places in Canada, is to start with the minimal approach.  With any luck, that is all that will be needed.

Written by: Carlo Laforge, Heritage Conservation Adviser.

If you have any technical questions related to the conservation of your historic place, submit a comment in response to this post. We would be happy to prepare a response.

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