Concentration of CO2 in the Atmosphere

Windows in Winter: A Cool Little Microclimate, Part 2

Nate Gusakov

It’s a rough life being a window here in the Northeast U.S. You’re expected to let light in and out, heat in but not out, and, in addition, you have to morph from being a wide-open hole in the wall into transparent, blizzard-proof armor many times a year, and you’re usually supposed to look good while doing it! That’s a lot to ask. In the last issue I wrote about the convection loop that can develop near our window panes in the wintertime, carrying cold air down onto the couch and tricking us into thinking that the window is leaky. This column is about another (perhaps more troubling) issue with windows in winter: condensation on the bottom of the panes.

To understand the phenomenon, we can return to the basic principle of condensation: Condensation occurs when warmer, moist air meets a cooler condensing surface below its dewpoint. The inside environment of a house has many direct sources of moisture that are constantly dumping water vapor into the air: our breath, evaporation from showers and laundry, cooking, etc. As this warm moist air makes its way down the windowpane (there’s our old friend the convection loop again), it gets cooler and cooler until it just can’t hold that water vapor anymore and releases some of it on the nearest available condensing surface: the bottom of the pane.

The classic, curving-up-the-side shape of this condensation (see picture) has to do with the surface temperature of the different regions of the pane of glass. The edges of the pane are colder – they are closer to contact with the frame material which acts as a thermal bridge, conducting outside cold into the glass. Why then does the condensation only form at the bottoms of the panes and not all the way around the edge of each one? Now that’s a good question. I think the answer lies in the orientation. If the window were to be completely horizontal but still to the exterior (i.e. through the flat ceiling or floor, looking at the outside), I think it would condense equally around the edge of each pane. Because most windows are vertical, the good ol’ convection loop drives the process downward. In addition, gravity draws any condensation at the top of the pane down towards the warmer center of the pane where it likely re-evaporates before really becoming visible. Conversely, at the bottom of the window gravity is drawing any condensation farther down toward the colder edge and so the beads of water grow in size as they’re pulled downward. The real trouble begins when all this condensed moisture settles in at the bottom of the window as liquid water and then stays there consistently throughout the winter. If the window frames are wood, this water will first stain and then help rot the wood. Regardless of the frame material, this water will serve as a great little petri dish for mold.

So, what’s to be done? Proper household ventilation is a critical first step—help all that moisture get out of the house in a safe way instead of milling about and condensing on things! Even drying the air won’t help all the time, though. Regardless of ventilation, humans generally become uncomfortable as a house’s air gets too dry. Federal recommendations (ASHRAE) are to keep indoor humidity levels between 30%-60% for best health, and many folks become uncomfortably dry as levels approach 40%. I’ve noted significant condensation on our windowpanes (modern double-pane) even when a hygrometer right there on the windowsill was reading 30% RH! The next simplest answer is to warm up the surface of the window pane. Triple-paned windows do this by adding yet another insulating glass and gas sandwich layer between the inside pane and the outside air. Installing plastic window film (see last issue’s column) will perform a similar function for a fraction of the cost. There are even products on the market that plug into a wall outlet and stick onto the bottom of the window pane to warm up the glass with electrical resistance heat. At the very least, be sure to check for any staining or mold growth and clean it well with a fungicidal cleaner on a regular basis.

As building science continues to develop, and we continue to build tight houses that hold moisture better than the drafty old farmhouses of a few generations ago, new answers to the problem of window pane condensation will be developed. You got one?

Nate Gusakov is a BPI-Certified auditor, home performance contractor, and energy consultant for Zone 6 Energy in New Haven, VT.

Caption (Photo coming):

Condensation on a window. Ozgu Ozden, Unsplash (

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