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Concentration of CO2 in the Atmosphere

The Window In Winter: A Cool Little Microclimate

Side-by-side office windows. Window on right has thermal shade drawn, window on left is uncovered. Photo: Zone 6 Energy.

Nate Gusakov

“We really need new windows. There’s always a draft coming through them!” I hear this a lot from clients who are looking to improve the comfort and efficiency of their home. In certain cases, this is probably true (e.g. old single-pane ‘barn sash’ windows or malfunctioning windows that won’t shut and lock anymore). Of course, it is always important to be reaching for more efficient comfort; however, I rarely end up recommending new windows as a top priority.

Even a modern, Energy Star-approved window won’t have an insulating value any better than about R-5. While R-5 is good for a transparent window (and better than the R-2 to R-3 of older double-pane windows), it still falls short of the R-25+ insulation that will likely be found in the walls of a modern house. This means that whenever the outside temperatures are cold, these windows are radiating exponentially more heat to the outdoors than the walls around them. In addition (and more important, for this particular article), the surface temperature of the inside pane of glass is going to be much colder than the surface temperature of the wall around it.

The result is that this rectangle of very cold glass acts as a little refrigerator for the air that is directly next to it. This air loses its heat through the cold glass, becomes denser than the air around it, and sinks. More warm air comes from above to replace it and is subsequently cooled by the glass chilling plate (or window pane, as we like to call them) and also sinks. Voila, we have a convection loop! When you sit down on your couch on a cold winter night just below the living room windows and feel an annoying cool draft on the back of your neck, it’s probably not a leaky window—it’s probably the convection loop of cooled air from the window’s microclimate washing over you. So, what’s to be done about this? Ultra-high efficiency triple-paned windows certainly can help, as the innermost pane is a good deal warmer than the second pane (which would now be in the middle of an argon gas sandwich). These windows are expensive, however, and replacing windows is a significant undertaking.

Thermal blinds are a great answer– they usually cost $75-$150 per window. If diligently pulled at night and on cold dark days, they will serve to block the convection loop from developing, presenting a much warmer surface area to the inside of the house.

Pros:

  • Aesthetically pleasing (design choices available)
  • Retain access to windows
  • Can be left installed year-round and offer light and privacy control

Cons:

  • A little expensive
  • Often forgotten
  • Still allow interior humidity to reach cold glass window pane (more on this in the next issue)

Another answer is to install the thin plastic film that you tighten with a hair dryer. If you do it carefully, it makes an excellent ‘third window pane’ on the inside of your window. The thin plastic won’t radiate heat the way a glass pane does and will again present a much warmer surface area to the inside of the house.

Pros:

  • Very inexpensive
  • Still lets light in
  • You don’t have to remember to pull the blinds
  • If sealed well, prevents humidity from reaching cold glass window pane (more on this next issue)

Cons:

  • Can be punctured easily
  • Hard to seal on certain window types
  • No access to the window through the winter

If you think you need new windows but just can’t afford them or aren’t sure yet, try one of these methods this winter. You just might find that the drafty old windows are suddenly performing much better than you thought they could.

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

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