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Improving Window Performance

Due to marketing, we tend to believe replacement windows are our best option for dealing with thermal transmittance and solar heat gain.

Better windows certainly help, but significant and cost-effective solutions are in our control by the way we handle any window at our house.

When windows are replaced, selection criteria depends on a number of factors

Customer  Performance Targets for Windows

Prepare House for Improving Windows

Before improving windows, control moisture to avoid condensation.

Manage Indoor Moisture Levels.  Air leakage around windows is important, but is taken care of with air sealing.  Windows are a great indicator of indoor moisture issues that must either be corrected or managed.

Methods. Condensation on windows is corrected by increasing their thermal resistance (R-value) or lowering indoor relative humidity.  Ventilation is used to manage controllable moisture. 

Manage Moisture at Source. High humidity must be dealt with at its sources.  It comes in through the building shell, ductwork, and migration through building materials.  Air sealing reduces air leakage. 

Vapor barriers prevent evaporation, such as on the crawlspace floor.  Vapor retarders and house wraps help too.  Peak performance from a cooling system acts as a dehumidifier while cooling the house.

Dehumidifiers. Dehumidifiers work, and may need to be part of the final solution, but should never be used as the primary means of controlling moisture because they consume a lot of energy!  When a dehumidifier fails, moisture issues return instantly.

Indoor Relative Humidity. Relative humidity inside the house should be maintained at 35-40% for comfort and never exceed 50% for a prolonged period of time to avoid mold growth. 

At this level, winter time temperatures need to get to 35 degrees Fahrenheit with single-pane windows for condensation to occur.  It won’t take much before it snows.  At this temperature, dual-pane windows require 65% indoor humidity before becoming a problem.  When temperatures outdoors are near freezing, the heat is probably on, which lowers indoor relative humidity in the process.

Improve Window Performance

Install Window Treatments and Windows.  Install window treatments and window according to manufacturer’s instructions, building codes, and best practices.  When windows are replaced, be sure to insulate and air seal the gap between the frame and rough opening.

Post Window Installation Inspection. Window replacements and upgrades should result in reduced air leakage.  As such post window installation testing should be performed, in part, to ensure combustion appliances are safe enough to live with.

For home performance contractors, final testing is mandatory when combustion appliances are present.  The concern is not so much as the windows as it is the reduction in air leakage. The concern is ensuring the areas fireplaces, water heaters, and furnaces are in are not depressurized to the point of causing smoke to backdraft into the house.

Where there are incentives or certificates offered, the sponsor (e.g., utility company) usually requires final testing too.  Their considerations are to verify the characteristics of the windows.

We are ready, willing, and able to perform third-party testing.

In Conclusion . . .

Windows form part of the thermal barrier for the house.  Short of replacing windows, there are a variety of ways to improve window performance that are useful, regardless of the window installed.

Adding a second window pane is the fastest way to cutting heat transfer through the window in half.  A 50% reduction is huge, and works everywhere.

Improving window performance have the side benefits of reducing fading due to heat, visible light, and UV light.

Here’s to a more comfortable and energy efficient house!

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