In climates where temperatures dip in the winter, air infiltration can be a major cause of heat loss and a contributor to high energy bills. Complaints of drafty windows can diminish your customer’s overall satisfaction with their new home or remodel project. Different types of windows, even windows made by the same manufacturer, perform differently when it comes to preventing air infiltration. Do you know which operable window is the most efficient?

It’s not just about the glass
Glass coatings on modern windows are excellent at managing heat loss and heat gain, but neither U-Factor nor Solar Heat Gain Coefficient Ratings have any impact on how well a window will prevent air movement. When it comes to air infiltration, the real difference between one window and another isn’t the glass, but rather a window’s components and how well it’s assembled.

Which types of windows perform best?
Fixed windows have no moving parts and are the tightest windows available, but a non-operable window isn’t always an option and you may be choosing between three popular operable window styles.

Casement Windows
Casement windows are better at stopping air infiltration than any other type of operating window because of the tight seal that can be achieved around the entire sash. Jeff Siverhus, product manager at Marvin, explains, “The locking mechanisms on Marvin casement windows are designed to pull the sash in tight, which creates a consistent compression of the weather stripping around the entire perimeter of the sash.”

What if your customer demands the efficiency of a casement window but wants the look of a double hung? Add a simulated check rail (a thick single horizontal grille or divided lite) to a casement window, which will give it the appearance of a double hung from the street.

Double Hung Windows
“Though they can still be very energy efficient, double hung windows are less airtight because they do not have the same locking mechanism that sucks the entire sash in tight like a casement window does.” says Siverhus. “Also, compared to casements, there are simply more seams and linear feet of weather-stripping that are susceptible to air infiltration.”

If ventilation from the top sash isn’t needed, consider a single hung window to reduce the opportunity for air to infiltrate the perimeter.

Slider/Glider Windows
Sliding windows, often referred to as gliders, are also considered “hung” windows with similar air infiltration properties as double hungs (in the case of Marvin, careful engineering means gliders can perform as well as double hungs). It’s important to note, however, that because they aren’t equipped with a check rail engagement or sill interlock, they can be more susceptible to air infiltration.

According to Siverhus, one way to improve the air infiltration performance of a slider or glider window is to choose a configuration with one fixed sash.

What to Know About Air Infiltration Ratings
Air infiltration is rated by the industry standard ASTM E283 air infiltration test, which measures how many cubic feet of air passes through a window per minute in relation to the size of the window. Looking to meet ENERGY STAR minimum requirements? Look for numbers lower than .30 cfm/ft2. The difference in ratings between operable windows will vary by manufacturer, but to add context, Siverhus notes that for Marvin, all three of the window styles mentioned above are well below minimum requirements for air infiltration, with the Ultimate Casement Window coming in at less than 0.01 cfm/ft2.

If you want to make an apples-to-apples comparison, ask your window dealer for the air infiltration ratings on the windows you’re considering buying for your next project.

Keep in Mind - Windows are only part of the solution
Even the highest quality windows will have very little positive impact on comfort levels in a house that has no weather resistive barrier, poorly insulated walls, and that is riddled with attic air bypasses. Avoid disappointing your customers by explaining that windows are only one component of a host of efforts and improvements that will contribute to the overall energy efficiency of a home.

A Marvin dealer can help evaluate various window options to determine which will perform best given the individual needs of your next project.