How To Fix Window Air Leakage and Heat Loss

How To Fix Window Air Leakage and Heat Loss

Brennan Windows is here with another installment of your window and door shopping consumer guide. Today, we focus on factors that affect heat and air leakage.

If you find you constantly have air leakage or want to find ways to reduce your energy bill, continue to read this blog to see if these factors are causing you to lose heat in the winter or have air leak in or out of your home.

Why You Lose Heat

There are several influences that add up to losing heat through window components, all of which follow one basic law of nature: heat energy moves from warmer areas to colder areas. There's no way around this natural principal, but there are ways to slow the process down. The principal heat transfer process in windows are radiation (not the bad stuff), conduction and convection. Air leakage is also a huge factor in heat loss as well.

Radiation, Conduction and Convection

Heat often moves to the cooler outside pane and is released outdoors as it is absorbed by the inside pane of a double-glazed window. Heat loss can take place through the glazing by means of radiation; across the spacer material (the space which separates the two glazing layers) at their edges and through the frame of the window by conduction; through the movement of air in the space between the two glazings (by convection); and between the moveable or operable frame components (by air leakage) (Fig. 25).

Heat loss through radiation represents nearly two-thirds of all total heat loss from a standard window. Ordinary glass readily transfers heat to colder surfaces (ie. has a high emissivity), you can reduce you radiation loss by lowering the emissivity of the glass (this is where the term low-e, low emissivit, comes from). Heat loss through conduction occurs primarily through the edges and frame of the window's units. Recent technology has improved window material construction and designs that more effectively use insulating materials, dramatically reducing conduction heat loss, so replacing windows in your home for newer ones should automatically reduce your conduction heat loss. Convection loss occurs beacuse of the air movement between the space of multi-glazed windows. If the space is too small, heat loss by conduction in the air is significant; if the air space is too large, too much still air will begin to rise from heat on the warm interior side and fal as it is cooled on the cold exterior side of the window, causing more heat loss to the exterior cold side. The ideal space between glazings that minimizes convection heat loss is half an inch to two-thirds an inch. Some windows also use other gases, such as argon or krypton, in the space to reduce convection heat loss. If your window does use other gases, optimum spacing are likely to be different.

Air Leakage

Air leakage is a significant contributor to high energy bills year round. Most air leakage from operable (windows that open) windows occur between the window's sash and frame, or the meeting rails of a sliding sash (Fig. 26). Bigger windows tend to leak less air per unit air. Air leakage also may occur in poorly constructed fixed windows between the insulated glass unit and the frame. Even though fixed windows are completely sealed, holes are still required to effectively drain rainwater, giving them room to potential air leakage. Windows with the lowest leakage rates, regardless of type, tend to be fixed windows though. Operable windows come in many types, but awningcasement or similar types of windows (with a closure mechanism that pulls the sash against a compression gasket) typically hae the least rates of air leakage. Air leakage can be a big problem if windows are poorly or carelessly installed in the rough opening. If the space between the outside perimeter of the window frame and the rough opening isn't sealed with either caulking or foam insulation, air will leak through it, so be sure (especially if you do it yourself) to insulate and seal the space before attaching the window trim. Or use a reliable and professional window installer like Brennan Windows to have ease of mind that it is properly installed.


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