How to Locate Air Leakage in Windows
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How to Locate Air Leakage in Windows

Many older homes, and all too often modern homes, can have drafty windows and frames that will increase energy costs in both the summer and winter. Next to stopping water, preventing unconditioned air from entering the home is the second biggest way to improve indoor air comfort and prolong the life of your home and windows can be a major source of air leakage. This article will describe how to locate air leaks around windows.

Many older homes, and all too often modern homes, can have drafty windows and frames that will increase energy costs in both the summer and winter. Next to stopping water, preventing unconditioned air from entering the home is the second biggest way to improve indoor air comfort and prolong the life of your home and windows can be a major source of air leakage. Moisture laden air entering the home in summer months can lead to wood rot, peeling paint, and mold and mildew growth along with the associated increase in cooling costs. Cool, dry air entering in the winter can cause wood flooring and trim to shrink and crack and cause you heating system to work overtime.

Air leakage is noticeable during a windy day when air is forced into gaps and cracks in the building envelope and you hear the air blowing or feel the draft. This is sometimes referred to as “wind-effect”.  Wind effect occurs as air moves over the top of the building and the moving air increases the pressure on the windward side of the building.  As the air passes over the building it entrains air on the leeward side into the air stream, creating a negative pressure on the leeward side of the building.  This pressure difference between the windward and leeward sides of the building drives infiltration. 

Another type of air infiltration is due to stack-effect leaks. The stack-effect can create a greater energy loss than those due to wind. Stack-effect leaks are caused by warm air rising through the house in the same manner hot air rises through a chimney and out through openings in the exterior walls or attic space. Energy is wasted during the heating season as heated air is lost and cold air is brought in on the lower level of the home to replace it. Drafty second floor windows are a common source for stack-effect leaks.

Before you can stop the air flow through your home, you need to locate the source of the leaks.

Locating Air Leakage

Wind-effect leaks are easy to find on a windy day. You can purchase an inexpensive duster or smoke pen or a smoke generator and pass it along the window frames and sashes. The smoke or powder may be drawn into the window or blown away from the window depending which way the wind is blowing. A lit candle can also be used in the same manner and notice any flickering or fluttering of the flame as you slowly more it along the window perimeter. Use caution when working with an open flame. Mark any air leaks with painter’s tape or a pencil mark.  

Smoke generator

Stack-effect leaks are tougher to locate. The two main areas in which to concentrate your search are the foundation and the attic. The stack-effect can be reduced by eliminating air infiltration through second floor windows. Inspect the foundation from the inside and look for cracks in concrete or gaps in cinder blocks above ground level. Mark any defects with tape or a pencil so you can caulk or repoint the joints later. Also check for gaps between the foundation and the framing sill plate that sits on top of the block or concrete. Poorly fitting basement windows are another common source of problems.

Stack-effect in the winter

Gaps around window frames can be filled with fiberglass insulation, low-expansion foam, or caulk depending on the size. You may need to remove the trim around the window to completely seal the gap. Air leakage around window sashes can be corrected by installing new weatherstripping or replacing the existing weatherstripping with a similar type.

Air leakage around window can be made worse when high power exhaust fans are used in bathrooms or kitchens. If you have a large range hood or downdraft exhaust fan in your kitchen, you might want add an outside air intake to bring in some make-up air so you don’t depressurize your entire house.

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