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Winter is the time we rely on our windows to keep the cold air out and the warm air in. Sadly (and coldly) this doesn’t always happen. When cold weather comes, thousands of you will be asking, “Why are my windows cold and drafty? There can be many reasons for air infiltration leaks in windows.

1. Window thermal transmission (R value or U value)

A decent window with Low-E insulating glass (U=.33) will still allow about 4 times the energy loss compared with the insulated wall next to it. (R=12 or U=.08). To improve window thermal conductivity change the glass or add a Low-E film to the glass.

To reduce the feeling of having cold & drafty windows, place the heat source near the bottom of the window to allow the heat to rise along the face of the window. For much more about window thermal conductivity, go here: Best Windows for Energy and Comfort

2. Window caulking

Take a look at the caulk around the outside your windows. If there are openings, even small ones in the caulk, you may have window air leaks that will be causing loss of heat and energy. Seal any gaps in caulk with silicone sealant. If the caulk is really bad, have it cut out and replaced by a skilled caulker. For more about window caulk and failures, go here: Window Sealant Failure

Failing window perimeter caulk is coming away from the brick mortar, allowing window air leaks and water into the building
Failing window perimeter caulk is coming away from the brick mortar, allowing window air leaks and water into the building

If you feel air infiltration coming around the window frame on the interior side, you are probably feeling what is known as “cavity air” (air inside the wall). To fix this type of window air infiltration, caulk the interior perimeter window with paint-able latex paint and put gaskets behind all cover plates on electrical outlets and switches.

Gap in window between sash and frame will allow window air infiltration leaks into the building
Gap in window between sash and frame will allow window air infiltration leaks into the building and cost you money in unnecessary energy costs

3. Window adjustment

If a window is not square and level in the opening, gaps can occur between operable parts (sash) and fixed parts (frame). These gaps can allow excessive air infiltration and make your windows cold and drafty. If you see uneven gaps around sash, you might need to have the window reinstalled level and square, which might also involve recaulking.

If, even after squaring the windows, drafty gaps still exist between sash and frame, you may need to replace the weatherstripping with a larger version. I begin to explore this subject in #9 below, Window weatherstripping and gaskets.

Window glazing leaks bring too much air infiltration into the building while also allowing water which can damage the window and glass.
Window glazing leaks bring too much air infiltration into the building while also allowing water. The water leaves stains but the air infiltration is usually invisible

4. Window glazing leaks
Sometimes the gap between the glass and the surrounding frame or sash is open enough to allow air leaks. Big benefits will come from sealing this gap at all four sides. This caulk joint, called a “cap bead” will not only reduce air infiltration, but will also prolong the life of your windows by keeping water out, reducing water damage and extending the life of your insulated glass seals. For more about insulated glass failure, go here: Insulated Glass Seal Failure

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The wood-clad window industry uses the term “accessories” for the various strips of metal that are used to transition between adjacent windows (mullion connectors) and also around the edges of the windows (trim extenders). These accessories usually have a little returned edge that snaps into a groove in the window – the accessory groove. When these accessories are snapped in to their groove, they look neat but beware, the tight fit does not repel water or air infiltration.

In my experience, wood-clad window manufacturers and installers often fail in the task of sealing the accessories to the window. This is especially true at the “mullion connector”, which is a U-shaped channel that connects adjacent windows to one another. To fix this source of air infiltration, seal all edges of vertical and horizontal mullion connectors to the window. It is not easy to do this neatly, so use a skilled caulker and the best silicone caulk.

Mullion connector between windows is often loose and unsealed, allowing air infiltration leaks
Mullion connector between windows is often loose and unsealed, allowing cold & drafty air and destructive water through the windows

6. Unsealed wall penetrations

Unsealed light fixture can bring cold air infiltration into the building
Unsealed light fixture can bring cold, drafty air infiltration through the windows

Unsealed light fixtures, electrical outlets, exhaust vents and pipe penetrations on the outside of the building can bring cold air and even water into the wall. This cold air then flows through the wall and finds the easiest path into the building, like an unsealed interior seam around the windows. That air will be a draft you feel when sitting in your rocking chair next to the windows, spoiling the prosaic scene. Why do you think granny had a blanket on her legs? Yes, it’s true. It is because of window air infiltration.

On aluminum windows cold air in the wall cavity can also chill the frame and increase chances for condensation. To fix excessive air infiltration in the wall cavity which can easily come through the windows, have a skilled caulker inspect and seal all penetrations in the exterior walls.

7. Window and wall mounted air conditioners

In an elaborate study, by NREL, National Renewable Energy Laboratory it was found that although air infiltration coming through the air conditioner itself was “fairly modest, roughly 1–2 cfm under normal operation. Infiltration resulting from installation in a window, following manufacturer instructions, was significant. The measured installation leakage was equivalent to a 27–42 square inch hole in the wall.”

To fix that crazy amount of air infiltration, use caulk between mounting flanges of the air conditioner and the adjacent part of the window or wall. If a double hung window sash is raised in order to receive the air conditioner, take extra steps to seal the top edge of the lower sash to the face of the upper sash, first with an expandable foam followed by the best duct tape available. Seal nooks and crannies with silicone caulk.

8. Interior storm windows and films

Commercial interior storm window for condensation reduction
A good commercial interior storm window can be highly effective for reducing or eliminating condensation

A very good answer to window air infiltration leaks as well as poor thermal conductivity is to add a new window to the interior of the existing. Sometimes called an “interior storm window”, these products will find increasing acceptance in larger commercial and institutional buildings where a permanent, engineered solution is desirable. Any type of window can be fitted with an interior secondary window and older curtainwalls especially benefit. To learn more about this exciting option, see this article: Building Energy Benchmarking and Window/Curtainwall Retrofit

9. Window weatherstripping and gaskets

Now we have arrived at what I really want to talk about. Window weatherstripping and gaskets are the strips between operable and fixed parts of the window. When the sash is closed, these are intended to be compressed, blocking the flow of air leaks through the building. When these are worn or missing, you can lose far more heat, and feel far colder near the window, than from any other cause. Attacking this problem takes some skill and knowledge, but the benefits can be huge.

Windows that slide like double hung windows, sliding windows and sliding doors use fuzzy weatherstrips that allow the sliding motion while maintaining a seal. These are called “sweep weatherstrips” or “pile weatherstrips”. Windows that compress their weatherstripping, such as casements, awnings and hopper windows use compression weatherstrips, also called gaskets. These window gaskets are usually compressible bulbs of various shapes.

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Types of window weatherstrips, sweep-type and bulb-type
Sweep weatherstrips, used on hung and sliding windows and bulb weatherstrips, used on casement, hopper and awning. Tip left image shows a plastic fin embedded in the pile for a better seal.

Read an actual case study where we replaced weatherstripping in an apartment building in Arlington Heights, IL: Replace Weatherstripping on Drafty Windows

Resources – Detecting Air Leaks

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