Window Sweat: What Wet Windows Are Trying to Tell You

Sloppy, wet, sweaty windows .. they’re a common complaint during cold winter months.

Condensation on the inside of your windows during the winter may indicate that you should look at upgrading your windows to function withe LEED efficiency. More likely, however, water on the inside of your windows is a symptom of a completely different problem with your home.


What Causes Window Sweat?

The physics of window sweat are simple. In the winter, the inside surfaces of even good quality windows are likely the coldest surfaces in your home. The air inside your home will naturally form a convection current cycle (visible, at right) against these cold surfaces.

As colder air sinks, warm air replaces it.

As warm, moist air comes into contact with the colder interior glass surface, the air drops below dew point, depositing moisture on the glass.

As the convection current continues over time, more and more moisture is deposited on the glass until your window sills become a sweaty mess.


Window Sweat: Condensation Resistance

Sample NFRC Window Label

The National Fenestration Rating Council (NFRC) has developed a standard window performance decal that you will find on any new window assembly sold in the United States.

When it comes to window sweat, the number to pay attention to on the NFRC sticker is the Condensation Resistance Factor.

The NFRC condensation resistance number is expressed on a scale of 1 to 100. The higher the number, the greater the resistance to condensation and sweating. If you live in a cold climate and are planning to replace your window(s), pay attention to this number.


Window Sweat: The Air Quality Story

Excessive moisture forming on interior window glass may be pointing to an issue with the ventilation of your home. In fact, most homes with relatively modern window assemblies that are showing interior window condensation in the winter have more of an issues with the air quality in the home home than the quality of the windows.

More and more, homes are being built using tighter, more efficient construction techniques. A tighter building envelope is great news when it comes to energy efficiency, but only if indoor air quality can be maintained through adequate ventilation.

In the old days, enough energy robbing cracks and poorly sealed openings provided unintentional ventilation to homes. As we’ve gotten better at sealing our homes, we’ve created another problem associated with bad air unless specific measures are taken to ensure adequate ventilation.

Moisture from showers, respiration, and cooking tends to build up in our homes during the winter, causing window condensation, mold and mildew growth, and possibly even rot.

Not only will moisture build up in a home with inadequate ventilation, but household cleaning chemicals, off-gassed chemicals from plastics and other synthetic items, allergens, mold spores, dust, radon gas, and a lot of other nasties can also create unhealthy indoor air.


Window Sweat: The Home Ventilation Solution

If you have problems with window sweat, your windows may be at fault, but maybe not. The only way to be certain is to have an independent professional energy auditor evaluate your windows along with the air exchange and air leakage of your home.

Depending on the tightness of your home envelope, an auditor may recommend installing an energy exchanging mechanical ventilation system such as a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV). In some cases, a simple whole house fan and vents will provide adequate fresh, healthy indoor air.

A few years ago, I installed a Heat Recovery Ventilator in my own home. A years-long issue with sweating windows in two upstairs bedrooms and a bath vanished within a day of turning on the system.


Original content from Green Building Elements, by David Arthur. David is a LEED-AP, Green Building Consultant, and rabid fan of all things sustainable. He is the editor of

Leave a Reply