Getting a clear view on sweaty thermal windows

  • Sat Feb 23rd, 2019 1:30am
  • Life

Nailing It Down

By Dave Murnen and Pat Beaty

When you start counting the baths, cooking, indoor plants, wet laundry, the dog’s water dish, the rain-soaked dog, wet jackets and hats, and on and on, you’ll quickly get the idea that there is a mini climate in your house — and it needs to be dealt with, or you’ll get a visit from the mold fairy!

Your heated house is constantly changing these sources of water into airborne vapor and will allow them to change back into water where a cold surface meets the warm, moisture-laden air, like a toilet tank or windowpane.

The trick is to control the “incoming” sources of this moisture and balance the rest with vented fans and other types of ventilation. When you do this, your comfort goes up and the excessive moisture on the thermal windows goes away.

Humidity in the house should be 45 to 55 percent. This creates comfort and livability year-round. In excess, it is bad for the house.


One of the questions we are often asked this time of year is: “Why do I have moisture inside my thermal-pane windows?”

If the question refers to moisture between the panes of glass, where you can’t wipe it away, then the window’s thermal air seal has been compromised and the glass portion of the window unit needs to be replaced. Your local glass company will be your best adviser and most direct link to replacing the unit under any applicable warranty.

But, if there are beads of moisture on the inner side of the glass and you have been wiping it up, then you most likely have a high humidity problem to solve. Here are some clues to help you hunt down the sources.

First, ask yourself if all the windows in the house have beads of moisture on them, or if it’s just a few specific ones and only at specific times.

If only a few windows are affected, note where they are located. Are they anywhere near a source of moisture, such as the bathroom, kitchen or laundry room?

If yes, then ask yourself when the moisture shows up. For instance, is it after someone takes a hot shower, or after you have cooked a steamy meal on the range? If either of these scenarios shows any promise, you should check out the use and function of your range hood and the bathroom fan.

To properly remove moisture from the house, exhaust fans need to be vented to the exterior of the house through proper piping and vents with flappers. Are they there? Are they being used? Are they being used long enough after cooking or bathing to get rid of all the moist air?

Experiment a little at a time by leaving the bath fan on for 15 minutes after the first shower is over. Did it help?


If you still have a problem, check your clothes dryer’s hose and connection to the vent system for leaks. Do you see lots of lint in the room?

When properly hooked up, a dryer will vent all its moist air to the exterior of the house. Turn on the dryer and feel all the pipe connections for air leaks. Are all the hose clamps tight? Any pinhole leaks or tears in the pipe? Is the flapper vent outside the house clear of lint and moving freely? Is there a kink or sharp turn in the hose? If the system is plugged with lint, it’s also a fire hazard.

What if your dryer hose goes under the house? Follow it. If it ends anywhere but outside the house or foundation, it is likely the source of hot, moist air — which will invite bugs and cause expensive damage.

A proper system would consist of smooth pipe that has been screwed together and has all the joints heat-taped. The pipe will have a slight slope to drain to the outside, preferably wrapped with insulation and supported with straps every couple of feet. The pipe will be connected to a flapper vent after passing through the foundation wall. The flapper needs regular cleaning to open and close freely, keeping cold air and critters out.


One thing to remember is that each of us, including the dog, can put out something like a quart (more or less) of moisture per night. Several people sleeping in a closed-up room means lots of moisture. Opening a window will help reduce moisture in the air.

A much more dangerous source of moisture is a gas appliance, such as a gas furnace or hot water tank that is improperly vented or has a blocked or damaged vent. Besides exhausting plenty of moisture, these appliances give off deadly carbon monoxide gas.

Another clue can be headaches or nausea when at home, but feeling normal when away. Your local gas company field man will be your best expert, so call now!

It is recommended that every house has a carbon monoxide detector. They’re relatively inexpensive and definitely worth the money for you and your loved ones’ health.

Dave Murnen and Pat Beaty are construction specialists at NeighborWorks of Grays Harbor County, where Murnen is executive director. This is a nonprofit organization committed to creating safe and affordable housing for all residents of Grays Harbor County. For questions about home repair, renting, remodeling or buying, call 360-533-7828 or visit 710 E. Market St. in Aberdeen. Our office is fully ADA-compliant.