All About Homes Residential Inspection Service

I own a four year old house with dual-pane windows. Last week when I was washing the windows I noticed that some moisture had gotten between the glass panels on a couple of the windows. I thought it was from using the hose, but when I went inside to check the glass panels they were absolutely dry. I also saw dry water stains on the sheet rock around the top and sides of the windows. Obviously, this has been going on for some time. Any ideas about where the moisture is coming in from? T. Butler, Castro Valley

Your question actually deals with two separate issues, the first being the moisture between the glass panels, and the second being the staining of the window jambs. Let’s discuss the window panels first.

The moisture that was between the glass panels was not from washing the windows. Dual pane windows are basically two pieces of glass that are separated by a thin metal strip at the edges and sealed together at the factory. Between the glass panels is a space that is filled with an inert gas or air. This space and the gas that fills it, is what gives the window its insulating qualities.

An inert gas is used between the glass panels because it does not have a combination of hydrogen and oxygen molecules that can mix and form condensation. Condensation between the panels causes staining and fogging that cannot be cleaned. So as long as the window seals stay intact, the windows serve their intended purpose and remain fog free.

Occasionally the seal around the glass panels breaks, the inert gas leaks out and air enters in. Water vapor can then form. This normally occurs during the early to mid-morning, and in the evening when the temperature outside is either warmer or colder than the temperature inside of the house. Water vapor will condense on the insides of the glass panels and run down the glass.

Once the temperature stabilizes, the moisture evaporates and the window looks fine. Over time, the window will begin to fog and stain between the glass panels, preventing the ability to see through it. This is usually when people begin to realize that their window has a bad seal, and needs to be replaced.

You may be fortunate because most window manufacturers offer a five to seven-year warranty on the seals between the glass panels. Contact your builder and try to find out who actually manufactured the windows and call their customer service department and ask what their policy is on replacing the windows. If for some reason the windows are not covered, you can call a local glass company and pay to have them replaced.

The water staining that you see around the jambs of your windows is usually the result of moisture penetrating the exterior siding and/or trim around the windows and getting behind the flashing seal. This normally occurs when the flashing has been improperly installed or has a tear in it. Let me explain.

Any opening in the exterior of a building is an invitation to water leakage and this includes doors and windows. Under the exterior siding is a paper or plastic type of membrane that prevents any moisture from getting through the siding and into the wood framing members. Window openings create a break in this membrane, and any breaks must be properly flashed and sealed next to the window frames for the building to remain water tight.

Windows are usually installed days and sometimes weeks before the siding goes on a building. The strips of flashing paper that are installed around the window and eventually “woven” into the waterproof membrane under the siding, often get torn or curled. This makes it difficult to get a watertight seal between the flashing paper and the waterproof membrane under the siding. Breaks in this membrane cause leakage into wall cavities and window jambs.

Most of the time this leakage can be prevented by keeping any separations between the window frame and the exterior siding or trim, well caulked. The caulking joints around window and doors should be inspected every year because of the effects of weathering and building movement. Caulking sometimes lasts for only two or three years before it has to be repaired or replaced.

If the leakage is more severe, it usually indicates that the flashing underneath the exterior siding will need to be repaired. This is a much bigger job and should be done by a professional.

John R. Schneider is a licensed general building contractor and a certified residential code specialist. He is president of All About Homes, a residential inspection company, and has been performing code and construction consultations since 1985. Readers may address their comments to John Schneider, 24326 Mission Blvd. Suite 7, Hayward, Ca 94544. Schneider will answer questions of general interest in the paper. He reserves the right to edit the letter for brevity and clarity. Readers are encouraged to contact a competent contractor or code consultant for specific information regarding questions they may have about their homes.