What a Fogged Double Pane Window Is Telling You

Stand outside on a humid morning and look at the glass from an angle, then do the same from inside, because only one side tells the truth. The breakdown sitting on austin replacement windows heather fortner lists the signs in order for windows in Austin, TX, and the first one is always moisture trapped between the panes, which cannot be wiped off from either side.

What fog between the panes means

A double pane unit is two sheets of glass with a spacer bar around the edge and a sealant that locks the air space shut. Fog or a milky film between the sheets means that seal has failed and outside air is getting in. Once moisture is inside, the desiccant in the spacer is saturated and cannot be reset. The unit does not heal itself, and the haze usually gets worse every season.

Why heat cycling does it

Seal failure follows movement. In Austin a west wall can go from a cool night to a surface too hot to touch in one afternoon, and the glass, spacer and frame all expand at different rates. A vinyl window moves more than a fiberglass window, so the sealant takes more stress in vinyl. After many summers of that, the gasket at one corner lets go. Sun exposure on a south wall or in the open stretches near Zilker Park speeds the same wear.

The argon fill leaves with the seal

A good unit is filled with argon, which slows heat moving between the panes. Once the seal fails, the argon leaks out and room air replaces it. The window then performs worse on its u factor even if the fog is faint. A low e coating on the inside surface can also be attacked by the moisture, leaving stains that do not clean off.

Replace the glass or the whole window

A fogged unit in a frame that is still square, sealed and free of rot can often take a new insulated glass unit alone, without disturbing the trim. That works well on a wood window or a clad window with original sashes that still run smoothly. If the frame is warped, the balance is failing, or several windows on the house are fogged in the same year, a full replacement window is the cleaner repair, because the next failure is already built in.

What to check before deciding

Look at the frame corners for gaps, press on the sill for soft wood, and try the sash on its track. Look for fog on several windows, not one. Houses near the Balcones Fault, where clay soils swell and shrink and shift the framing slightly, sometimes fail first at the windows that rack. A fogged double hung that still opens easily is a glass repair. A fogged one that sticks is a replacement.