Window warranties: it's all in writing, but you have to read the fine print.

So much about selecting a good window comes down to the warranty," says Chris Mathis, of MC Squared, a building-science consulting company based in Asheville, N.C. Mathis focuses on windows, doors, and skylights, serves on the ASHRAE and ASTM code committees as well as the International Energy

Conservation Code Committee of the ICC, and was a founding board member and the former director of the National Fenestration Rating Council. He insists that regardless of window materials and features, the warranty should be one of the first window-selection criteria to consider. "If a window fails in a few years, it doesn't matter how good or strong or energy efficient it started out to be," Mathis says. "About 30% of the replacement windows installed are replacing windows that are only about seven years old. This speaks to how poor many of the units out there are."

The most common window failure is the seal on an insulated glass unit. Once the seal blows, the window fogs up between the panes. This can cause a drastic reduction in window R-value and may oxidize the low-E coating, turning it into a high-E coating that absorbs heat rather than reflecting it. In the worst cases, the pane actually deforms, bowing inward, as a gas fill, such as argon, escapes.

Mathis recommends choosing a window with a minimum 20-year warranty on the insulated glass, which most of the big window companies offer. "There's a reason some of the big companies have been in business a long time. They've been in it long enough to figure out what works," Mathis says.

Although the bigger window companies have a known presence, he remarks, they only have 10% to 12% of the market. "There are so many window makers out there," he says, underscoring that it doesn't take much to get into the window manufacturing business and capture local markets with cut-rate pricing. But these are the companies, Mathis cautions, that may not be around in three or five or seven years when the glass fails.

WARRANTY WORDING

Mathis recommends staying up late at night if that's what it takes to read the fine print on any window warranty. Many warranties cover a lot up front, but that coverage quickly declines when all the details are spelled out.

Some warranty types and language to look for include:

* "Non-prorated" warranties, which will cover the entire purchase price of the window for the term of the warranty. Remodelers are familiar with prorated warranties on roofs. But a well-made window shouldn't gradually degrade with exposure as rapidly as roofing materials do, so there is little justification for such a warranty structure.