Window Glass for Denver's Altitude and Sun

Most replacement window decisions come down to frame material and style. On the Front Range, the glass deserves just as much attention, for two reasons: altitude and sun.

The altitude problem

Most modern windows use insulating glass units: two or three panes sealed together with a gap of air or gas between them. Glass manufacturer Cardinal IG explains that when a sealed unit is built at low altitude and then installed at a higher one, such as Denver, the panes bow outward into a "pillow" shape, because the pressure sealed inside is higher than the outside air pressure. If the pressure difference is large, the glass can crack or the edge seal can fail early, letting moisture in and fogging the glass.

Andersen's altitude guidance adds that smaller windows are most affected, because small panes are stiffer and can't flex to relieve pressure, which puts more stress on the glass and seal.

Capillary tubes and the argon trade-off

The common fix is a capillary tube: a very thin tube in the spacer that lets pressure slowly equalize. Cardinal recommends capillary tubes for high-altitude installations and says it doesn't use large-diameter breather tubes.

There's a trade-off. Cardinal notes that argon easily diffuses through a capillary tube, so it doesn't sell high-altitude argon-filled units. Andersen says that using a capillary tube eliminates the argon gas fill and slightly lowers thermal performance. Some manufacturers offer extra low-E coatings to make up the difference.

Practical takeaways:

• Ask every bidder how the manufacturer handles altitude for your elevation. Foothills homes, like those in Evergreen, sit higher than Denver.
• Ask whether the quoted performance ratings are for the high-altitude version of the glass.
• Check that the warranty covers glass at your elevation. Some manufacturers limit coverage without capillary tubes above certain altitudes.

The sun problem

Denver's thin, dry air means intense sunlight. West- and south-facing windows can overheat rooms, fade floors and furniture, and drive up cooling costs. That's where low-E coatings come in: microscopically thin metal layers that reflect heat while letting visible light through. Coatings vary. Some let more solar heat in (useful for winter warmth on south walls); others block more of it.

Reading the NFRC label

ENERGY STAR says certified windows are independently tested, certified and verified by the National Fenestration Rating Council (NFRC), with criteria that vary by climate zone. The NFRC label shows:

• U-factor: how well the window insulates. Lower is better.
• Solar Heat Gain Coefficient (SHGC): how much solar heat comes through. Lower blocks more heat.
• Visible Transmittance (VT): how much light comes through. Higher is brighter.
• Air leakage and condensation resistance, on some labels.

Some cities set minimums. Northglenn's permit page, for example, lists a required U-factor of 0.30 or less for replacement windows. Your contractor should confirm local requirements.

Matching glass to orientation

Many homeowners use the same glass throughout for a consistent look, but you can ask about different coatings by orientation.

Costs

HomeGuide's window replacement guide (updated February 5, 2026) puts replacement at $450 to $1,500 per window on average. Upgraded glass packages, triple panes and special coatings add to that. Get the glass specification in writing on every bid so you can compare.

Historic homes

If your home is a Denver landmark or in a historic district, replacement windows need design review, and the guidelines encourage storm windows over original windows. See our Capitol Hill windows guide.

For more on frames, installation and permits, see our windows guide and the Northglenn area page.