Triple pane windows are energy efficient but expensive says study

Although highly-insulating triple pane windows are very energy efficient, they are expensive and can take decades to recoup the investment, says a new report from energy efficiency experts at one of the US Department of Energy’s (DOE’s) National Laboratories.

Sarah Widder, a researcher from the DOE’s Pacific Northwest National Laboratory (PNNL), in Richland, WA, presented the report at the Buildings XII Conference in Clearwater Beach, FL, recently.

The meeting was sponsored by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).

As wall insulation has improved immensely, windows have lagged behind and are now considered the weak spot in the “thermal envelope.”

The technological challenge is to improve insulation and solar heat gain, so as to reduce winter heat loss and summer heat gain through the windows.

In their report, the researchers describe a series of experiments carried out at PNNL, where they compared the energy efficiency of two homes.

At first the homes were identical, but gradually, the researchers changed one feature at a time in one of the homes, and then worked out the difference in their energy performance.

Installing triple pane windows to replace old ones

In one particular experiment, the PNNL team studied the effect of replacing aluminum-frame double-pane windows – common across the US – with newer, highly-insulating triple pane windows.

They found that the triple pane windows reduced energy use by 12.2%, a not unsubstantial amount when you consider that windows represent only a small proportion of a home’s “envelope” say the researchers.

But when they compared this with the very high cost of the highly insulating windows, they calculated it could take between 23 and 55 years for the reduction in energy to make up for the investment.

However, the team suggests the triple pane windows also bring some attractive benefits, and people should consider these when deciding whether to invest.

For example, two benefits the team highlights are comfort and health.

Co-author Graham Parker, a senior staff engineer at PNNL, says:

“The windows cut down dramatically on cold air radiating from the windows and they reduce temperature variations in the home, where some areas will be much warmer or cooler than others. They also nearly eliminate the formation of condensation on the inside of the window which can lead to mold growth and unhealthy indoor air. It’s hard to put a dollar value on comfort and health.”

Triple pane windows also cool the house in the summer

The team also found that the windows can reduce energy consumption (of cooling) by nearly 25% in the summer. This is due to their low-emissivity (low-e) coating that reduces the amount of heat coming in from the sun’s rays.

They conclude that although highly insulating windows may not appear cost-effective in today’s market, the prices are likely to reduce significantly with manufacturing improvements and increased market penetration.

Also, because these windows produce more consistent and uniform interior temperature distributions, it means they may enable more centralized duct design and smaller HVAC (heating, ventilation, and air conditioning) systems.

Thus by enabling shorter, more centralized duct systems and smaller HVAC systems, they could bring the additional cost savings as part of a “high-performance building envelope,” either in new builds or as retrofits.

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