Consumer's Guide – To buying Energy-Efficient Windows and Doors

Catalogue No.: M-92-156/2001E
ISBN: 0-662-27162-9

Section 8 –
The Benefits of High-Performance Windows

The technology of high-performance windows may be impressive, but the benefits are equally compelling. And these benefits extend beyond the more obvious ones.

8.1 Savings in Heating Costs

First and foremost, high-performance windows are energy-efficient. They offer immediate savings on home heating costs. Depending on the house design and the existing levels of efficiency in the rest of the building, switching to high-performance windows should yield nine to eighteen percent reductions in space heating costs (Fig. 41).

8.2 Savings in Cooling Costs

Concerns about energy efficiency are not limited just to the heating season. In many parts of Canada, summertime heat requires space cooling. High-performance windows work equally well at keeping the heat out in the summer months. The same low-E coating that keeps infrared (heat) energy inside the home in the winter keeps unwanted heat out in the summer. This keeps the interior cooler and cuts down on the need for air conditioning.

Studies have shown that, for most of Canada, it is still appropriate to choose your windows on the basis of the ER number. Exceptions to this rule are those few locations where air conditioning costs are high relative to the amount spent on space heating. In these few cases, consider west-facing windows with a lower solar heat gain potential.

8.3 Increased Comfort

In houses with conventional windows, air leakage, drafts and radiative heat loss all contribute to occupant discomfort which the heating system must try–usually with only limited success–to overcome. But high-performance windows are better insulated and maintain a much higher surface temperature on the interior glazing. This characteristic, together with effective weatherstripping and proper installation, makes the window "feel" warmer to the occupant.

8.4 Higher Humidity Without Condensation

Improvements in comfort extend beyond the reduction in drafts and cold spots near windows. During the winter, outside air is very dry and can significantly lower the relative humidity (RH) inside. This in turn can lead to annoying static electric shocks when touching doors or light switches, as well as dry throats and other irritants.

If you try to compensate by humidifying the air, then once you get above about 40 per cent RH, there is the risk of condensation forming on the windows. This reduces visibility and can lead to deterioration of the frame components and mould growth.

High-performance windows can change all this. The higher inside glass temperatures and improved thermal performance of edge spacer and frame components allow much higher RH levels inside (Fig. 42). This can reduce if not eliminate RH-related problems associated with poor windows. Since higher humidity levels are possible with better windows, this means reduced static shocks, improved health, and healthier plants. Fig. 42 shows the relative humidity at which condensation will start to form at the centre of the glass of different types of glazing. However, condensation will usually form at the edge of the glass at a lower relative humidity.

8.5 Lower Sound Transmission

The heavy gas fills in high-performance windows, which reduce conductive and convective heat losses, also reduce sound transmission from the exterior to the interior. The greater the number of panes, the better the sound absorption. Sound attenuation is complex, depending on frequencies and other factors; some benefit may be realized with high-performance windows.

8.6 More Daylight

The advent of high-performance windows is allowing larger glazing areas to be incorporated into house designs, in both new construction and renovations, without the penalty of either high heat loss in the winter or high heat gains during the summer. This not only enhances energy efficiency and improves the view, but it also lets in more daylight, which may lead to reductions in the use of electric lighting.

You save twice when you take advantage of this natural light: first, you save on electricity used for lighting; second, because a conventional light bulb uses only 15 percent of its energy for lighting and wastes the other 85 percent as heat, you lower the home's cooling load in summer. This is especially the case in larger buildings.

8.7 Increased Passive Solar Potential

Conventional south-facing glazing offers, at best, a break-even proposition in terms of balancing heat gains and losses. In other words, the energy that south-facing windows gain during the day through solar inputs is about equal to the energy they lose through radiation, convection, and conduction heat loss during the night.

High-performance windows are changing all this. With the right selection of low-E coatings and gas fills, it is now possible for most windows to gain more energy during the day than they lose at night.

With window frame areas kept to a minimum, it is now possible to have even larger window areas and still obtain net energy gains. In addition to saving on space heating costs, this can give the house a brighter, more open feeling.

8.8 Reduced Mechanical Complexity

On a cold winter night, conventional windows (RSI 0.35 or R-2) lose about ten times as much heat as an equal area of a reasonably well-insulated wall (RSI 3.52 or R-20). It is for this reason that architects and heating contractors have been forced to locate heating registers, convectors, and radiators directly under windows. This compensates for the high heat loss and air leakage in close proximity to these windows.

Lower air infiltration and the reduced conductive heat loss of high-performance windows may make the practice of perimeter heat distribution less important. It is now possible, thanks to high-performance windows, to locate heating registers on interior walls, either at the floor level, or near the ceiling. This reduces the length, diameter and complexity of heating duct layouts. Discharge outlets on an inside wall near the ceiling may provide more comfort, especially with air conditioning.

Reducing the length of duct or piping runs in the home saves on capital material costs as well as installation time, in either new homes or renovations. Shorter duct runs with smaller diameters can also mean smaller fans and less energy to run the fans. In some cases, cost savings may offset the additional costs of upgrading windows.

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