Windows

Windows in Lethbridge:

Windows are an important consideration in reducing heat loss from a home. Designed well, they also allow for passive gains from sunlight entering the home.

The main difficulty in assessing the economics of upgrading windows in a home is in the wide range of styles and materials used to make them – styles like bay windows, casement windows, and so on; frames made of aluminum, vinyl, wood or fiberglass; gas used to fill between panes; and, specialized treatments like low-e coatings to keep heat from leaving the home or solar heat gain coatings to limit radiation from entering the home.

We will use the Lethbridge climate data for modelling and will use costs for typical windows published by local manufactuers. We will make clear the many assumptions that have been used to assess the upgrading of windows from 2-panes to 3-panes.

Windows are an important feature in a living space, as they allow for desirable natural light and the movement of fresh air. Windows are also a significant source of heat loss and may be the source of uncomfortable drafts in the winter.

Heat Loss:

Windows are rated for heat loss based on the RSI-value. A low RSI-value has more heat loss than a high RSI-value. For comparison, a typical insulated wall in a home has an RSI-value of 3.5 m2oC/W (or R-20 in Imperial units). A double-pane window may have an RSI-value of 0.37 about one-tenth the insulating value of the wall in which it is mounted. A high-performance double-pane window may have an RSI-value as high as 0.68. A triple-pane window with gas fill between the panes has an RSI-value of 1.33, which has 3.5 times more insulating value than the standard double-pane window. It should be kept in mind, however, that even a high-performance triple-pane window has an insulating value of about one-third of the wall in which it is mounted. In other words, in new buildings only install windows where they are needed and as small as aesthetically possible.

Using Lethbridge climate data, the following chart shows the monthly energy loss for each square meter of windows in a home. It also shows the three types of windows discussed above. It is clear that the better windows reduce heat loss substantially. But is it worth it?

 

Cost Evaluation:

As noted above, assigning a cost to windows involves many factors from window-style, frame materials, gas fills and specialized treatments. Based on rule-of-thumb estimates from window manufacturers, there is roughly a 15% premium for triple-pane windows over double-pane windows. The cost of a double-pane window is, on average, $750/m2. Therefore, an upgrade to triple-pane windows would cost $115/m2.

It should be noted that this economic evaluation is assuming a new build (or an existing building where the windows are in need of replacement). To justify the replacement of functioning double-pane windows with triple-pane windows in an existing home would mean using a cost of $865/m2, which would not be economically justifiable based on energy savings alone.

For economic payback, the annual heat loss from a double-pane window (ignoring July and August) is 1.25 GJ/m2. The high-performance triple-pane window would have an annual heat loss of 0.35 GJ/m2, for an energy savings of 0.9 GJ/m2.

Assuming a cost of natural gas of $16.76/GJ, the savings for upgrading to triple-pane windows is $15.90/m2 each year. This results in a simple payback of: $115/m2 / $15.90/m2/y = 7.2 years.

If one were to add the social cost of greenhouse gas emissions at $50/tonne, the payback would be reduced to just over 6 years.

[It should be noted that if one were to replace functional double-pane windows with triple-pane windows at the full cost of the triple-pane windows, the economic payback would be 55 years. This is longer than the expected life of the window.]

Other factors:

In addition to reducing heat loss, triple-pane windows reduce noise and improve home comfort by reducing drafts.

Furthermore, where the windows are well positioned, the home can take advantage of passive heat gains from sunlight. The chart below shows the heat gains from windows facing the four cardinal directions, north, south, east/west.

You will first notice the north-facing windows have very little potential for passive solar gains due to our northern latitude – the sun simple does not shine through our north-facing windows. As such, these windows should ideally be minimized in a home design.

The east/west-facing windows have most of their heat gains in the summer months when the sun is rising or setting. This is exactly the type of heat gains one does not want in the home as it simply adds to the air conditioning demand. On the other hand, there are some desirable heat gains available in the winter months. Some designers will plant deciduous trees on the ease and west sides of the house as they can shade in the summer and allow some sunlight through in the winter, once the trees have shed their leaves. Landscaping like this can also help lower unwanted air movement through the home by blocking wind, which is particularly true in Lethbridge with predominant west winds.

Finally, the south-facing windows offer substantial opportunities for heat gains in the winter months and, with proper eve design or awnings, limit heat gains in the summer. In fact, a south-facing window in Lethbridge can allow enough passive heat gains to offset the heat loss over a year. Properly designed south-facing triple-pane windows can actually become a net heat generator for the home. Enthusiasts of using the sun to heat a home may refer to the many websites focused on passive design like the Passive House Institute (https://passivehouse.com).

Summary:

For new home builds or homes in need of replacement windows, choosing to upgrade from double-pane to high-performance triple-pane windows will cost roughly $115/m2 and will have an economic payback of 6 to 7 years, making it an affordable option that will reduce greenhouse gas emissions by roughly 15 tonnes CO2 for a standard home over the 30-year life-cycle of the window.