Benefits Of High Performance Windows
Babyfoot Developments has chosen Alpen Fiberglass Series 525 windows for its high performance construction projects.
How do high performance windows work?
Coatings for West Facing windows. Low solar gain Low-e coatings. These coatings work by extending the spectral selectivity characteristic of a conventional high solar gain low-e coating (South Facing Windows), bringing it from outside the solar spectrum to the edge of the visible portion of that spectrum. The infrared portion of the solar spectrum (accounting for ~53% of the energy from the sun) is reflected by the coating back outside the home. The reflected solar IR radiation returns to the outside and is thereby prevented from entering the cool interior of the home. The result is a major reduction of the solar heat gain through the window without significant loss of visible light transmitted through the glass. The new coating reflects almost half of the incident solar radiation back outside while still allowing a large percentage of daylight to enter.
There are two numbers which all window manufacturers use to tell how good their windows are in admitting both solar heat and light. The visible transmittance, sometimes called VT, gives the fraction of incident light that is admitted. Look for this number to be high, ideally 50-80%. The other number is called the solar heat gain coefficient, SHGC.
Look for this number to be as low as possible for applications with undesirable solar exposure, ideally below 0.4 (or 40%). By keeping VT high and SHGC low, you can ensure that adequate daylight will be admitted while still blocking significant quantities of solar radiant heat gain. SHGC measures the total amount of solar energy transmitted into the room. . The heat rejection ability of window coating can be as high as 80 percent.
For our ultra energy efficient home project in the GTA, our average SHGC for non-south facing windows is ~ 20%, meaning that 80% of the solar energy is rejected. On south facing windows, that are shaded by the roof overhang during the summer months, we have a SHGC of 50%. This ensures adequate passive solar heat gain during the winter months, where the sun angle is below the overhang.
The U value, also known as the coefficient of heat transmission(W/m2 C) gauges how well a material allows heat to pass through. U-value ratings generally fall between 0.20 and 1.25. The lower the U-value, the greater a product’s resistance to heat flow and the better its insulating value. The inverse of (one divided by) the U-value is the R-Value.
Windows & Babyfoot Developments Ultra-Energy efficient Home
In a typical home, windows, including patio doors, represent about 10% of the envelop surface, and 25% of a home’s heat loss, while the external walls, account for ~90% of the house envelop surface with an equivalent heat loss. To put this in perspective in a 2012 OBC built home, the typical R value for a 2×6 external wall is of ~R22. While a window meeting OBC 2012 guideline will have an R value of ~ 2. So, if you see an energy efficient home project with a disproportionate amount of North or East facing windows, smile. You are sure to receive a Christmas card from the local utilities for your loyal contributions.
The windows Babyfoot has chosen are Alpen http://bit.ly/TrBf8J. These fiberglass windows possess an R value of up to 5.9. These double glazed windows, with a suspended thin film, create a window with an internal double Argon filled cavity providing superior condensation resistance. NFRC has a great explanation on how the condensation resistance factor is calculated (Condensation Fact Sheet). Fiberglass frames were chosen because of the material’s thermal stability, ensuring extremely low air leakage values through our envelop.
The thermal resistance of the windows, including their ability to minimize air infiltration is crucial in energy efficient home design. An important part in respiratory health is to maintain adequate moisture (40-50%) within your home throughout the Canadian winter season. In most new built homes it is a challenge to maintain 30% RH without triggering high levels of condensation on the windows and ultimately promoting mold growth.
Window installation must be performed according to manufacturer’s instructions. For high-performance windows, using wood shims to align the window within the framing cavity was simply not an option that we considered. Window installation will be the subject of a more detailed blog.