Choosing windows for a Passive House or low energy house project will bring with it many options and questions to be answered. This is where a Passive House Consultant helps you determine the ideal combination for your particular project and climate.
The number of panes of glass partly determines the R-value (or insulation value) of your window.
A single pane window has r-0.9 so has very little insulation and will fog/frost easily. Although this may look pretty, it will lose a lot of heat and make your home uncomfortable.
Beyond single pane, windows with multiple panes of glazing are called Insulated Glazing Units (or IGU’s).
These windows achieve a higher insulation value by trapping dry air or gasses between two or more sheets of glass.
A double pane window has an R-value of ~2 or 3 so it will insulate better than the single pane used in old houses. But it will likely not meet any of the energy codes in colder climates.
Triple pane windows have R-values between 3.2 and 8 depending on coatings and gas fill
Quad pane windows achieve R-values up to r-14, keeping the warmth in to maximize your comfort and minimize heat loss.
Some of the best performing windows utilize thin films within the IGU to provide separation of the gas layers without added weight of multiple panes of glass. Some of these units use 2 glass layers and 4 film layers to achieve up to R-20.
Basically, the more layers of glazing you have, the better your window will insulate from the cold.
There is a choice of gas fill for windows. Some options are: Air, Argon, Krypton, Xenon
Most windows use argon as it is a low cost fill that has 34% lower heat conductivity than air.
Krypton and Xenon are higher priced alternatives that a few manufacturers use to achieve higher performance. Krypton achieves 63% lower heat conductivity than air and Xenon is 79% lower heat conductivity than air.
If you choose Krypton or Xenon, be prepared to pay more though. These rare gasses will add roughly $100 to the cost of a window if they are even avaialble from your manufacturer.
Glass surfaces come either clear or coated. Clear glass lets the most light through but coatings can help reduce heat loss by "reflecting" heat back in to the house in cold climates or back outside in hot climates. Some coatings are designed to block light so a room is not flooded with too much light as well as heat.
However, the more coating layers there is in a window, the less Solar Heat Gain the unit has so this is useful in warm climates to keep the heat out. This heat gain is beneficial in cold climates to harvest “free” energy from the sun. Properly balanced and shaded windows will reduce overall energy usage for heating that needs to be covered by other means. A Passive House Consultant achieves this proper balance through careful computer modelling of the house.
Depending on the climate, season and location, a window may need to be shaded to reduce the solar gains during the middle of the day when extra heating is not needed. This can be achieved through simple overhangs built in to the building, or with moveable shading devices.
Window frames come in a variety of materials, construction, colours and sizes. Some of these factors are architectural/aesthetic choices and some factors can greatly affect the overall energy usage of the home.
The classic material used for hundreds of years is wood. It is simple, durable (when maintained properly) and can be well insulating using modern construction methods incorporating air or foam cavities within the frame.
Some wood frames are clad on the exterior with aluminum, creating a very durable product that requires little maintenance.
Modern fibreglass frames with insulated cavities are an excellent choice for frames as the fibreglass will expand and contract at the same rate as glass. These frames provide excellent support and will last for a long time as well.
Insulated PVC frames provide low cost, low maintenance frames. One big advantage is that the mitred corners are heat sealed/welded together providing an airtight seal superior to other frame types.
Aluminum conducts heat very well and therefore is unsuitable for window frames. An exception is aluminum cladding on the outside of an insulated wood framed window where the aluminum cladding provides a low maintenance durable finish.
Regardless of the type of material used, insulated frames are the best option to choose as the frames can represent up to 1/4 of the total window area. Installing glazing units that have R-10 in a frame that is only R-3 can result in a total R-value for the window of ~R-7.