Fenestration is the placement of windows and doors in the exterior shell of the building. Ideally the following percentages of glass area to floor area should be met. (The actual percentages used in the Greeny Flat are shown in the photos).
- South wall 7-20% (depending on the amount of Thermal Mass – discussed in Step 8)
- North wall <4 %
- East wall <4 %
- West wall <2 %
Use the best insulating (lowest whole-window U-value) windows that you can afford. Double glazing costs more but the advantages in 1) improved energy efficiency, 2) improved comfort, and 3) reduced condensation (i.e. less chance of mould) are priceless. The material that the frames are made of can also greatly affect the overall performance of the windows. In the Greeny Flat we used double glazing throughout with aluminium frames which do not have the best energy performance but have the advantages of being very durable, less expensive, locally made, fire resistant, and no-maintenance.
For the north windows we used clear glass, i.e. no coatings on the glass, as this allows more of the heat from the sun into the house in the winter. The windows are shaded in the summer so we are not concerned about too much heat entering in summer (see the section on Eave Overhang).
For east, west, and south windows we used glass with a “Comfort Plus” low-E coating that reduces both the heat gain in summer and the heat loss in winter.
For west windows look for ones with the lowest SHGC to reduce heat gain in the summer.
When choosing and placing windows it is important to consider which windows really need to open. Fixed windows are not only less expensive but they seal a lot better, perform a lot better and have less to go wrong with them than openable windows. On the other hand it is important to provide openable windows in the right place for cross ventilation in summer. This will be covered more in Step 9 but it is worth noting here that not all windows need to open. Of the ten panes of glass on the north wall of the Greeny Flat, only three of them open, the rest are fixed.
Don’t forget to add insulating blinds or curtains. Top-down-bottom-up cellular blinds (double or triple cell) provide good insulation as well as excellent control of sun, views and privacy. These can be combined with regular curtains for a more traditional look as well as a second layer of insulation.
Be aware that while insulating blinds can help keep the inside of the home warmer in winter and cooler in summer, they also have the effect of making the windows themselves colder on a winter’s night because less heat from the inside of the home can get to the windows. This can create a serious condensation problem on the windows if the house is not adequately ventilated. Another advantage of top-down-bottom-up blinds is that they can be left a little bit open at both the top and the bottom which allows a bit of warm air to move behind them and can help reduce condensation.