When wishing to minimise the use of network energy in a building there is no point in having excellent insulation and high thermal mass if there is no free solar energy entering. Yes, there is some gain from the sun-exposed walls and roof conducting their gained heat to the interior but this is a slow process – better to have the sun enter directly through windows and then trapping it.

While the window construction factors – using single/double/triple glazing, argon-filled or not, e-glass or not, tinted or not, frame material, etc. – are important in determining the thermal performance within the building, perhaps as important is the location and orientation of the windows elements themselves within the exterior walls. A standard double-glazed, argon-filled, e-glass aluminium window in a code-compliant minimal insulated timber-framed south wall may produce a lesser thermal performance for its room than a high thermal mass room with its single-glazed timber framed window orientated to receive maximum sun exposure.

For any given wall orientation and window type, it is the proportion of the window to wall area that influences its effect on the spaces behind: too much and there is excessive heat loss when the solar energy is not entering, and the outdoor temperature drops; too little and insufficient solar radiation is able to enter. Confounding this is the other uses of windows. They bring in daylight so that energy is not needed for artificial lighting, they give connection to the outside world, and also allow for particular scenery to be appreciated – requirements that are often at odds with the thermal performance aspects of windows. The design answer comes from the best juggling of the conflicting needs. There is no standard guideline, no one-answer-fits-all. 

While double glass and e-coatings can improve the performance of windows, the frames must also be considered, especially as the performance of the glass increases. Heat is lazy and takes the easiest path. These days, timber-framed windows can be fabricated from specially treated timber which, I am told, is as dimensionally stable as aluminium. This offers a substantial reduction in thermal bridging problems so that the window unit works better as a whole. 

The other type of window that can significantly improve solar performance, especially for buildings that have walls with minimal sun exposure, (such as being close to neighbours or in narrow valleys), are skylights, roof windows and clerestories. These can make very significant increases in the passive heat gains but again must be carefully positioned and constructed to make the most of their potential.           

Tinted glass, or tinted overlay, is often used to control daylight glare from a window, but it is also sometimes promoted as providing improved thermal insulation. While the interior heat gain in a room with a north wall receiving full sun will be reduced, this does not mean that tinting will improve the thermal performance of the whole building. As a window’s orientation moves away from the full-sun north wall it will reach a balance point where the conductive heat losses from the room exceed the reduced radiation gains from the tinted glass. From this point on, it is necessary to reduce and then remove the tinting if the overall solar gain is to be maximised.

On the plus side, for real buildings inhabited by real people, the Building Code and the NZ Standard do not make any allowance for curtains. In my opinion this is sensible because when the building is complete there is often no money left for curtaining, except some lightweight fabric only sufficient for privacy. By properly installing curtains, the thermal performance of the window portion of the wall can be improved many times over that achieved by fiddling with the glass unit. Also using thermal curtain fabric does not mean that a better thermal performance is gained for the room; often it will be less than that from well-fitted traditional multi-layered, full length curtains. 

Bigger windows mean less insulated wall; smaller allows more insulated wall. So where is the balancing point? Through Ecorate Ltd – Architect I provide a thermal simulation analysis, as a design tool, which gives an objective measure of the thermal performance of the proposed building arising from different window arrangements, rather than relying on guesswork. Starting with the base results it is then an efficient process to quickly explore the effects of different window types, sizes, locations, and window/wall ratios to find a practical and aesthetic best-fit.