Windows - hard to get right

Windows – this is often where the comfort of occupants is decided.

There’s so much to take into account: heat loss and heat gain, for winter and summer; views; daylight access; privacy. All of this needs consideration for the geometry, optical and thermal properties of the glass, as well as the performance of edge-spacer and frame. Next is the interaction between window, environment, and other building elements. Orienting window areas is not as easy as simply working with the cardinal directions. Contours, shading objects, and self-shading need to be considered, which links to how the window is positioned in the wall or roof section. This will have an impact on shading, but also on getting optimum thermal protection. Moreover, installing windows so as to keep wind and water out, and warm internal air in, is crucial. The High-Performance Construction Details Handbook https://passivehouse.nz/hpcd-handbook/ has good explanations and i.a. best practice examples for these matters. Per square meter, windows are by far the most expensive elements in the building envelope, and to reduce costs and complication, it’s a good idea to first and foremost consider reducing the window area. All remaining windows need proper thermal protection for winter and summer. Not just good glazing, but also warm edge-spacers, and frames constructed to have at least the same thermal properties as the glass. And for most windows, external moveable shading is the only effective solution to deal with summer overheating.

The one thing you don’t need to worry about if you have a whole house ventilation system is using your windows to let in air and with it pollution, noise, and critters. That windows are unsuited for ventilating houses is somewhat old news. At a House of Commons Select Committee on the poor indoor condition in English houses in 1835, George Birkbeck testified that he never encountered a building that was well-warmed and ventilated. Asked whether he attributed the lack of performance to the “want of practical knowledge on the subject”, he replied:

“I do; heating and ventilation, especially the latter, seldom entering into the mind of the builder when he projects his building; he begins as if he did not know that ventilation could be necessary; he trusts to the doors and windows, to neither of which belongs the business of ventilation. The doors admit the occupants to the chambers; the windows the light; and apertures ought to be introduced to admit air for ventilation as regularly as the other openings.”