What is the most efficient disposition of windows to prevent thermal loss and improve lighting?

Question

Glazing is expensive and causes thermal loss. With those two considerations in mind, what configuration of windows is the most efficient in terms of reducing the area used by windows and maximizing a room brightness?

For example, are windows in the upper part of a wall more efficient in capturing sunlight or in the bottom half? or, is there a considerable difference between many elongated rectangular windows and one big pane of glass?

Are there any equations that describe this relationship?

All of these questions assume that windows are placed in the sun-facing wall of a building, but I would also like to know about the efficiency (understood in terms of thermal loss and lighting) of skylights.

Answer 1

There are whole books on passive design.

Lawrence Livermore has a simulation program that can help with design.

Some quickies: for a south facing window, triple pane is net lower than double pane. You lose more from the extra glass than you gain from insulation. It’s pretty close though. On the shaded (north) side if the house triple pane is a win.

Gas leaks out of sealed units. Typically 5-10 years. Argon fill may be worth it. Xenon not so much. Do they make windows that can be refilled in situ?

The year is not symmetrical. April 21 is one month after spring equinox and you want heat. August 21 is one month before fall equinox and has the same sun, but heat is the last thing you want. Consider crankdown awnings that allow you to adjust.

There is an edge effect with windows. Both the window frame and the rough opening are locations for higher heat loss. Consider the window to be 6” larger all the way around for heat loss, but only its real size for heat gain. I would not install a window smaller than 3 feet on a side except in a bedroom or bathroom.

Skylights have higher losses. They aren’t close to normal (right angles) to the sun in winter, unless you have a very steep roof running east west. They get covered in snow. Air temp at the ceiling is warmer than room as a hole. More loss.

But the biggest reason is that it affects the whole roof design. You can’t put 2 feet of cellulose insulation by a skylight.

Look at clerestory windows for a compromise solution. These still are the highest point in the room, so will have higher temp air. (And triple pane may be worth it.) But in our climate they are close to normal to the Dec 21 sun (13 degrees above the south horizon at noon) and so still do some gain, as well as provide daylight in the northern half of the house. In addition they are more insulation friendly to the roof as a whole.

Answer 2

This depends primarily on whether you're in a heating or cooling climate. In many places you want to heat the house in winter but cool it in summer, but people also live where they almost always want heating or cooling.

The key search term here seems to be "passive" - I did a couple of searches for "window design passive heating" and "...cooling" and got useful links.

https://www.yourhome.gov.au/passive-design/passive-solar-heating (Australia) https://www.energy.gov/energysaver/energy-efficient-home-design/passive-solar-home-design (États-Unis)

There are various equations for the detailed design stage that relate location, orientation and so on to heat gain/loss and light levels as well as structure etc. But those typically aren't very useful for "what should I do" questions so much as "if I do this what happens".

For example: the difference between a given area of glazing as multiple panes vs a single pane is often cost rather than performance, at least until you get into panes big enough to cause structural problems (you can build a glass house, but...). There are edge effects from the window frames but those are generally minor for moderately sized windows. Windows are sometimes deliberately recessed to get shading, but that's a decision made during detailed design.