Commentary: Choosing Windows for Green Building

Lead Author: Dr. Jeff Howe

Publish date: 06.01.2008

 

 Old News is Good News!

 

Recently I was involved in a debate with a very knowledgeable window expert who insisted that casement windows were absolutely the best choice for a home today given the major concerns with energy use and the environment. This led to a group discussion about why double hung windows even exist anymore anyway, given their apparent weakness in resisting air infiltration. Apparently, with seven extremely knowledgeable window people in the room (including three window manufacturers and four building material experts), I was the only one old enough to remember why double hung windows were created in the first place.

 

To understand why different window types exist, I use the following summary as a “rule of thumb” on how to choose a window type. Basically there are six considerations for picking the right window:

 

1. IF air infiltration and insulation value are the primary concerns (e.g. A north wall in the north - a south wall in the south) - a wall does much better than a window. Note: A window researcher once made the astute observation, “you better be sure you know what you're trying to do before you design a hole into a perfectly good wall!”

 

2. IF sunlight and a view are also of concern (e.g. A south wall in the north, - a north wall in the south), then a fixed window is the next best choice (to a wall).

 

3. IF airflow is desirable (e.g. You want/need to open the window) then you have another question you have to answer, which is: Are heating and cooling a major concern?

 

4. IF heating and cooling ARE a major concern (E.g. Minnesota) then you need to have a window that opens and closes vertically (heating and cooling air are vertical issues - e.g. hot air rises, cool air falls) and a consideration should be given to the proximity of the window to the ceiling and or floor depending on the window's location in the building! (Generally windows should be close to the ceiling on second floors and close to the floor on the first) In these situations double-hung windows are the ideal.

 

5. IF heating and cooling are NOT a major issue (e.g. Hawaii, parts of CA, AL, LA, GA, SC, TX,) then windows that open horizontally and directionally are valuable. Both casement windows and sliders effectively meet this need. The difference between casement and sliders is generally “degree” of need. Egress casements can provide the greatest open areas for high volume airflow purposes.

 

6. Finally, IF airflow is a concern but persistent strong weather patterns and/or highly exposed locations also exist (e.g. Seattle), then awning windows are an option.

 

Overall, in much of North America, given the prevailing winds, the best window solution is to put double hung windows on the east and west walls, fixed windows on the south walls, and no windows on the north walls. Obviously that does not account for local geographic characteristics such as mountains or lakes, or specific house design issues such as shape and other internal functions being served (e.g. specific needs of kitchen and bath).

 

So, not only are double-hung windows important, but they are the best choice in a great many situations. In fact, if people took greater advantage of the seasonal adjustments that can be made on the air-flow through a house by using double hung windows we might need fewer mechanical supplements that demand energy. For example, in the summer in the north you can open the lower part of the window on the west side of the house and the upper part of the window on the east side of the house and thereby use the natural flow of air generated by nature. Now how's that for green building – and no mechanical engineering degree needed!

 

Dr. Jeff Howe

June 2008