Fenestration is a fancy architectural term that means “openings in the outside of the building.” You will see this word a lot when you read ACCA Manual J or when you are doing a Manual J load calculation.

Fenestration loads include heat losses and gains through windows, doors, skylights, etc., and they can make up a significant portion of the overall load. They may also be the biggest variable load throughout the day as the sun moves across the horizon.

There are two big things to watch for when entering fenestration loads.

1. Look for tags on doors and windows that say NFRC (National Fenestration Rating Council) certified. (Visit nfrc.org for more information.) If they have this mark, it means that the entire door or window was rated, including the glass, frame, etc., over the entire rough opening rather than the glass only. If it is not NFRC-certified, then you are better off using ACCA Manual J tables.
2. Use the full rough opening of the doors or windows you are entering into Manual J rather than just the part you can see. Many doors and windows will be pretty standard or at least consistent across the building, so once you get one rough opening and U-factor, you will often be able to use it over again. The gif below shows some items you will need to find the U-factor of a window.

There you have it. Try to use the word “fenestration” next time you play Scrabble with your grandma for extra awesome points.

—Bryan

P.S. — We have a 3D animation that shows how to do a Manual J block load calculation. It also has information about fenestrations and U-factor. You can watch it at https://hvacrschool.com/videos/manual-j-load-calculations-3d/. Matt Bruner also wrote about field data for windows in Manual J load calculations in his tech tip at https://hvacrschool.com/manual-j-field-data-windows/.