U values-difference in energy loss?
I know that code in our area, South Central PA, requires windows and doors to have a rating of U.35 or less for energy efficiency in new construction.
We’re trying to decide whether to use French doors with a U.31 NFRC rating or French doors with interior blinds that have only a U.36 NFRC rating.
How much heat loss is involved between a U.31 and U.36 rating on a window?
Any thoughts?
Replies
The heat loss depends on the size of the window/door and the temperature difference between the indoors and the outdoors. If the French doors measure 36 square feet, and the temperature difference between the indoors and the outdoors is 65 degrees (70 degrees indoors and 5 degrees outdoors), then the heat loss from the interior to the exterior will be 842 Btus per hour for a door with a U-factor of 0.36, versus 725 Btus per hour for a door with a U-factor of 0.31. The heat loss through the door with a U-factor of 0.31 will be 86% of the heat loss through a door with a U-factor of 0.36. Of course, many factors affect glazing choice, including orientation (is it facing south? west?), climate, glazing solar heat gain coefficient, size of roof overhang, etc.
Thanks for the input.
Both doors are on the north side under an overhang. They are standard Reliabilt doors from Lowe that are 72 x 80 inches R.O.
We are trying to make energy efficient choices where we can keeping factors such as cost in mind. This building is not a house but a large finished barn/workshop/garage, etc.
I guess this answers my question. Although the doors with the interior louvers would make having privacy on this side easier since we have 2 doors we would be loosing 234 BTU's an hour.
This seems like a lot.
234 BTU per hour sure sounds like a lot, doesn't it?
Since Natural Gas spot prices are 8.065 or so per MMBTU, that means that you'd be losing
234 x.000001 x 8.064 = $0.00189 per hour.
But don't forget, the furnace is only 80% efficient, transmission and all the other misc charges. Might take it to .01 per hour.
Point well made.
Guess I'll just buy the ones I really wanted with the internal blinds.