Anyone ever hear of a window, or unit of insulated glass, as having different winter and summer U values? 


This wasn't on an NFRC sticker.  It was on a glass manufacturer's spec sheet. 


They listed it like this:


Winter - Air / Argon  0.35 / 0.31

Summer - Air / Argon 0.32 / 0.28


Our program requires a U value of 0.30 to receive an incentive rebate.  So obviously, homeowners and contractors want to average the summer and winter Argon fill U values to come up with an average of 0.295 and call it good.


I think it is just a piece of insulated glass that they want to have installed/retrofitted into their existing wood windows, and get an incentive for that. 


Views: 4492


Reply to This

Replies to This Discussion

The winter U-value is what is shown on the NFRC labels, and should be used for code compliance and incentive programs. It is calculated at -18C (about 0F) outside, and 21 C (about 70F), and a roughly 15 mph wind speed. The summer U-value is calculated with conditions where it is warm outside and hardly any wind. This means that the U-value will be a bit better, but it is not a representative condition for heat losss. The Solar Heat Gain Coefficient (SHGC) is more important during periods with solar gain. So think U-value for heating, and SHGC for cooling.

WIndows and Daylighting Group
Lawrence Berkeley National Laboratory

Currently in communication with this gentleman:

Andre Anders, PhD
Leader, Plasma Applications Group
Lawrence Berkeley National Laboratory
1 Cyclotron Road, MS 53, Berkeley, California 94720, USA
Tel. 510-486-6745, Fax 510-486-4374

He's circulating the information among his associates and will be in touch following the holiday. Will probably find it's way Christian at LBNL, who has responded since last I checked in.

BTW, Christian's advice to use the winter number is probably not what you wanted to hear regarding the available incentive. If 0.30 U is a common cut off # for incentives, I would expect manufacturers to work to that figure, although in a manufacturing environment, such changes may be a challenge. Perhaps a bit of latitude from incentive providers is in order...who is going to notice a U difference of 0.01 ?

Thanks for surfacing this mystery.
Steve, Adam,

Andre forwarded your question to me, that is why I responded here. Many manufacturers have products that meet U=0.30, since that was the cut-off for the IRS tax credit.

If you are looking for even better windows, you can take a look at this website, where you can find listing of manufacturers that meet U=0.22 and less.

Thank you for the replies! Our program does require NFRC stickers, so this guy is going to be out of luck anyways.

But I was curious about the different winter and summer U values anyways. Thanks for the response and clairificaiton!




  • Add Videos
  • View All


Latest Activity

David N. Armington liked John Poole's discussion Two Part Epoxy and Repair of Structural Wood
17 hours ago
David N. Armington joined John Poole's group

Historic Home

Historic and vintage homes are significant to our cultural heritage, yet often lack energy…See More
17 hours ago
Seth Romme is now a member of Home Energy Pros
John Nicholas replied to Rob Madden's discussion Blower Door Testing on Energy Star v3 home
"So you have a slab on grade home, duct work in the attic?"
Diane Jackson posted a photo

"Drive Home"

Addison Homes spent a fun day with a film crew from the National Association of Home Builders. We…
Paul Raymer posted a blog post

Healthy Home Evaluator Training

Fall River, MA     October 13, 2016…See More
Diane Chojnowski updated an event

Better homes video series at Seventhwave

October 24, 2016 to January 26, 2017
Profile Iconseoeleczo and Parfait Pouliotte joined Home Energy Pros

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service