Energy Star climate zones and windows
We are getting ready to replace a bunch of windows in our 1901 house, and are getting quotes on Marvin Elevate (fiberglass outside, wood inside, which match ones we put in an addition over a decade ago and have been quite happy with). It turns out that Seattle is in what Energy Star calls the Northern climate zone (same as lots of very much colder-winter/hotter-summer places, e.g., Minnesota), so the Elevate windows do not qualify for Energy Star in our zone. The Infinity line (all fiberglass) do. In the Essential line (also all fiberglass, but not the same as the Infinity line, contrary to what some people tell you), the picture windows qualify, but the double-hungs do not.
How much do I need to worry about this in Seattle? Obviously better efficiency is better, but I don’t know exactly what meaning to attach to a point or two (or rather a hundredth of a point or two) difference in U-factor. The windows that are being replaced are 1980s metal-framed ones with shot seals that are very cold in winter, and we are expecting a considerable upgrade in comfort and appearance regardless.
The requirements in the Northern zone:
A U-factor of 0.27 or less (any SHGC is allowed)
OR
A U-factor of 0.28 AND an SHGC of 0.32 or more
OR
A U-factor of 0.29 AND an SHGC of 0.37 or more
OR
A U-factor of 0.30 AND an SHGC of 0.42 or more
Marvin Elevate double-hung: U-factor 0.29 SHGC 0.31; picture window 0.30, 0.33
Marvin Essential double-hung: 0.30, 0.33; picture window 0.28, 0.34
Marvin Infinity double-hung: 0.28, 0.33; picture window 0.27, 0.34
(This is all off the energystar.gov website and was a little bit of a pain to compile. I hope it’s accurate.)
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Replies
Irene, don't worry so much about the Energy Star guidelines unless you are trying to go for certification. What is more important is that the windows meet the requirements of the Seattle Energy Code Table 402.1.3 ( https://www.seattle.gov/documents/Departments/SDCI/Codes/SeattleEnergyCode/2018SECResidentialChapter4.pdf ) which allows a max U-value of 0.30. So your Marvin Elevate windows meet that requirement.
Since you are looking for comfort and efficiency upgrades, are you set on the double hung style? They aren't as effective when it comes to air sealing as casement or tilt-turn windows.
I would also suggest looking at Fenstur Windows from BC ( https://www.fensturwindows.com ) They are clad-wood windows that have a much higher performance per dollar than the Marvin. They might be cost competitive to the Marvin Elevate with better performance.
It does sound like you want to match the windows on the addition, so that might not be an option depending on the adjacency of the windows.
I'm an architect in Seattle, so happy to provide localized advice on window selection.
Thank you! As far as I recall, we went with mostly double-hung windows in the addition because that's the style that would originally have been used, and the aesthetic case for putting them in the older part of the house is even stronger. The reason I was looking at the Energy Star tables was that I was hoping the windows would qualify for the federal rebate, but unfortunately they don't (they would have to meet the Most Efficient standard, which is even more stringent than the one I quoted). Not that the rebate amounts to very much against the total price, but we seem to keep missing our chances at rebates, and it's kind of annoying.
Re the increased comfort and efficiency, I really meant that the current windows presumably have such awful U-factor and infiltration numbers that 0.3 has to be a huge improvement. And of course any 1901 house could use a ton of upgrades in the way of air sealing, insulation, etc. (we have done some of that, especially on the basement walls and the attic, but far from all we might), so it's going to be leaky anyway.