Hi,

I am looking to build a new home about 2600 sq ft in the Boise, ID area. I am trying to as green/energy efficient as i can afford. Our builder is putting in .35 u-factor window i trying to figure out if I want to go with .35 or upgrade to .30 u-factor windows. Is there a bigger savings difference between these two to justify the $750 upgrade to .30 u-factor?

Thanks.

In your area, I've seen lots of recommendation to go with a U-factor between .32 and .30

It's not JUST about "energy efficiency", it's also comfort level. If the windows bleed heat, your place gets cold.

Also, there are other considerations.

Solar Heat Gain
Where and how windows are being placed.
Visibe Transmittance.
Air Leakage
Condensation Resistance.

There's other things people could probably talk about better than I can.
But don't simply base your decision on U-factor alone.

Current IRC code-max for windows in your climate zone (5B) is U0.32, but lower is better. U0.35 is really nowhere near good enough.

https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

For a NEW house in Boise's climate a double-low-E double pane high SHGC >0.5 glazing (eg Cardinal's LoE180 +i89 ) on the north and south facing windows and a double-low-E with an SHGC of 0.3 or less on the east and west facing windows will be "worth it" in the end, both on comfort and energy use. With Argon fill they will come in around U0.22 - U0.26 depending on the glass spacing and the coating type for surface #2.

All the big window vendors have this type of glass, but you have to seek it out. IIRC Anderson calls it HeatLock or something, I forget what Pella calls it, but there are many window vendors with double low-E options. They all have a low-E coating on the interior side of the exterior pane (called surface #2 in window-geek speak), and a hard-coat (indium tin-oxide) coating on the surface that is in contact with the room air (surface #4). The hard coat low-E on surface #4 reflects the room heat back into the room, and reflects your body heat back at you when standing next to it on a cold day- it's VERY nice. But it also lowers the temperature of the glass, which is an issue in climates much colder than yours, since condensation would form on the window, which neutralizes the low-E effect. You'll see window condensation if dwells at -8F to -10F for hours on end frequently whereas a single low-E U0.30-ish window won't get much condensation until it's well into the negative double digits. But that's not normal winter weather for Boise- at least not frequently enough to matter.

The double low-E glass is definitely going to be a cost-adder, but in your climate it's basically triple-pane type performance at a MUCH lower price point, delivering much higher comfort levels.

Take a peek at the U-factors and SHGCs of the double-low-E glazing in the tables on page 12-14 (p15-17 in PDF pagination) in this document:

http://www.cardinalcorp.com/source/pdf/Technical_Glass_Guide_Web.pdf

If using different glazing types based on which direction the window faces it's useful to make them different sizes, to make sure they don't hang the high-gain windows on the west side (raising the peak cooling load) and the low-gain windows on the south side (blunting the wintertime energy use offsets.) If it's too late for that, use higher-gain glass everywhere, and buy exterior shades if it turns out the west facing glass ends up cooking you in July/August. In summer the high angle of the mid-day sun causes most of the heat to be reflected away, and it's easy to shade them with roof overhangs, but in the afternoon & evening the sun angle is quite low with very low reflectance, and difficult to shade without VERY deep overhangs.

As far as I know ever window manufacture (us and Canada) uses guardian glass but call they each call it by a different tinting/coating name. Thus no window manufacture has better glass then another.

My house would be facing East (front door facing east) the majority of the window will be facing South and west. The build is actually going to be in Canyon County, ID. I called Canyon County, ID building official and he indicated that Idaho did not adopt the 2015 energy recommended changes and that code is .35 u-factor.

Dana1.
Would your recommendations of u-factor and SHGC still apply if my house is facing east?

No matter which side the house faces, the don't change- the windows on the west side still face west, and would be better to have minimal area and low SHGC.

If you have a lot of east facing glass (on the front of the house) it's better to have a SGHC no higher than ~0.3 or it will roast you out of bed on summer mornings. It's nice to get an AM hot blast in January, not so much in July.

Just because U0.35 is code-max (=the crummiest window that's still legal to install in Canyon County) doesn't mean that's what makes SENSE to install in a custom built house that you intend to actually live in (as opposed to tract homes built on spec or multi-family rental housing, etc.) Even IRC 2015 code-max U0.32 or the propsed "upgrade" to U0.30 isn't going to provide much of a comfort or energy use improvement. It's really more about the comfort, not the net-present-value-of-future-energy-savings, and in your climate a U0.22-U0.25-ish double low-E is going to be the sweet spot. It'll be more than a $750 upcharge, but it'll also be more than a $750 upgrade in creature comfort, winter or summer.

From a lifecycle NPV against energy savings point of view see the zone 5 row in Table 2, p.10 of this document:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

They're recommending ~U0.24 with an SHGC less than 0.50 as the generic sweet spot for windows, not tuned to which direction it faces. Within, say the Anderson 200 series, some of the Low-E PassiveSun w/ Heatlock double-hungs come pretty close to filling that bill at U0.26 and an SHGC in the mid to high 0.40s:

https://www.andersenwindows.com/~/media/aw/files/technical-docs/performance/performance-windows-patiodoors-nfrcratings--200series.pdf

Within the Anderson 400 series some of the Low-E4 PassiveSun w/ Heatlock casement windows come in at U0.24 also with SHGCs in the 0.40s:

https://awwebcdnprdcd.azureedge.net/-/media/aw/files/technical-docs/performance/performance-windows-patiodoors-nfrcratings--400series.pdf

The glazing may not be what's on the shelf at the local box store, but this isn't very exotic glass.

Pella's "AdvancedComfort Low-E" double pane glass is in the low 0.20s at center glass with coatings on surfaces #2 & #4 and would deliver whole-window U-factors in the right range.

http://media.pella.com/professional/adm/Fiberglass/3M_F2_Performance.pdf

The difference be between a U-0.35 window and U0.30 single low-E coating on surface #2 or #3 can barely be felt, but it can be measured in the fuel bill.

But the difference between a U0.25-U0.30 single low-E window and a U0.25-ish (or lower) window with a second coating on surface #4 is something that's easy to feel standing next to it even when it's only 40F outside, and DEAD OBVIOUS when it's 10F outside.

So, be specific about the glazing type, see what the up-charge is then decide if the extra comfort is "worth it", while still factoring in the lifecycle energy savings. A dual low-E window is nowhere near as expensive as a triple pane, but in your climate it delivers comparable comfort.