Hi, I’m confused about what glass to use on a huge expanse of windows in my upstate NY house. The glass area is basically two whole walls facing SW (205 degrees) and NW (315 degrees). Roof overhangs are short, exterior shades are not an option, and neither is planting more trees. I’m trying to do what I can with window selection and interior solar shades. The glass is a mix of fixed pane and sliding doors. The new windows will likely be Andersen (contractor’s preference and less costly from his suppliers). They offer their low-E4 glass in double pane and triple pane, both optionally with a low SHGC coating called Smartsun and a coating designed to reflect room heat back inside called Heatlock. The room currently gets a lot of glare and heat on sunny days even in the winter. Would this suggest Smartsun glass, even though I’m in a climate with significant heating load in the winter as well as cooling load in the summer? If cost is not a limiting factor, should I go for triple pane for comfort? What about Heatlock? Assuming unlimited budget is the best glass triple pane with both Smartsun and Heatlock? And in real world where upfront cost probably is a limiting factor, which of the three options—Smartsun, Heatlock, and triple pane—offers the least bang for the buck? Is it worth a little financial stretch to add it? Many thanks to anyone who can advise.

It is best to avoid or minimize North and West facing windows. Any chance you can reduce or eliminate some windows and provide longer overhangs to shade the South facing windows?

Posted By sailawayrb on 04 Jul 2018 04:57 PM
It is best to avoid or minimize North and West facing windows. Any chance you can reduce or eliminate some windows and provide longer overhangs to shade the South facing windows?

No, I wish it were possible to do either of those things, but unfortunately neither is possible.

Yes, you definitely want to limit the SHGC to under 0.30 on huge west facing glass, even in upstate NY.

Whether a low SHGC double pane w/ low-E4 is going to work for you depends on just WHERE in upstate NY. While you can hit ~U0.20 with a double-pane using the pyrolytic hard coat on surface # 4 it lowers the temperature of that glass. If you're at elevation somewhere in the 'dacks where it dwells for many hours per winter at -15F or colder several times per winter you can expect copious window condensation during those hours with a double pane, but not with a U0.20-ish triple pane. For some guidance on that, see p22 (p25 in PDF pagination) of this document:

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

That table presumes a 70F indoor temp. If you keep it at 65F indoors those RH numbers need to be adjusted downward a bit. For most of upstate NY the 0F column would apply, but for cooler spots such Saranac Lake it's worth scanning the -20F column for potential condensation issues.

Cardinal's LoĒ³-366 + i89 with argon fill in a dual pane would work fine in most of upstate NY, with an SHGC of 0.27 and U-factor of U0.20. I'm pretty sure Anderson uses Cardinal's glass, but have re-named it for their own marketing purposes (Smartsun, perhaps?) But there are similar products from other vendors.

That’s extremely helpful. Thank you so much for taking the time to offer an explanation and opinion. And that link is great—lots of really useful data and illustrations that help me understand them. I see why the LoE366+i89 argon double pane units makes sense in this application. I’m at low elevation at the southern edge of upstate NY, so while temperatures do sometimes dip below zero farenheight, it’s rare that they stay there long. Which reading that table tells me those IG units, especially with 3/8” or more spacer, should rarely experience condensation at comfortable indoor humidity levels around 35-40%. And I believe that LoE366+i89 does equate to “Smartsun” and “Heatlock” coatings in Andersen’s marketing-speak.

Sound like you're in one of the US zone 5 (or maybe even zone 4, Westchester) counties? If you're in one of the green or yellow counties on this map there is effectively no condensation risk with a double low-E double pane:

https://energycode.pnl.gov/EnergyCodeReqs/images/new_york.png

In the blue counties near a border a green county the risk is still too low to matter too. At elevations of 1500' or higher in the Catskills or Adirondacks (eg Saranac Lake, Lake Placid) there may be some condensation risk, but even there it can be managed by ventilation to bring the indoor RH down.