Hi All, I got my hands on a pretty sweet house near the Twin Cities that I’m working on. Built in ‘73 and has 2x4 construction. The siding was replaced in 2014 so I’m not able to really do a deep energy retrofit the way I would like, but I can make it better. The windows in this house are absolutely shot. I’ve already air sealed and insulated the place about as well as I can (2 ach, R-20 walls, R-60 attic) and the windows are the last and most expensive part. I’m not crazy about vinyl windows in this climate and I would rather not do wood. You guys have any suggestions on good fiberglass or composite windows? I’m familiar with a few of the bigger name brands (Pella, Marvin, Andersen, Fibertec) but I don’t know any others. The bigger name brands of fiberglass can cost 2-3x as much as a vinyl. Anyone here fans of fiberglass windows?

Casements or double/single-hungs?

In your climate whatever the vendor the sweet spot is a double-pane double low-E with a high solar gain, with low-E coatings on surfaces #2 & #4. Andersen calls it "Low-E SmartSun(tm) w/ Heatlock(tm)". Pella calls it "AdvancedComfort Low-E". Cardinal Glass LoE180 + i89 glass can be specified with some of the smaller vendors.

The down side of that type glass is that at temperatures south of -15F there can be condensation on the room side surface (#4) unless you keep the indoor humidity extra low, but the center-glass U-factors can hit U0.20 with argon fill, which is triple-pane territory. When the condensation forms the emissivity soars (to the emissivity of water), but in the Twin Cities that can be avoided 99% of the time in your location by ventilating with an HRV under dehumidistat control, keeping the indoor humidity under 35%. If you hold the line at 30% RH indoors you're good for -20F outdoors without center-glass condensation with LoE180 + i89 glass. See the table on p22 (p25 in PDF pagination) in this document:

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

Casements and fixed. Doing a sleek contemporary look with fewer opening windows. All black.

Also the difference in u-value doing that double low-e coating didn’t change a lot. Marvin’s all fiberglass got to .25 and Pella’s Impervia .24. I suspect that minor difference is the stainless steel locks in the Impervia vs aluminum in the Marvin. That double coating drops the condensation resistance about 10 points in both brands. Just doing the regular single low-e coat has a .29 (not even energy star) in Marvin and .27 in Pella (just makes energy star).

Another factor — a couple of these are north facing windows. I know this isn’t ideal for energy efficiency but I’m rehabbing an already built structure.

In other words, changing the glazing’s SHGC on these N windows isn’t going to accomplish much as there’s no direct sunlight.

Thoughts?

I always over the years got the feeling US manufactures don't know how to build windows for the north....

From leaking seals, to cracked glass, to shitty latches, to not sliding nice, mildew, and fading...

We went with an all fiberglass window out of Canada in Toronto. We made a weekend trip out of it and toured the plant and had a great time.

The manufacture is Inline. Cost was a lot less and were good to work with. We did a 15' and 13' double sliders, tilt outs, and lots of casements.

Not a single problem....and the house had a ACH of .79...

Good Luck, this is one of the harder choices in a house. They also make to any size and use all the top glass. Don't know if you know but almost all the glass is made by one manufacture (due to patents - Guardian Glass?) so Pella, Anderson, and all use the same glass they just change the name.

Posted By strategery on 21 Sep 2019 04:37 PM
Also the difference in u-value doing that double low-e coating didn’t change a lot. Marvin’s all fiberglass got to .25 and Pella’s Impervia .24. I suspect that minor difference is the stainless steel locks in the Impervia vs aluminum in the Marvin. That double coating drops the condensation resistance about 10 points in both brands. Just doing the regular single low-e coat has a .29 (not even energy star) in Marvin and .27 in Pella (just makes energy star).

Another factor — a couple of these are north facing windows. I know this isn’t ideal for energy efficiency but I’m rehabbing an already built structure.

In other words, changing the glazing’s SHGC on these N windows isn’t going to accomplish much as there’s no direct sunlight.

Thoughts?

It's the comparatively high SGHC, not just the U-factor that makes LoE-360 + i89 glass such a winner. A high SHGC still delivers a measurably bigger net energy gain even on north facing windows(!). That's true especially (but not exclusively) when facing open high reflectivity snowscapes, less so if it's in the shadowy dark woods.

I believe Pella's AdvancedComfort Low-E used in the Impervia series is in fact one of Cardinal LoE-xxx + i89 products (but it is not LoE-360- the SHGC is too low.) If not Cardianal, a competitor's similar product, since it has a hard-coat low-E on surface #4, as well as another coating on surface #2. See p.17:

http://media.pella.com/professional/adm/Clad-Wood/C_ProductPerformance.pdf