NE and SW facing window selection in Climate Zone 6
The majority of the windows in our home face Northeast and overlook a roofed deck facing the lake and mountains. We get filtered sunlight, as the mountains block the earliest morning rays and the sun is further filtered by the roof over our deck and by a number of very tall trees. Summers are warm and sunny (though require no air-conditioning), and winters are cold and overcast. We are replacing leaky double-pane vinyl windows with fiberglass, triple-pane windows. My initial thoughts were to use all lowE 180 glass to maximize Visual Transmittance to brighten up the rooms and the view, recognizing we are getting little passive solar from the NE.
1) Is there any reason why I shouldn’t use 180 glass facing the NE?
We do have a few windows facing southwest, and these do get much more direct sun. Unfortunately, we may need to use double-pane slider windows (likely Marvin Ultimate fiberglass windows) for these for egress purposes. The initial thought was to use 270 glass given they are SW facing and possibly i89 on the 4th surface. These are going to be outie windows resting on a 3-inch wood-framing window buck, as we are adding exterior Rockwool.
2) How worried would you be about condensation on an i89 surface coating on a double pane in climate zone 6 sitting out on a window buck (which would make it more prone to condensation)? Winter temps here have been quite variable, but winter lows rarely hit -10 F. Average lows in January are between 10 to 15 degrees F. I could use a triple pane from either Marvin or Weathershield, but they would be aluminum-clad wood, which I don’t necessarily love from a longevity perspective (but maybe I shouldn’t be that skeptical).
I don’t have a HRV system in the home, though we are looking into this, as well, as lowering humidity in the winter is the other side to this equation.
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Todd,
This link - https://www.greenbuildingadvisor.com/question/marvin-ultimate-vs-pella-architect-windows - some notes that may be of interest to you.
There have been many discussions on combinations of window coatings and probably as many opinions. Search the questions with various key words like the coating numbers and see what pops up.
Regarding the I89 coating, I have seen notes that some cleaning agents can affect it since it does sit on the room facing surface. The position of the window as an outie shouldn't increase the risk of condensation above the inherent risk of using I89. However, if you plan on shades or curtains that further isolate air flow to the window, then I would guess your condensation risk would go up. The I89 coating creates a cooling effect on the interior side glass similar to that of putting a shade between you and the window. You feel warmer, the glass doesn't.
I had very well fitted honeycomb shades in one house that caused the double panes to cool enough to result in icing at the perimeter. While the room was warmer from covering the windows, the room heat no longer kept the glass above dew point. We often got icing along the window glass at the spacer contact area. The aluminum cladding on the windows exacerbated the conduction of winter temps to the spacers, so the edges of the IGUs got freezing cold.
It has been 30 years now since I bought those windows, so maybe current cladding methods are less directly conducting. The cladding did hold up well, just not a good trade-off for thermal performance.
Visible transmittance and solar heat gain questions are a slippery ones. I would contend that for your circumstances of overcast winters, you should make any selection based on summer sun/heat loads. Typically, the SHGC rating will also affect visible transmission. For shaded windows in the summer, the additional heat gain from high SHGC choices might not be too bad. Thermally, I would focus on U value first for comfort. Scour the questions with the key words SHGC and visible transmission for more reading.
In my particular case, I face very high sunload in the summer and window orientation that does not favor winter gain, so I went with the lowest SHGC. The visible transmission is about 43% which sounds terrible, but it is not noticeable unless you open a window and compare the view through the glass. Our winters tend to be very sunny as well, so the low visible transmission provides relief from snow bounce. Your environment sounds very different so different choices will likely be made.
Thanks, that is helpful. I keep going back and forth re-thinking my decision on the high SHGC, high VT in the summers. Our summers here are short but pleasant, but highs can hit the 90s with more intense heat that comes later in the day for about 2 months. Most of that won't be hitting us directly, as our larger windows will be protected by a 15 foot overhanging roof over the deck (not yet installed) and sit facing the northeast behind a curtain of trees that are taller than our home. I keep rethinking it, but I don't think I can screw it up with the high SHGC windows with this orientation.
We are looking at Accurate Dorwin windows for most of the home, so:
SG400 glass (what we have speced)
U=0.16 for the whole window
.48 for SHGC
.56 for VT with SG400 glass.
An alternative was the SB60 glass, with
0.14 U factor,
.28 for SHGC
.48 for VT.
I don't know what the difference is between a .48 VT v. a .56. The U-factors were close enough that I considered that difference of 0.02 to be negligible.
I keep going back to a quote by Martin: "Cold-climate homes need windows with a SHGC in the range of 0.39 to 0.65." and "There is no danger of a north window leading to summer overheating, so the bottom line is: in a heating climate, be sure to select high solar heat gain windows for the north side of your house.."
https://www.greenbuildingadvisor.com/article/high-solar-gain-glazing
For those egress windows (really, only 2 of them), I'm looking at U-factors of 0.29 or .23 or so with the i89. Those are obviously in a different class entirely, but egress is egress. One of the windows has curtains, the other doesn't. They can probably just stay open in the winter.
I would be cautious about using 180 on northeast windows, but it sounds like you won’t ever be getting direct sun.
I would not use i89 in a climate that cold. I89 results in lower interior glass temperature, which may be further reduced by being an outie window. You risk condensation issues unless you keep humidity very low, don’t use window shades and have radiators directly under the windows. In your climate, triple pane is a much safer way of achieving target u-value.
I live in zone 4a with i89 everywhere and have a few issues when it’s below 20 - the worst is a small double hung in a bathroom, which gets so dripping wet that I may have to replace the sashes. Ironically, the 12’ double sliding door has had no issues whatsoever.
With all the consideration about the windows and glass it is a shame you are using just a wood window buck for the installation.
Have you looked at Thermalbuck?
https://thermalbuck.com/