Choosing Between Window Glazing Options
Glazing selection by a novice! How would you do differently?
To-be homeowner, building a south-facing, two story, residential house in Hartford (Windsor County), Vermont (CZ 6A). Am following advise in the “pretty good house” guide and efficiency Vermont High Performance Base options. My building insulation features are Slab and frost wall: R15, Wall: 2×6 stud with mineral wool (R23) + ZipR sheathing (R-12), Attic: blown in Cellulose (R60) Air-tightness < 2 ACH50.
Knew nothing about windows before reading GBA two months back!Now, looking forward to make an educated decision! LOGIC tilt-turn windows (https://logicwd.com/) are my preferred choice, because of their low U value, high Visual Transmission, and variable SHGC options (window specifications attached, U value 0.15 ~0.16). Alphen windows are my second choice.
I have great view to South, moderate to North, East-west views are Forested (No visual appeal, other than the Fall season). Here is how I am specifying my windows:
Floor area: 2500 sq. ft
South wall: 145 sq. ft. (5.8% of floor area) Mostly picture windows; Will have overhang on both stories (2.5 ft depth place at 1.5 ft high from window top border). Selection criteria: High-to-Moderate SHGC windows (Glazing 180/Clear/180 option?)
East-West wall: 105 sq ft combined (4.2% of floor area) Mostly Tilt-turn windows; Selection criteria: Low SHGC windows (Glazing 272/Clear/272 option?)
North Wall: 80 sq ft (3.2% of floor area): Mix of picture and tilt-turn windows, Selection criteria: Lowest U, Highest VT (Glazing (272/Clear//180 options?)
My total glazing to floor ratio is 13.2%. Since first time working with windows specs, I am looking forward to opinions. What would you do differently?
Regards,
Rajib Roy
Edit:
1. Updated my location and window views that matters to visual appeal.
2. updated overhang to the south windows.
Excerpt from comments:
1. Robert Opaulch (comment #1)
a. Southern high SHGC windows only can take advantage of solar heat gain in clear winter days, and Vermont gets a fair amount of overcast days in winter (38% sunshine in my location in January) vs. more sunshine in summer (58% in July). Site specific conditions like tree cover and surrounding structures play a role solar shine/shade.
So in this case, reconsidering total glazing area to optimize Solar Heat Gain around the year, minimize construction cost and energy demand is recommended. Bob provided a wealth of reasoning and tools to help me decide.
2. Kyle R (Comment 3): Advised that Logic 272/CL/272 (U 0.15, SHGC 0.24, VT 0.39) are noticeably dark due to low Visual Transmission (VT). Sacrificing tiny U value for better VT such as 180/CL/180 (U 0.16, SHGC 0.38, VT 0.47) would be better.
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Replies
You seem to be on the right track already, picking higher solar gain windows on the south side and calculating the square footage of window vs. floor space. Just guessing from the information that you provided, you will not need auxiliary space heating in rooms facing south during afternoons during sunny mid-winter days. Not that Vermont is all that sunny mid-winter. But do the math.
I’d suggest that you model the heat losses and gains of your proposed home, main rooms and their windows. Given your plans, you seem to know that in Vermont, you will have far more wintertime solar gains and daylighting from south-facing windows than other directions, assuming they are not shaded from the sun. Two good resources to help are:
• Weatherspark.com has great charts and climate data, including solar insolation, cloudiness, and temperatures during the year for any location. Not knowing your location, I’d guess that you’d get not much direct sunlight during winter months (mostly overcast winters in VT). But you’d still get more daylighting and solar heat gains from south-facing windows than other directions, which you must know since you are specifying higher solar gain windows for your south-facing windows. Model your gains vs. losses to decide whether for your location and solar shading, its worth higher heat losses vs. higher solar heat gains (and daylighting).
• More details about solar gains, geographic location, and window orientation:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
• Note that wintertime solar heat gains only occur if there is nothing blocking the sun from shining on your windows (e.g., tall trees, adjacent buildings). The track of the sun in Dec-Jan is quite low in the southern sky. (Rough rule of thumb: Objects to the south of your building must be twice as far away as their height, including any ground slope, to provide your home’s interior with clear midday sunlight in Dec-Jan.). Note the angle of the sun on Dec 21 solar noon for your location, vs. the location of trees, buildings, etc.
• You can compute the solar gains through your proposed windows by using the data available on this excellent web site: http://www.susdesign.com/windowheatgain/index.php
Here are some questions and suggestions:
1. Would you consider reducing the total square footage of windows even more? Windows lose a lot of heat during your cold winters, and your walls far less. Not to mention the cost of larger windows are quite high compared to the lower cost of comparable wall area. Passivhaus/PHIUS uses this strategy to minimize window area as yet another way to get extremely low heat loss numbers.
2. If you align your window sizes with the stud wall framing, you might reduce the amount of framing in your walls a little, and reduce your labor a little. Maybe you did this already.
Others will tell you to ignore the direction of your windows or always use higher U-factor windows. Do the math. You have to put your windows somewhere, so putting them on the south side usually works better for northern locations (and north side for predominantly hot humid climate in southeastern USA locations), assuming no shading from the sun. Of course, consider your home’s views in various directions, and the aesthetic appearance of your home’s exterior.
That is great info Robert.
One item I take a bit of issue with is window sizing. Windows are the soul of the house, size them to give you the views you enjoy. A bit larger is an energy penalty that is worth the cost.
You do have to take a bit of care with larger west facing windows as it can overheat the house in the afternoons, some exterior shading is a good idea. East facing doesn't seem to be as big of an issue as the solar gain tends to be when people are out of the house.
Thank you Bob!
Influence of cloud cover and surrounding tree and building cover was not in my consideration. In addition to sundesign tool (https://www.susdesign.com/windowheatgain/index.php), I plan to use the BEOpt energy modeling tool to do a parametric study of variable glazing options (to educate myself).
However, a hypothetical study with the Sundesign tool assuming 100% sunshine year round estimates more heat gain in December (20 kBtu/sq.ft.) than June (11 kBtu/sq. ft.). Feels like counter intuitive! Is it because of sun angle variation? Am I missing something?
Thanks again,
Rajib
In December/January, you will get a LOT of solar heat gain from south-facing windows, since the sun is lower on the southern horizon, shining directly on south-facing windows mid-day when sunlight is strongest. In June, the track of the sun goes from east to above the roof to west. So minimal solar heat gain from south-facing windows in June and July, since the sun is striking the roof midday, not the south side of the house.
East and west facing windows get a LOT of solar gain in the morning and afternoon, not much at noon (solar noon when the sun shines on the roof).
Look at Table 6 "Monthly Solar Heat Gain" in the "Quantitative Look at Solar Heat Gain" article:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
You would have to compute the solar heat gains for your own location (this example is northern USA, farther south than your location in Sweden).
Anyone reading this Q&A: Enter your latitude etc. into the "Sustainable by Design" web site:
http://www.susdesign.com/windowheatgain/index.php
which will provide you with a table of solar heat gains per hour and per day for south, east and west at times of the year you want computed. (Great web site!)
^^^^That is great information Robert. Thank you for sharing.
You are already looking at very good IGUs, I would maximize VT with the 180/Clear/180 for all windows. I have 272/CLR/272 ( manufacturers mistake) and they are noticeably dark. In your climate and with your window area I don’t think minor differences in u value and SHGC are going to be noticeable.
Thank you Kyle for bring this important point.
The difference in U values are so small that higher VT is preferred choice! What type interior shades are you using (curtains, blinds or something else better suited)?
Regards, Rajib
In the bedrooms we have cellular blinds. Everywhere else we have no window treatments. We live on a heavily wooded lot.
Make sure you don't mix glazing types in areas where they'll be visible at the same time -- one will look noticeably darker than the other and it looks "weird". If on the same wall, different interior rooms will prevent this difference from being noticed inside, but you'll still see it outside. On corners, it can be less of an issue, since one side of the corner will probably naturally see less light than the other anyway, but it's still noticeable.
It looks like you're planning on triple pane windows. I recommend you get the full depth glazing option (1-3/8"), which gets you maximum performance for little difference in cost (usually). The thinner triple pane IGUs sacrifice a lot of performance to fit into thinner frames, sometimes to allow an "upgrade" from a double pane to a triple pane in the same frame. Make sure you're getting triple pane windows designed to be triple pane windows.
I think with triple pane windows there isn't as much to be gained with the more aggressive low-E coatings compared with double pane windows, so I like to stay with Cardinal's Low-E 180 coating. Cardinal's i89 interior coating can still add a bit of benefit here, and will probably end up getting around the same performance improvement as the more aggressive coatings, assuming a full thickness triple pane IGU, but will less impact on VT.
I really think the importance of VT is underestimated in many cases, especially if you have a good view to look at through the windows.
BTW, I completely agree with Akos about the importance of windows too! Energy efficiency alone is NOT everything! If you have too few or too small windows, it feels like you're living in a box.
Bill
Bill,
Thank you for the advice on using consistent glazing option for the same room and wall. This is very important! Great tips on VT and coatings. Appreciate it.
Rajib
Early on it is easy to say you like tilt turn. The choices get much harder once you have the bids on the table in front of you and the window number is 4x the box store window number. Not that the box window were really an option in your mind it is just that that number is so darn big.
When I modeled my house I found it made almost no difference what direction a window faced every window was an energy looser and the bigger the window the more it lost. Your mileage may vary.
I think the smart move is to build your own BEopt model enter your cost per square foot of each window and coating and run the numbers.
https://www.nrel.gov/buildings/beopt.html
If you go down the BEopt road be sure to find and watch the YouTube training videos.
Walta
Hello Walter,
you are right about significant price jump in efficient windows vs box store options. Vermont energy code specifies U 0.28 or less (Energy start Northern zone windows).
To save on cost, I am using 95 sq. ft Tilt-turn windows and 235 sq. ft. Fixed/Picture windows.
You are correct that windows have weakest insulation in building envelop. I plan to perform BEOpt parametric modeling to optimize cost (construction and energy). Thanks for the advice.
Regards, Rajib
I'm staring at those numbers right now. No big box quotes, but the European tilt and turns in aluminum/wood or aluminum are 4X a quality vinyl quote. Aluminum/vinyl are 3X and full vinyl tilt turns are 2X. I'm in northern climate zone 5 and I'm going highest U factor and easy on the SHGC. In my current house, I have 8-10 degrees temperature difference in south rooms versus north during sunny periods in both winter and summer. It's very uncomfortable, not so much the absolute temperature, but the huge variation between rooms. I'm looking to specify windows and hvac to try to minimize that.
Great breakdown of window frame options and price. Please keep us posted on your decision, would love to learn more.
Rajib
Rajib,
Depending on overall budget, I will settle on two options. Either a fiberglass fixed and casement system in triple glazed. U value for casement are 0.17 and 0.14 for fixed panes. If the budget allows, European tilt and turns in some aluminum configuration. Full aluminum has the best look with slim frames. U values are around 0.14. I just don't like the chunky build and exposed hinges of the more value oriented full vinyl tilt and turns.
Fiberglass is about 1.5x vinyl and aluminum is about 3 to 4X. It's quite a jump, that's why I think most settle on fiberglass as an upgrade over vinyl. As I mentioned, I'm keeping all SHGC values around 0.30 to try to even out temperatures in different rooms.
A fixed window with an awning window (essentially a casement window on it's side) under it, is another option you may want to consider depending on your requirements.
Bill
Yes, that would be the spec for the kitchen sink window! The ones I haven't decided on are the basement window wells. Sliders would probably work best, but I dislike them.
Thank you Allan,
I am leaning toward Logic Tilt-turn options (do not want casement in the first floor decks). As you've mentioned casement with top-hinge is a great option for ventilation. I will keep fiberglass frames in mind. Thanks again.
Rajib
We did all triple pane, tilt turn windows with the same .49 SHGC, primarily to avoid any extra screwups with windows coming from Europe. They've been fine. Lots of south facing glass heat the house nicely on sunny winter days, but it never gets uncomfortable. Might hit 75° on a sunny day like yesterday, when the outside temperature only got to high teens. But it never gets any warmer than than. Three foot overhang avoids summer heat pretty well.
As everyone knows, you really only get one chance to get windows right.