Hi, I'm wondering if anyone here has built, lived-in or visited a house that has a mix and match of glass types (i.e. Cardinal glass). For example, in my new house I'm considering LoE180 on the south windows (with overhangs) while using Cardinal 366 on the east and west windows. With an open floor plan, it will be possible to view both types of glass from the family-living room area.

I looked at a samples of the 270 (standard, they did not have any 180) and 366. The salesman said we should go outside and view them in the daylight, which we did. To be honest, I could not see all the much difference between them (in terms of VT). But this was with sunlight streaming all around us (we were OUTside) - so I wonder if it will be markedly different inside.

The upcharge for the special glass isn't all that much. It is an energy advantage. But would it be a visual nighmare? Of course this is a personal thing - what bothers one is fine for another. But if anyone has some experience on this I would be all ears.

Thanks!

What is the actual visual transmittance of the two types of glass you want to use in the specified glazing assemblies? I can begin to see a difference of about 8% between two assemblies. It also can change a little based on sun and viewing angles, and, as you mention above, inside vs outside.

Now, I am talking about being able to see a difference, no matter how small. As you note, above, it is up to you to decide if that difference means something to you, and or the structure you are putting it on. I'm building a contemporary home with large windows and corresponding visual planes although there is some offset of the planes and when faced with a 10% difference in the glazing VT, simply chose to keep the glazing on each side of the home the same rather than risk having someone perceive the differences. The architect was sensitive to it. He wins awards so he knows something. I'm not sure it would have been an issue for us, alone.

Best as I can figure there is as much as a 16% - 20% difference between the two types of glass you mentioned. Depending on how it is used, it will be seen and affect the look of your structure. Only you can say if it will matter. The only way to be sure is to get the glass rep to come up with some actual samples for you to position and look at from different angles. As long as you are setting up a real-world test, you might as well invite as many friends as possible to view it. You will get different opinions from them.

ICFHybrid,
Thanks for your info/experience. BTW, also thanks for the response on my Q about specific window brands in another thread - it did a lot to help me understand the differences.

Re: VT, for the 2-pane windows I’m considering the info is:

366 w/ Argon: 44%
272 w/ Argon: 49%
180 w/ Argon: they don’t provide the data but do offer the 180 glass …
IG w/ Air (by way of comparison): 56%

The 366 would be for a west facing window, the 180 for a south facing patio door - in the same room - just a few feet from each other. This will be the location in the house with the most contrast in the smallest distance.

I don’t think the window rep/dealer have ever had a customer make this glass request before so viewing the sample 6”x6” glass 366 sample next to a “normal” 272 demo window is about the best mock-up I’m going to see.

So I guess I’ll have to decide, dive in with an order and hope for the best.

So I guess I’ll have to decide, dive in with an order and hope for the best.

Exactly, but do take the time to do the comparison with whatever samples you can best get together.

It's most likely that the 5% difference between the 366 and the 272 won't even be perceptible under most conditions.

I think that the 180 might have quite a bit of difference from the other two, though.

BabyBldr-

I used Pella Designer high solar heat gain coefficient (SHGC) windows on the south, east, and west sides of my (heating-only) house, and low SHGC on the north side. The visible transmission (VT) values are 0.52 for the high SHGC, and 0.48 for the low SHGC. I cannot see a difference in the transmission characteristics. However, I did chicken out in the living room that has two north facing and two east facing windows, all closely spaced. I used low SHGC for those four windows.

Most scientists consider eyes as logarithmic detectors so that they can operate over a very wide dynamic range (one billion to one, but that is with the help of the iris). For a logarithmic detector, the 8% difference in VT for my windows corresponds to a 1.8% difference in perceived brightness, so less than I am able to notice. On the other hand, the SHGC's are 0.49 and 0.28, and it is easy to FEEL the difference in heat gain in direct sunlight on windows facing the same direction.

Lee Dodge
www.residentialenergylaboratory.com
in a net-zero source energy modified production house

it is easy to FEEL the difference in heat gain in direct sunlight on windows facing the same direction.

Lee, how many seasons have you had in that house now? Would you do the same thing with the high SHGC windows again?

Up here by the 49th parallel, we see very little sun on the East and West faces during the Winter when you are most needing the solar gain. In contrast, during the Spring and especially the Summer, you get treated to early sunrises and sunsets that really smoke the East and West sides. Consequently, we are using high SHGC glass on ONLY the South face.

What do you notice about that low-lying morning and afternoon sun in the Summer on your East and West sides?

ICFHybrid-

I moved into this house just over one year ago. I am at 38.5 deg N, and am in a sunny, high-mountain valley. You asked if I would use high solar gain windows on the east and west side if I were to have the opportunity to revisit that decision. Yes, for me it has worked out well. HOWEVER, it is necessary to be willing to use shades on the east and west sides to block the summer sun to avoid overheating in the summer. Overhangs (24") on the south side take care of shading without lowering the shades on that side. I installed light-blocking, double-cell cellular shades with side seals on the east side, and light-filtering, double-cell cellular shades with side seals on the west side. (The light-blocking shades are better at blocking the summer sun. They are also significantly better insulators in the winter, according to my testing.)

Using high solar gain windows allows my windows to be net energy producers rather than losers, according to Lawrence Berkeley National Lab's RESFEN model. According to that model, the annual heat losses through my windows are as follows:
N 3.17 MMBtu (net loss)
E -1.41 MMBtu (net gain)
S -4.28 MMBtu (net gain)
W -1.42 MMBtu (net gain)
The gain on my south side would be higher (-7.35 MMBtu), except that my neighbor erected a giant carriage house on his detached garage that shades my south windows in the winter after 1330. If I had used low solar gain windows, the net energy exchange on the east and west sides would be losses instead of gains. Admittedly, this assumes that RESFEN is only accounting for solar gains when they are needed, and not during the summer. Detailed calculations showed an estimated annual total natural gas use for heating plus domestic hot water of 25.2 MMBtu (7354 kWh), so the window contributions are significant. Actual use over the first year for heat and hot water was 22.6 MMBtu (6630 kWh). (Hot water heating energy was reduced by a solar thermal system.) Therefore, the passive solar gains were predicted to 31% of the value of the actual net usage of 22.6 MMBtu (6630 kWh). The natural gas energy use was more than offset in terms of source energy by excess PV energy generated by a 3.15 kW system. More detailed data are available on the web site below.

Note that in this sunny environment, ADDING windows on the south side actually reduces energy consumption by the house, and for that reason, a window on the south side was added to the standard floor plan for this production house. This result is in contrast to advice often given in this forum to reduce window areas in houses to save energy.

Lee Dodge
www.ResidentialEnergyLaboratory.com
in a net-zero source energy modified production house

Admittedly, this assumes that RESFEN is only accounting for solar gains when they are needed, and not during the summer.

So, RESFEN doesn't give you what the extra energy requirements for Summer cooling might be?

When engineering this passive solar, we found that the windows and glazing were substantially give and take. If you added windows to realize the solar gain, you needed to offset that with cooling expenditures, ad nauseum. Obviously, there are exceptions, the best one being placing the window (eaves, etc) where high Summer sun can't enter, but where lower Winter sun can. Taken to extremes, however, that results in oddly placed windows that don't serve their aesthetic purpose as well.

Exterior shading is another one, but who wants to have shades blocking their windows for the majority of the Summer?

In general terms, we found that achieving energy savings of 15%-20% using passive principles was relatively easy, but getting to 30% took quite a bit more work and after that it required some sophisticated techniques (and increasing tradeoffs in upfront cost and intangibles) to get closer to 40%. Of course, this is only one climate type and set of weather conditions.

ICFHybrid-

Sorry to mislead with my response. Yes, RESFEN gives both heating and cooling requirements. In my case, I do not use air conditioning, so I focus on the heating requirements. My comment was that I am depending on RESFEN to be accurate in assigning the loads to heating and cooling appropriately, which I assume that it does. The highest indoor temperature that I have seen this summer without A/C is 77 or 78 F (25 C), about where I would set the thermostat if I had A/C. I would prefer to lower the shades on the east and west sides to avoid direct solar radiation in the summer in most cases whether I had low or high solar gain windows, as it produces too much glare to be in direct sun. This is a very sunny climate -- they brag about 300 or more days of sunshine a year. Most of my windows are on the south and north sides, and those shades stay up except at night, and provide lots of light without the glare. During the winter when the sun is lower, I use the east and west windows to add heat, as long as the sun is not directly glaring in the eyes of the inhabitants.

Looking at my numbers in a different way, the passive solar reduces my heating needs by at least 24%. That is for a standard production house with the addition of one window on the south side, care in designing the overhangs, care in selection of the windows and shades, and the addition of higher insulation levels than the production house. Since I don't need A/C, there is really no trade-off with cooling costs except for using the shades to block the direct summer sun on east and west sides.

Lee Dodge
www.ResidentialEnergyLaboratory.com
in a net-zero source energy modified production house

I do not use air conditioning

Oh, that's interesting. Sorry, it was just my assumption that you used air conditioning there.

Well, you bring up something which I don't think any of this team discussed much. My energy engineers do mostly commercial where cooling BTUs are a real thing that you have to pay for. We installed AC capability as we have about 5 days a year when you might want it to sleep well, particularly if you have elderly "guests", but we certainly aren't going to "buy" every BTU of cooling indicated. We have a high atrium that will probably exhaust 90% of the excess via thermal stack effect without any cost other than a little juice to open the atrium windows. The atrium venting was not included in the calculations. I suppose as long as one doesn't go overboard, you can continue to add windows, realizing the solar gain during heating periods and not having to worry much about the unwanted gains.

What do you do about ventilation during the high indoor temperature periods? Open a window or do you run a ventilator without recovering heat?

The solutions for summer cooling that are available here (a dry high-mountain valley at 2180 m or 7150 ft.) are not at all universal in North America. We are currently in our hottest time of the year (July). Average lows for July are 8.9C (48F) and average highs are 28.3C (83F). It has been much hotter than those averages the last few days, with a low today of 15C (59F) and a high of 36C (97F) yesterday. So the strategy here is to open windows at night and cool the house to a target value of 19 C (67F), close the windows about 0800, and turn on ceiling fans if the house gets too warm. The only fresh air is from the HRV, which actually serves a purpose in the summer, cooling the hot outside air with the cooler indoor air across the heat exchanger. The house is well insulated (R-60 ceiling insulation, R-27 walls, triple-pane windows), but not shaded at all (since I want the winter heating), and the maximum indoor temperature has been OK, especially with the ceiling fans, about 25 C (77F or 78F). Current conditions are 32.8C (91.1) outdoors, and a dry comfortable 23C (73F) indoors with only the HRV using electricity (not much electricity with ECM motors). Lighting is all fluorescents, and refrigerator is very efficient, so not much added heat load to the hosue. I realize this approach will not work for a majority of folks in North America as it does not cool off enough at night for this strategy, but it is a very energy efficient approach well adapted to this area. It also makes use of the insulation package and HRV year around. It is dry enough here that I am planning on adding a whole-house humidifier. I am trying to think about how I can get a "swamp cooler" (air conditioning by water evaporation) effect in the summer without adding too much to the heating load in the winter. Concerning your question about opening your atrium windows to let out the hot air, one question is whether the outside air is cooler than the air in the atrium. If so, then there should be a driving force for the airflow, and a reason to desire the air exchange. And of course, moving air has a cooling effect for the inhabitants. Lee Dodge www.ResidentialEbergyLaboratory.com in a net-zero source energy modified production house

Concerning your question about opening your atrium windows to let out the hot air, one question is whether the outside air is cooler than the air in the atrium

No problem there. We originally had three motorized openers planned across the top of the atrium, but when we finally got around to calculating what the flow might be there, we were able to do away with two of the openers. The flow was remarkably large due to the height (about 33'). All the heat in the main open living area has access to the atrium and we rarely see 90F outside. With even a few degree difference up there we will have pretty strong flow.

O.P. here, back with an update. I did indeed order the mix-n-match windows.
- Cardinal Low E 180 on south side of the house
- Cardinal Low E 366 on the east and west sides of the house
- The north has only one window and I went with Low E 270 on that one

As to the concerns over differences in the V.T. - no problems, IMO. I can stand in the house and look out the south patio door (6' wide) then turn my head 90 degrees and look out the dining room window to the west and not notice any appreciable difference in the two glass types. From the outside of the house standing at the corner to view both sides, same thing - no appreciable differences. Perhaps someone more visually sensitive would be bothered, but for me, it is fine. And I'm really glad I took the plunge, so to speak. I had never even really heard of this idea until I read about it in a GBA blog. But my initial impression (we're just finished framing/roofing so not living in the house yet) is that the 366 glass is a big help, especially with the hot summer sun that seems relentless on the west side of the house.

The sales guy at the window store wasn't familiar with the glass options or what they meant, since "everyone" just gets the standard. So he read aloud the glass options in the drop down menu of his order entry program and that was how I was able to confirm that the windows I was ordering were indeed available with the special glass. I told him which glass options to pick for which windows and we were in business. When the windows arrived the stickers called out the glass type along with the other typical info. I did not get high performance windows, just casement or awning, double pane, argon filled with the special request for the glass.

Thanks to the posters above for sharing info and experiences.

BabyBldr-

Thanks for the feedback on the windows. Glad that you took the plunge. Your analysis on the lack of visual difference corresponds to what I have also observed for my windows. I think that you will be happy with the mix of windows, especially if you have an overhang on the south side to reduce summer solar heat gain. My high solar gain windows account for a significant part of my winter heating, making it worth the time to work with the window suppliers to educate them about what they should have already known.