I have published a report on the advantages of using high solar gain windows in predominately heating climates, and the problems with the newer IECC codes regarding high solar gain windows at: http://www.residentialenergylaboratory.com/window_codes.html I would welcome any suggestions for improvements or clarifications that I can include in updating this report. BTW, what groups make up the IECC?

Lee,

Here is some feedback from my friend that is a engineer in the window industry.

Me?  In any enviroment where you are going to need heat in the winter I would use high solar gain windows facing south - keeping in mind that I would also have adequate overhangs to protect against heat gain in the summer and if no overhang then I would include some sort of exterior shading.   I would never use high solar gain on the west side windows even if they would probably be  an advantage in deep winter - if you lived in the upper 1/3'd of the country - but otherwise the heat gain in summer would be a major disadvantage. 

The problem with general prescriptions (code, or otherwise) is that no house is a general house, every house is a PARTICULAR house, on a particular site with particular shading factors, etc.

There are many decent energy use modeling tools such as DOE2/BEopt etc to help figure out the particulars when making design decisions regarding a real situation. Building code mins/maxes are a really crude tool for this, but will usually (but clearly not always) lead to a thermal design that's cost-effective in a 15-25 year net-present-value on energy cost savings. But for any design you actually care about it's better to model it, and get a code variance if you can demonstrate that a higher U-value higher solar gain factor window will result in lower energy use than a code-max lower-U model when installed in the particular direction, house, & site as-designed.

Overhangs on south facing windows are overrated for controlling summer heat gain on south facing windows, since the peak insolation periods occur at sufficiently high incident angles that the difference in refractive index of plain old sodium glass & air result in total (or nearly total) exterior reflection- it's a fundamental of optics. Literally half or more of the summertime radiant gain through south facing windows is from scattered & reflected light- the reflectivity of what you see out that window is at least as important as any overhang-shading factor. Operable exterior shades do a far better job of controlling the unwanted heat gain, since it reduces radiation from all angles. Many PassiveHouse designs have huge south facing glazing fractions with minimal or no overhangs, yet don't roast the occupants in summer. Overhang so some good, but not nearly as much as many designers credit them with. (Again, model it.)

WindowsonWashington-

Thanks for the feedback on the report. Here are some comments that I have to your comments.

You are suspicious that the authors had a preconceived notion of their conclusions. When I originally looked at this problem, I was trying to decide what windows to put in my house. After looking at it for this area, the computer model RESFEN indicated a 15% reduction in heating costs using high-gain windows instead of the low-gain windows that were standard for the production house. However, I could not find any high solar-gain, double-pane windows that would meet the U <= 0.35 spec for zone 6 at high altitude (generally no inert gas fills). Indeed it was hard to get anybody to quote high solar gain windows, as they all wanted to sell low-gain windows. Since I was in another state, it would have been difficult to sit down with a building inspector and show him the tradeoffs and try to convince him about glazing theory. I ended up paying $6500 extra for triple-pane windows to guarantee that I could meet the U spec and still get high solar gain, something I'll never pay off, but they are nice-looking windows. The actual price of coatings for high-gain versus low-gain are almost the same.

As noted in the report, the house was based on my own house, and assumed to be a single story house with insulated basement, 149 m2 (1600 ft2) on the first floor, and to be very well insulated. The house was assumed to be symmetric in terms of windows, having a total window area of 6.20 m2 (66.7 ft2) on each side, which would correspond to four windows on each side, each measuring 1.04 m x 1.50 m (41” x 59”). If we assumed the house was 40' x 40' and had 9' ceilings, then the glazing area would be 18.5%. The windows in my house do not exactly match this configuration (less total glazing area), but to avoid any criticism of bias, I chose equal window areas on each side.

You said, "Low U factor glass is typically going to be warmer than higher U factor windows in winter..." but this is certainly not the case. There is a dramatic increase in the radiation coming through the high-gain windows that can be felt in standing inside and feeling the suns rays. I have the same size windows but with different coatings on the east side of my house. Also the rooms with high-gain windows fell much warmer because they are. At night, I close the shades like most people, and there is no difference in comfort level.

You said, "...but I think that the word 'significant' used in the report may be (or may not be) a bit of a stretch." That may be true overall, but in my case it was a 15% difference, which is significant if you are trying to reach net-zero source energy. Even if we said that in the country as a whole we could save 3% energy by proper choice of windows, that would be a heck of a lot of gallons or CCFs of fuels. And the high-gain windows often use hard coats rather than soft coats used in many low-gain windows, so durability might be better.

Thanks again for taking the time to look at the report. I appreciate your comments.

Dana1-

Thanks for the comments. You said, "But for any design you actually care about it's better to model it, and get a code variance if you can demonstrate that a higher U-value higher solar gain factor window will result in lower energy use than a code-max lower-U model when installed in the particular direction, house, & site as-designed." I think that is crucial to my argument. You and 19 other people in the U.S. would be willing and capable of going to the county inspector and have the confidence that you could convince them of the advantages of using high solar gain windows. The other 300 million of us will plod along doing what we're told and follow the code. We should have smarter codes that encourage folks to save energy without having to have degrees in physics and be willing to fight the system.

You said, "Overhangs on south facing windows are overrated for controlling summer heat gain on south facing windows, since the peak insolation periods occur at sufficiently high incident angles that the difference in refractive index of plain old sodium glass & air result in total (or nearly total) exterior reflection- it's a fundamental of optics." According to Fig. 5 of http://gaia.lbl.gov/btech/papers/31727.pdf, it looks like a single glass window at my 38.5 degree latitude at the highest summer elevation (23.5 deg) would result in an angle of incidence of 75 degrees, and the solar transmission would be about 75%. At slightly lower angles off the summer solstice, the transmission increases rapidly. With more panes and coatings the transmission would be less, but for houses like mine adding an overhang is simple, and if the overhang width is computed correctly (http://www.susdesign.com/overhang/), it can pretty much eliminate the direct solar radiation when you don't want it. I use internal shades when needed to further block the scattered light, but, like most folks, I do like to keep an eye out on the mountain peaks when I am around. You are a fan of operable external shades, and I like the concept, but I must admit to never having seen one in person. They might be hard to get approved in neighborhoods in the U.S. What is the cost?

Thanks for taking the time to reply.

Lee: Thanks for the LBL reference paper!

But you overestimate me- even I can't convince some code inspectors of ANYTHING, no matter how well armed I am with the published facts and models. But I've been able to negotiate at times. (Like the time I agreed to swap the toilet for a low-volume flusher in order to be able to put in a drainwater heat recovering unit. I won't even try to figure out what the guy was thinking, other than he was bustin' my chops and then some.) Smarter codes would surely help, but smarter code-inspectors wouldn't hurt either, eh? ;-)

Looking at the same figure 5, note that it's a normalized transmittence. If you look at figure 2 you'll see that at 0 degrees the transmittance is less than 1, and drops to ~30% @ 75 degrees, or about half the 0 degree transmittence. And U0.5 double panes can still be relatively high gain at low angles. A no-coatings high-gain double window would deliver sub-20% transmittance at ~75 degrees, and in typical cases the radiated gain from the scattered light of the surroundings would higher by a factor of about 2 or more due to the more favorable transmittance angle. Where overhangs are easy to design in it'll cut down some, but it's easy to mentally overestimate the fraction that it's really cutting, and underestimate the scattered light gain.

Cutting 25-50% of the problem with an overhang surely helps, it's just too easy to convince yourself that it's fixing 75-80% of the problem when it's not. With any kind of low-E coating on either pane the value of that overhang sinks even further.

Exterior shades come in a variety of transmittances appearances & costs. These are see-thru, fairly economical, cut more than half the light/heat s, and are available at some box-stores (some people hate the look though):

http://www.coolaroousa.com/view-product.do?product_id=654080070806.

http://www.builditsolar.com/Projects/Cooling/RollupSunShades.htm

There a shiny aluminum variants too, but most people (and condo associations) hate those even worse! :-)

Dana-

I have to tell you that I had a different idea on what operable external shades might be compared to what you provided links for. Those links describe something like a pull-down shade that requires going outside to pull them down, and then hopefully the wind will not tear them up. I can see these being used on a window or two on the south or west sides, but I cannot imagine using them on every window in the house. Plus, even though my house is a single story, the windows are high enough up that I could not reach most of the shades without a ladder. And that type of shade would be problematic in the winter with snow on the ground and the cold winds blowing. I was hoping that "operable" meant that they could be operated from inside the house.

I was picturing something like a metal roll-up shade that I have seen that rolls up and stores at the outside top of the window, but that somebody had figured out how to make look decent. I have also found that interior shades work much better (2x or 3x) as insulators (not sun blockers) if they have seals on the sides to cut down on convective air currents across the windows (http://www.residentialenergylaboratory.com/r_value_cellular_shades.html). This would be even more the case with outside shades; they would need some type of side seals to be a good insulator in the summer or winter.

You see those faux shutters in New England that are just nailed onto the house. It would be nice to be able to have something like that which would be hinged, and closed and sealed from the inside. Maybe they were hinged in the old days, but, they might have required opening the windows to operate them, and they would not have sealed out the winds.

A disadvantage of the interior shades is that they tend to increase condensation on the windows if they are a good insulator since they reduce the innermost glass surface temperature, and this would not be the case with exterior shades. However, I believe that for my application, the interior shades are still the most practical solution to a summer sun blocker and insulator, and a wintertime insulator.

Posted By Dana1 on 21 Nov 2011 07:03 PM
 
Exterior shades come in a variety of transmittances appearances & costs. These are see-thru, fairly economical, cut more than half the light/heat s, and are available at some box-stores (some people hate the look though):

http://www.coolaroousa.com/view-product.do?product_id=654080070806.

http://www.builditsolar.com/Projects/Cooling/RollupSunShades.htm

There a shiny aluminum variants too, but most people (and condo associations) hate those even worse! :-)

Any winds above 20MPH would tear those things apart. I personally am NOT a fan of exterior window shades that are made in that fashion. For exterior use, I would get something that is made for high winds and wear & tear, like these:

Rolling Shutters

Do HSGW allow damaging UV rays to come into the home? The rays that fade carpeting & fabrics?

Lbear,

I just sent you a private message through this forum.

Posted By Lbear on 03 Dec 2011 04:15 AM
Do HSGW allow damaging UV rays to come into the home? The rays that fade carpeting & fabrics?

Soft-coat low-e coating will control UV transmission a bit better.

Total fading potential is both a function of UV light and visible light.

Single pane float glass will allow about 70% of the UV spectrum in whereas double pane hard coat will let in less than 30%.

2 coat soft low-e will drop that number below 20%

Lbear-

All the low-e windows block most of the UV rays that are particularly damaging in fading. I am on the road and do not have time to check the data for my Pella triple-pane windows that include both low SHGC (north side) and high SHGC windows. However, I remember something like 95% UV blocking for the high SHGC and 98% for the low SHGC, or at least, the low SHGC had higher UV blocking than the high SHGC. But all the low-e windows block not only some of the IR, which corresponds to their low IR emissivity, and most of the UV.

To all-

It is critical that code and code officials start to understand and allow the advantages of high SHGC windows because the window manufacturers sell windows that meet code. It is true that there are window manufacturers that supply windows with all options of SHGC but not enough do it make the market competitive. I am looking for such windows for the houses I build, including one for myself, but there are very few vendors in my area so shipping alone becomes an issue. The tax credit pushed windows to a u-value of .30 or lower with a low SHGC so the window manufacturers followed suit. I live in Iowa and I know the value of south-facing windows in the winter-time. It's sad that the general building community does not. What's even worse in my area is that developers typically orient streets north and south so it's hard to find a lot that has a predominant side facing south.

Posted By swcasey on 07 Dec 2011 07:59 PM
To all-

It is critical that code and code officials start to understand and allow the advantages of high SHGC windows because the window manufacturers sell windows that meet code.

Code...building science unintelligent...say it ain't so. 

Cardinal makes some nice coating packages and they make quite few peoples glass.  I would suggest you talk to them.

My window buying experinece was similar to that of swcasey. My calculations showed that I would benefit dramatically with high SHGC windows. I talked to a number of suppliers, and they were only interested in supplying low SHGC windows. In my high-altitude area, argon fills are generally not available (Serious Windows and a few others are exceptions), and it is difficult to meet the over-aggressive U-value maximum spec in the IECC code (even more so for Energy Star) with high SHGC windows. So saving energy is made more difficult becase of poorly thought out energy-saving codes!