Does the sun have to shine directly onto a window in order to have an effective or noticeable SHGC? During summer on south facing windows, can the solar gain cause excessive overheating with windows that have a SHGC of > 0.40 when outdoor temps reach 95F?

What is the definition of "excessive overheating" in this case?

There is a certain amount of heat that will be passed to the interior by nature of the 95F outside air warming the window (where the U-value governs) and a certain amount of energy that will be passed by the direct solar gain (where the SHGC governs) depending on sun angle, aperture, etc. What all this does when it gets inside is a function of the inside configuration and materials.

If you live in a sun-intensive climate, outside scattering of the solar energy becomes more significant. For example, even if you have huge overhangs and the sun can't get directly in, a nice white concrete sidewalk directly outside could help the rays get reflected in. I could also see where having a reflective roof section could facilitate a way into second floor windows that might be above it

Posted By ICFHybrid on 14 Aug 2012 09:55 AM
What is the definition of "excessive overheating" in this case?

There is a certain amount of heat that will be passed to the interior by nature of the 95F outside air warming the window (where the U-value governs) and a certain amount of energy that will be passed by the direct solar gain (where the SHGC governs) depending on sun angle, aperture, etc. What all this does when it gets inside is a function of the inside configuration and materials.

If you live in a sun-intensive climate, outside scattering of the solar energy becomes more significant. For example, even if you have huge overhangs and the sun can't get directly in, a nice white concrete sidewalk directly outside could help the rays get reflected in. I could also see where having a reflective roof section could facilitate a way into second floor windows that might be above it

Well, I talked to a homeowner who tried the whole passive solar thing in the same area and he is having serious overheating issues. The area is Northern Arizona (Chino Valley). He placed windows with a high SHGC on the southern exposure (SHGC = 0.49 & 0.62) and although he has overhangs (24"), he claims that in the late spring through early fall he is experiencing overheating. That whatever gains in the winter are completely lost in the summer due to the heat gain and his running of the AC units to keep things cool. Even with running the AC all day to cool it, he says that southern end is way too hot to be sitting in during the daytime.

So I am thinking of just going with a SHGC of 0.37 / U-Value 0.13 to play it safe on the south facing windows of my home.

I think the climate of the area and the elevation (4,800 feet) is such that high SHG windows are a dangerous proposition. Being it's the high-desert area of Arizona, it's a heating and cooling climate. Unlike Flagstaff which is a heating dominated climate and Phoenix which is a cooling dominated climate, Chino Valley is both heating & cooling. It can see temps in the mid to upper 90's during summer and the intense desert sun makes for 330 days of the year being mostly sunny. Even in winter daytime highs are in the mid 50's. 

 In the end if I go with a SHGC of 0.37 on the south end it will still provide some SHG during winter but during summer it will not be an oven like a 0.49 or 0.62 window would be.

You can't just shotgun it by picking an overhang like 24". You have to shoot the sun angles for your latitude using the relative locations of the tops and bottoms of the window as well. Then there is the issue of glazing area and what sort of a surface the radiation impinges on once it is inside. The very reason low SHGC windows are specified is primarily because of southern climes. I would be very careful trying to make a passive solar in Arizona versus one in northern latitudes. All of these factors will be much more sensitive down there.

What about the westerly windows?

In any case, the solution now is to block the gain from those windows entirely during the period in which it is too hot. Outside sunscreens come to mind.

In the end if I go with a SHGC of 0.37 on the south end it will still provide some SHG during winter but during summer it will not be an oven like a 0.49 or 0.62 window would be.

Have you done the calculations to see what the results are or is this another guess?

I doubt your homeowner has done the full heating/cooling math on his south facing windows to determine that it's a "complete loss", but BeOpt and DOE2 would model the energy use consequences reasonably well. It may be that his over-glazed for the optimum energy use, or that the house has insufficient thermal mass for the amount of glazed area, or in fact it could be just fine, and he hasn't really calculated it. There are better (usually proprietary) models available for optimizing passive solar for both heating & cooling season, and if he was just winging it on rules of thumb it's unlikely that he found the sweet-spot.

South facing windows don't have nearly the summertime gain of east or west facing windows due to the much higher solar angle on the south side resulting in reflecting a much larger fraction of the energy. That can be further mitigated by designing in appropriate overhangs that yield summertime shade on those windows while still yielding full south-sun in winter.

Overhangs don't work on the east and west facing windows though, since the sun is always lower in the sky in the morning and afternoon than at midday. The gain of west facing windows have a higher impact on overheating and peak loading though, since the wall & roof gains and outdoor air temps are lower in the AM than in the PM. Wintertime gains from east & west facing windows are negligible compared to the south windows though, and minimizing or eliminating the west facing windows to lower the peak AC load will often be closer to optimal from both an overheating and total energy use point of view. Minimizing or eliminating east facing windows would also lower the total annual energy use, but has a much lower effect on the peak loads.

Ultimately the ideal SHGC of the south facing windows depends on the total glazed area, the thermal mass, and the thermal loads of the house. It's not a one-size fits all "0.37 is better than 0.62" kind of deal. Model it.

Edited to add:

At any gain coefficient, in some locations the reflected light from the surroundings can account for as much as 50% of the gain through south facing windows. The reflectivity of the landscape in view counts (a ripe field of wheat or a salt flat is much more reflective than a forest or desert basalt formations, etc.)  But that's a secondary effect relative to the fundamental modeling issue.  But the power-boost of snow reflected sun is HUGE, and can be factored into the calculations in areas with persistent mid-winter snowpack.

Wintertime gains from east & west facing windows are negligible compared to the south windows though

The arguments for letting the Easterly sun in a bit are 1) that the East windows are never going to roast you, and 2) people who work outside the home like to have a bit of sunlight come in in the early morning. Those are never going to work for the Westerly windows, though.

Posted By ICFHybrid on 14 Aug 2012 05:51 PM
You can't just shotgun it by picking an overhang like 24". You have to shoot the sun angles for your latitude using the relative locations of the tops and bottoms of the window as well. Then there is the issue of glazing area and what sort of a surface the radiation impinges on once it is inside. The very reason low SHGC windows are specified is primarily because of southern climes. I would be very careful trying to make a passive solar in Arizona versus one in northern latitudes. All of these factors will be much more sensitive down there.

What about the westerly windows?

In any case, the solution now is to block the gain from those windows entirely during the period in which it is too hot. Outside sunscreens come to mind.

In the end if I go with a SHGC of 0.37 on the south end it will still provide some SHG during winter but during summer it will not be an oven like a 0.49 or 0.62 window would be.

Have you done the calculations to see what the results are or is this another guess?

The 24" overhang is the limit per code due to prevailing winds (uplift). One can try and go more than 24" but it gets very costly, engineering and construction wise.

The westerly windows are low SHGC.

The HOA in his community does NOT allow exterior sunscreens.

I believe he will try and install some sort of interior window screening or tints.

As far as the calculations go, he had his design calculated and he contacted the rep who ran his window calculations/SHG and the rep told him that it might be one of those "in theory" issues. Whatever that means. I guess he is trying to say that in theory the program showed a net gain with a high SHGC but in reality it is not working like it is supposed to. I've seen this happen in the engineering field. Engineers run calcs and put it to paper but at the time of building the home/structure, thing come into play that make the engineer come out to the field and change the calcs and/or design.

I told him to try and get a refund from the guy who did his calculations but the contract has fine print that prevents this.

One can try and go more than 24" but it gets very costly, engineering and construction wise.

The materials might be a bit more, but if you already have an engineering package, it should cost no more to do the calcs for the bigger overhang.

I believe he will try and install some sort of interior window screening or tints.

It'll do something, but it won't solve the problem.

The program showed a net gain with a high SHGC

Do you mean that it showed a net energy savings between heating and cooling with the high SHGC windows? That is very possible, but you also have to run a space by space analysis over the entire year to make sure that no one space gets too warm or cold at any one time.

Well, here is what I have for my design:

18 windows totals:
uPVC Triple Pane Windows - Triple Sealed - Warm Edge Spacers - Gas Filled
Air Infiltration = 0.03 cfm - DP70 - Grade 10 Forced Entry

Window Options include = SHGC of 0.37 - 0.49 - 0.62 / U-Values of 0.13 - 0.14 - 0.14 / VT = 55 - 70 - 73

SOUTH FACING = 12 windows / 308 square feet of windows / 1 operable window
NORTH FACING = 1 window / 42 square feet / fixed glass
EAST FACING = 2 windows / 66 square feet / 2 operable windows
WEST FACING = 3 windows / 72 square feet / 3 operable windows

Latitude = 34.45'25N   Longitude: 112.27'24W  Elevation 4,750 feet - Chino Valley, AZ

Slab-on-grade 2 story ICF Home w/ stucco exterior - R23 Walls - R50 Roof (non vented)

Lbear,

Have you determined the amount of expansion the uPVC windows will experience during a day that starts out cool and then ends up hot.  Some of my homes with uPVC double-hung, south facing windows are difficult to open under these conditions.  For this reason, I no longer use vinyl windows.

If I lived in Arizona, I think my first assumption would be that there was a period in Summer during which I wanted ZERO direct radiation coming in the windows, particularly the south-facing ones. and design the apertures accordingly. During that period, it would be best for the house to function as a cool, dark shelter giving respite from the Summer heat. At some point in Spring and Fall, as you move either forwards or backwards into the heating season you want to start admitting and retaining enough insolation to try and avoid using your heating system, but no more.

Up here, in the Pacific NW, the assumption was that you couldn't have enough sun in the Winter and then slowly start restricting it as the season got warmer. Whatever you were left with in high Summer was just something you would have to deal with some other way. Another reason that works is that up here, you can't count on having direct sunshine every day, even in Summer, so there is some built-in moderation there.

Posted By Alton on 15 Aug 2012 07:53 AM
Lbear,

Have you determined the amount of expansion the uPVC windows will experience during a day that starts out cool and then ends up hot.  Some of my homes with uPVC double-hung, south facing windows are difficult to open under these conditions.  For this reason, I no longer use vinyl windows.

Good point. What I was told and what I have read, I need to leave 1/4" gap on each side of the window frame/buck to allow for thermal expansion of the window unit. Intus uses uPVC (a better form of PVC) and they use a steel tube channel throughout their window frames. This steel channel really gives the window strength and rigidity.

The 6 operable windows that I have are all Tilt&Turn (reverse casements). These windows have built in clearances to take into account any thermal expansion. So whether cold or hot, the windows are designed to operate without issues. They are also adjustable after install. The T&T windows lock in like 7 places with their pins and use 3 gaskets to keep them sealed.

This video shows some of the points I mentioned: IntusWindows     IntusWindows2

My backup choice is aluminum clad/wood but the pricing is quite a bit more for those windows.

Posted By ICFHybrid on 14 Aug 2012 06:23 PM

Wintertime gains from east & west facing windows are negligible compared to the south windows though

The arguments for letting the Easterly sun in a bit are 1) that the East windows are never going to roast you, and 2) people who work outside the home like to have a bit of sunlight come in in the early morning. Those are never going to work for the Westerly windows, though.

Thost argument for east windows work in temperate or mixed climates, but in hot sunny places with high cooling loads letting in the morning sun just kicks off the daily-roast early.  Even if it doesn't measurably change the peak-load of the afternoon it adds to the energy-use numbers.  It's still  possible to daylight an east-facing room without a lot of east-facing window area or high solar gain. 

Most rooms on an east side would have some northern or southern exposure to tap for daylighting, and with sufficient overhangs or awnings on any south facing glass the gain from that can be minimized.  The very oblique angle of the morning sun on N & S glass reflects the bulk of the radiant energy, yet still produces decent lighting levels.

Thost argument for east windows work in temperate or mixed climates

Yup. Wouldn't do it in a hot zone. You want to reserve every bit of nighttime coolness in a hot zone.

Posted By ICFHybrid on 16 Aug 2012 05:53 PM

Thost argument for east windows work in temperate or mixed climates

Yup. Wouldn't do it in a hot zone. You want to reserve every bit of nighttime coolness in a hot zone.

On the north end, it really doesn't matter on what the SHGC is because it never really gets direct sunlight. Agree?

I wouldn't make the mistake of using high SHGC glazing on the North in a hot and sunny climate simply because of the scattering/reflectance issues touched on here. In a gray/cool climate, some people might be wary of anything that serves to reduce the light through the glazing.

Posted By Lbear on 17 Aug 2012 01:57 AM

Posted By ICFHybrid on 16 Aug 2012 05:53 PM

Thost argument for east windows work in temperate or mixed climates

Yup. Wouldn't do it in a hot zone. You want to reserve every bit of nighttime coolness in a hot zone.

On the north end, it really doesn't matter on what the SHGC is because it never really gets direct sunlight. Agree?

It's really site-specific:  If the view out the north window is a large house or building with heat-rejecting low-E windows or a white-sand desert you could have VERY significant reflected heat & light radiating at you from the north.  If it's dark rock & trees, not so much.

Posted By Dana1 on 17 Aug 2012 10:12 AM

It's really site-specific:  If the view out the north window is a large house or building with heat-rejecting low-E windows or a white-sand desert you could have VERY significant reflected heat & light radiating at you from the north.  If it's dark rock & trees, not so much.

So in your opinion would you consider the below terrain to be reflective? The soil is light colored (bottom right corner of photo).

That's a fairly medium non-reflective background- the plants are doing their best to suck up as much of the solar sprectrum as they can make use of without overheating. Fields of ripening wheat would be more than 2x as reflective, dunes white sand would be more than 4x.

You can have everything else working for you, but if there is a light-colored sidewalk or patch directly in front of your windows, you can still get smoked at times.