Hi,

I live in Mountain View, CA.  My house is heating only, with no AC (its pretty cool here).  I am looking for replacement windows and have found alot that are low U and low SHGC.  I am trying to find a good supplier that also has high SHGC and low U windows for the south facing side of my house.  I found on Cardinal their LoE-180 window glass, but I can't seem to find any company that uses this?

My wife is less interested in the U / SHGC factors and more concerned with the look/feel/style, etc.  So, i doubt I can mix and match the south window style/company with the rest of the windows and need a single supplier.

Does anyone know of window suppliers I could acess here that might have low U windows with both high SHGC (0.50 or above) and low SHGC windows?

Thanks,

John

John-

I don't know of suppliers specific to San Jose, CA, but I used Pella Designer windows in my house in Colorado, and mixed high and low solar gain windows on the same house. I used high solar gain on E, S, and W sides, and low solar gain on the north side. I am in a heating only climate and do not have A/C. These triple-pane windows have the following characteristics:
High SHGC: SHGC = 0.49, U = 0.31, VT = 0.52
Low SHGC: SHGC = 0.28, U = 0.29, VT = 0.48
I do not notice a difference in the appearance of the low and high SHGC windows from inside or outside. These windows are relatively expensive, and for that reason I do not necessarily recommend them in preference to other windows. I chose triple pane windows so that I could have high SHGC windows and still meet the U <= 0.35 requirement in this zone. The house was being built remotely from where I was living, so that it was awkward to talk with the county inspectors to justify cheating on the U<= 0.35 requirements to get the high SHGC that were much more efficient for passive solar heating. The U<=0.35 is a poor code requirement in this area since it discourages folks from using high SHGC in this sunny, cold area.

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

I live in Mountain View, CA. My house is heating only, with no AC (its pretty cool here). I am looking for replacement windows and have found alot that are low U and low SHGC. I am trying to find a good supplier that also has high SHGC and low U windows for the south facing side of my house. I found on Cardinal their LoE-180 window glass, but I can't seem to find any company that uses this?

I am in the same general area. Wondering what you decided to do and if you are satisfied with the window choice. Would love to hear any advice.
Thank you.

It appears that you are going the correct route. According to the heat loss program for houses that specializes in windows, RESFEN, the heat losses for a well-insulated house with high solar gain, low-e windows on all four sides in the San Francisco area could save roughly 35% in heating costs compared to the same house with low solar gain, low-e windows (http://www.residentialenergylaborat...codes.html).

You might contact Cardinal Glass and ask them which manufacturers use their LoE-180 glass in their windows.  It can take some effort to track down suppliers for high solar gain windows.  Serious Windows is certainly a manufacturer that will supply high solar gain windows, but their prices are very high, and some people worry about the longevity of their heat mirror (plastic) films although they offer (or used to offer) a lifetime warranty.

Posted By Lee Dodge on 06 Mar 2012 11:44 PM

...You might contact Cardinal Glass and ask them which manufacturers use their LoE-180 glass in their windows... 

I know that Marvin Integrity All Ultrex (fiberglass framed windows) are offered with Cardinal LoE-180 glass.  If you download the Architectural Detail Manual (ADM) from here you can see the performance data for the LoE-180 glass.

Pella also offers a similar glass package in their fiberglass Impervia line.  Pella calls it "NaturalSun".  Here is the performance data for their single hung windows:  http://www.pellaadm.com/userdocs/documents/F2SH_GP.pdf

Posted By arkie6 on 07 Mar 2012 12:45 AM

I know that Marvin Integrity All Ultrex (fiberglass framed windows) are offered with Cardinal LoE-180 glass.  If you download the Architectural Detail Manual (ADM) from here you can see the performance data for the LoE-180 glass.

Pella also offers a similar glass package in their fiberglass Impervia line.  Pella calls it "NaturalSun".  Here is the performance data for their single hung windows:  http://www.pellaadm.com/userdocs/documents/F2SH_GP.pdf

The Pella ratings are not that great IMHO. The Pella SHGC rating of .45 or higher have a U-factor of .35 or higher, which is not that great of a U-Factor. During winter and in the daytime sun you will get good SHG but then at night the windows will work against you and convection will take over and start to suck all of the interior heat, literally right out that same window. It's 3 steps forward and then 3 steps back. At night or during the a cloudy day with low solar gain, those windows will totally work against you.

If you look at any Net Zero Home or  any LEED Home, if they utilize Passive Solar, the U-Factor has to be in the  0.15 -  0.22 range. The heat loss from a U - 0.18 window is 51% LESS than a U- 0.35 window.

Net Zero Home - Passive Home

Also, that single hung has an air infiltration of .8 - 1.0 at 75Pa. A casement or awning window has an infiltration of .05 at 75Pa. For a Net Zero/LEED home, they recommend a rating of  .3 or lower. The single hung has > .8, which means it leaks 2 to 3 times more air than a casement or awning.

I wasn't providing a window recommendation, just pointing out some nationally available brands of windows that I have come across in my research of fiberglass framed windows that have high solar heat gain glass available.

Yes, Serious Windows are quite expensive. The North side of the house (very shaded with redwoods) is all aluminum clad that was updated 12 years ago so I would like to match as closely as I can the look. I am just researching the difference between fiberglass and aluminum clad and still trying to understand what would be right for the South side of the house which currently has the original single pane paper thin 1950's windows. It may also be that we will have to use different glass for different sets of windows. One part of the South wall is shaded, while another gets intense sun. It's very tricky to optimize everything (U-factor, SHGC, aesthetics and price!) in this climate with the large temperature differences (e.g. today we woke up to 45F but it is predicted to get to 72F).

I'd appreciate any pointers to articles/opinions about fiberglass vs. aluminum as that is probably the first question I need to research and answer for our needs and then I can start calling around to see about the Cardinal glass LoE-18.

Thank you so much for your time and effort from this novice!

arkie6-

Thanks for tracking down some specific high solar heat gain coefficient (SHGC) windows.

dljmth-

In your climate zone, if you have solar energy to work with on some windows, then the windows can provide net energy gains rather than losses, so you are wise to consider high SHGC windows. I ran some numbers in RESFEN for a 1600 sq. ft. house, 66.7 sq. ft. of windows on each of 4 sides, that is well insulated to emphasize the effect of window selection. The insulation package is the same one described in the link that I provided earlier. You currently have some single pane windows, so I ran them on all 4 sides of the house. Lbear mentioned a window with a U-factor in the range of 0.15 - 0.22 Btu/hr-ft^2-F, and I had a Serious window in the data base with U = 0.19, SHGC = 0.22, so I ran that one. arkie6 provided a reference to Pella window specs, and the most suitable high SHGC double-pane was U = 0.31, SHGC = 0.54, so I ran that one. Since you don't use AC, I quote the results for annual heating energy only, and the whole house heating cost for natural gas (fuel cost only):

Single pane, aluminum: 18.7 MillionBtu, $171.52
Serious, U=0.19, SHGC=0.22: 11.5 MillionBtu, $105.26
Pella, U=0.31, SHGC=0.54: 7.0 MillionBtu, $64.58

Note that in all cases if you have unblocked solar radiation, the south-facing windows provide a net gain, NOT a net loss of energy, so you are wise to consider high SHGC windows. In the case of the Serious and Pella windows, the south, east, and west windows are all predicted to provide net energy gains, not losses. The RESFEN results suggest that there is no reason to mix and match high and low SHGC windows in your climate if you have solar radiation available. Since the solar radiation is irregular, you could choose to mix and match. In my house, I have both, and I cannot see any difference in the appearance.

The RESFEN results suggest that the heating costs associated with the windows are not that great, so I would not worry too long and hard about window selection. Just don't overspend on windows since there is not that much gain to be had. Those costs presented above will appear low relative to your actual costs because the house modeled is modest in size and very well insulated, and the costs do not include natural gas monthly connection fees.

Posted By dljmth on 07 Mar 2012 01:11 PM
Yes, Serious Windows are quite expensive. 

I am about to get a quote on SW. I had a previous quote on a different home for the 600 series window with Q10 glass. For a 30 x 48 they were $300 per, for a 72 x 60 they were $420 per window, delivered. For 25 windows, the total was $9,700 delivered. The U-Factor on these windows ranged from 0.14 - 0.21. To me that is very reasonable and far from "quite expensive" for a great performing window.

I will get another quote for a different set of windows and should have the numbers back in a week. I will post them here, if they are expensive I will eat my words but if they are anything like the above quote, they are far from expensive and actually beat out Pella and Anderson, or any of the mega box store window manufacturers.

Posted By Lee Dodge on 07 Mar 2012 02:16 PM
arkie6-


Single pane, aluminum: 18.7 MillionBtu, $171.52
Serious, U=0.19, SHGC=0.22: 11.5 MillionBtu, $105.26
Pella, U=0.31, SHGC=0.54: 7.0 MillionBtu, $64.58


The RESFEN results suggest that the heating costs associated with the windows are not that great, so I would not worry too long and hard about window selection. Just don't overspend on windows since there is not that much gain to be had. Those costs presented above will appear low relative to your actual costs because the house modeled is modest in size and very well insulated, and the costs do not include natural gas monthly connection fees.

Lee -

What would the RESFEN results show if you ran a Serious Window: U = 0.22, SHGC = 0.54  (which is what the Passive House had in Oregon)

The SHGC of 0.54 would be identical to Pella but with a 27% lower heat loss due to the U= 0.22 of the Serious vs. the 0.31 of the Pella.

Lbear-

You said: "If you look at any Net Zero Home or any LEED Home, if they utilize Passive Solar, the U-Factor has to be in the 0.15 - 0.22 range."
Therefore, I picked out a window closer in U-value to the middle of that range (0.19), rather than at the end of the range.

I will let you exercise the model for other sets of parameters. That might drive home the point that solar heat gain coefficient is more significant than U-value in heating dominated, sunny climates.

By the way, I do not agree with your statement about the window requirements for a net-zero energy home. As one counterexample, I would offer the Habitat for Humaniity Net-Zero House in Denver that was designed by engineers at NREL who used "Double-glazed, low emissivity (U-value = 0.30 Btu/h-F-ft2) high solar heat gain coefficient (SHGC = 0.58) glass was chosen for the southern windows" (from http://www.nrel.gov/buildings/pdfs/43188.pdf). This was a net-zero energy house that was designed with incremental costs in mind. They show an incremental cost of $28,054 over the standard practice house in this area.

Posted By Lee Dodge on 07 Mar 2012 04:46 PM
Lbear-

You said: "If you look at any Net Zero Home or any LEED Home, if they utilize Passive Solar, the U-Factor has to be in the 0.15 - 0.22 range."
Therefore, I picked out a window closer in U-value to the middle of that range (0.19), rather than at the end of the range.

I will let you exercise the model for other sets of parameters. That might drive home the point that solar heat gain coefficient is more significant than U-value in heating dominated, sunny climates.

I am no expert by any means but I thought that a window with a high SHGC and a high U-Value would be counterproductive. In wintertime the south facing windows might see only 6 - 8  hours of direct sunlight (of course depending on Lat/Lng). This would mean that for 16-18 hours, the SHG for those windows are ZERO and are working as a net loss if they have a high U-Value as convection is the energy robber. A windows with a U-Value of 0.30 will lose heat from the home at a 30% higher rate than a U-Value window with 0.20. This would equate to 16-18 hours of energy loss at a 30% higher rate.

Is there a point of diminishing returns on SHG vs. U-Value? If a window had a SHGC of 0.60 and a U-Value of 0.50, would it mean that SHG is still the significant factor and the U-Value has little to no meaning? Once again, I am no expert, I am just trying to understand this as something is just not making sense to me. I will continue this point in the next post...

Posted By Lee Dodge on 07 Mar 2012 04:46 PM

I will let you exercise the model for other sets of parameters. That might drive home the point that solar heat gain coefficient is more significant than U-value in heating dominated, sunny climates.

By the way, I do not agree with your statement about the window requirements for a net-zero energy home. As one counterexample, I would offer the Habitat for Humaniity Net-Zero House in Denver that was designed by engineers at NREL who used "Double-glazed, low emissivity (U-value = 0.30 Btu/h-F-ft2) high solar heat gain coefficient (SHGC = 0.58) glass was chosen for the southern windows" (from http://www.nrel.gov/buildings/pdfs/43188.pdf). This was a net-zero energy house that was designed with incremental costs in mind. They show an incremental cost of $28,054 over the standard practice house in this area.

Denver is a heat dominated sunny climate but there is a catch to this.

I did look up that study home you referenced and I found this important statement they made: (emphasis mine)

"... heat mirror windows on the north, east, and west and double-pane high SHGC low-e windows on the south. The southern window area is enlarged for solar gain.

The indoor winter temperatures averaged about 68F; however, the indoor temperature commonly oscillated by about 6 to 8 (degrees) F because of solar gain on sunny winter days. During the summer, the indoor temperatures tended to track outdoor temperatures and often exceeded standard comfort conditions. The indoor temperature remained lower than outdoor temperatures during hot sunny periods, which indicates the southern overhangs prevented overheating. On two occasions the indoor temperatures exceeded 90oF for several hours. The homeowner reported that indoor temperatures were similar to those in a Habitat Metro Denver standard practice house next door. She was coached to open the windows during the cooler evenings and close them during the hot days. However, security concerns prevented her from opening the windows at night."

What really struck me was the following, "The homeowner reported that indoor temperatures were similar to those in a Habitat Metro Denver standard practice house next door." This in essence states that the home performed poorly during summer. It performed on par with a standard build home. While one can debate as to why it performed poorly in the summer I would say it is safe to state that the high SHGC and high U-Value windows backfired during summer months. Even with the overhang, as overhangs are not 100% effective, especially during spring and fall months when it can be hot outside and the sun is at a bad angle and contributing to a heating load when a cooling load is needed.

Was the project a failure? Of course not but it was not a success either. A home that is comfortable in the winter but miserable in the summer (90F+ interior temps), is not a success. Installing an A/C unit would drop the interior temps but the home would no longer qualify as a net zero home unless other counter measures were done. Here is where the high SHGC and high U-Value windows come into play. In my opinion, the windows are hurting the home. The high SHG and high U-Value is great in the winter but is a killer in the summer. A window with a high SHGC and a high U-Value, in this case 0.30, will not perform as well in the summer as a window with a < 0.20 U-Value.

Thanks for all the input.

A home that is comfortable in the winter but miserable in the summer (90F+ interior temps), is not a success. Installing an A/C unit would drop the interior temps but the home would no longer qualify as a net zero home unless other counter measures were done. Here is where the high SHGC and high U-Value windows come into play. In my opinion, the windows are hurting the home. The high SHG and high U-Value is great in the winter but is a killer in the summer. A window with a high SHGC and a high U-Value, in this case 0.30, will not perform as well in the summer as a window with a < 0.20 U-Value.

I am no expert by any means but I thought that a window with a high SHGC and a high U-Value would be counterproductive. In wintertime the south facing windows might see only 6 - 8 hours of direct sunlight (of course depending on Lat/Lng). This would mean that for 16-18 hours, the SHG for those windows are ZERO and are working as a net loss if they have a high U-Value as convection is the energy robber. A windows with a U-Value of 0.30 will lose heat from the home at a 30% higher rate than a U-Value window with 0.20. This would equate to 16-18 hours of energy loss at a 30% higher rate.

This is exactly my confusion. Not sure what makes the most sense in our climate. Today was a great example. We had a beautiful 74F day and up until about 11am I kept the shades open to let in the sun. Then it started getting really warm in the house so I closed the shades and will keep them closed through the night because the temp will drop to 45F and the window coverings help (these are still the single pane, leaky windows mind you) - but the effect is the same. Alternatively, we've had really cold days where despite the sun shining, there is so much cold air coming into the house (yes, I know these are leaky old windows) that we close the shades so we have some insulation. Hot days (although there are few) could mean it will get really hot inside the house with a high SHGC. Cold days, it can still be cold in the house with poor performing windows. Maybe we'll just still need to rely more on shades/curtains to control the heat loss in the winter and the blazing heat on those few really hot summer days.

As for brands, turns out there is a Serious dealer less than a mile from us so I will go check them out tomorrow. Still not sure about fiberglass vs. aluminum.

On a side note, I find it frustrating that I can't go to a window vendor and show them my plans and have them tell me what I need. It's disappointing that I have to tell them. Not a lot of value-add.

Posted By dljmth on 08 Mar 2012 01:02 AM
Thanks for all the input.


This is exactly my confusion. Not sure what makes the most sense in our climate. Today was a great example. We had a beautiful 74F day and up until about 11am I kept the shades open to let in the sun. Then it started getting really warm in the house so I closed the shades and will keep them closed through the night because the temp will drop to 45F and the window coverings help (these are still the single pane, leaky windows mind you) - but the effect is the same. Alternatively, we've had really cold days where despite the sun shining, there is so much cold air coming into the house (yes, I know these are leaky old windows) that we close the shades so we have some insulation. Hot days (although there are few) could mean it will get really hot inside the house with a high SHGC. Cold days, it can still be cold in the house with poor performing windows. Maybe we'll just still need to rely more on shades/curtains to control the heat loss in the winter and the blazing heat on those few really hot summer days.

As for brands, turns out there is a Serious dealer less than a mile from us so I will go check them out tomorrow. Still not sure about fiberglass vs. aluminum.

On a side note, I find it frustrating that I can't go to a window vendor and show them my plans and have them tell me what I need. It's disappointing that I have to tell them. Not a lot of value-add.

Where are you located?

Therein lies the balance between high SHGC and a low U-Value. Along with window size, placement and southern exposure angles. These all play a critical part in passive solar technology.

You can have a window with a high SHGC of 0.50 or higher and then that same window have a U-Value of > 0.40. This type of window will bake you in the day and then freeze you at night. Especially if the outdoor temps drop a lot (40s and colder) at night, this type of window creates a reverse campfire effect. That is why it will always feel colder sitting or standing next to those types of windows, especially at night or when it is cold outside.

That is why HVAC vents are placed at windows and doors, to help with moisture/condensation control and because these are your biggest heat loss areas (windows).

In regards to that test home in Denver, they dropped the ball and in a round about way admitted to it but blamed it on Denver's summers getting hotter. Denver's summers can get very hot, all they needed to do was look up the weather history on the area. Of course they were focused on heating the home during winter and then when summer came, the poor owner was baked out of the home with interior temps reaching 90F+. The solution they gave? Open the windows. The owners response was that it was UNSAFE to leave open windows at night in central Denver. I found that funny...

The moral of the story is that very few climates are 100% heating only climates. Most areas in the USA are a mixture of both (heating and cooling) with an emphasis on heating if you live in the northern portion of the continental USA. That doesn't mean that one will not need A/C in Green Bay or Chicago, as they can see 90F+ temps in the summer.

What technology now offers is a window with a high SHGC and a low U-Value. This allows one to have their cake and eat it too. Yes, they are more money but technology costs $$$.

Where are you located?

Sunnyvale / Mountain View area in Northern California near San Jose.

Posted By dljmth on 08 Mar 2012 01:22 PM

Where are you located?

Sunnyvale / Mountain View area in Northern California near San Jose.

It's a mild climate with a lot of sunshine but it does get some extreme high/low temps every so often. You have seen temps as high as 109F and as low as 19F but overall it is very mild.

You basically want to get moderate solar gain in the winter but you have to be careful because you are not a heating dominated climate. Depending on your homes position to the suns angles, overhangs, insulation, you don't want to get too crazy on high SHG windows. Or you will end up doing what you are doing now, closing the shades in winter during the daytime to keep from overheating the home. Or you will have to do what the Denver resident was told to do, "open your windows at night" to cool down the home.

You are in a mild climate so you have to keep that in mind while choosing windows. Remember, you said that you have to draw the shades in winter during the daytime to keep from overheating the home. I bet the windows you currently have are letting in tons of SHG. So much so that you are getting baked inside during wintertime. The windows lack good U-Value so at night all of that interior heat goes right out the window and you get cold at night.

Your average high in winter is 58F and your average low is 42F. You can also see 70s during winter, as you are right now. Which leads to the problem you are encountering right now with high SHG making the home too hot during the daytime in winter. Unless you want to play the curtain game you are playing now, you have to be conservative on your SHG window choice.

NOAA SOLAR CALCULATOR

Posted By Lbear on 07 Mar 2012 09:56 PM

Is there a point of diminishing returns on SHG vs. U-Value? If a window had a SHGC of 0.60 and a U-Value of 0.50, would it mean that SHG is still the significant factor and the U-Value has little to no meaning? Once again, I am no expert, I am just trying to understand this as something is just not making sense to me. I will continue this point in the next post...

Just going by the numbers, there are five things that combine to define window performance: (1) U-value (thermal conductivity), (2) solar heat gain coefficient (SHGC), (3) outdoor air temperatures through the year, (4) solar insolation, and (5) latitude.  (Wind speed is another factor, but is usually left out of the analysis, and window leakage is important, but there are usually no specs for leakage other than a maximum spec.)  It would be difficult trying to select the two window parameters U-value and SHGC for various locations, BUT the hardworking engineers at the U.S. national labs have made it easy for us, at least those of us in the U.S.  Free software, specifically RESFEN (Residential Fenestration, written at Lawrence Berkeley National Laboratory) and BEopt (Building Energy Optimization, written at National Renewable Energy Laboratory and using other Dept. of Energy computing engines) take into account those 5 parameters listed above, and do all the thinking for you.  This is not difficult; you just read the results.  For window tradeoffs, RESFEN is easier to use.  For complete detailed house analysis, BEopt is prefererable.  Both these models have a library with many window specs given, but you can also add any window spec that you want into eiither program.  (There are many other computer models that could also be used.) 

Lbear, you would like things to be simpler.  Move to Germany where the sun never shines in the winter (exaggeration intended), and you can just rely on U-value.  That is part of the reason that the Europeans focus so strongly on U-value.  To do that in cold, sunny parts of the U.S. can be folly, although that fact has not stopped Energy Star and IECC from passing some (in my opinion) silly specs for windows that discourage passive solar heating.

Posted By Lbear on 07 Mar 2012 10:05 PM

What really struck me was the following, "The homeowner reported that indoor temperatures were similar to those in a Habitat Metro Denver standard practice house next door." This in essence states that the home performed poorly during summer. It performed on par with a standard build home. While one can debate as to why it performed poorly in the summer I would say it is safe to state that the high SHGC and high U-Value windows backfired during summer months. Even with the overhang, as overhangs are not 100% effective, especially during spring and fall months when it can be hot outside and the sun is at a bad angle and contributing to a heating load when a cooling load is needed.

There is no debate about why it performed poorly in the summer.  If you close up a box and put it outside with no ventilation day or night, it will approximately follow the outdoor temperature but be moderated by the fact that it is insulated, and it will achieve a higher average temperature due to solar heating.  You blame the heat on the high SHGC windows, but the statement was that the indoor temperatures were similar to the standard practice house next door, NOT higher than the standard house that presumably did have low SHGC, low-e windows.  

The well insulated house could easily be fixed to be more comfortable by using a work-around to overcome the stubborn occupant.  Just put a whole house fan that brings in fresh air at night (after 8 PM) until it reaches 66 F in the house and then regulates to that temperature until 9 AM when it shuts off for the day.  It would need to roughly seal like a poulltry house fan with louvers, and be sealed up in the winter.     

So let me offer a different counterexample to your statement "If you look at any Net Zero Home or any LEED Home, if they utilize Passive Solar, the U-Factor has to be in the 0.15 - 0.22 range."  My house is a net-zero source energy house, and the windows have U=0.29, SHGC=0.28 (north and part of east) and U=0.31, SHGC=0.49 on the other sides, with no gas fill in either due to the high-altitude limitations.  This is a production house poorly aligned for passive solar, but it certainly has met net-zero source energy so far (http://www.residentialenergylaborat...y_use.html).  This house does not include air-conditioning, and last summer temperatures were in the 90's on some days.  I think the highest indoor temperature that I saw was 76 F (24 C).  I DID open windows at night, and closed them in the morning, and also closed shades on the east and west sides at appropriate times of the day to limit solar gain, very standard practice in Colorado, except in the ghettos apparently.  (The south side is mostly shaded by the overhang.)  Well insulated homes work well against summer heat if you have cooling at night and ventilate the house to make use of it.  This is not a passive house, since it uses active solar systems, and the inhabitant is not passive in that he can open windows and shades when necessary, but it meets net-zero source energy. 
 
So I think this same strategy could be used in the San Francisco (Mountain View) area.