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 Post subject: What kind of performance do I need?
PostPosted: Mon Apr 16, 2012 8:48 pm 

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I'm still shopping for windows, but I'm wondering about the various glass packages. I live in Dayton OH where we have many more heating days than cooling days. But my wife and I like a cool house, so we run the AC more than most. We have a total of 20 windows, but 12 of them have direct southern exposure with very little shade. One small window is on the west side of the house, none on the east and the remaining 7 windows are on the north side.

The current windows are 20 year old aluminum clad wood windows from Louisiana Pacific that have seen better days. Currently things really heat up unless we keep the blinds pulled on the south side.

Should I be looking at windows with really low SHGC for the south side given our southern exposure and desire to keep the house cool? Or will that mean giving up too much heat gain in the winter months? And how concerned should I be about VT numbers? I've noticed that VT goes down (naturally) as the SHGC numbers go down. Is a VT of say .34 going to be a mistake on a window with southern exposure? The south side faces the backyard, the north side faces the street; I'd still like to be able to see out the windows at night!

Am I over-thinking this? :D

Thanks,
Randy


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 Post subject: Re: What kind of performance do I need?
PostPosted: Mon Apr 16, 2012 9:03 pm 
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You are not over thinking it at all, I commend the thought that you are putting into this purchase. :D For maximum efficiency, you actually want the highest solar gain on the south side of the home (in your climate). I do understand your desire for a cool home in the summer months, but the standard low-e coating on a high quality window will provide a substantial benefit in that regard, perhaps more than you would expect. I would look for a shgc of .25 or higher (much higher for maximum passive solar benefit), and a VT of no lower than .40.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 6:14 am 
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Bravo for an informed poster. :D

Dayton is over 6:1 heating to cooling degree days.

While you like the home cool, you should still focus on a higher SHGC glass for the South facing walls as you will get more free solar energy in the winter months and standard 2 soft coat Low-e will reduce the solar heat by more than 2 fold.

That VT number is way to low. If you are talking about a window without grids, the VT should be 0.50 and higher.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 8:55 am 

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Sometimes you may want to go with a lower SHGC glass even on south windows. Some rooms, especially with a lot of windows get to warm even with shades down and the ac on. The windows on my south side almost always have the curtains down and would not benefit much from a high SHGC glass if at all.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 9:35 am 
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Todd,

The benefit to higher range SHGC glass is the fact that it allows for passive solar heat gain during the heating months.

Standard Low-e coatings (2 coat like Cardinal 272 or Guardian 7138) already have SHGC rates that are less than half that of standard annealed glass.

Couple that massive reduction in solar heat with what will also be a somewhat reduction in actual glass size and you are left with a window that will only let in about 40%, or less, solar heat as compared to it existing unit.

If you look at the passivhaus design concept, they usually orientate the home to face South and place large exposures of glass in that wall. Dark colored floors and good thermal mass items in those room collect the solar heat during the day and let it off slowly when the sun is down.

During the summer months, blinds are pulled down to limit the amount of heat gain.

Even still, most passivhaus designed homes will have glazing in the South wall that is well, well above the SGHC numbers that you see in a typical replacement window (i.e. 0.25-0.30 in double pane).


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 10:00 pm 

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Thanks for the feedback. I've got two estimates scheduled for Thursday from local Soft-Lite dealers. It appears the Imperial LS dual-pane with Super Spacer (u-factor=.27, SHGC=.28, VT=.51) would be a good fit, or should I consider one of the other glass options from Soft-Lite? I think one of the triple-pane packages might be overkill.

I told my wife you guys said I wasn't over analyzing it, but I don't think she believed me. :)

Thanks again for the help!


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 11:01 pm 
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I think you are on the right track.

You are already miles ahead of most consumers so bravo for doing your research.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 11:21 pm 
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I agree, that would be a very good choice. :D


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 Post subject: Re: What kind of performance do I need?
PostPosted: Tue Apr 17, 2012 11:42 pm 

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I don't think we are talking a passivhaus here. With 12 of 20 windows facing south it is very easy to overheat a room with direct sunlight and little shading. I also don't know how big these windows are or how many are in a given room or what is in there to absorb the heat.He may be trading off some real world comfort in order to save a little on his heating bill.I don't think he'll be seeing much in savings from .28 to .22 SHGC.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 12:18 am 
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toddinmn wrote:
I don't think we are talking a passivhaus here. With 12 of 20 windows facing south it is very easy to overheat a room with direct sunlight and little shading. I also don't know how big these windows are or how many are in a given room or what is in there to absorb the heat.He may be trading off some real world comfort in order to save a little on his heating bill.I don't think he'll be seeing much in savings from .28 to .22 SHGC.


No, we are not talking passivhaus here and hence the reason that I previously pointed out that passivhaus designed homes specify much higher SHGC range glass. SHGC in the range of 0.28 to 0.22 would never even enter the conversation.

It was also pointed out that in order for the passive solar equation to work, there needs to be thermal mass in the South facing rooms to take full advantage.

More information would be helpful in this case but based on what are likely going to be standard sized windows, the better choice for his climate would the the SHGC of 0.28 vs. 0.22.

Heating a home (assuming that we are talking about a home in Dayton with a 6:1 HDD to CDD ratio) is almost universally more expensive that cooling a home given the cost of producing Btus from gas, oil, or electricity. Cooling a home and/or controlling some of the radiant heat is cheaper and easier.

A SHGC of 0.28 is going to be at least, if not more, 2X more effective at blocking radiant heat from the sun than his current windows. Depending on the size of the wood window and the size of the insert, additional glass loss may bring that total radiant heat reduction to 2.5X or more. That will likely be more than enough of a reduction for the client to feel cooler in his home.

The poster is going to note some considerable reduction in radiant heat straight away and going to and SHGC range of 0.22 would be more commonplace for homes with a 50/50 split on HDD to CDD.

The best interpretation that I could gather from study I read on zero energy showed that a home with 60% Southern exposure in a Minneapolis climate showed that the window with the higher SHGC and less efficienct U-Factor (SHGC = 0.53 and U-Factor = 0.37) was still closer to zero energy than the more efficient window and lower SHGC (SHGC = 0.30 and U-Factor = 0.34).

While Minneapolis is a bit more HDD to CDD dominant, Dayton is closer to that climate than South Carolina. We are also still at what they would consider the low range of the recommend spectrum of SHGC in talking about a 0.28.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 1:30 am 

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What kind of glass is in the original windows? How much do think he would save going going from .22 to .28 in his bill? I have been in rooms that get to hot even with standard low-e glass and would take the trade off.I'm sure the poster would be fine with the windows proposed,just don't want to see him unhappy with his product.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 7:54 am 
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toddinmn wrote:
What kind of glass is in the original windows? How much do think he would save going going from .22 to .28 in his bill? I have been in rooms that get to hot even with standard low-e glass and would take the trade off.I'm sure the poster would be fine with the windows proposed,just don't want to see him unhappy with his product.


I would venture a guess that he has either single strength double pane or single strength single pane with a storm. Both net roughly the same SHGC.

It would be an impossibility for me to venture a guess as to what he would save as his home is not a laboratory and there are far to many variables and undefined conditions.

I am basing my recommendations on scientific studies of zero energy windows and everything tested indicates that in Heating degree day dominated climates, it is better to go with the higher SHGC range glass for more passive solar.

In their recommendations, they were recommending going from a SHGC of 0.30 to 0.53. I would certainly be hesitant given his complaints of heat in recommending a SHGC of 0.53 without making him fully aware of that windows performance and the amount of radiant heat it will allow.

In this case we are talking about a window at 0.28. That is below the lower end of the scale in the study I am referencing and such a huge reduction over what he currently has that I don't think it makes any sense to go further below that.

If you are spending $6 to heat your home vs. $1 to cool it, wouldn't you concentrate on the free heating aspect of it and "tune" the drastically reduced summer heating with shades and blinds while still allowing for the free wintertime passive solar.

I think the customer is on the right track and if he wants to add any additional details, I am sure we can hash this out further but a window with a 0.28 SGHC would be my recommendation over the 0.22.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 8:26 am 
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I would agree with WoW, and I think that the key for SHGC in most homes is moderation, meaning .25 to .35 is the ideal range to prevent that intense heat, while still allowing some passive solar. You are balancing comfort and efficiency in that range, and I think when you stray too far one way or the other you are sacrificing something. BTW, these comments apply to the location of the OP, other regions will differ.


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 5:20 pm 

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First, I'm glad I could start such a lively debate and I truly appreciate everyone's time and opinions.

Yes, I have what I believe are clear glass dual pane windows, circa 1991.

The rear of the house faces south (slightly south-west actually).

I have a two story with a walkout basement. Two DH windows are in the basement; they're both 42"x55". One of the two basement windows is under the deck and is shaded most of the day. The other is in full sun till late afternoon.

The first story is the problem. No shade EVER; bakes in the sun all day. It's has two DH 38"x54", two casements 24"x61", one picture 48"x61", one DH 37"x38", one DH 33"x38". There's also a 5' patio door that I'll replace someday, but I has internal blinds we keep closed most of the time. The first floor is where we spend the most time and where most TV viewing occurs. Unfortunately the TV faces the windows. We have wooden blinds, but keeping them closed is the problem. We have a nice view out the back and my wife likes to enjoy it.

The second floor only has has three windows: two DH 37" x 54", one 33"x38". They receive a little shade from the overhang. They all have blinds that are kept closed most of the time.

This afternoon is a bright sunny day, but it's only about 70 degrees. AC is on. At 5:30 pm the two basement windows were mostly shaded, the 1st floor was in full sun, and the 2nd floor windows were about 35% shaded. It's about 69 degrees in the basement, about 74 on the first floor. It's probably 5 degrees warmer on the second floor.

My infrared thermometer shows a temp of over 90 degrees on the basement sash rails near the glass. I got 108 degrees on the 2nd floor casement and picture windows on the edge of the sash!

The info at http://www.efficientwindows.org/city_all.cfm?id=30 reinforces what you guys have said about wanting a higher SHGC. Looking at the variance in total costs, though, I'm not sure that the added winter heat gain with a medium solar-gain window is worth keeping the blinds closed in the summer and making the wife unhappy. The again, I suspect anything I choose will be a HUGE improvement over what I have now!

Thanks,
Randy


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 Post subject: Re: What kind of performance do I need?
PostPosted: Wed Apr 18, 2012 6:25 pm 
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Do you have grids?

Those windows are of modest size with the picture being the larger unit.

The internal mini-blinds are a great option but I wish that Soft-Lite had tilt and raise vs. tilt only.

Given the modest sizes of the openings, standard Low-e (SHGC around 0.25-0.28 in most cases) is going to provide considerable reductions in solar heat.

If you have grids, the SHGC numbers usually drop by about 0.03 points.

That large a temp shift in floors would have me take a look at the insulation levels in the attic as well.

Think about your car windows. They are cast usually with a hard coat low-e that does not provide the same drop in solar heat gain and you can feel a considerable different between the window being open and being in the up position. The delta on your new units will be even greater.

If you are still concerned at the end of the day, go with triple pane on the back of the home. If you are going to suffer via the loss of passive solar, you might as well have the greater U-factor out of the unit. You will loose about 10% more VT by comparison as well.


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