Browse Forums Windows & Doors 1 Nov 20, 2008 2:57 pm I just went to the bother of looking up different SHGCs of different windows and finally realised what should have been obvious: double glazing dramatically lowers SHGC.
Examples: Dowell 3Clr single, 6.4, 0.74 Dowell 4EA single, 4.5, 0.61 Dowell 3/12/3, 3.9, 0.57 EcoClassic 5/10/5 3.5, 0.56 Compare the top one with the bottom one, U drops by 45% and SHGC drops by 25%. Does anyone have a feeling for how much this effects their usefulness in passive solar design for cold climates? If conductive/convective heat loss is reduced but solar gain is also reduced then you need to make sure the heat you avoid losing outweighs the heat you avoid gaining. Perhaps there is a case for leaving north-facing windows single glazed? If they are going to give you most of your heat input (including heating) and only a (perhaps sizeable) minority of your heat loss, then if you double glaze you have might have a net heat loss. Am I missing something? Anyone know if this ever works out this way in practice e.g. as modelled in software? P.S. Anyone know what EA and EG stand for in the WERS glazing types? Re: U-value vs SHGC & double glazing 2Nov 20, 2008 3:50 pm Hi Russell,
clear double glazing reduces the solar gain about 5%. But single glazing will make you loose what you have gained very fast at night time. Therefore single glazing is not advisable in any direction of the house. Single glazing = no insulation, a hole in your thermal envelope Re: U-value vs SHGC & double glazing 3Nov 20, 2008 4:15 pm russell Does anyone have a feeling for how much this effects their usefulness in passive solar design for cold climates? If conductive/convective heat loss is reduced but solar gain is also reduced then you need to make sure the heat you avoid losing outweighs the heat you avoid gaining. Perhaps there is a case for leaving north-facing windows single glazed? If they are going to give you most of your heat input (including heating) and only a (perhaps sizeable) minority of your heat loss, then if you double glaze you have might have a net heat loss. Am I missing something? Anyone know if this ever works out this way in practice e.g. as modelled in software? P.S. Anyone know what EA and EG stand for in the WERS glazing types? In a heating climate the overall impact of SHGC is far outweighed by the loss of U value. That is why people like Burbank use double glazing on every side in every orientation. EA is a proprietary brand of Low-E glass (energy advantage). EG is the same but it slips my mind. Peter from Ausdesign may be able to give you model data. Incidentally, Low-E glass is a one way solution... if you number the glass surfaces from the outside, 1, 2, 3, 4, then in a heating climate we place the active surface of Low-E on surface 3, where it keeps the heat inside the home and in a cooling climate on surface 2 where it keeps the heat outside the home. The ideal window fitted with Low-E would have a swivel sash that could reverse the effect between winter and summer.. But notice that the active surface of Low-E is always between the panes - never on the outside. There are 2 types of Low-E, hard & soft. One time only soft was available and it was being used on single glazing in the belief it was as good as double glazing. Well it wasn't and the surface was very prone to damage, so it got a bad name. Also it never works in a single glazed situation as it needs to be adjacent to still air to be effective. In double glazing it is fine and works very well. "ECOECO" At 'EcoEco', we design windows, we design the best windows, we do it for you, so that when you’re happy we are happy. Tel. 1800 326 326 Re: U-value vs SHGC & double glazing 4Nov 20, 2008 4:34 pm Thanks for that --
EcoClassic Also it never works in a single glazed situation as it needs to be adjacent to still air to be effective. In double glazing it is fine and works very well. I was wondering about that-- from a physics perspective if a portion of the absorbed heat is not re-emitted, once the temperature of the glass rises as far as it's going to, the asborbed energy it will have to be conductively dissipated somehow -- all things being equal half of it will be into the house. Double glazing would largely prevent that. So, is that effect reflected in the WERS? What air movement, if any, is present in the testing? Re: U-value vs SHGC & double glazing 5Nov 20, 2008 7:26 pm russell e a feeling for how much this effects their usefulness in passive solar design for cold climates? If conductive/convective heat loss is reduced but solar gain is also reduced then you need to make sure the heat you avoid losing outweighs the heat you avoid gaining. Perhaps there is a case for leaving north-facing windows single glazed? If they are going to give you most of your heat input (including heating) and only a (perhaps sizeable) minority of your heat loss, then if you double glaze you have might have a net heat loss. This can be modeled but the conventional wisdom by passive solar designers is that you can get adequate storage in your thermal mass despite the lower radiant gain with double glazing. Mathematically, the radiant heat flux on a sunny day is an order of magnitude higher than the convective loss through a single glazed window. e.g. 1000 watts/sqm vs 100 watts/sqm. You are charging your house with 10 x more heating energy than is lost in that same period of time. This is the whole concept behind passive solar design. However, to make it work you need the right amount of exposed solar mass for the amount of glazing. It can be a big problem if you don't have enough. During the day, the air temperature will rise to uncomfortable levels because there is no excess mass to store the energy. When night falls, air temperature rapidly declines through the uninsulated window and you have no heat capacity to offset this loss. Re: U-value vs SHGC & double glazing 6Nov 20, 2008 7:49 pm russell I was wondering about that-- from a physics perspective if a portion of the absorbed heat is not re-emitted, once the temperature of the glass rises as far as it's going to, the asborbed energy it will have to be conductively dissipated somehow -- all things being equal half of it will be into the house. Double glazing would largely prevent that. Here is a good overview of how low-e glass works. http://www.awa.org.au/documents/HowComf ... sWorks.pdf it is spectrally selective. It has a different emissivity for long wave (far-infrared) radiant heat that is usually absorbed or emitted from furnishings and people to that of short wave (near-infrared) solar radiation. Emissivity follows the Stefan Boltzman law where the rate of radiated energy is proportional to the fourth power of the temperature of the object (in this case the window). There is some contribution by conduction but not much due to the air gap. I cannot find a reference in WERS to say this effect is incorporated in the U value or SHGC. Theoretically, it is an independent parameter of the thermal characteristics of a window. Re: U-value vs SHGC & double glazing 7Nov 20, 2008 8:02 pm dymonite69 This can be modeled but the conventional wisdom by passive solar Mathematically, the radiant heat flux on a sunny day is an order of magnitude higher than the convective loss through a single glazed window. e.g. 1000 watts/sqm vs 100 watts/sqm. You are charging your house with 10 x more heating energy than is lost in that same period of time. Right, so your net gain during the day is e.g. 900 W/m2. Then, switch to double glazed, your solar heat gain drops to ~750 W/m2 due to lower SHGC, and your convective heat loss drops to 55, so your net gain is 695 W/m2. So you'd be better off with single glazing in the middle of the day Ok, night time changes all that. Say for argument's sake that convective losses stay at 100 W/m2 24 hours a day -- outside temperature drops but so does inside temperature. Solar gain drops to zero, so net heat gain is -100 W/m2 for SG, -55 W/m2 for DG. Say there's 6 hours of good sunlight and take the rest as zero. Over a 24-hour period you then get SG: 900 W/m2 x 6 h - 100 W/m2 x 18 h = 3600 W-h/m2 DG: 695 W/m2 x 6 h - 55 W/m2 x 18 h = 3180 W-h/m2 So under those assumptions, the net daily energy flux for the north-facing windows is greater in the single-glazed case. Still, I've plucked those assumptions out of pretty thin air. Let's say daytime is 19C inside, 13 outside, night time is 16 inside, 2 outside. So, temperature difference of 6K daytime, 14K nighttime or roughly double the convective loss. And let's say we only average 4 hours equivalent of full sun SG: 900 W/m2 x 4 h - 200 W/m2 x 20 h = -400 W-h/m2 DG: 695 W/m2 x 4 h - 110 W/m2 x 20 h = +580 W-h/m2 There comes your big benefit for DG. So which of my two sets of assumptions is closer to the reality? Re: U-value vs SHGC & double glazing 8Nov 20, 2008 8:15 pm russell So you'd be better off with single glazing in the middle of the day Ok, night time changes all that. So under those assumptions, the net daily energy flux for the north-facing windows is greater in the single-glazed case. Still, I've plucked those assumptions out of pretty thin air. There comes your big benefit for DG. So which of my two sets of assumptions is closer to the reality? Are you trying to send yourself mad? Sophisticated energy rating programs can do this for you. They take the climate data - air temp, insolation, number of sunny days for your locale. It then calculates an hour by hour day by day energy flux in your household including the effect of thermal mass. It will then map the temperature change throughout a 24 hour period, calculate how much energy is required to bring it into a comfortable zone and summate it. Low air temps, poor solar insolation or lots of cloudy days will defeat the passive solar design. There is no 'free heat' to store and the cold outside just sucks out whatever you have. In that case you are obligated to insulate everything including the best double/tripled glazed windows you can find. Re: U-value vs SHGC & double glazing 9Nov 20, 2008 8:20 pm dymonite69 Are you trying to send yourself mad? Sophisticated energy rating programs can do this for you. Are there any that are freely downloadable, reliable, and reasonably easy to use? Re: U-value vs SHGC & double glazing 10Nov 20, 2008 8:35 pm russell dymonite69 Are you trying to send yourself mad? Sophisticated energy rating programs can do this for you. Are there any that are freely downloadable, reliable, and reasonably easy to use? An engineering saying - Cheap. Fast. Good. Pick two. I once played around with RESFEN once. It's from the US. The problem is to find a US city that has a climate that most resembles your own town. I used Oakland, CA (it's across the bay in San Francisco) to mimic the Adelaide Hills. You get to choose if you want a slab for the floor which I assume is modeling thermal mass. The trouble with all of these programs is that you are never quite sure what assumptions they are making unless you have the complete technical manual. Furthermore, it depends how detailed you are with entering the data. Usual rule of computing - GIGO. Garbage in, garbage out. Anyway knock yourself out: http://windows.lbl.gov/software/resfen/resfen.html If it really is that important just pay someone do to it. I think it is about $200-300 per simulation. Re: U-value vs SHGC & double glazing 11Nov 20, 2008 8:52 pm Cool, downloading RESFEN and will have a fiddle.
It's a pity AccuRate costs money (and a LOT of money). Apart from being against the public interest, as a matter of principle , being made by the CSIRO it ought to be free to Australian citizens. If only I could charge hundreds of bucks for every computer program I wrote while working for CSIRO . P.S. Fenstration, hey, there's a word you don't hear every day... I can feel a rap coming on ... something to do with radiation penetration of my fenestration, insulation, installation, calculations, frustration... Re: U-value vs SHGC & double glazing 12Nov 20, 2008 9:32 pm russell It's a pity AccuRate costs money (and a LOT of money). Apart from being against the public interest, as a matter of principle , being made by the CSIRO it ought to be free to Australian citizens. If only I could charge hundreds of bucks for every computer program I wrote while working for CSIRO . You are paying time for the dude to translate the plans into data for the program and then analysing it for you. Re: U-value vs SHGC & double glazing 13Nov 20, 2008 9:42 pm dymonite69 You are paying time for the dude to translate the plans into data for the program and then analysing it for you. Oh yes I understand that. I just mean the software itself costs money to obtain. All intellectual property of the US Government is in the public domain, but the Australian government takes the opposite approach. They take our tax dollars and use them to produce things we have to pay for a second time. Re: U-value vs SHGC & double glazing 14Nov 20, 2008 10:14 pm OK, had a fiddle with RESFEN. I chose Seattle WA as having very similar temperatures to where we're building.
Single glazing definitely increased heating costs. BUT so did south-facing glazing area (remembering this is for the northern hemisphere) regardless of whether it was single or double glazed... which seems to go against what we're told. For example the Your Home manual says of heating climates "It is possible for an advanced window’s energy gains to exceed its losses, even if it faces south." Perhaps Seattle has poorer insolation than most southern Australian climates? Re: U-value vs SHGC & double glazing 15Nov 20, 2008 10:22 pm EcoClassic There are 2 types of Low-E, hard & soft. One time only soft was available and it was being used on single glazing in the belief it was as good as double glazing. Well it wasn't and the surface was very prone to damage, so it got a bad name. Also it never works in a single glazed situation as it needs to be adjacent to still air to be effective. In double glazing it is fine and works very well. So does single pane Low-E not work at all or does it perform to some degree. I assume there must be a measurable benefit hence the variance in glazing values on single paned windows. Peter Clarkson - AusDesign Australia www.ausdesign.com.au This information is intended to provide general information only. It does not purport to be a comprehensive advice. Re: U-value vs SHGC & double glazing 16Nov 23, 2008 11:50 am ausdesign So does single pane Low-E not work at all or does it perform to some degree. I assume there must be a measurable benefit hence the variance in glazing values on single paned windows. Hi Peter I conceed... you are right. I decided to check a few results in WERS and it would appear that my information is wrong. WERS shows that identical windows glazed in clear and Low-E show a reduction in U value of around 1 to 1.5 for Low-E versus clear in a single glazed situation. In all material published outside Australia I find comment "not recommended in monolithic applications", and I cannot find any U values comparing Low-E in single glazed situations. It appears to be used only in double glazed units in the UK and USA. That may be due to the prevalence of double glazing in those countries and possibly due to the difficulty in cleaning the exposed active surface in a single glazed situation. "ECOECO" At 'EcoEco', we design windows, we design the best windows, we do it for you, so that when you’re happy we are happy. Tel. 1800 326 326 Re: U-value vs SHGC & double glazing 17Nov 23, 2008 12:00 pm Wouldn't it also be the case in single glazing use that unless people are very unusually meticulous about cleaning it frequently and properly, it's going to end up dirty and having the same emissivity as normal glass? "Now, supposing a house to have a southern aspect, sunshine during winter will steal in under the verandah, but in summer, when the sun traverses a path right over our heads, the roof will afford an agreeable shade, will it not?" -- Socrates, ca. 400 BC Re: U-value vs SHGC & double glazing 18Nov 23, 2008 12:03 pm russell Wouldn't it also be the case in single glazing use that unless people are very unusually meticulous about cleaning it frequently and properly, it's going to end up dirty and having the same emissivity as normal glass? I guess you're right Russell, had not thought of that. "ECOECO" At 'EcoEco', we design windows, we design the best windows, we do it for you, so that when you’re happy we are happy. Tel. 1800 326 326 Re: U-value vs SHGC & double glazing 19Nov 23, 2008 12:45 pm Quote: Hi Peter I conceed... you are right. I decided to check a few results in WERS and it would appear that my information is wrong. WERS shows that identical windows glazed in clear and Low-E show a reduction in U value of around 1 to 1.5 for Low-E versus clear in a single glazed situation. In all material published outside Australia I find comment "not recommended in monolithic applications", and I cannot find any U values comparing Low-E in single glazed situations. It appears to be used only in double glazed units in the UK and USA. That may be due to the prevalence of double glazing in those countries and possibly due to the difficulty in cleaning the exposed active surface in a single glazed situation. If the WERS assumes still air on the surface of course it shows a lower U value However double glazed low E is still performing better, plus the low E surface is not designed to be exposed to wear. There is a good reason why others don't recommend it. Re: U-value vs SHGC & double glazing 20Nov 23, 2008 4:33 pm windowexpert If the WERS assumes still air on the surface of course it shows a lower U value I am perplexed to why the U value should change with a coating that alters radiant energy transfer. Surely this is an independent property of the glazing. U value is a concept derived from a physical concept called thermal conductivity. It relates only to heat transfer by conduction and there is linear relationship to the temperature gradient across the pane. Radiant energy follows the Stefan Boltzman law where energy transfer is proportional to the fourth power of the temperature of the object. These are two terms which are independent of one another and cannot be combined into one expression. For instance during the daylight hours average solar radiation passes through a normal window @ 1000W/ W/m2. There is also some radiant energy passing out but nothing of the same magnitude. The conductive transfer is dependent on the relative temperature gradient between the inside and outside temperature but probably would not exceed 200W/m2 (and probably much less). Nonetheless, on a sunny day the net energy transfer is predominantly inward and mainly radiative. At night time the net energy flows are mainly outward (no solar radiation and the outside temperature also falls). Thus a U value that incorporated radiant heat transfer would essentially be a nonsense. This is one of the reasons I decided to go overseas for my double glazed windows. 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