DG glazing - have I got it right?

I'm going a little insane and in circles trying to make sure I've got the glazing for our new house right and would appreciate some feedback. We are building solar passive and all windows and doors will be double glazed, we are in cold area and zoned as heating for passive design, although summer heat is quite high too. That's the easy bit, now for the part that is troubling me

My thinking so far after much research and double guessing is:
Northern windows/doors (18-20mm IGUs): 4mm clear/10-12mm Argon/4mm Low-E
Southern windows/doors (18-20mm IGUs): 4mm clear/10-12mm Argon/4mm clear
(there are no windows/door on Eastern or Western sides)

Windows/doors are aluminium framed (hoping to afford the thermally broken) and the frame can support up to a 24mm IGU.
* Should I have Low-E on the southern side as well? I had intended it to be used to reduce heat loss rather than keep heat out. That said it should be internal for that use?
* Should I ditch the argon fill and go for the 24mm IGU instead? I've read 11-13mm argon fill is the optimum but would I get better results with the larger IGU with air fill?
* There are three windows on Northern side that are in bedrooms, should I drop the low-E from them?
* Would you go for thermally broken over argon fill and low-E if cost is getting out of control? What would you do if guesstimates on thermally broken were double?

TIA

Argon fill is expensive and doesn't offer that much over air fill. 24mm is way to wide an air gap - convection currents will occur. 16mm is ideal.

The frame material has a big influence e.g. UPVC / wood is more insulative than aluminium - not just frame construction e.g. thermal break

If you want aluminium, you could get the builder to add trims on the inside of the frame to provide more insulation.

Ideally you want to minimise south facing windows anyway - IGUs still have much lower insulation than a well insulated wall. Remember that in Summer, the sun rises in the SE and sets in the SW.

Hi Dragon girl, try sending a PM to Ed @ eco classic, he knows his stuff when it comes to windows!

Thanks for the replies.

Dymo I'm not sure I understand your trims comment re the alu frame (sorry I'm more brain dead than usual tonight as the wee one is cuting three molars atm). Do you mean slipping some insulating material between the metal frame and the brick wall? We are building an insulated double brick envelope, can we reduce heat transfer between the frames and walls by wrapping some, I don't know, rubber? insulation board? that sort of thing around the frame so that they don't touch or is most of the issue the heat transfer between the warm inner side of the frame with the cold outer side?

Your thinking is correct. the best airgap (or argon) is 12 mm - more is worse (especially 24mm ), and less is worse for energy efficiency. Check WERS residential u-values, ignore the u-values which include Low-E, it is meaningless in your situation. Thermal break is only of any value if the u-value is better than the standard aluminium, and our standard window beats most thermally broken windows. Our thermally broken window is about 70% dearer than standard, but most would be 400% dearer at least. Argon adds about 0.2 of u-value, so ignore it unless your house energy rating demands it. It is not so expensive but it does deplete over the years. Repeating, don't go for thermally broken unless it is more efficient, most aren't.

I would stick with low-e on the North facing windows, it's more difficult to control later.

Yes, you could stick timber on the inside of the aluminium frames, that would create the effect of a composite timber/aluminium window and improve the u-value, one manufacturer does exactly that... but it may detract for the aesthetics.

Ed

I was very interested in reading your comments about optimum air gap in DG windows. Could you point me to where I can find more information on this, please (or provide further explanation)?

currently planning new house with DG windows & want to get it right too!!!
many thanks in advance

Sorry, I don't have the authority with me... but it comes from a study by ASHRAE reproduced in the ASHRAE Handbook in the 80s or 90s. I have it at the office. Among other things, it's what we designed our windows around, hence it ranks 1st out of 10,000 in WERS.

Email me if you need more...

Thanks
Ed

Cheers Ed. So the Low-E on side 3 (which I think is the inner side of the inner pane, please correct if wrong) won't interfere with the solar gain to the slab we are trying to achieve with the windows? Also is your standard window the uPVC? We've seen some pics on the net of PVC windows and haven't liked the way they look. Will drop you an email too.

Yes, that's side 3. Low-E will interfere with solar heat gain, you have to decide what's most important, keeping the SHG out or using the SHG to advantage. Our most popular window is aluminium and the energy efficiency rivals our PVC window. Out PVC range is on hold while we changeover to new profile systems...

Thanks
Ed

What's sensible for temperate coastal Sydney? Timber double glazed with 6mm air gap? Or aluminium/timber composite with 12mm air gap?

And what's best to cut noise through windows facing the street? Different thicknesses of glass --- eg 6mm facing street, 4mm facing into the room --- or something like laminated glass on the street side?

thanks, B

Hi B

Timber is a better insulator, but 12mm is perfect and 6mm is very poor. I would go for aluminium with 12mm air gap.

Yes, you are right, different glass thicknesses is best. Laminated is neither here nor there, it adds a little but not as much as the price. Glass mass is number one.

Ed

Thanks Ed!

B

hmmm better check my supplier ..hope i got it right....

Hi there,

About the argon gas - agree with Ed, marginal cost outweighs marginal benefit. They don't even bother to use it in Russia, and their winters are much colder. Triple glazing for cold climates is much more effective than any gas, if money is no object. If someone really really wants gas, xenon performs better than argon and krypton, while argon is least expensive.

About the optimal width of air-gap for maximum thermal insulation, I don't want to say 12mm is wrong - it's not, and doing that would send the wrong message. The sources I have, which include New Scientist the magazine, a glass supplier in Australia, and wikipedia, various books, recommend 16mm. Other sources do also report 12mm and many others report in-between values. The way they test these things, results can vary - and some variables include size, how vertical is the unit, whether gas is used, etc. Basically, don't get stuck on a certain value of air gap, sometimes there are other priorities such as bushfire compliance, security, acoustic insulation that also have to be met.

In regards to acoustic insulation, the bigger the air gap, the better the performance. However, glass thickness is also important - the thicker the glass, the better the performance. I would say glass thickness precedes air-gap in importance. Laminated glass is worth it - every cent! If air gap is precious little eg less than 6mm, it would be better to just purchase the thickest glass to fit in the system and just use that single glazed eg 12.38, 10.38, 8.38 etc or whatever the unit accommodates.

If you have a large window on the sunny (west or north west side) get low-e glass or tinted glass, make the window smaller, or consider not having any windows at all on the west. Similarly, get rid of your skylights and the louvres - they tend to be less energy efficient than windows.

Worrying about 1-2mm of air-gap is a bit like trying desperately to re-wire your aircon to make it work harder, all the while you have your windows open. Just get some decent windows and forget about it, enjoy life!


Cheers,

My source (12mm is optimum) is a multilevel scientific study by ASHRAE, it's what we based our window design on and it is why our aluminium window is number 1 out of 10,000 aluminium windows in WERS. In fact if we vary the air gap (air and Argon are almost identical and Krypton is better at smaller gaps) I can see the result in the window rating software Therm which comes out of Lawrence Berkeley Laboratory and which forms the basis of all the window energy ratings systems in Australia (that is window and house) - so let's agree to differ. I think it's very important. But it can be overridden by other priorities as you say.

Edit... I would have greed once about laminated, that is until I had a window tested at RMIT, with and without laminated. 1dB was the difference. Now although the scale is logarithmic, we can only differentiate 10dB. So much for theory.

Ed

Ed, can you please state what qualifiers you are using to claim number one position? Using WERS for VIC, aluminium & double glazing you aren't even in the top 50 when sorted by U-value.

Unfortunately you cannot filter by glass type as many companies seems to be using quite exotic films or something to get a low U-value

Your analysis is correct, but you must look deeper.... On this page http://wers.net/AdvResidentialSearch you choose all manufacturers, aluminium (note not composite timber/aluminium and not thermally broken aluminium) awning, double glazed. The first page is full of composite windows which have been misfiled, so don't count those, the next page has the first truly standard aluminium awning window with low-e and that's EcoClassic. It is preceded by a Southern Star fixed window, but fixed windows are rated in much larger sizes so rate better than awning and it is fixed, not awing... Then the EcoClassic aluminium awning window appears as: -

EcoClasssic Group Pty Ltd | EcoClassic Aluminium Casement Window - Double Glazed
ECO-011-05 4EA/12Ar/4 51% 68% 2.7 0.50 0.56 5.00

The first window with clear glass is on page 17 by A&L, but it uses 3mm glass and that can't be in full height or below transoms - so thicker glass will reduce the U value... and in any case the U value is the same as ours, but we use 4mm minimum glass.

A&L Windows - SA | A&L Aluminium Awning Window - Double Glazed
A&L-004-01 3/12/3 43% 66% 3.5 0.55 0.57 0.30

Then we have on page 19 (as all windows with the U value of 3.5 are in alphabetic order), EcoClassic. And all those preceding have Low-E and/or tint... so the EcoClassic aluminium awning is definitely the most energy efficient aluminium window in WERS.
EcoClasssic Group Pty Ltd | EcoClassic Aluminium Awning Window with Thermally Broken Sash - Double Glazed
ECO-007-02 4/12Ar/4 45% 60% 3.5 0.53 0.54 5.00
EcoClasssic Group Pty Ltd | EcoClassic Aluminium Awning Window - Double Glazed
ECO-001-02 5/10/5 42% 67% 3.5 0.56 0.60 0.15
EcoClasssic Group Pty Ltd | EcoClassic Aluminium Awning Window - Double Glazed
ECO-001-01 4/12/4 41% 68% 3.5 0.58 0.61 0.15

I hope my explanation makes sense... and if it was not true I am sure the competitors would sue me...

EDIT... By the way, you can tell the glass type by reading under the Glazing ID column... like this one... 6.38CP/8Ar/3 - this means Comfortplus (Low-E), 8mm Argon, 3mm clear. The EcoClassic 4/12/4 means clear glass (not Low-E). EA means Energy Advantage (Low-E) not laminated. etc.

Thanks
Ed

Thanks Ed, been a little busy of late hadn't gotten around to replying to your helpful post.

While I agree with your analysis, it seems that about 50 companies can claim "No. 1 in WERS" depending how they wish to define "No. 1".

For example if I select "Thermally Broken Aluminium", Awning, then Dowell Windows come out well on top with a U value of only 1.7 using glass of type: 4SolB/12Ar/4Clr
No idea what 4SolB is??

Why wouldn't I want thermally broken given it has such a better U value?

I think the WERS scheme is a good start, but the field is still confusing for the layperson trying to find a reputable & reasonable priced window supplier with approriate thermal performance.
I think there should be better filtering & explanations on the different types of glass & some sort of cost comparison. The WERS site does not seem to even list the glass codes used!

If anyone else claims to have the BEST ALUMINIUM DOUBLE GLAZED AWNING WINDOW in WERS, then I challenge them at the ACCC - so name them here if you like (but I don't think you can)... Go check out the Dowell product before you buy

And if you check each manufacturer's WERS page you can hover over the glazing and each will be revealed.

Ed

Ed, I believe what you are saying & aren't trying to prove otherwise - my point is that the WERS site can be misleading & extremely difficult to determine what actually matters, etc, when trying to be "thermally responsible". I know nothing about windows & do not wish to tender for quotes from 30 different companies. Was hoping to use the WERS site as some sort of a shortlist, but it just seems to make the process even more confusing!

And to be pedantic: I'm sure the ACCC could say your signature is misleading "No. 1 in WERS" when you actually mean "No. 1 in Aluminium Frame Awning style in WERS"