Double Glazing - Air Gaps - What's Best - Myth Vs Truth

Arm yourselves with the truth, give those sales guys a hard time!!!!

Over and over I hear that 16mm is the best air gap, I hear this from customers who have visited suppliers who only have the 16mm gap (24mm glazing space), and a few suppliers who have argued this with me (briefly ) so I thought it may be useful to show the ASHRAE study which proves that 12mm is the optimum gap, not 16mm... (American Society of Heating Refrigeration and Air-Conditioning Engineers)

The graph below shows the "x" axis as the gap in inches (0.5 inches is 12.5mm) the vertical "y" axis shows the centre of glass U factor (or U value). The curves at the top of the graph refer to the outer pane and the lower curves refer to the inner pane.

You will notice that the curves for air/argon become elevated AFTER 0.5 inches, meaning that the U factor increases after 12mm. and the window becomes less energy efficient. Proving 12mm is the optimum gap for double glazing using Argon gas or air.

Also of note the impact of Krypton gas, often touted as "the best". It's very expensive but it has an optimum impact around 0.3 inches or 8mm. and is less effective at gaps higher or lower than 8mm.

You can see that in all cases, at smaller gaps than optimum the U factor increases (gets worse) sharply.

Also as you can see, air gap (or Argon gap) is critical for energy efficiency and triple glazing has little value if the gaps are small (so double glazing with 12mm gaps is better than triple glazing with 6mm gaps).

Many local window manufacturers use gaps as low as 4mm in their double glazing as they have a very narrow glazing space, their windows being designed for single glazing - (check out the gaps you are being offered as air is free - today anyway ).

Remember 12mm = optimum and air is free, so when you buy double glazing you want a gap as close to 12mm as possible - you are already paying for the glass.


Ed




Copyright ASHRAE 2001

Thanks so much for that - it's nice to hear actual facts about the science behind windows and insulation

Just a quick question along the same lines though - with Argon, I've heard that it tends to leak out at the rate of around 1% a year. Is this "old" truth or no truth whatsoever.

You're welcome...

Studies show leakage is around 15% in 20 years. But the Argon concentration starts out 75% - 90%. Of course some double glazed units leak more which shows up as condensation and they get replaced.

On the ASHRAE graph the Argon does play a role but it is minor compared to the whole, i.e double glazing in an energy efficient frame.

Our EcoTech aluminium window tested with air ranks 1st in WERS against all windows with Argon. And with Low-E it ranks 1st, 2nd, 3rd and 4th against all windows with Low-E and Argon (that's 1,400 windows). And ours has the 12mm gap.

WERS is an independent assessment.

Ed

What is myth and what is truth is not proven by some ASHRAE graph.
According to DIN EN 674 the optimum airgap with Krypton is 10mm, the optimum with air 15mm, the optimum with Argon is 16mm.
My software says roughly the same.
4/16/4 lowE Argon is a Ug 1.1 glass unit and it is regarded as state of the art of double glazing and it is used widly in central Europe if triple glazing is not used.
If 12mm would be only a fraction better than 16mm they would use it in Germany and Europe. A system that can take 24mm can be easily used with 20 mm by using different beads.
But some systems can NOT take more than 20mm and those who use
this systems naturally have to bring along all possible reasons to make
the 4/12/4 look the best option.
We are free to use the best performing airspace as our system can take glass from 4-36mm and we use beads accordingly.
If 12mm would be better I would use it but you would have to bring good evidence to prove DIN EN 674, Fraunhofer Institute of Physics, IFT, The Passive House Institute, all European glass Manufacturers and all window manufacturers wrong.

ASHRAE is THE respected authority... even higher than you

Yes... that's why we designed ours to be 12mm. You need 24mm for reasons of thicker glass (for sound attenuation) and wider air space for venetians. But you need 12mm air space for optimum energy efficiency QEF. That's why EcoClassic uses 12mm - unlike your window, ours is designed by us, we didn't need an excuse for 12mm, we designed it to be 12mm. If the rules of physics change we will redesign it.

Present your data... not your window design software, your empirical data to support your argument.

Oh and check our WERS output - best in class...


Ed

Thank you for the interesting data and standards.

Judging from Ecoclassic's graph, the difference in U-value between 8mm air gap and 12 mm air gap is about 8%. From 12mm to 18mm the difference is even less. The results plateau at the 8-10mm gap (the steep part of the curve < 8mm is where you definitely don't want to go)

To put things in perspective changing from R 2.0 to R 2.5 ceiling batts is a 20% improvement.

A leaky poorly insulated house with gaps around badly fitting doors and open vents will easily defeat the advantage of an insulated glazing unit.

If I was building a Passivhaus then a triple glazed argon would be the way to go. Apart from that I would ensure that everything else to reduce energy leakage before forking out the cash for one of these units. 10mm vs 12mm vs 14mm is just incremental tweaking. Good quality windows won't solve your problems if your overall construction is rubbish.

Thanks Dymonite,

8% is still a worthwhile pursuit since it will cost zero for the air. And at 6mm gap which is common, it will be 25% saving or so...


That is 20% for heat lost through the ceiling only.

Only true if you don't try to recover the heat lost by adjusting your heater, or if it's not on a thermostat.

So if you run your home on a thermostat, or you adjust the heating to maintain the same temperature then the windows will leak say 5kW per hour on a cool/hot day and that's whether or not your house leaks like a sieve.



The answer is, if you want the ideal, then you must do as you say, but don't discount incremental improvements - they all make a difference. So if you want a different temperature inside to outside, then every watt saved is a watt saved.

Ed

You designed your windows with 12mm gaps as standard because of the ASHRAE graph?? You must be kidding yourself
You truly believe ASHRAE is more credible than DIN EN 674, the European window industry, IFT, Fraunhofer Institute or the Swiss Instutute of Glas (see attachment)

2 things...

1 It's more credible than you...

and as I said

2 Check our results in WERS...

Oh - and feel free to show your empirical data...

ASHRAE is the data behind Australian AND New Zealand window ratings (but of course you don't agree with that - and you're the expert!!!)

Yes I truly believe...

Ed

Can I believe? YES I CAN!!!

Just to be clear - YES I did... it's a hoot isn't it...



Yes I do... If it's not European it's no good in your book, so what is your problem with races other than European? Do you think the Swiss or Germans or Austrians are somehow superior beings to USA (and China I recollect)? The population of the USA closely matches that of Northern & Western Europe (where you draw your information) - (China's is larger than both put together by a factor of 2). The USA (& China) have lots of quite clever chappies (and chappesses ) you know... (as does Australia and NZ).

Standards? Standards are just that, not necessarily right, just standards. But the data supplied by ASHRAE, that's different. It is empirical data, laboratory measured by a world renowned authority (just ask Dr Peter Lyons).


Ed

The movement to highly energy efficient building methods has started in USA and China. The Australiens were the first to use double glazing and you have invented the double glazed PVC window 40 yars ago.
Your system has been installed in millions of applications around the world and you are the ONE
Why don't you teach those dumm Europeans who used single glazing until less than ten years ago??

Just as an offside comment:-

Do you two have to get into a p!ssing competition every single time?

Disappointing you think it's a competition... I thought I won ages ago

If windowexpert - or anyone - presents misinformation (as I see it) I will present the facts (as I see them).

Is that not reasonable? I try to remain unbiased... maybe I fail.

Ed

Actually I think the sexual tension is palpable...

You are never unbiased, you bend everything around to suit your products whether it is outside beads or spacer of IGU,s
I don't need to do that because I can do the glazing either way and I can use any size spacer, plus I have no commercial interest here.
I don't claim to have reinvented the window, I am the boring guy who
uses long term proven systems, but it is in the clients interests to be assured that their window have stood the test of time in a variety of climates and conditions.
I understand that the expertise of Rehau, Kommerling, VEKA and so on is nothing compared to your multinational operation with own labs and
multimillion dollar development budgets.

You win with the opinion of one institution against many others and you base your product on this, gosh you are so incredible smart

I won't waste my time anymore for discussions with you.
Now you are free to spread your true gospel and nobody will question your statements anymore
Those who think you are the one who knows it all should do as you say . And others who want some straight talk without commercial interest are welcome to send a PM as they have done in the past

Of course I bend everything to suit my products, that's because I based the products on the knowledge I have, knowledge researched over time, derived from respected authorities, in the USA, in China and here in Australia. And it's this knowledge that I argue with you.

I presented a graph, it's based on real data, not just hearsay. You poo poo the authority on the basis of the German's, the Swiss are higher authorities you say. First of all you have not presenetd any facts, just your own words, secondly we can't even see the text you refer to, so we are supposed to just accept what you say.

My facts are there, present the facts behind your theory, then we can have a discussion, until that happens there is no argument.

If you want to move in that direction I would like to see your thermal analysis of a window using 12mm gap and 16mm gap - with and without beads (as the beads are the only difference apart from gap). And I would like to see the Standards references you mentioned and any empirical supporting data.

I am happy to joust with you, don't take offence. But don't expect me to back down or concede unless you present facts.

Ed

Does anyone have any figures or graphs on the following:

12mm gaps in glazing and air conditioning vents installed in the ceiling
24mm graps in glazing and air conditioning vents

and the two without the airconditioning vents.

Thanks.

from my perspective who is 'winning' is irrelevant as the informtation and counter discussion - IMO - is what makes the forum excellent, I am capable of taking the facts/marketing and comments and make my own call - lovn google.

As for the sexual tension, thats a visual I didn't need .. more love and less war

I concede, I don't have the biggest transom...

Jesus, I'm trying to have breakfast.