Green Building Forum - What is the best u-value obtainable with double glazing?

What is the best u-value obtainable with double glazing?

Mike George — Wed, 14 Jan 2009 19:05:37 +0000

Client has been advised 1.2W/mK but has not [so far] been given specific details. Anyone know how low you can go?

Tripple glazing is not an option. Frames are Oak

What is the best u-value obtainable with double glazing?

Mike George — Wed, 14 Jan 2009 19:11:06 +0000

Just been forwarded this by the Client http://www.alternativerooms.com/page18.html Anyone heard of it or used it?

What is the best u-value obtainable with double glazing?

rogerwhit — Wed, 14 Jan 2009 19:47:59 +0000

Roughly seems to equate to Planitherm soft coat with warm edge & 16mm argon, and I guess that's the value for glass only not whole window necessarily unless the frame is damn good.

What is the best u-value obtainable with double glazing?

CWatters — Wed, 14 Jan 2009 20:02:20 +0000

They say they use Argon so it's possible to improve them further by filling with krypton or Xeon instead. That is if you can find someone with a cylinder. If they have to buy one for your job it will probably be too expensive. Google suggests contacting Clayton Glass...

http://www.fenestration-uk.com/News/NewsItem.aspx?id=2840
http://www.claytonglass.co.uk/

We had to use krypton filled units in some metal framed windows. Everyone told me it was very expensive but they meant "expensive in relation to Argon". The absolute cost was acceptable compared to the alternative.

What is the best u-value obtainable with double glazing?

Mike George — Wed, 14 Jan 2009 20:03:03 +0000

Thanks Roger. Presumably the surface area of both frame and glass must be taken into consideration to get an average u-value for bregs?

What is the best u-value obtainable with double glazing?

rogerwhit — Wed, 14 Jan 2009 20:31:32 +0000

I'm no expert on calcs Mike but sources I've seen suggest approx 1.2 as mentioned for Ug relating to Uw 1.5 with a timber frame.

What is the best u-value obtainable with double glazing?

James Norton — Wed, 14 Jan 2009 21:14:00 +0000

Nordan N-Tech Low Energy (as opposed to N-tech passiv) do 1.2 Uw with argon, low E, warm spacer + thermally broken frame

J

What is the best u-value obtainable with double glazing?

jamesingram — Wed, 14 Jan 2009 21:23:29 +0000

Got some Oak windows from a german supplier , they quote Ug 1,1W/m2K on the spec. from what I can make out the unit are 4/16/4 argon fill
I ask my translator tomorrow for more info
cheers Jim

What is the best u-value obtainable with double glazing?

caliwag — Wed, 14 Jan 2009 21:47:06 +0000

So how long do these inert gases stay in the unit. I recall a so-called expert suggesting little more than 7-8 years and then diminishing.
So, is this just a ruse to get big areas of glass through building control or green thinking?

What is the best u-value obtainable with double glazing?

jamesingram — Wed, 14 Jan 2009 21:58:45 +0000

some talk about that here
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1100&page=1

What is the best u-value obtainable with double glazing?

Mike George — Wed, 14 Jan 2009 22:00:32 +0000

Thanks all for the help. I can look into this more tomoorow now.

Posted By: caliwagSo, is this just a ruse to get big areas of glass through building control or green thinking?

Unfortunately, yes, I believe it is a ruse, at least the gas part of it. Not sure it makes that much difference to the u-value though as argon isn't that much better than air. Every little helps for the Whole house metthod of calculation though.

What is the best u-value obtainable with double glazing?

rogerwhit — Wed, 14 Jan 2009 22:35:31 +0000

Loosely, I might suggest that 90% argon as against air might add 0.2 to Ug when new. A warm edge spacer say 0.1. And yes the argon may be fugitive, but is cheap at outset & will be effective for a while - might we say for the sake of argument that it will add an average of 0.1 to Ug over the life of the sealed unit, ie before the seals fail altogether & the unit mists?

What is the best u-value obtainable with double glazing?

rogerwhit — Wed, 14 Jan 2009 22:44:08 +0000

As an aside, it is possible to slit open failed dg units & recover & clean the glass, manually, but it's time-consuming. I can envisage a simple bench-mounted machine to do the slitting, and feel that that's what should be happening in glazing shops. One day ...

What is the best u-value obtainable with double glazing?

fostertom — Wed, 14 Jan 2009 23:23:10 +0000

I though that recycling glass by re-melting was so OK eco-wise, that laborious recovery was pointless? Specially as old glass is much more brittle, hard to work, than new? True/false?

What is the best u-value obtainable with double glazing?

rogerwhit — Wed, 14 Jan 2009 23:49:48 +0000

'Old' glass? If a unit made with new glass fails in 20 yrs, that's not very old. And I was postulating NON-laborious recovery, Tom! But it would need a comparative audit of energy inputs.

What is the best u-value obtainable with double glazing?

fostertom — Thu, 15 Jan 2009 15:42:17 +0000

Well, this is interesting. As I understand it, glass begins going brittle from the moment it's cast - glaziers much prefer 'new' or fresh glass to work with. I've found cutting old glass very tricky and liable to shatter. Indeed, audit wd be interesting - anyone know, straight off?

What is the best u-value obtainable with double glazing?

Sally M — Thu, 15 Jan 2009 17:14:42 +0000

C Watters mentions Clayton Glass. This is where I intend to get my glass from. They have good factory policies in place for efficiency but still transport the materials from China.

I have been quoted U values for the centre pane as:1.1
The timber frames are being made by a separate company.

I don't know if this helps?

What is the best u-value obtainable with double glazing?

rogerwhit — Thu, 15 Jan 2009 17:34:21 +0000

The production of a kilo of new glass seems to be responsible for a kilo of CO2 emmissions, Tom, mainly ascribed to the energy used. (Bath University: Inventory of Carbon & Energy, V1.6a, Hammond & Jones)

What is the best u-value obtainable with double glazing?

Mark Siddall — Thu, 15 Jan 2009 19:08:16 +0000

Hi Mike,
The best 2G that I've heard of is CP of 1.1 W/m2K by Solarglass. Not widely available in the UK (needs to be imported.) As far as I know a CP of 1.2W/m2K is the UK best available. The frame is now the weakest link so keep with windows as large as possible and where possible minimise the transoms and mullions.

Mark

What is the best u-value obtainable with double glazing?

biffvernon — Thu, 15 Jan 2009 19:34:18 +0000

Posted By: fostertomSpecially as old glass is much more brittle, hard to work, than new?

I think that may be an urban myth.

What is the best u-value obtainable with double glazing?

fostertom — Fri, 16 Jan 2009 09:25:27 +0000

OK, glad to know that biff, but are you sure? You should know, long time since I had my experiences with cutting glass.

What is the best u-value obtainable with double glazing?

fostertom — Fri, 16 Jan 2009 09:26:41 +0000

Posted By: rogerwhitproduction of a kilo of new glass

What if it's recycled glass - or rather if the new window glass is new for tech reasons but the old glass gets recycled for less demanding duty?

What is the best u-value obtainable with double glazing?

Tuna — Fri, 16 Jan 2009 10:56:55 +0000

Can someone confirm these quick calculations? I was just trying to work out what effect the improvements in U-value would have on our self build.

A difference of 0.1 W/mK gives me these estimates:

Assume 20m2 of window (180m2 house). Average annual temp is ~9 degrees for the UK. Internal temp maintained at 20 degrees.

Heat loss due to 0.1 w/mK = 0.1 x 20 x (20-9) = 22W

Heat loss per year = 22 x 24 x 365 = ~192 kWh

CO2 emissions (assuming gas heating at 0.194Kg/kWh.) = 37 kg
Assuming average household emits 9 tonnes CO2 a year, this is equivalent to 0.4% of household emissions

Cost saving (assuming gas at 4.2p per kWh) = 192 x 0.042 = Â£8.09

So, going to windows with a U value of 1.2 from more commonly available 1.3 saves 37 kilos of CO2 a year, or about eight quid.

Is that right?

What is the best u-value obtainable with double glazing?

fostertom — Fri, 16 Jan 2009 11:04:21 +0000

Posted By: TunaAverage annual temp is ~9 degrees

It's not the annual average, but the average through the heating season, that you're interested in - so lower. Look up degree-days.

What is the best u-value obtainable with double glazing?

Tuna — Fri, 16 Jan 2009 11:22:03 +0000

OK, well repeating the calc assuming we suffer the average winter temp of 4.5 degrees and heat our home all year round, gives

31W -> 271kWh -> 52kg CO2 (0.6% of annual household emissions) -> Â£11.38 per year

Obviously, that's not the case in real life, but it gives an upper limit to the savings to be made.

This is back of envelope maths, so forgive the hackery - I just wanted to get an idea of the order of magnitude we're looking at.

What is the best u-value obtainable with double glazing?

fostertom — Fri, 16 Jan 2009 11:35:14 +0000

What if you assume that crude oil price will rise 10-fold within 6 yrs of resumption of economic activity, so all-fuel delivered prices rise 5-fold, at today's money-value level? And compare medium-quality 2G @ 1.3U, with medium-quality 3G @ 0.9U at little extra cost, or top-quality 3G @ 0.75U or less. So far just about payback/ROI - throw in also the building's capital value uplift, if the windows are just part of a comprehensive zero- or near-zerofuel programme.

What is the best u-value obtainable with double glazing?

Tuna — Fri, 16 Jan 2009 11:56:25 +0000

Well the carbon saving won't change, and the price saving was just to work out some figures - in practise our house will be heated by a combination of wood, solar and LPG so the actual saving is smaller. Another way to look at it is that 0.1U difference equates to approx 0.75 kWh/day additional load on your heating system during the heating season.

Comparing med quality 2G with 3G gives 0.4U, so four times the figures = 200kg CO2 or Â£45 fuel price saving. What were your conclusions on cost / m2 for 3G as opposed to 2G?

I'm not going to start working out figures based on theoretical future fuel prices - it's pretty meaningless (how valid are the price assumptions, do you include compound interest on savings made, or that they're reinvested elsewhere in the building envelope etc. etc.)

This really isn't about cost - my interest was just in trying to quantify the benefit of improving your glazing. I've found selecting glazing and suppliers for our self build a very difficult process, so anything that gives additional context is welcome.

What is the best u-value obtainable with double glazing?

fostertom — Fri, 16 Jan 2009 13:04:23 +0000

Posted By: TunaWhat were your conclusions on cost / m2 for 3G as opposed to 2G?

I found a year ago that med quality Scandinavian-style 3G (4-12-4-12-4 in solid timber frames) was little if any extra on 2G 4-16-4 in the same frames - but only a minority of the 'Scandinavians' 2G frames were capable of accepting the 3G units (not to be confused with e.g. 4-6-4-6-4 3G). Latest Passivhaus-grade 3G designs cost more - not sure how much; these have 4-16-4-16-4 in thermal-break timber frames.

Posted By: TunaI'm not going to start working out figures based on theoretical future fuel prices - it's pretty meaningless (how valid are the price assumptions, do you include compound interest on savings made, or that they're reinvested elsewhere in the building envelope etc. etc.)

Well, we all need to take a view on all that, because it's the key 'will we or won't we' factor in the whole eco/carbon revolution. Rocketing prices will create property-value differentiation (aka added 'value') that will drive and finance the change, putting payback and ROI, let alone regulation and treaty, in the shade as drivers. You'd better have good reasons for believing fuel prices won't rocket, because if that change-driver doesn't materialise, IMHO we'll miss the boat to transform the world to live within its energy (i.e. solar) current account instead of eating its energy seedcorn/capital, and a return to Dark Ages is the prognosis.

What is the best u-value obtainable with double glazing?

PaulT — Fri, 16 Jan 2009 15:10:34 +0000

Curtains.

Curtains reduce the heat loss through windows (Particularly radiant heat exchange with the restof the room and convection currents)

They also improve your thermal comfort in highly glazed rooms...

SAP has a curtain factor of adding 0.04 to the thermal resistance (so window U = 2, effective 1.85)

My view is that curtains are not so much as an improvements, more a mitigation factor as the steady state heat loss indicated by the U value is an underestimate for windows (because they are the weakest link the convection currents are more pronounced and the surface boundary is compromised - resulting in up to 20% more heat loss, etc)

----------------
Solar gain
Triple glazed units with larger section frames will provide less solar gain and natural lighting
The GLazinfg will also transmitt less light, also providing less soalr gain and natural lighting

These factors also need to be included in net energy balance calculation (and will vary significantly by location and orrientation)

------------------
Insulated shutters and high spec double glazing
My prefered option
Less heat loss at night than triple glazing (tbc)
More Solar gain durring the day
More natural daylight
An adjustable sun shade for summer to reduce overheating
....
An adaptable building?

-------------------------
I used THERM to produce some U values for double glazing and different edge/frame options (Over Christmas).

(Please note this is just a theoretical model - small windows are much more dependent on the Frame)

Standard Timber Frame window U = 2
+Argon Fill U = 1.8
+Thermal spacer U = 1.5
+Insulated Frame U = 1.2 (the best I could get with DG).

Krypton filling (etc) may iprove very slightly.

--------------------------
Question?
- PVC frame with insulation stuffed inside it (Can do very easily witth EPS) - V.Cheap
- Solid timber with U = 3 for the frame

I have access to a PVC window factory and will be able to stuff insulation inside the frames! (PVC is European sourced 5 chamber construction and recyclable)

Does the argument that "the embodied energy of insulation is insignificant compared to the energy saved over decades" also apply to very cheaply turning standard PVC frames into insulating frames?

What is the best u-value obtainable with double glazing?

CWatters — Fri, 16 Jan 2009 15:46:20 +0000

Posted By: Tuna
So, going to windows with a U value of 1.2 from more commonly available 1.3 saves 37 kilos of CO2 a year, or about eight quid.
Is that right?

Sums look correct to me.