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

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.

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

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

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?

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.

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 ...

'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.

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?

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

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

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 = £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?

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) -> £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.

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 £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.

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)


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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)

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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?

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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.

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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?