Target U-values… Cost/benefit sense check… What am I missing? 🤷🏻‍♂️

[Oz07]

With windows and doors is there a ballpark figure per m2 with mid range 2g vs. mid range 3g? @craig

 

Do you run into a similar issue with glazing and payback periods some of these top spec windows costing a lot. Especially as they wouldn't last as long as a wall. 

 

If having a house with average wall u value 1.8 there must be a certain u value window you can go for before it becomes uneconomical. Excluding the benefits of reduced noise etc etc. Average 4 bed with 30m2 glazing I bet ai could work it out. 

[JohnMo]

4 minutes ago, Oz07 said:

house with average wall u value 1.8 there must be a certain u value window you can go for before it becomes uneconomical

Would suspect it's down to the wall to window ratio and that will vary by room and by house.

 

Example

Our living room has nearly 30m2 of glazing and about 10m2 of external wall in total.  So going for high performance 3G wins the day, every day. For clarity our living room has more heat loss than the rest of the house in total. So we upgraded to to 3G.

 

I's all a balancing act something's make sense in one build and no sense in another. Generalising is choice decision.

 

 

[Crofter]

2 hours ago, Oz07 said:

If having a house with average wall u value 1.8 there must be a certain u value window you can go for before it becomes uneconomical. Excluding the benefits of reduced noise etc etc. Average 4 bed with 30m2 glazing I bet ai could work it out. 

1.8 is horrendous for a wall (you don't mean 0.18 do you?) but improving your existing walls is much harder than retrofitting better windows.

None of this is rocket science. It comes down to surface area and u value.

[craig]

12 hours ago, Oz07 said:

 

With windows and doors is there a ballpark figure per m2 with mid range 2g vs. mid range

 

Windows are tested at a standard size of 1230x1480. However, for a 1000x1000 window (1 m2), I’d be expecting 1.2Uw to 1.4Uw for double and triple should be 0.7 to 0.85Uw. There really isn’t any excuse for a double being 1.4Uw unless the frame is s**t and the glass is the same.

 

If triple is around 0.85Uw, it’s not the best Uf value (frame) and sitting around 1.0Uf with 0.5Ug glass or higher Ug (0.6/0.7 for example) but a better Uf.

Edited 13 hours ago by craig

[Oz07]

7 minutes ago, craig said:

Windows are tested at a standard size of 1230x1480. However, for a 1000x1000 window (1 m2), I’d be expecting 1.2Uw to 1.4Uw for double and triple should be 0.7 to 0.85Uw. There really isn’t any excuse for a double being 1.4Uw unless the frame is s**t and the glass is the same.

 

If triple is around 0.85Uw, it’s not the best Uf value (frame) and sitting around 1.0Uf with 0.5Ug glass or higher Ug (0.6/0.7 for example) but a better Uf.

I meant price like allow roughly £x per m2 for mid range 2g vs. 3g. 

[Oz07]

10 hours ago, Crofter said:

1.8 is horrendous for a wall (you don't mean 0.18 do you?) but improving your existing walls is much harder than retrofitting better windows.

None of this is rocket science. It comes down to surface area and u value.

Sorry yeh .18

[craig]

Ahh, that depends. It’s very open to a lot of things. Somewhere in the £400/£800 region system / supplier / manufacturer dependent I would suggest. 

Edited 13 hours ago by craig

[Oz07]

12 hours ago, JohnMo said:

Would suspect it's down to the wall to window ratio and that will vary by room and by house.

 

Example

Our living room has nearly 30m2 of glazing and about 10m2 of external wall in total.  So going for high performance 3G wins the day, every day. For clarity our living room has more heat loss than the rest of the house in total. So we upgraded to to 3G.

 

I's all a balancing act something's make sense in one build and no sense in another. Generalising is choice decision.

 

 

Yeh obviously all a decision based on individual house. I bet the average lounge would have 20m2 of wall vs 5m2 glazing if you think of a 5x3m box and 2 sides being on external wall. 

 

You went 3g throughout not just in that one room?

[Oz07]

1 minute ago, craig said:

Ahh, that depends. It’s very open to a lot of things. Somewhere in the £400/£800 region system / supplier / manufacturer dependent I would suggest. 

So like double per m2 between 2 and 3g?

So with 30m2 glazing 12k 2g or 24k 3g if my sums are right?

[craig]

Just now, Oz07 said:

So like double per m2 between 2 and 3g?

So with 30m2 glazing 12k 2g or 24k 3g if my sums are right?

As I say, it’s a very open question. What glass specification, float, laminated, toughened, locking/non locking handles, fixed, turn only inward/outward, tilt and turn, reversible etc.

 

Double is likely to be somewhere between £300/£600 and triple £400/£800 but a lot depends on spec, manufacturer discounts (which can vary even with the same manufacturer) supplier markups and so forth.

[craig]

I’ll give you an example in an hour.

[Oz07]

1 minute ago, craig said:

I’ll give you an example in an hour.

No thats fine its more like 25% not 100% then. Interesting I might play with some numbers and AI later but we know numbers aren't its strong point

[craig]

16 minutes ago, Oz07 said:

No thats fine its more like 25% not 100% then. Interesting I might play with some numbers and AI later but we know numbers aren't its strong point

 

It's cool, so I just put a Rehau Synego 1000x1000 Upvc (white/white) window together as well as an aluminium window and timber alu window. uPVC exact same spec same spec for all, no change other than glass. Spacer bar is a warm edge spacer, glass for double is exactly the same (1.1Ug toughened 4/20/4). Triple is 0.5Ug 4/18/4/18/4 toughened, none have extras or locking handles.

 

Double

Fixed - £250 per m2

Tilt and turn - £350 per m2 (exposed hinging)

Tilt and turn - £395 per m2 (concealed hinging)

 

Triple 

Tilt and turn - £410 per m2 (concealed hinging).

 

Aluprof aluminium, double

Fixed - £350 per m2 standard RAL inside and out.

 

Figures are rounded up for simplicity.

 

Timber aluminium (triple)
Gutmann Tilt and turn  - £775 per m2, finger jointed pine, standard clear lacquer, standard RAL.

Edited 12 hours ago by craig

[JohnMo]

12 minutes ago, Oz07 said:

might play with some numbers and AI later

Then ask it the same question tomorrow, and you may get the polar opposite answer

[Oz07]

[Nickfromwales]

On 16/02/2026 at 22:25, Iceverge said:

White uPvc fascia and soffit and black guttering and down pipes whilst we're at it. 

Sounds odd on paper, but that looks very nice in real life  

[Oz07]

I also ran the numbers on single glazing even though you wouldn't be allowed it in UK regs. Interesting how big a difference in u value between single glaze and 2g vs not so big a difference between 2 and 3g (if the numbers are accurate) 1/2/3 5/1.2/.7

 

I might approach my next place differently instead of fabric first ill do economic fabric first. 

[Crofter]

9 hours ago, Oz07 said:

Sorry yeh .18

Ok that's much better. 

If your walls are already at 0.18 then improving the glazing will see a more substantial return because they are now the weak point, by far. 

People don't always realise that even the best windows money can buy will lose three times as much heat as a BRegs minimum wall.

And cheap 2G will be up to uW 2.0 which is appalling.

Everybody will find their own happy place on the cost/benefit curve, but personally I wouldn't ever aim for less than 1.4 as you should be able to find that for about the same price as the 1.6/1.8/2.0 crap.

1.1 is widely available at sensible prices. In my experience the prices only tend to rise markedly once you go below that.

[Oz07]

45 minutes ago, Crofter said:

Ok that's much better. 

If your walls are already at 0.18 then improving the glazing will see a more substantial return because they are now the weak point, by far. 

People don't always realise that even the best windows money can buy will lose three times as much heat as a BRegs minimum wall.

And cheap 2G will be up to uW 2.0 which is appalling.

Everybody will find their own happy place on the cost/benefit curve, but personally I wouldn't ever aim for less than 1.4 as you should be able to find that for about the same price as the 1.6/1.8/2.0 crap.

1.1 is widely available at sensible prices. In my experience the prices only tend to rise markedly once you go below that.

Yeh i dont think I would consider triple if I could get sensible 2g at 1.1 u value. Can you still pass new b.regs sap with 2g in England though?

[Crofter]

4 minutes ago, Oz07 said:

Yeh i dont think I would consider triple if I could get sensible 2g at 1.1 u value. Can you still pass new b.regs sap with 2g in England though?

I don't know about the English rules but AFAIK they should just specify a u value.

Good 2G can be better than poor 3G. Not necessarily cheaper, though.

[JohnMo]

1 hour ago, Oz07 said:

2g at 1.1 u value

You need a really good frame, or a good one and Krypton gas filled.

[Crofter]

I'd be surprised if you could get 2G to outperform 3G without it ending up costing more. 

 

When I was recently shopping for new windows, I was offered a Krypton option and it pushed up the price much more than simply going to 3G.

[craig]

23 minutes ago, JohnMo said:

You need a really good frame, or a good one and Krypton gas filled.

Double glazing should be 1.1Ug or 1.2Ug argon filled 26/28mm, there is no excuse for it being higher. If it is, I’d be looking elsewhere.

Edited 46 minutes ago by craig

[JohnMo]

20 minutes ago, craig said:

Double glazing should be 1.1Ug or 1.2Ug

Sorry I was talking U_w, so frame and glazing (U_g and U_f).