U values for windows and doors on new build

[Caroline]

Hello everyone

 

I have searched the planning portal and can't find what the requirements are for U values for newbuild windows and doors.  Can anyone direct me or help please?

 

Thank you.

Caroline

[G and J]

Do a google (or equiv) search for "building regs u values for new builds" and you'll get lots of choice

 

Eg. 

 

2025 U-values includin0.18 W/m²K for walls, 0.13 W/m²K for floors, and 0.11 W/m²K for roofs. Windows and doors have a target U-value of 1.2 W/m²K, while the maximum allowable U-value for these elements is 1.6 W/m²K. These U-values are part of a 'notional building' specification and require the use of energy modelling software to demonstrate compliance, which also considers other factors like air permeability and heating systems. 

[Caroline]

On 29/08/2025 at 11:13, G and J said:

Do a google (or equiv) search for "building regs u values for new builds" and you'll get lots of choice

 

Eg. 

 

2025 U-values includin0.18 W/m²K for walls, 0.13 W/m²K for floors, and 0.11 W/m²K for roofs. Windows and doors have a target U-value of 1.2 W/m²K, while the maximum allowable U-value for these elements is 1.6 W/m²K. These U-values are part of a 'notional building' specification and require the use of energy modelling software to demonstrate compliance, which also considers other factors like air permeability and heating systems. 

Thank you for your reply.  Our pre SAP came out very good apparently 300% more than the 10% requested.  Don't think the 1.2 windows rather than the 1.0 on pre SAP will be an issue but waiting for the company who carried out the report to get back to confirm.  God knows why architect put 1.0 when 1.6 is allowed!  Everything has a knock on effect and take so much time to rectify.

[Crofter]

I'd be wary of simply googling this. 

I was looking on to it recently and Google returned the 2022 Scottish regs, which have been superseded. I was only alerted to this because I'd been chatting to another forum member who had mentioned needing to reach lower numbers to comply. I had to dig a bit deeper to get the current numbers. 

 

Where are you located? It might make a difference.

[Crofter]

2025 Scottish regs are on p.458 of the Domestic Buildings Technical Handbook and put windows and doors at 1.2

The wall, floor, and roof values are all lower than those quoted above which are presumably for England?

 

[Caroline]

5 hours ago, Crofter said:

I'd be wary of simply googling this. 

I was looking on to it recently and Google returned the 2022 Scottish regs, which have been superseded. I was only alerted to this because I'd been chatting to another forum member who had mentioned needing to reach lower numbers to comply. I had to dig a bit deeper to get the current numbers. 

 

Where are you located? It might make a difference.

I am in the South East of England (Greater London area)

[SteamyTea]

I think that windows can be specified 2 ways.

Just the glazing, and the mean value of the frame and glazing.

Our expert @craig will know the details off by heart.

 

As a general rule, a certain amount of thermal element 'swapping' is allowed. So if your windows are not up to regs, extra wall insulation can be included, somewhere, to compensate. I think there are limits imposed in this i.e. minimum standard.

 

There is also a raft of contributory rules about security and safety that affect windows now i.e. one rule says no more that a set height, while another says no less than the same height.

 

Never trust an architect to get the thermal elements of a building right, decide yourself if you want a better performing building or one to just the minimum building regulations (which these days are not too bad in paper, but still don't perform that great in the real world (mainly airtightness and thermal bridges).

[craig]

U Values, the bain of my life. Something you need to be careful with. Most well known European suppliers will give you the Uw, others will give you the G value.

 

To break it down.

 

Uw is the whole window value tested according to the industry standard and is all four sides of the frame, glass including spacer and and feature bars/astricals/georgian bars.

 

G is just the glass, no frame.

 

Be careful, as some suppliers will give you the calculated unit performance including G value which does change based on window size.

 

However, for PHPP for example, you want the actual G value as shown on the test certificate i.e 0.53 - if unsure ask the supplier/manufacturer and if they are unsure, just ask for the glass certificate from the glass supplier or ask for the specification and add it to Euroglas/Guardian or Saint Gobain glass calculator. That will give you the answer.

Edited 19 hours ago by craig

[ADLIan]

I assume you mean Ug value for glass only U-val. G value of the glazing is very different.

[craig]

I do, you are absolutely correct, I was on my phone and I call that the "fat finger syndrome". Thank you for picking that up.