Window provider U-values a bit crap?

[Moonshine]

My preferred window provider has given me the following schedule for window selection u values, with the total U-values below (highlighting mine).

 

To me it seems there are some windows that actually don't met the regs for a new build.

 

The frames are aluminium, and its the frame performance which is causing the poor, are these typical values of alu frames?

[markc]

A solid alu frame without thermal breaks will give very poor figures

[Dutch]

https://www.justdoorsuk.com

[Moonshine]

6 minutes ago, markc said:

A solid alu frame without thermal breaks will give very poor figures

 

its an ALUK OPTIO 58BW-ST, which from the section drawings indicates that it has thermal breaks in the frame, i would have though this would have helped the performance, (the 1.9 Uf windows are fixed frames, and the 2.3 Uf windows are openable)

 

[ProDave]

Why do you want solid aluminium?  Would aluminium clad timber be an alternative to look at?

[Russell griffiths]

solid aluminium will be cold to the touch, I used to have ally in a previous place, always a cold feel to the window. 

I would go ally clad timber if you want better performance, you won’t achieve it with just aluminium. 

 

[markc]

The frames shown are not thermally broken.

an example would be

 

[Bonner]

I have similar frames, it looks like these are the thermal breaks. Pretty sure nobody makes unbroken  aluminium frames in this country.

 

[Moonshine]

3 minutes ago, Bonner said:

I have similar frames, it looks like these are the thermal breaks. Pretty sure nobody makes unbroken  aluminium frames in this country.

 

yes, they are the thermal breaks

 

how do you find them in situ? what window spec did you have?

 

Do they feel cold to the touch internally?

Edited September 8, 2021 by Moonshine

[Bonner]

Sorry meant I am having similar frames, Origin windows but not fitted yet. Don’t think you can see the thermal breaks in situ, only on a x-section sample. I think they do feel cold because metal conducts heat away from your hand so efficiently. 

[Ronan 1]

I'm having full Aluminium windows and have values of 1.4 as an average think worst was 1.7 of the top of my head. There tipple glazed too though not sure that makes much of a difference 

[Moonshine]

I have had a quote back for Rational aura plus windows, both double and triple glazed, they are a few grand more than the alu quote but the u-values are so much better, as below

 

 

Also i like the idea of wooden windows better.

 

Its £850 between the double glazed Rational option and the triple glazed, so i think i am going to go triple glazed

[Geoff-Belgraver]

Not  quite within this post, but for Victorian double-glazed wooden sash windows, does anyone know the ideal way to use the U-value? Should I just use the area of the sash, or include the box as well? E.g, the sashes in my office are 1.68m2 and with the boxes and timber at top & bottom they are 1.89m3

[SteamyTea]

On 08/09/2021 at 16:59, markc said:

The frames shown are not thermally broken.

an example would be

 

Does anyone know what material is used as the thermal break, and what sort of separation it gives, and over what surface areas?

[Iceverge]

@Moonshine

 

U values are indeed not stellar. It's the frame U value rather than the glazing that's the issue. Unsurprisingly it's the smallest windows that have the most frame area vs glass that are the worst. 

 

Here is a screenshot of my PHPP to give an idea of what range of frame U values are normal.

[markc]

15 minutes ago, SteamyTea said:

Does anyone know what material is used as the thermal break, and what sort of separation it gives, and over what surface areas?

The ones I have seen are thermally broken with a slide in rubber/urethane type material. Separation distance doesn’t really matter as the heat or cold cannot jump any gap from one piece of aluminium to an adjacent member.

@Iceverge, that’s an interesting and useful comparison.

[SteamyTea]

4 minutes ago, markc said:

Separation distance doesn’t really matter as the heat or cold cannot jump any gap from one piece of aluminium to an adjacent member.

I think it does matter, it is why thicker wall insulation reduces thermal losses.

It is also why not only the thickness is important, but also the contact area.

[Nick]

On 25/09/2021 at 10:53, markc said:

The ones I have seen are thermally broken with a slide in rubber/urethane type material. Separation distance doesn’t really matter as the heat or cold cannot jump any gap from one piece of aluminium to an adjacent member.

@Iceverge, that’s an interesting and useful comparison.

 

Modern thermal breaks are mostly polyamide. In high spec windows they are supplemented with lots of additional foam insulation stuck to / around them to improve u-values and minimise room for convection currents.

 

On 25/09/2021 at 10:59, SteamyTea said:

I think it does matter, it is why thicker wall insulation reduces thermal losses.

It is also why not only the thickness is important, but also the contact area.

 

The depth of the polyamide section definitely does matter. To get better results from aluminium windows you need to increase the polyamide depth. You can get aluminium frames that give you far superior u-values to Rationel, but they will be 100mm+ depth (most of which is thermal break) and therefore cost a lot more.

 

Nothing wrong with the windows in the OP. Good system but they are 58mm depth and mainly designed for replacement of existing windows so not high-spec. Your window sizes are probably on the small side if you're getting those u-values.

 

The psi value shown on the calculation also indicates that the supplier is using solid aluminium spacer bars in the glazed units (or doesn't know how to programme their software). In 2020 you should be getting a warm / thermally improved spacer bar in everything.