Cost effective total window replacement

[Oz07]

8 hours ago, Mike said:

A key benefit of triple glazing is that the inside air temperature next to the glass will be a few degrees higher than it would be next to a similar double glazed unit. If you have underfloor heating, then the closer that temperature is to the target room temperature, the slower the convection currents are in the room. Which creates a more uniform temperature gradient from floor to ceiling, so you don't need the thermostat as high (which is where there's a cost saving). It also removes condensation risk even when temperatures drop well below zero.

 

However, if you don't have UFCH then the first of these benefits goes 'out of the window' as you'll have significant convection currents anyway. And if you do have UFCH + well-specified double glazing (argon filled, low-e, good weather seals, well-insulated frame), then the temperature gradient can still be very good.

 

In short, it's preferable to choose double glazing in a well-insulated frame, rather than a triple glazing in a standard / poorly insulated frame. U_w is the best guide for that.

 

And what is a reasonable target to aim for? Excluding passive house craziness 

[Mike]

7 hours ago, Oz07 said:

And what is a reasonable target to aim for? Excluding passive house craziness 

Building Regs Part L gives a minimum requirement based on a hypothetical window size. However the standard just rules out the worst performers and is easily beaten by windows with a solid timber frame. It's also not very helpful unless your windows happen to be that size; windows with a lot of glass or a lot of frame would have a significantly different values. Which is why, for a proper comparison, it needs to be calculated per window using the manufacturer's data (some may calculate it for you):

 

Uw = (Uf x Af + Ug x Ag + ψg x Lg) / ( Af + Ag )

 

where:

Uf = frame U-value

Af = frame area

Ug = glass U-Value

Ag = glass area

ψg = Psi value (thermal bridging) of the glass-edge spacer*

Lg = perimeter of the glass

 

*I should have mentioned the importance of this - it's the spacer that keeps the 2 panes of glass apart. Cheap windows use aluminium spacers that are very conductive; good windows use low-conductivity plastics that significantly cut the condensation risk at the edge of the glass.

 

For an illustration of what can go wrong, see this post:

 

 

 

Edited 9 hours ago by Mike

[Oz07]

2 hours ago, Mike said:

Building Regs Part L gives a minimum requirement based on a hypothetical window size. However the standard just rules out the worst performers and is easily beaten by windows with a solid timber frame. It's also not very helpful unless your windows happen to be that size; windows with a lot of glass or a lot of frame would have a significantly different values. Which is why, for a proper comparison, it needs to be calculated per window using the manufacturer's data (some may calculate it for you):

 

Uw = (Uf x Af + Ug x Ag + ψg x Lg) / ( Af + Ag )

 

where:

Uf = frame U-value

Af = frame area

Ug = glass U-Value

Ag = glass area

ψg = Psi value (thermal bridging) of the glass-edge spacer*

Lg = perimeter of the glass

 

*I should have mentioned the importance of this - it's the spacer that keeps the 2 panes of glass apart. Cheap windows use aluminium spacers that are very conductive; good windows use low-conductivity plastics that significantly cut the condensation risk at the edge of the glass.

 

For an illustration of what can go wrong, see this post:

 

 

 

I think most units for new windows are specced with warm edge spacers nowadays aren't they?

 

Interesting that timber normally beats upvc. How do the smaller manufacturers (particularly timber with big profile manufacturer back up) calculate their u value? From my experience they just quote the unit u value.  I know the accoya performs well thermally. Are other timbers the same?

[craig]

23 minutes ago, Oz07 said:

I think most units for new windows are specced with warm edge spacers nowadays aren't they?

 

It depends where you are sourcing your products, alu spacers are still fairly common unfortunately.

 

26 minutes ago, Oz07 said:

Interesting that timber normally beats upvc.

 

Regarding timber beating uPVC, a good quality uPVC frame with 4/5 chambers will beat a timber aluminium window on performance.

 

26 minutes ago, Oz07 said:

How do the smaller manufacturers (particularly timber with big profile manufacturer back up) calculate their u value?

 

They will have testing done on the system or at least they will be using the tested performance of the profile from the supplier. The program they use (excel, winpro etc) will perform the relevant calculation and supply the Uw, some won't even display it and just say AA Rated for example.

 

29 minutes ago, Oz07 said:

I know the accoya performs well thermally. Are other timbers the same?

 

From recollection, the lambda (λ) are pretty similar across the relevant timbers With oak being slightly higher. 

[Oz07]

@craig I was talking about small joinery manufacturers. They aren't getting testing done on their system if the bulk of their work is bespoke. 

 

Decent upvc beats timber then? Even without any special glues for the units or filling in the chambers. 

 

I honestly haven't seen alu spacers on newbuilds in over a decade. I reglazed some of a 30+ yr old timber conservatory the other week and that was 4:6:4 aluminium spacer

[Mike]

1 hour ago, Oz07 said:

Decent upvc beats timber then? Even without any special glues for the units or filling in the chambers.

No, uPVC needs additional insulation of some kind to beat timber. Timber just starts off with an advantage, but can be enhanced too - for example with cork inserts or with foam insulation between the timber and an aluminium external shell. For examples of what can be done, see https://database.passivehouse.com/en/components/list/window

 

However uPVC is often chosen as a cheap option and, as usual, you tend to get what you pay for.

 

Edited 3 hours ago by Mike