Roof windows and overall roof U-value

[Inkster]

I have an extension starting soon. It will have a vaulted roof. I'm trying to decide whether to use Fakro U8 quadruple glazed or U6 triple glazed roof windows. The U8 U-value is 0.58 and the U6 U-value is 0.81.

The roof (excluding the roof windows) will have a U-value of 0.15.

The walls will also have a U-value of 0.15, with a triple glazed French door and window with U-values of 0.8.

With 2 x U8 roof windows, the overall roof U-value will be 0.22.

With 2 x U8 roof windows, the overall roof U-value will be 0.26.

The U8 windows would cost a total of about £650 more than the U6 windows.

I'd like your opinions on whether or not this is too much extra money to spend for a 0.04 improvement in roof U-value.

Thanks in advance.

 

[freddiemercurystwin]

I wouldn't spend the extra cash, it'd take years to claw back the extra. Instead devote your efforts into making sure the extension is as air tight and tightly insulated as possible and make sure your existing house is to a reasonable practicable standard in terms of heat loss and air tightness too.

 

[Inkster]

I wouldn't spend the extra cash, it'd take years to claw back the extra. Instead devote your efforts into making sure the extension is as air tight and tightly insulated as possible and make sure your existing house is to a reasonable practicable standard in terms of heat loss and air tightness too.

Thanks Freddy. My builder is big into insulation and air tightness and is going to work on the air tightness of the rest of the house when the extension is finished. I've already put insulated plasterboard on the remaining outer walls of the house.

With regard to my original post, will the 0.04 difference in U-value in the roof make much of a difference overall, going forward?

 

[freddiemercurystwin]

No.