Working out a U value

[Ollie78]

Hello

I have been asked to do a job making some windows and need to calculate the U value for the proposed design. I will be using heritage Double glazed units that give a U value of 1.2 ( 4/8/4 krypton filled ) so I know that value at least.
Utile or Accoya timber and probably AQ21 or similar for sealing the openers, they will be a flush casement style with divided lights mostly 6 panes each section. They want trickle vents as well.
Would normally make 44mm sashes/casements or thereabouts in a 75mm frame but can go thicker if need be .

Its the classic case of they need to meet building regs but also be "conservation" due to the location etc. Normally I am more concerned with the conservation appearance than the actual values so have not had to calculate this before but I am sure someone here has.
Hoping there is a calculator somewhere I can enter in parameters and it will spit out a value.

Thanks

Ollie

 

[Doug71]

I had this problem a while ago, the old local building inspector was happy to work on just the centre pane U value but the new one wants to know the total U value of the whole window

I looked on line and found a few firms who would do it for me at a cost or there were some free calculators but it's more complicated than you would think.

Some firms like Whitehill include U value certifications for their window systems but it's no use for people like me who just do one offs of everything.

In the end someone on this forum kindly helped me out, what I remember from it was the U value of the wood sections was less than that required for a window as was the U value of the glazing so basically any shape or size window would be okay.

 

[baldkev]

https://www.ukworkshop.co.uk/forums/joinery-cabinet-making.36/
Dougs thread

 

[Ollie78]

I had this problem a while ago, the old local building inspector was happy to work on just the centre pane U value but the new one wants to know the total U value of the whole window

I looked on line and found a few firms who would do it for me at a cost or there were some free calculators but it's more complicated than you would think.

Some firms like Whitehill include U value certifications for their window systems but it's no use for people like me who just do one offs of everything.

In the end someone on this forum kindly helped me out, what I remember from it was the U value of the wood sections was less than that required for a window as was the U value of the glazing so basically any shape or size window would be okay.

I have found the ones online that you mention, you find them and they look great then you discover they want money for the secret formula.

I was considering the whitehill "System" just because you get a guaranteed U value, I already use loads of their cutters in the more traditional method so I gave them a call to see about the system. Well its a nice zero setup system with everything you need but its 10 to 12k and my moulder isn`t big enough so thats another wedge on top. Not really possible at the moment.

Ollie

 

[HOJ]

This is an area I looked into years ago, and on the basis of my research decided not to take on anything that needed to be proven, consequently I don't get involved in new builds or standard replacement windows, sticking with Listed and buildings in conservation areas, where the authorities are less concerned with compliance.

The cost of providing quantified evidential calculations was prohibitive as each type and style and size of window would have needed to be individually assessed.

The nearest I got was to produce a simple spreadsheet to do the calcs, but this was not an "approved" solution therefore not allowed to be presented as evidence of compliance.

 

[Jacob]

Hello

I have been asked to do a job making some windows and need to calculate the U value for the proposed design. I will be using heritage Double glazed units that give a U value of 1.2 ( 4/8/4 krypton filled ) so I know that value at least.
Utile or Accoya timber and probably AQ21 or similar for sealing the openers, they will be a flush casement style with divided lights mostly 6 panes each section. They want trickle vents as well.
Would normally make 44mm sashes/casements or thereabouts in a 75mm frame but can go thicker if need be .

Its the classic case of they need to meet building regs but also be "conservation" due to the location etc. Normally I am more concerned with the conservation appearance than the actual values so have not had to calculate this before but I am sure someone here has.
Hoping there is a calculator somewhere I can enter in parameters and it will spit out a value.

Thanks

Ollie

The U value of the wood is going to be lower than the glass but the area small, so I'd just calculate it as all DG for which you have a value, and forget about the wood.
There's loads of info if you just google "U Value"

 

[Ollie78]

The U value of the wood is going to be lower than the glass but the area small, so I'd just calculate it as all DG for which you have a value, and forget about the wood.
There's loads of info if you just google "U Value"

Almost too much information, it was giving me brain ache yesterday. I am thinking your method is as good as any but if they will accept this I don`t know.
Thanks @HOJ for the spreadsheet idea, not that I am claiming to know how to work one of those! I am like you in trying to stay away from new build stuff and more in the area of replacing and upgrading old stuff.
This one is a new build in a conservation area so the worst of both.

I think that if there is a listing or a heritage component then the regs should be adjusted/ignored to accomodate this as long as it is not a safety concern.
It is very tricky to please both sides, they are also wanting trickle vents in otherwise "cottage" style windows which are just over a metre high they will look horrible. I am suggesting a slim type of vent above the head of the window so its much less visible.

Thanks for all input.

Ollie

 

[Jacob]

Almost too much information, it was giving me brain ache yesterday. I am thinking your method is as good as any but if they will accept this I don`t know.
Thanks @HOJ for the spreadsheet idea, not that I am claiming to know how to work one of those! I am like you in trying to stay away from new build stuff and more in the area of replacing and upgrading old stuff.
This one is a new build in a conservation area so the worst of both.

I think that if there is a listing or a heritage component then the regs should be adjusted/ignored to accomodate this as long as it is not a safety concern.
It is very tricky to please both sides, they are also wanting trickle vents in otherwise "cottage" style windows which are just over a metre high they will look horrible. I am suggesting a slim type of vent above the head of the window so its much less visible.

Thanks for all input.

Ollie

Unless they are draught proofed all round they can be seen as "leaky" windows and won't need vents. You might have to argue the case! And there are other ways of doing it as you suggest.

 

[Doug71]

For the trickle vents on the outside you can sometimes get a bit creative and put a hole in the underside of the head drip then put one of these over/in the hole if you don't want to use the normal plastic vents.

https://www.ironmongerydirect.co.uk...6WViF4og_jZv8BfFTdtczFv-ocYwhWEgaAuNnEALw_wcB
Bradshaw joinery hides them nicely in the frame here if you don't mind the extra work



When I converted my place I didn't want trickle vents on the casement windows I made, the building inspector was fine with me not using them but did suggest I screw some of the covers on the outside so it looked like I had them then I could remove them off once it was passed off