Too much glazing

[Polstar]

Does anyone know how to perform an area-weighted U-value calculation?

I have seen the equation in ADL1B but it doesn't make much sense to me!

Do i work out the total area of walls and the total area of windows....and then what??

 

[tony1851]

It's a 2-stage process;

First, you take your extension as proposed and work out the areas of the floor, walls, roof and glazing. Knowing the U-value of each of your elements, you then work out the total heat loss from your extension, and then work out the average U-value.

Then you do the same calculation for what they call a 'notional' extension, which is the same size as yours but with the maximum permitted area of glazing.

If you have more glazing than normally permitted, you will probably find that your average U-value is worse than that of the 'notional' extension.
In that case, you will have to reduce the U-value of some other part (eg by adding more insulation to the roof) to bring your U-value down.

It's a time-consuming process, often involving going backwards and forwards through the calculations, to arrive at the right figure. And if you think that's complicated, just hope they don't ask you for a SAP calc............

 

[Polstar]

Thanks Tony, that is a much clearer way of explaiing it than anywhere else i have seen!

I spoke to the BCO and he pointed me in the direction of another method that they accept. I have worked out the floor area of the whole property and the total area of the openings. The total area of openings works out less than the 25% of the total floor area. I hope this will get me through...

 

[barry69]

I assume that when working out the area of glazing, you take the area of the whole window including frame and not just the visible glass area?

 

[cjard]

Barry, the glass will have a different U to the frame. You could either calculate the average for the glass+frame (if the frame's u is worse, do this) or just assume the frame is as bad as the glass (if the frame is better) and use the glass U. As a result you'll have a calc where the final U value is worse than the built building should be, so if it's still above whatever target, youre OK. If it's below the target, maybe going back and working in the frame's better U will help.. But it it's that knife edge close, put another inch of kingspan somewhere