Calculating U values for single storey extension

[davphi338]

Hi,

I'm in the process of planning a single storey extension. I want to add more window area than the permitted 25% of internal floor area. I have spoken to Buidling control and they say I can offset other areas to gain more glazing i.e add more insulation in walls, floor or ceiling. They gave me the following figures for U values, 0.35 walls, 0.25 floor, 0.25 roof, 2.2 glazing per sq m. I presume these are the min U values required for each element.

I have calculated that I need an extra 1.96 sqm glazing so after deducting the U value of the decrease wall area from the increase glazed area gave me a figure of 3.62. I then divided this figure by the internal floor area of 17.2 sqm to give a figure 0f 0.21. This is where now I came to a stall. My drawings quote the insulation for the floor of 85mm of Celotex, I am considering putting 100mm of Celotex but cannot find any information as to check if 15mm of extra celotex will increase the U value by 0.21.

If anyone has any input to whether I am on completely the wrong track it would be most appreciated.

 

[jeds]

80mm of Celotex in a floor gives a U-Value of 0.20 W/m²k. 100mm gives 0.17. The actual U-value required for a floor is determined by the ratio of the perimeter of the room to the area - known as the perimeter/area ratio. (p/a) Once you work this out look up the ratio in Part L1B and it will give you the minimum U-value.

If you calculate the total heat loss with 25% windows/doors and then repeat the calculation with your desired percentage - and then make a compensating adjustment - you won't be too far away. But bear in mind that strictly speaking BC may (should) require calculations to be carried out by an approved SAP assessor and on approved software - but some BCs are more relaxed than others.

The U-values given to you by BC are the minimums.

 

[davphi338]

Thanks for the reply Jed, a quick question regarding the perimeter/area ratio. I am knocking through my kitchen and dining room and constructing a single storey extension, when I calculate the p/a ratio do I take the measurement of all four sides or just the external walls of the new extension.

 

[philipdayton]

Another thing to consider is the actual window U values.
The figure of 2.2W/m2K is a maximum, with soft coat glass and
Argon filled units, window U values of 1.4/1.5W/m2K are achievable
with double glazing

 

[jeds]

Just the external of the new extension. The area should include the whole thing but the perimeter should be only the heat loss walls. i.e. don't include the wall adjacent to the existing house wall.