Anyone know how I calculate the U-value for underfloor insulation for a house extension. I know you first divide the perimeter by the area and can then look up on manufacturers web-sites to determine the thickness of material needed to achieve the specified U-value. In my case though, this leads to a very high p/a value and I'm not sure that the calculation is applicable in this way. The floor is a suspended timber floor with air-space below it down to the oversite concrete.
Too keep things simple, lets say I have built a rectangular extension on the back of my house of say 6m by 2m. This gives a perimeter of
16m and an area of 12m, therefore p/a = 1.3 which is off the scale on the web-sites I have looked at so far.In reality only one of these dimensions is on an external wall because one of the 6m lengths is along the existing back wall of the house, one of the 2m lengths adjoins my neighbours extension, and the other adjoins an existing extension which has been built on the side of the house and projected 2m beyond the existing back wall. So if I only need to consider external perimeter, the calculation is now 6/12 = 0.5 which is a much more reasonable value.
To complicate matters further, the existing house has no underfloor insulation and I am only doing this to achieve building regulations on the extension. The actual extension is L-shaped and makes the p/a value look even worse.
Finally, I have about a dozen cavity wall insulation batts left over. Is there any reason why I can't use these providing they achieve the required u-values ? I realise I will have to support them with chicken wire or something similar but is the fact that it is a fibrous material likely to cause problems ? I suspect not as I have been led to believe that others have used ordinary insulation rolls as normally used in loft spaces.