Perimeter / Area (P/A) Ratio - internal room areas or ignore internal walls completely?

[oranjeboom]

I know that it's measured by dividing the exposed perimeter by the floor area, but should the internal area be obtained by measuring all the separate rooms of the house (e.g. work out each room area and then adding them up to get total internal floor area), or should the internal area be measured by ignoring any internal walls?? I have added a pic below for clarity where the internal wall thicknesses would be an exaggerated 2.5m!

 

 

e.g. house above has an exposed perimeter of 25metres (ignore the neighbours wall). How should the internal area be measured - simply 12x4m or work out each room area first? The former method will ignore the width of any internal walls. So the length of the internal area is 12m or 9.5 multiplied by 4m.

 

38 vs 48m!!

 

Of course I've exaggerated the widths of the walls, but there could be a big difference in reality if your structure has a lot of internal walls that you either ignore and measure 'through' as in the blue measurement or you measure each individual room.

 

I've seen people do it both ways, but they can give very different ratios (0.68 vs 0.54 in this example). Many of the insulation sites are very vague on this also.

 

 

 

 

[Ferdinand]

It depends on the purpose ... and eg whether voids and space above stairs etc count.

 

Perhaps it also needs to depend slightly on the best answer for your purposes ?.

Edited May 1, 2018 by Ferdinand

[oranjeboom]

Purely for my own heatloss calcs, so the most accurate method is favoured in this case!☺️

[A_L]

3 hours ago, oranjeboom said:

Purely for my own heatloss calcs, so the most accurate method is favoured in this case!☺️

 

The correct area is 12m x 4m = 48m².

[oranjeboom]

23 minutes ago, A_L said:

 

The correct area is 12m x 4m = 48m².

Thanks! That will help with my end result then.

 

And with the exposed perimeter, is it correct that I use the external (exposed) wall (i,e, outside face) rather than the internal (exposed) wall. So in example above the perimeter  would be 25metres rather than 22ish metres?

[A_L]

14 minutes ago, oranjeboom said:

And with the exposed perimeter, is it correct that I use the external (exposed) wall (i,e, outside face) rather than the internal (exposed) wall. So in example above the perimeter  would be 25metres rather than 22ish metres?

 

The floor perimeter must also be measured on the inner face of the external walls (heat loss walls only).

[oranjeboom]

Thanks!