All other things being equal I would say 28% cheaper for a mid-terrace assuming rectangular shape twice as long as it is wide. could say 30% cheaper, outside walls loose a lot of heat.

I read somewhere that the calculations were fundamentally flawed because there is air leakage between the houses. Not sure if this would apply to new housing though.

Isn't there usually a cavity between the houses? I think the problem is convection within that cavity taking heat to the loft and roof so direct thermal conduction and air leaks to the cavity matter.

Tony, Victorian non insulated solid brick walls are what I have in mind too.

Thats very interesting. How did you calculate that number?

My calc goes something like this :- for a terrace two long walls no heat loss, front and back = two units of heat loss, ceiling = 2 more and floor another one = total 5, then semi = solid party wall = front and back 2 again, party wall nil, flank wall 2, ceiling 2 and floor 1 = total 7; all very simplistic (7-5)/7 = about 30% A new terrace with cavity, now party walls loose lots as only a half brick walls = 30% more than a solid party wall semi.

Cavities were a big mistake

I guess it depends on if they are ventilated or not, Jonti - and if ventilated, do you recover the heat - so called zero heat loss walls.

Full fill cavity walls tend to be better as there is less convective loss

Regards

Barney

Until recently it was not allowed to fill party wall cavities -- NHBC rules and sound/fire rules too?