The reason that good airtightness is so seldom achieved in the UK is that few in the construction industry have the slightest knowledge of what they are discussing with thermal envelopes and so it risks being a case of the blind leading the blind.
Progress can be made:
A Victorian solid-walled house in Hereford has been retrofitted to a leakage rate of 0.9 ac/h at 50 Pa as part of the work to bring it up to roughly the Passivhaus Standard.
A mid 20th C cavity-walled house in Oxford has been retrofitted to achieve 3 ac/h @ 50 P. This is the level required by law in new Swedish detached houses 35 years ago i.e. from January 1978.
Similar retrofit work to the first project was being done in Canada in the late 1970s. I’ve been informed by this in my own practice and recommendations to clients since 1980.
But thermal retrofits are expensive and it seems clear that in towns CHP/DH is cheaper, plus lesser improvements in insulation and airtightness as time goes on and as parts of our buildings are replaced e.g. new roof tiles. I don’t think a Passivhaus retrofit is very likely to stack up financially unless the fuel being displaced is oil or LPG.
David Olivier BSc MASHRAE
ENERGY ADVISORY ASSOCIATES
Lower Orchard House
KINGTON HR5 3LX
T (01544) 340647