It was a well attended and interesting visit last night to Steve’s house in Brighton where he’s installed an MVHR system to a 1930’s semi D without the usual whole house retrofit.  The house has cavity wall insulation and new double glazed windows, so reasonably airtight and insulated.  The MVHR was installed to combat condensation damp caused by insufficient ventilation, i.e. no back ground ventilation/trickle vents in the new windows, closed off chimneys and insufficient mechanical extract in the kitchen/bathroom.  The MVHR has made a huge difference, sorting out the mould and keeping RH around 50%.  However, would the house have been alright if the background ventilation and mechanical extract had been resolved?  Was the MVHR required or a luxury and with a relatively leaky house does the heat energy saving outweigh the electrical energy of the fans?  Even though this is a DIY project you would normally need to factor in several thousand for installation.  My feeling is that truly to keep moisture levels down, especially when drying clothes on dryers, requires such a high level of air change, the benefits of MVHR probably do make sense, they certainly felt like they did at Steve’s house.