The UK's first passivhaus leisure centre

[edit] My home is EPC B with continuous heat recovery and recirculation of interior air. Consequently we have low heating bills and no sign of any condensation or mould.

How does airtight home guard against condensation and mould?

Thank you.

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I think in terms of Passivhaus it isn't just the air tightness that helps guard against condensation and mould, it is the combination of: Super insulation levels, reducing the internal surface temperatures differences, particularly of the triple glazed windows. Air tightness, preventing cold draughts and warm air leakage. And finally high efficiency heat recovery which replaces stale moist air with drier fresh air, whilst retaining a high proportion of the heat.

To benefit from the higher efficiencies and reduced risks of condensation and mould, ideally one has all three. If one has not so good U-values, colder surfaces and colder window internal surfaces, along with not such a tight fabric, in particular drafts, there is a greater risk of condensation, likewise with wall performance and if persisting over a prolonged period potentially mould. This will also though be very dependent on how the building is used, ie the levels of cooking, drying, washing and the occupants' habits of opening windows.

It is worth noting also, that there is some evidence to suggest that thermal comfort is improved when all internal surfaces are a similar temperature because humans notice the temperature differences, ie feeling a particularly cold patch may lead the heating being turned up throughout, because of sitting near a cold window surface, for example.

Certainly if one has heat recovery and recirculation of interior air, the risk of mould is significantly reduced in comparison to a building with none of the above (as it would be with daily natural ventilation). The level of air tightness will impact how much work that recirculation unit is doing (and electrical energy being used). In the same way that positive input ventilation can help mitigate moisture issues in older buildings (but without heat recovery). It is worth visiting the UK Passivhaus Trust webpage for further guidance, but I hope that helps in the meantime.

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