For the final instalment in our mini-series of Passivhaus hacks, Kate Jacobs has asked our panel of experts how to make our homes breathable – and to explain thermal bridging …

Because the Passivhaus ideal is all about incredible insulation and airtight construction, incorporating the right ventilation is an absolute essential to avoid problems including condensation and mould. In the non-Passivhaus home though, ventilation is generally less of an issue, although the more insulation and draught-proofing you add, the more important ventilation becomes.

At its simplest, ventilation can be a matter of briefly opening a window; in the bedroom when you wake up, or after a bath or shower. “Older homes just weren’t built for four people having a daily shower, so there can be problems with condensation,” says Anaïs Bléhaut of daab design. It’s worth considering reducing water-related activities: shorter showers, drying washing outside or in an isolated, non-condensation-prone room and simmering a lidded pan rather than boiling away with the lid off. Sometimes, we’re guilty of not using our kitchen and bathroom extractors when we should, so it’s good to make sure they’re always turned on when they’re needed, even if that means putting up with a bit of extra noise. Edward Farleigh-Dastmalchi of fardaa also advises “making sure that extractors are working efficiently; keep them clean and maintain any vents and filters.”

If extra ventilation is required, trickle vents are often a good choice. These are generally fitted at the top of a window frame, allowing fresh air to circulate through a room. They are closeable so that air flow can be stopped during very cold or windy weather, but it’s advisable to open them during the day at least, even in winter.

Of course, these ventilation methods will lead to heat loss, so it’s always a balancing act. That’s why Passivhaus design incorporates heat recovery, typically with Mechanical Ventilation Heat Recovery (MVHR) system, which pushes extracted air through a heat exchanger so that the heat is retained and pumped back into the house. Companies such as Vent-Axia offer retrofit MVHR systems that work in existing homes, as well as localised units for problem areas such as kitchens and bathrooms.

Thermal bridge-free design

Despite the slightly confusing name, thermal bridges are perhaps better thought of as cold bridges: weak points in the envelope of a building where heat is conducted between the interior and exterior. This might be a structural element of the house, such as a steel balcony, or something less obvious such as curtain wall supports that compromise the performance of the wall insulation. Sometimes there will be signs that something has gone wrong, such as condensation forming on a cold patch of wall.

These issues are designed out of a Passivhaus buildings from the start but, in existing homes, we can still eliminate as many of them as possible. Fergus Knox of Knox Bhavan explains: “Thermal bridges often occur when insulation is not continuous. It’s important to insulate areas such as eaves and parapets. We also use timber studs, which are less conductive, in retrofit projects, to reduce cold bridging.” Farleigh-Dastmalchi also advises caution when fitting new insulation such as cork panels, “Use specialist plastic insulation fixings rather than metal, or attach with a specialist mortar from a company such as Mike Wye, who are brilliant for sustainable construction materials.”

When making any major new additions such as steel beams to support an extension, or new windows, Bléhaut advises that it’s important to get them carefully checked over at the design stage. Ask searching questions of the architect or contractor to be sure that new elements are being integrated into the fabric of the house without creating unwanted thermal bridges.

For our full series of Passivhaus hacks see:

The Green Series #1: an insiders’ guide to insulation

The Green Series #2: how to control heat and draught-proof your home

The Green Series #3: how to make your home airtight