Ventilation is very important and most of the housing stock in the UK relies to a greater or lesser extent on incidental draughts for ventilation. We know that my house is very air tight so it will need ventilation. I designed in a passive house type ventilation system, a Genvex, one of the better systems. This is fully ducted using rigid ducting, 200, 150 and 125mm in diameter and each room has either an inlet or extract terminal or both.

Ventilation rates are still open to question and some claim that 25m3/person/hr is required. We find that as there is a fully mixed large volume of air in the house we can live with less than 40% of that between us with no perceivable loss in freshness. We can control the fan speeds in one percent increments from 10% to 100% generally we run at between 35% and 45% with higher rates immediately after showering or when family are visiting.

The higher the ventilation rate the bigger the heat losses are and ventilation heat losses are my biggest losses even though I have a very efficient heat recovery system.

Read more of my pre-build advice on ventilation

You need air leakage of less than 3ACH@50Pa before an MHRV shows any benefit. Above that, and you’re just adding to the ventilation losses (even with the heat recovery), plus there’s the running cost of the fans.

To know if it’s too airtight or not, you need to know that actual air change rate in litres/second and compare that with recommended values based on the occupancy level.

For example, when I did the analysis for Tony’s house using hot2000, this is what it came up with for ventilation requirements:


Kitchen, Living Room, Dining Room 3 rooms @ 5.0 L/s: 15.0 L/s
Utility Room 1 rooms @ 5.0 L/s: 5.0 L/s
Bedroom 1 rooms @ 10.0 L/s: 10.0 L/s
Bedroom 2 rooms @ 5.0 L/s: 10.0 L/s
Bathroom 2 rooms @ 5.0 L/s: 10.0 L/s
Other 2 rooms @ 5.0 L/s: 10.0 L/s


F326 Required continous ventilation: 60.000 L/s (0.43 ACH)
Central Ventilation Supply Rate (): 60.000 L/s (0.43 ACH)
Total house ventilation is Balanced
Gross Air Leakage and Ventilation Energy Load: 22733.789 MJ
Seasonal Heat Recovery Ventilator Efficiency: 81.565 %
Estimated Ventilation Electrical Load: Heating Hours: 1635.585 MJ
Estimated Ventilation Electrical Load: Non-Heating Hours: 98.895 MJ
Net Air Leakage and Ventilation Load: 6191.642 MJ

See if your airleakage test results give the express it in litres/second, then you’ll see if you need an HRV or not (I suspect not)

Paul in Montreal.