When Bebington, our Passive House in Weetwood, Leeds, was tested, it achieved 0.42 air changes per hour. The Passive House limit is 0.6. But what do those numbers actually mean, and why do they matter so much?
What airtightness measures
Airtightness is about uncontrolled air leakage: warm air escaping and cold air getting in through gaps in the building. Those gaps are rarely obvious. They are around window frames, where floors meet walls, behind sockets, around pipes and at the eaves.
Leakage wastes heat, causes draughts and can carry moisture into the structure, where it condenses and causes damp. A leaky house also makes ventilation with heat recovery much less effective, because air bypasses the system.
How it is tested
A blower door test fits a powerful fan into an external doorway and pushes or pulls air until there is a pressure difference of 50 pascals between inside and outside. The fan then measures how much air it has to move to keep that pressure. The more air it moves, the leakier the building.
The result can be expressed in two ways:
- Air changes per hour (ACH or n50): how many times the whole volume of air in the house leaks out each hour at 50 pascals. This is the Passive House measure.
- Air permeability (q50): cubic metres of air leaking per hour for every square metre of the building's outer surface. This is the measure used in UK Building Regulations.
How Bebington compares
- Passive House limit: 0.6 air changes per hour
- EnerPHit (retrofit) limit: 1.0 air change per hour
- Bebington result: 0.42 air changes per hour, with an air permeability of 0.50
For comparison, Building Regulations in England allow a new home an air permeability of up to 8. Bebington's 0.50 is many times tighter than that legal limit.
How we got there
Airtightness is not achieved by one product. It comes from a continuous airtight layer around the whole home, specialist tapes rather than sealants at every junction, a service zone so electricians and plumbers do not puncture the layer, and a test before plastering so any leaks can be found and fixed. We cover the lessons in 3 things we learnt building a Passive House Plus home in Leeds.
Why it matters for you
Whether you are building new or retrofitting an existing home, good airtightness means fewer draughts, lower heating bills and a healthier building. If you are planning a project in Leeds or Yorkshire, talk to us about how we would approach it.
