Passivhaus

Thermal Envelope

The thermal envelope is where most of the work happens. High-performance insulation across the walls, roof and floors cuts heat loss to a fraction of a standard build, which is what lets a Passivhaus home hold a steady temperature without much heating in winter.

Consistent Indoor Temperature

The same insulation works the other way in summer, keeping heat out, so a well-insulated envelope is as much about preventing overheating as retaining warmth. The payoff is a consistent indoor temperature through the year and very low running costs.

Triple Glazing

Triple glazing is effectively a requirement at this level of performance. Three panes of glass with insulating gas between them cut heat transfer far below what double glazing manages, which is why Passivhaus windows reach whole-window U-values of around 0.8 W/m²K or better. 

Low U-values

In practice that means no cold spots by the glass, no draughts spilling off the windows, and better sound insulation as a bonus. The window keeps its inner surface warm enough that you can sit beside it in winter without feeling the cold come off it.

Airtightness

Airtightness is one of the least visible parts of a Passivhaus and one of the most important. It means sealing the building so air can't leak uncontrolled through gaps in the walls, roof and floors, which takes careful detailing rather than a single product: the base plaster, membranes, tapes and junctions all have to work together. 

Remove Draughts

Passivhaus sets the bar at 0.6 air changes an hour at 50 pascals, far tighter than the regulations. Reaching it removes draughts and uncontrolled heat loss, and it's what lets the ventilation system work properly, because you can only control air movement in a building that isn't leaking it everywhere.

MVHR

An airtight house needs a deliberate way to bring in fresh air, and that's what MVHR does. It continuously extracts stale, damp air from kitchens and bathrooms and supplies filtered fresh air to the living spaces and bedrooms, and as the two streams pass through the heat exchanger, most of the warmth from the outgoing air is transferred to the incoming air. 

Constant Fresh Air

A good unit recovers well over 75% of that heat, often closer to 90%. The result is a constant supply of fresh, filtered air, which matters for air quality and for anyone sensitive to pollen or pollution, without throwing away the heat you've paid for.

ASHP

What little heating a Passivhaus still needs is best supplied by an air source heat pump. It draws heat from the outside air, even in cold weather, and uses it to warm the house, and it can run in reverse to cool in summer. Because it moves heat rather than burning fuel to make it, it's several times more efficient than a gas boiler, and on a building this efficient the demand it has to meet is small to begin with. 

Enhanced Energy Efficiency

We house the unit carefully, in this case behind cedar louvres, so it sits quietly with the architecture rather than being bolted to a wall as an afterthought.

Managing overheating

A very well-insulated, airtight building can overheat if solar gain isn't controlled, so this is designed in from the start. It begins with orientation, keeping unwanted sun off the glazing, and adds shading where it's needed: external shutters and screens that stop the sun before it reaches the glass, and overhangs sized to block the high summer sun while still letting the lower winter sun reach in to warm the rooms. 

Comfortable Indoor Climate

High-performance glazing helps too. Together these keep the interior comfortable in summer without resorting to mechanical cooling.

Eliminating thermal bridges

A thermal bridge is a point where heat bypasses the insulation, a junction between wall and floor, or a poorly positioned window, and it causes both heat loss and a cold internal surface. Those cold surfaces are where condensation forms and, over time, mould. Avoiding them means continuous insulation across the whole envelope and meticulous detailing at every junction, including where the windows sit within the wall build-up. 

Avoiding Mould

Get it right and the building keeps a stable temperature, uses less energy, and stays free of the condensation and mould that affect colder, leakier construction.