Passivhaus (or passive house) is one of the most demanding energy standards in the world. It was developed in Germany, and the Passive House Institute in Darmstadt still sets the criteria. Full certification on an existing Oxfordshire home is rarely practical, but the thinking behind it is useful for any renovation or extension, and it can make an older house noticeably warmer and cheaper to heat.
What is a passive house?
A passive house is designed to need very little energy for heating or cooling, while staying comfortable and well ventilated. It does that through the building fabric rather than through bigger heating systems. The headline criteria for a new Passivhaus are:
- Space heating demand of no more than 15 kWh per square metre per year, or a peak heating load of no more than 10 W per square metre.
- Airtightness of no more than 0.6 air changes per hour at 50 pascals, confirmed by an on-site pressure test.
- Summer comfort, with temperatures above 25°C for no more than 10% of the year.
- Primary energy limits, which vary with the certification class.
For refurbishments there is a slightly relaxed standard, EnerPHit, intended for buildings where the existing architecture or conservation issues make full Passivhaus unrealistic. In a cool-temperate climate like ours, it allows a heating demand of up to 25 kWh per square metre per year and airtightness of 1.0 air changes per hour.
The five core principles
Most passive house design comes down to five ideas.
- Thorough insulation around the whole building envelope.
- Minimal thermal bridging, so heat cannot leak out through junctions, lintels and floor edges.
- Airtightness, so warm air is not lost through gaps.
- High-performance windows and doors, carefully installed.
- Mechanical ventilation with heat recovery (MVHR), which supplies fresh air while recovering warmth from the stale air going out.
Applying the principles to an older home
You do not need to build from scratch to benefit. Each principle can be applied, to a degree, when you renovate or extend.
Insulation, with care for old walls
Insulating roofs and floors is often the most straightforward step. Walls in older houses need more thought. Much of Oxford’s Victorian and Edwardian housing, and most Cotswold stone cottages, have solid walls built with lime mortar that absorb moisture and let it dry out again. Approved Document L itself warns that energy improvements to historic and traditional buildings should not cause long-term deterioration of the fabric, and names stone and lime-mortar construction as examples. Wrapping these walls in impermeable materials can trap moisture and cause damp and decay. Breathable insulation systems, such as wood fibre or insulating lime plasters, are often a better match, but they should be designed for the specific wall.
Airtightness
Sealing gaps around windows, doors, loft hatches, service pipes and floor edges cuts draughts and heat loss. In a renovation, the best opportunities come when walls, ceilings and floors are opened up anyway. An air test before and after shows what has been achieved.
Ventilation
Airtightness and ventilation go together. A tighter house needs a planned way to get fresh air in and moisture out, or condensation and mould follow. That might be MVHR in a deep retrofit, or well-placed extract fans and background ventilators in a lighter one. Building Regulations ventilation requirements still apply.
Windows
Windows are usually the weakest point of the envelope. New windows in an extension need to meet a U-value of 1.4 or better under current Building Regulations, and high-performance double or triple glazing can do much better. In a listed building or conservation area, secondary glazing or careful repair of original windows may be the right balance between performance and character.
Heating that matches the fabric
Once a house loses less heat, it can be warmed by a lower-temperature system. Current Building Regulations ask for new or fully replaced wet heating systems to be designed to run at a flow temperature of 55°C or lower where feasible. That suits heat pumps and larger radiators or underfloor heating, and is easier to achieve once the fabric has been improved.
Lessons for extensions and new builds
An extension is a chance to build to a much higher standard than the house it joins. Current Building Regulations already set limiting U-values for new elements (0.18 for walls and floors, 0.15 for roofs), and good design can go further:
- Design out thermal bridges at the junction with the existing house.
- Specify an airtightness line and test it.
- Use large south-facing glazing carefully. It gives free winter warmth but can overheat in summer without shading.
- Think about orientation, shading and cross-ventilation from the start.
For new homes, the Future Homes Standard regulations come into force in England on 24 March 2027, raising energy and ventilation standards and adding a requirement for on-site renewable electricity.
Is it worth the investment?
Savings depend on the house, what you do and future energy prices, so be wary of anyone quoting a payback period without looking at your home. What is certain is that a warmer, less draughty, well-ventilated house is more comfortable from the day the work is finished, and fabric improvements tend to last far longer than heating equipment. The most cost-effective time to improve the fabric is when you are already renovating or extending.
North Oxford Developments works on renovations, extensions and new build homes across Oxfordshire, with in-house teams covering roofing, insulation, plastering, flooring and Gas Safe registered heating work. We work alongside architects and energy consultants where a project calls for it. If you would like to make your next project more energy efficient, we are happy to talk through what is practical for your house.
