Wednesday, May 29, 2024

A Guide to Passive House Design in the UK

Passive house design focuses on creating extremely energy-efficient buildings that require very little energy for heating and cooling. The key goals of passive houses are to use at least 90% less energy for heating and cooling compared to typical buildings and to be comfortable for residents year-round. Passive house principles can be applied to new builds as well as retrofits of existing buildings. Here is an overview of passive house design considerations and requirements in the UK. 

High Levels of Insulation 

A core component of passive houses is high levels of insulation. This includes insulation in the walls, roof, floors, windows, and doors. Insulation helps minimise heat transfer and reduce thermal bridging. For walls, insulation levels usually need to be at least 300mm. Roof insulation is often 450mm or more. Insulation helps the building retain heat in winter and stay cool in summer. 

Airtight Construction 

Passive houses need to be extremely airtight to control air leakage. All gaps and penetrations in the building envelope must be sealed carefully during construction. Airtightness is tested with a blower door test and passive houses must achieve an air change rate of less than 0.6 air changes per hour at 50 Pascals pressure. Careful sealing, taped joints, and robust membranes help achieve the required airtightness. 

High Performance Windows 

High quality, multi-pane windows with insulated frames are used. Windows must have a whole window U-value of 0.8 W/m2K or less. Triple glazing is often required to meet this level of thermal performance. The windows specification balances minimising heat loss while still allowing in solar gains during colder months. 

Heating and Ventilation System 

Since passive houses are airtight, mechanical ventilation is required to provide fresh air and avoid humidity and indoor air quality issues. A heat recovery ventilation system is typically used. It transfers heat from outgoing stale air to the incoming fresh air stream to maintain energy efficiency. 

Some passive houses use small hydronic heating systems paired with a compact boiler or heat pump. Radiant floor heating systems work well by providing even heat distribution at low temperatures.  

A small wood burning stove can be useful for some rooms, such as a living room. The best small wood burning stove for a passive house is a pellet stove. Discover the benefits of a small wood burning stove that burns pellets by following the link. 

Limiting Thermal Bridges 

Careful attention must be paid to minimise thermal bridges that allow heat transfer through the building envelope. Common thermal bridge areas include corners, junctions between walls and roofs, and mountings for windows and doors. Limiting thermal bridges further improves efficiency. 

Solar Design 

Proper solar orientation and shading should be used to maximise passive solar gains in winter while controlling overheating in summer. Daylighting design is also important for reducing electric lighting demand. South facing glazing maximises solar gains during the heating season. Overhangs help control solar gains in warmer months. 

Achieving certified passive house status requires paying attention to all these details. Additional factors include limited energy demand for heating and cooling as well as primary energy usage thresholds.  

With good design and execution, passive house principles can deliver ultra-low energy buildings with excellent comfort.  

While passive houses require higher upfront costs, the long term savings are significant. 


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