Passivhaus Design Principles and Powered by Renewable Energy
Type: 2 storey detached
Construction: Highly insulated, airtight timber frame
Floor Area: Main House:191m2, Ancillary Accommodation: 45m2
Location: Chew Magna
There were two main drivers to the design of this new house: accommodation that would provide for the owners well into old age; and to do so in a manner which took note of the climate disaster and the need to reduce our carbon emissions to a minimum.
The former garage workshop on the site was dismantled and several components were reused here and elsewhere. The new building reuses the foundations of the workshop saving a large amount of carbon.
The project was designed and managed by the client-architect. Clients and family members were also involved in dismantling, ground exploration, site clearance, constructing staircase and internal doors and paint and oil coatings.
The designed thermal performance of buildings in the UK typically diverge from their actual performance, so the adoption of Passivhaus principles and tools which are known to work was an important starting point.
This is an indispensable tool for monitoring the effects of design decisions on energy performance.
Shape has a great influence on heat loss. The ‘form factor’ (ratio of heat-loss surfaces to useable floor area), can range from a compact flat (say 1.75) to a sprawling bungalow (say 5), and the flat will need much less insulation for the same performance as the bungalow. The shape of the Pump House is largely determined by the narrow site and the form factor is 3.35. A southern orientation helps to make best use of solar gain but this site results in an east-west orientation.
Off-site fabrication of panels makes for speedy construction on site. Full depth of wall and roof can be filled with insulation. Timber sequesters carbon helping to reduce embodied carbon.
Roof: 300mm glass wool between rafters, 35 wood fibre outside and 50 rockwool inside.
Walls: 235mm glass wool between studs, 60 wood fibre outside and 50 rockwool inside
Floor: 300mm XPS beneath slab, 100 each side of strip footings.
Doors windows and roof glazing have to have a very good U value and includes the frame. All triple glazed.
In a poorly insulated building half the heat can be lost through unintended ventilation. In a well-insulated house the proportion will be much greater hence the need for airtight construction and attention to detail. A continuous air-tight layer around the building is achieved with special fabrics, bonding tapes and sealants. Service penetrations, (pipes and cables), are sealed with grommets.
With airtight construction the circulation of air for health must be considered. The MVHR provides this through a system which draws moist warm air from the kitchen and bathrooms into the heat exchange unit while another fan draws fresh air into the unit. In the winter, the heat from the exhaust air is transferred to the incoming fresh air and this is delivered to the living rooms and bedrooms.
All lights are LEDs and all appliances A+++ where available.
Heat pumps use electricity to power the pump but don’t burn any fuel to create the heat. They just move it from one place to another. This provides us with domestic hot water and warm towel rails.
20 panels flush with the slate roof give 6kWp. A Zappi EV charger uses surplus solar to charge the car. The addition of a battery (4kWh) means more of the solar is used in the house and topping up with cheap night-time electricity makes for a cost saving.
Rainwater from the main roof is collected and stored in a 3,500 litre tank under the garden. This is used in flushing toilets, cleaning the car and in the garden. Typical UK use is about 150 litres per person per day. For us it’s 66 l/ppd.
Roofing, windows, steel beams, timber sections and concrete blockwork from the dismantled building were reused here and elsewhere. Timber from a sequoia taken down in the garden of our previous home was used in joinery in the new house. Use of GGBS in concrete in place of cement reduced embodied carbon.
The intention was to make a house which uses little energy and has a low output of operational carbon. Readings over the five years of occupation indicate that on average the house produced 920kWh more solar electricity than grid electricity used resulting in an average saving of 396 kg carbon each year. And this includes about 6,000 miles of car travel each year.
Oh, and it’s very nice to live in!