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Future-proof Passivhaus homes in Harpenden

The suburbs of Harpenden are becoming an unexpected haven of eco homes and home to a growing number of pioneering Passivhaus schemes. The Deerings, certified in 2017, set Passivhaus in motion for Harpenden, with projects in the local area now proving more ambitious for both new build and retrofit. 

Brambles Passivhaus Plus. Image credit: Tim Crocker

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New build: Brambles Passivhaus Plus

Brambles is a detached 3-bed home completed and certified in 2020. The all-electric home harnesses addtional renewable energy through solar PV and a reversible electric heat pump. Per square metre, Brambles saves 80% energy compared to a typical UK home.

Brambles Passivhaus Plus. Image credit: Tim Crocker

 

Brambles use so little energy, that it generates 3x more over a year than it imports from the grid. This house demonstrates how we can build today to meet the ecological imperatives of our time. It will protect against a changing climate and will last several times longer than a power station. As such it will be a gift to future generations, not a liability.

Bere Architects


Key stats

  • Construction: mixed: Timber frame

  • TFA:  150m

  • Certified: 2020

Brambles Passivhaus Plus. Image credit: Tim Crocker

Brambles Passivhaus Plus. Image credit: Tim Crocker

 

Construction

The home is constructed using a UK-manufactured frame made from Swedish engineered timber beams filled with cellulose insulation, chosen for its non-toxic properties.

Floor 

Concrete slab (150mm) power floated and polished as finished floor

EPS insulation (400mm)


U-value = 0.093 W/(m2K)

Wall 

Timber frame with cellulose insulation (310mm)

Mineral wool insulation (100mm)

Scottish larch rain screen cladding


U-value = 0.095 W/(m2K)

Roof 

Plasterboard ceiling, mineral wool insulation (45mm)

Vapour barrier board (12mm),

Timber frame with cellulose insulation (400mm)

Multi-ply bituminous membrane roof.


U-value = 0.097 W/(m2K)

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Predicted energy performance 

Airtightness

0.3ach @ 50Pa

Thermal Energy demand 

13 kWh/m2.yr

Primary Energy demand

129 kWh/m2.yr

Primary Energy Renewable

34 kWh/m2.yr

 

Key team

Brambles Passivhaus Plus. Image credit: Tim Crocker

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Retrofit: Harpenden EnerPHit Plus

Originally a leaky 1960’s detached dwelling, the existing 4-bed family home was retrofitted into an EnerPHit Plus in November 2021. As with an EnerPHit classic, the project prioritises a fabric-fist approach to drive down energy demand, but also includes additional renewable energy generation. The home produces as much energy as the occupants consume and provides enough energy to operate the building throughout the whole year.

Harpenden EnerPHit Plus. Image credit: Richard Washbrooke Photography

 

Retrofitting has cut our energy bills by 90 per cent. We have so much floor insulation we only need a very low level of underfloor heating, which is programmed to come on for just four hours at night when it’s cheapest. We don’t need any heating from late March until November. The standard appreciation on this house since we bought it would make it £1.3 million, but because of the retrofit it has been valued at £1.6 million. Estate agents have been asked if there are any houses like ours on the market.

The client, Harpenden EnerPHit Plus


Key stats

  • Build start date: January 2020

  • Construction: Cross-wall construction with cavity brick/block flank walls

  • TFA:  151m

  • Certified: November 2021

Harpenden EnerPHit Plus. Image credit: Richard Washbrooke Photography


Construction

Despite the challenges of retrofitting in a conservation area, the scheme successfully demonstrates the feasibility of achieving deep retrofit in keeping with the local vernacular. The existing structure consisted of cavity brick/ block flank walls which was externally insulated, with the original infill structure to front and back was replaced with site constructed timber insulated framing. As part of the summer comfort strategy, triple glazed windows include integrated blinds on southside of scheme. The ground floor was also replaced with a new insulated slab.

Floor 

New floor slab in main house:

  • Concrete slab

  • 160mm PIR insulation

  • Concrete screed with UFH

  • Floor finish

New floor slab to rear extension:

  • Concrete slab

  • 260mm PIR insulation

  • Concrete screed with UFH

  • Floor finish

Wall

Retrofitted Wall:

  • 200mm rigid wood fibre insulation

  • Existing insulated cavity wall

  • OSB board

  • Service void

  • Plasterboard and skim

 New Wall:

  • 80mm rigid wood fibre insulation, 300mm I studs with warmcel insulation

  • 18mm OSB

  • Service Void

  • Plasterboard and Skim

 

Roof 

Retrofitted pitched roof:

  • 30mm woodfibre sheathing

  • 240mm woodfibre insulation between rafters

  • 60mm rigid woodfibre board

  • Airtight membrane

  • Service void

  • Plasterboard and skim

Flat roof to extension:

  • 300mm rigid woodfibre insulation over joists, Smartply

  • Service void Plasterboard and skim

 

 

 

 

 

Predicted energy performance 

Airtightness

0.6ach @ 50Pa

Thermal Energy demand 

23 kWh/m2.yr

Primary Energy demand

84 kWh/m2.yr

Primary Energy Renewable

35 kWh/m2.yr

 

Key team

Harpenden EnerPHit Plus. Image credit: Richard Washbrooke Photography

 

Environmentally conscious clients were keen to give their 1970’s property a new lease on life through a holistic retrofit, and in the process, use their experiences to increase awareness among others. The main challenges of the project were to create an acceptable scheme in a conservation area, that aesthetically works in a row of almost identical houses. Harpenden EnerPHit Plus was the first Passivhaus project by the practice. It was also the contractor’s first retrofit project. Since this project, the practice has been working on several retrofit and Passivhaus projects.

Heather McNeil, Project Architect & Passivhaus Designer, AD Practice

 

Raising the bar for future-proof homes in Hertfordshire, these schemes are prime examples of self-build Passivhaus in the UK, providing optimum comfort and value, whilst driving down energy usage and fuel bills.

Feeling inspired? Discover more Passivhaus self-build schemes here. If you are interested in experiencing Passivhaus first-hand, the Passivhaus Open Days return this winter and offer an invaluable opportunity to experience Passivhaus projects for yourself!

Passivhaus Open Days Winter 2022

 

Further information

Brambles Passivhaus Plus

Harpenden EnerPHit Plus

The Deerings

Passivhaus goes Personal

Passivhaus Benefits

Passivhaus Powerhouses

Passivhaus Open Days 2022: Winter Edition

Brambles - iPHA Conference 2021

Harpenden EnerPHit Plus: 'Retrofitting has cut our energy bills by 90 per cent' - The Times

A beginner's guide to the Passivhaus Standard

1st October 2022


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