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The Woodhouse – Presteigne Passivhaus newbuild

A timber-framed newbuild home in Powys, designed to the Passivhaus standard and achieving an airtightness of 0.12 ach @ 50 PA, has now been occupied for two years. The project team included Architect Charles Grylls from Dempsey Decourcy Architects with Nick Grant as Passivhaus Consultant. Beth Williams from Build Collective was the structural engineer, Alan Clarke designed the mechanical services and Mike Whitfield Construction were the builders. 21° provided the triple glazed timber PROGRESSION windows and doors, as well as designing, supplying and commissioning the MVHR system for the project.


The project was built and designed to the Passivhaus standard, although the project is not currently applying for Passivhaus certification. The project team, who have worked together on a number of low cost timber frame Passivhaus projects. were keen to repeat details that have worked as much as possible to avoid ‘reinventing the wheel’. The team aim to continually refine their model and template to improve performance and reduce costs, simplifying designs as much as possible. This attention to detail and simplicity has borne fruit in that the build costs for the airtight shell for the project are estimated to be £1200/m2.

The house was built using timber frame I-beam for the roof, walls and first floor. An Isoquick insulated raft insulation was used for the foundations. Insulation within the timber frame was Warmcel recycled paper insulation. The house is clad in Douglas fir cladding, pre-painted by the client with Keim Lignosil paint.

The PROGRESSION range’s ‘frameless’ appearance very much fits in with the crisp minimalist visual language we often use on projects. The thermal performance of PROGRESSION is top of the range, which really helps us when modelling in PHPP. The ‘frameless’ aspect of the windows is also a bonus in terms of letting daylight in.

Charles Grylls - Architect

Working with Passivhaus structural engineer Beth Williams, the Ijoist timber frame is a refinement of previous designs that have evolved over a number of projects.  Another feature of the team’s approach to simplicity and keeping costs down is the use of fixed glazing for larger windows, although, obviously, ensuring there is an openable window in every room. The team has also developed good practice approaches to ensure good summer comfort with careful thought given to the positioning of windows for views and daylight.

The annual heating demand, according to the PHPP, is 14 kWh/(m2.a) with a specific heating load of 10W/m2. Space heating and hot water is provided by an air source heat pump, with radiators. Total energy consumption (electricity), including the freshwater borehole pump and workshop building, is about 3,100 kWh/year which equates to 20.6 kWh/(m2.a). With a primary energy factor of 3.1 this equals 63kWh/(m2.a) Primary Energy against the PHPP value of 71kWh/(m2.a)

I have said it before, but I really like the Progression windows and how they can be integrated into the wall with no exposed external frame. I have yet to have one leak in a blower door test but I know that if we have any issues that 21°’s after-sales support is excellent, as is their pre-sales technical help. Green Building Store are very much part of the team rather than just another supplier.

Nick Grant - Passivhaus consultant

Triple glazed windows and doors

21°’s Passivhaus-certified PROGRESSION triple glazed timber window range, with a whole window U value of 0.68 W/m2K was specified for the project, complemented with PROGRESSION triple glazed timber entrance doors. The windows were finished in a dark grey stain on the outside and painted RAL 9016 on the inside.

Our brief asked for the house to be as light as possible without compromising the requirement to maintain all year-round comfort. The choice of the PROGRESSION windows was perfect. In addition the careful positioning of the windows within the house have framed the surrounding landscape beautifully.


MVHR heat recovery ventilation

Alongside Mechanical Services Designer Alan Clarke, 21° also supplied, designed and commissioned the MVHR system for the project. A Zehnder ComfoAir Q MVHR unit was chosen along with rigid steel ducting.

We work with 21°s MVHR team regularly and know that they do a great job at designing quiet and efficient MVHR systems.

Charles Grylls

Project details


Charles Decourcy Grylls, Dempsey Decourcy Architects

Passivhaus Consultant

Nick Grant

Life changing homes