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MawsonKerr uses timber frame cassettes for Passivhaus home in Northumberland

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Dan Kerr

Temperatures are dropping, heating bills rising… our new series on energy efficiency starts by showing how England’s most northerly Passivhaus was built

Overlooking the Coquet estuary in Northumberland, Godwit House is built on a brownfield site on the edge of Warkworth, making it the most northerly certified Passivhaus in England.    

Big move

Airtightness and a very well insulated envelope contribute to the house's requirement for very little energy for heating, as well as helping it gain Passivhaus certification. 

The building is incredibly good at controlling air leakage, with an airtightness of 0.16 ACH (Certified Passivhaus standard is 0.6ACH or better and building regulations about 8 ACH). This, coupled with a very efficient MVHR, produces a comfortable and healthy internal environment and reduces energy use. 

How we did it

Factory-built timber frame cassettes filled with wood fibre insultation were used for the building envelope to give a great thermal performance and reduce embodied carbon. The inner sheathing board was also the airtightness layer and the outer board was breathable. Timber I-joists formed the structure in between.  The cassettes were craned into position to create the shell of the house in just a few days. For larger spans, engineered timber beams were used instead of steel, further reducing embodied carbon.       

  • Godwitt House.
    Godwitt House. Credit: Jill Tate
  • Godwitt House.
    Godwitt House. Credit: Jill Tate
  • Godwitt House.
    Godwitt House. Credit: Jill Tate
  • Godwitt House.
    Godwitt House. Credit: Jill Tate
  • Godwitt House.
    Godwitt House. Credit: Jill Tate

An Isoquick foundation was used as a great alternative to traditional strip foundations and concrete slab. This is an insulated base and upstand that sits on hardcore and creates the shuttering for the slab to be poured into. It reduces thermal bridging, helps airtightness details and is easy to install. Although the insulation is made from EPS (expanded polystyrene) it vastly reduces the concrete needed, as there are no deep excavations filled with concrete. It also reduces the spoil taken off site and there is no temporary shuttering to throw away, cutting the carbon footprint all round.  

The windows were high performing triple glazed with insulation externally overlapping the frame to reduce thermal bridging. A deep reveal was formed to help with shading and protection from the elements, and solar-powered integral external blinds were also used to the south facing windows to further control the internal environment during summer.

Externally, the walls use local stone and vertical larch cladding and the roof is a rubber waterproof layer with a sedum blanket over. Photovoltaic panels on the roof and a ground source heat pump using boreholes produce the very small amount of energy needed to heat the house.   

Junctions in the building were calculated for thermal bridging and we designed the floor-to-wall, wall-to-roof and corner junctions to make a positive contribution to heat retention rather than heat loss.   


Godwit House responds to both client and place and all the low energy design decisions were an integral part of this ethos. We shall find out how much of a saving there is on the heating bills soon!      

  • Panels craned in.
    Panels craned in. Credit: Mawson Kerr
  • Timber pannels.
    Timber pannels. Credit: True North
  • Timber frame.
    Timber frame. Credit: True North
  • Isoquick foundations.
    Isoquick foundations. Credit: True North


Architect MawsonKerr Architects
Contractor True North Construction
Structural engineer JCC Consulting
Timber frame PYC Frame
Windows Internorm


PH Performance -Actual


ACH@50pascals (≤0.6ach@50pascals)


Thermal energy demand



Thermal energy load




energy renewable demand (PER)



energy renewable generation (PER)



Elemental U-values








Windows (uninstalled)



Doors (uninstalled)



Window g-value




Timber frame



Timber frame



Insulated raft


Windows (uninstalled)

Alu-clad timber


Doors (uninstalled)

Alu-clad timber



MVHR – rigid ducting



Ground source heat pumps



PV array



Integral blinds




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