insulation and airtightness (technical)

Insulation and airtightness are essential components in reducing the need for fossil fuels.

Measuring zero carbon house by Passivhaus standards

zero carbon house is not a certified Passivhaus as no Passivhaus certification of existing buildings was possible at the time it was designed and there were no UK Passivhaus buildings constructed. (Passivhaus is the leading design standard, developed in Germany in the 1990s.) Nonetheless,  zero carbon house exceeds the Passivhaus standards for measuring heating and energy demands and airtightness in many respects. See the table below which compares the Passivhaus standards for new buildings with those of zero carbon house:


Standard PHPP target zch
Specific heat demand < 15kWh/sqm.yr 7.3 kWh/sqm.yr
Primary energy demand <120kWh/sqm.yr 41kWh/sqm.yr
Infiltration airtightness < 0.6ac/hr@N50 0.57ac/hr@N50
U-values, wall, roof, floor < 0.15W/sqm.K 0.08 – 0.11W/sqm.K
Glazing Ug < 0.80W/sqm.K 0.5W/sqm.K
Window Uw <0.85W/sqm.K 0.65W/sqm.K

External insulation

The walls and roof are 16 times better than the original building in their insulation standard and 28 times better in airtightness. Typical wall construction uses 280mm of insulation fitted to the outside face of both the new and existing masonry at the side and rear.  After negotiations with manufacturers it was agreed that all mechanical fixings could be designed out, as these would compromise thermal performance. The insulation is grey as it includes graphite to improve radiant heat transmission.  It is finished in a vapour-permeable external render with some colour accents.

Internal insulation

The front elevation of the original house is internally insulated to preserve its brickwork and stone features.  As there were no historic interiors, it is lined with timber battens, breather paper and 350mm cellulose insulation from newspaper.  Inside, a light timber framework independent of the external wall supports the variable vapour-permeability airtight membrane and lime plaster finish. Insulating wall ties maintain structural and thermal integrity.  The greater wall thickness is put to good use by forming window seats.  Insulation wraps down through the cellar and beneath the old and new floors.

Roof insulation

Above, a new 450mm void is framed out beneath the retained existing slate roof.  The new roof has 400mm of cellulose insulation between timber I-beam rafters, over-boarded with 100mm wood-fibre boards to give an improved decrement factor – that is slowing down the rate at which heat from external solar gains moves through the structure.


The triple-glazed opening windows have a slim 25mm external frame, which is covered with insulation externally.  Adjacent fixed windows are detailed as frameless stepped-edge triple-glazing sealed straight to the airtight membrane.


The airtight system of grommets, membranes and tapes was able to seal even the most complex existing and new junctions.  This has resulted in excellent airtightness results. It is 28 times better than the airtightness of the original building.

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