Paul McNally architect, passivhaus expert, director of the PassivHaus Architecture Company based in Cork, Ireland. Designer of completed award-winning commercial, residential and refurbishment certified PassivHaus projects since 2004.

Summer of 2018 sees us starting on site with our latest PassivHaus home, a new-build single family dwelling on the family farm in Ballygarvan, Co. Cork.

Contract negotiations lead us to look closely at the cost difference between the desired Passive House standard of energy performance and the legal minimum, Part L standard.

The following items were analysed as optional specification changes, if it were necessary to reduce the performance from passive to Part L;

Airtightness

3 m3/(m2.hr)@50Pa air permeability instead of 0.6 air changes per hour N 50, with one final test in lieu of 3 PH tests during the project.

Wall Insulation

Reduction of wall cavity depth from 250mm to 175mm, reducing the U Value from 0.123 to 0.171 W/(m2K).

Roof Insulation

100mm of mineral wool/timber structure was omitted, reducing the U Value from 0.117 to 0.155 W/(m2K).

Rooflights

Quadrupel pane rooflights were omitted for double-glazed units.

Thermal Bridge detailing

Simplification of window cill support, from our thermally broken detail, to a conventional precast concrete cill with rear-face insulation, similar to Part L Acceptable Construction Details.

Standard cavity metal wall ties in lieu of Ancon/Teplo low conductivity wall ties.

Armatherm FFR thermal breaks were omitted from steel to steel junctions in the slab and cavities where required.

Ventilation

Omission of the Heat Recovery Ventilation System for stand-alone Demand Controlled humidity -sensitive extract ventilation with dampened inlets.

Heating systems

Additional radiators would be required.

Structure

Narrower walls allowed narrower foundations.

Cost Difference

Byrne and Co. Quantity Surveyors for the project valued these potential performance reductions.

The net savings on the contract sum for this dwelling of 162.5 square metres was 5% of the total cost (including limited site works and services).

Fortunately this client did not need to execute the reductions and this is proceeding on site as a certified passivhaus. However, we note that this cost difference accords with other reported projects of a similar nature, i.e. one-off house.