Whole-life costs of a Passivhaus: Sensitivities of whole-life cost analysis for domestic Passivhaus buildings

Encraft’s Building Physics team recently published a technical insight that investigated the sensitivities which arise when modelling whole-life costs of Passivhaus dwellings.  The technical insight, published by Dr. Sarah Price and Helen Brown at Encraft, also shows the lifetime costs of Passivhaus homes are around 2 to 5% less than the lifetime costs of a similar home built to current building regulations standards.  However with certain specifications, for example – using all-electric heating and domestic hot water, Passivhaus homes can be up to 1% more expensive over their lifetime. Nonetheless, in majority of the scenarios examined (and even if it costs 10% more to build), a Passivhaus will have lower whole-life costs than a traditional new build.

The Passivhaus standard for building low energy homes is growing in popularity across the world. When compared to a typical new home, built to UK regulatory standards, Passivhaus demands superior insulation and air tightness, coupled with a mechanical ventilation system, all of which cost extra and can potentially increase build costs. The extra investment can be justified for those with a long-term interest in the building if lifetime costs are considered.

The choice of building services is especially important as a traditional gas boiler produces lower whole-life costs than an all-electric heating and domestic hot water system (such as the compact unit, or ASHP).  Not only is it cheaper to install, but the maintenance costs are lower, as is the cost of gas when compared with electricity. The insight also shows that metal ductwork in a ventilation system is favourable over plastic, even though the product is more expensive to buy. Because the replacement cost is so high, the much longer lifetime of the metal ductwork produces lower whole-life costs.

Whole-life costing analysis accounts not only for capital costs, but also for the total operational (fuel bill) and maintenance costs, which are summed over the lifetime of the building. Fuel price rises, tax and inflation can also be incorporated into the analysis when appropriate.  It is a method which can be used to compare a number of cases or scenarios, based on common assumptions, but it is not an exact science and is only as accurate as the information that is put into the model.  A series of scenarios should be analysed where data is unknown, for example, fuel price forecasts or tax so that the results are well understood.  Scenarios (or cases) can also be useful when a design team has a difficult decision to make regarding specification, for example, between a gas boiler and all electric heating.  Whole-life costing analysis is a valuable tool in building design, but the results need to be well understood and handled with care.

To read the complete insight, please visit the Encraft website www.encraft.co.uk