Designing a Passivhaus

3 August 2020 Kelly Newlands
Building to a Passivhaus level can reduce the heating energy requirements by a considerable amount, because reducing the heating or cooling requirements of a building means less energy is used in its lifetime, making it more sustainable. To achieve this, a building has to meet the Passivhaus Standard. This is defined as follows:
 
“A Passivhaus is a building, for which thermal comfort can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions, without the need for additional recirculation of air.”
 
Essentially, this means a building where all the heat losses have been reduced so much that it can be heated passively. In such cases, warmth is generated from people, household appliances and the sun, as well as mechanical ventilation with heat recovery (MVHR). All of these factors mean that the design of a Passivhaus needs to be carefully thought through to enable the standards to be met.
 

In numbers

Construction of a Passivhaus with SIPs
This means that, for the UK, the following should be achieved:
  • Space heating demand ≤ 15 kWh/m2/yr or space heating load ≤ 10 W/m²
  • Space heating cooling demand ≤ 15k Wh/m²/yr or space cooling load ≤ 10 W/m²
  • Primary energy demand ≤ 120 kWh/m²/yr (including hot water, space heating and cooling, fans, lighting, appliances)
  • Airtightness n50 ≤ 0.6ac/gr
The EnerPHit Standard (slightly lower requirements for retrofits) requires the following:
  • Specific heat demand of ≤ 25 kW.h/m²/yr
  • Primary energy demand of ≤ 120 kW.h/m²/yr
  • Roof, wall and floor U-values of ≤ 0.15 W/m².K
  • Windows and doors (including frame and glazing) U-values of ≤ 0.8W /m²K
  • Thermal bridging PSI values to be less than 0.01 W/m².K
  • Whole house mechanical ventilation with heat recovery (MVHR) which is ≥ 75% efficient and exhibits a specific fan power of ≤ 0.45 W.h/m³ and air leakage of ≤ 1.0 ach @ 50 Pa
 
As well as these numerical requirements, there is a demanding certification process which has to be gone through during the design and build phases, to ensure that the building is certified to Passivhaus standard. The Passivhaus Planning Package (PHPP) is a bundle of software and guidance notes. This is used in the design phase to ensure that a building will meet the required standards, and as part of the certification process.

The design

The efficiency of a Passivhaus relies upon a reduction of all unnecessary junctions – essentially, a box layout is the most conducive to achieving the standard. This obviously places limitations upon the design of the superstructure. A key characteristic of a Passivhaus is its ability to run from passive solar gain, so the fenestration needs to be carefully thought out, to enable the house to gain enough sunlight to warm it, but not too much that will overheat the building.

The path of the sun might influence the entire layout of the house, and to this end, the orientation of the plot needs to be taken into account. Elements that prevent overheating, like shutters or overhangs, may have to be integral to the building design, depending upon these factors. An MVHR system is essential to ensure air is kept fresh and circulated. The placement of these systems must be carefully thought out, bearing in mind that these systems can make noise, and that the interface between the MHVR and the kitchen extract system is critical. The Passivhaus Planning Package (PHPP) is an essential tool for designers; it will check that the designed property will not overheat once in service.
 
There are obvious benefits to designing to the Passivhaus Standard. As the push for carbon neutrality increases, so too will the demand for efficient homes. Low running cost is also a major benefit of the design, as the investment made initially (into premium insulation and glazing) will be more than earned back.
 

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