Designing to the Passive House Standard (PHS) W

With PHS, the temperature levels remain comfortable throughout the year and the background mechanical ventilation system provides good indoor air quality without drafts.

Contrary to popular belief designing to the PHS is not just for houses and its does not have to follow any particular aesthetic or construction method. I expect that almost any building type can be designed and built to PHS.

Co2 output is reduced when building to PHS when compared with UK Building Regulations.

In commissioning to the PHS you may have more capital cost, – these possible additional costs can be eliminated by the use of a regular product delivery team from designers to builders by serial developers such as housing associations. In most cases any additional capital costs are outweighed by the long term benefits you will have as a result of installing better windows and insulation as well as an efficient mechanical ventilation heat recovery ventilation (MVHR) system.

PHS can not only be used for new build, but also for conversion and retrofitting the PHS for this retrofit is called EnerPHit – it follows the same principles but is a little less demanding of the end product. The Passive House Standard is a pass or fail standard and thus does not grade near misses on “perfection” as would less rigorous grading systems such as the Standard Assessment Procedure (SAP) used in the UK the produce Energy Performance Certificates (EPC`s) under Building, Selling and Renting Regulations. SAP is used to assess properties on a 1-100 scale and rated via an Energy Performance Certificate (EPC) .from A to G ; a D is a typical OK housing stock, B is to new building regulations, anything less that D is poor whilst C is better than average. A house built to PHS would be much better that the normal top scoring A on the EPC scale as below.

Performance Gap in Practice – Example 1.
My experience of purchasing a new house from a developer showed that there was a big performance gap. Post purchase investigations proved that a designed SAP point of 79 value was actually 66. This gave the EPC rating difference of a D1 and a C1; here a D1 uses 150% of the heating energy of a C1 dwelling. It took over 400 hrs of complaining to get this and other defects put right! Home Owners should not be burdened with such poor standards.

The Standard Assessment Procedure (SAP) calculation method is used to produce the Energy Performance Certificate (EPC).

The Passive House Planning Package (PHPP) calculation method is used to justify the issue of a Passive House Standard Certificate (PHSC).

SAP/EPC and PHPP/PHSC have different approaches to the following:-

  • Climate data (geography)
  • Floor Area
  • Internal heat gains – may vary by a factor of 3
  • Window details
  • Shading
  • Ground details
  • Distribution system
  • Internal temperature 19.2degC or 20degC
  • Treated floor area may vary by 15-40%
  • Thermal Bridging
  • CO2 emissions calculation
  • Primary and Delivered energy calculations

– thus the direct comparisons may need to be taken with a pitch of salt.

SAP/EPC is based upon.(England Building Regs. Notional Dwelling from Table 4 AD L1A)

  • Construction materials and thermal insulation of the building fabric.
  • Air leakage and ventilation equipment.
  • Efficiency and control of heating systems.
  • Solar gains.
  • Choice of fuel for space and water heating, ventilation and lighting.
  • Space cooling.
  • Renewable energy technologies.

You need to input data relating to (figures illustrate notional dwelling that will be guaranteed approval)

  • Type of dwelling
  • Floors – ≤0.13 W/m2K (U-value)
  • Walls – ≤0.18 W/m2K (U-value) with Party Walls at 0.00 W/m2K
  • Roofs – ≤0.13 W/m2K (U-value)
  • Openings ≤1.40 W/m2K (U-value) (windows, doors, roof lights) max 25% floor area.
  • Ventilation (natural with extract fans)
  • Air tightness 5.0 m3/(m2)
  • Air conditioning – None
  • Main space heating (gas boiler 89.5% efficient)
  • Secondary Space heating – None
  • Hot water generation (Mains Gas – insulated pipework and cylinder if not combi-boiler)
  • Renewable technologies, including photovoltaic panels and solar water heating
  • Energy efficient lighting 100%
  • Thermal Mass Parameter – Medium (TMP=250)
  • Thermal Bridging – 0.05 W/m2K in place of default 0.15 W/m2K for calc.

SAP/EPC does not include cooking energy. In theory you could use a cooker to heat the house!

In the paper (A Comparative Study of the Effects of Thermal Mass in New Dwellings in Scotland by Janice Foster) SAP predicted the boiler heating load for both dwellings would be 20 kWh/m2/year. The actual consumption was 67 kWh/m2/annum and 89 kWh/m2/year.

Some choose to follow the PHS principles, but do not actually try to register for Passive House Certification as they do not have the full commitment to the very high level of quality control that is needed in order to deliver to a PHS; in these circumstances the performance gap will be narrowed and lower heating bills are to be expected.

The PHS is based upon providing a comfort level for the occupants of the building as well as reducing the primary energy use to a low level; 15kwh/m2/yr for heating and ventilation; just 1/3 of the 2016 Zero Carbon House Standard set at 46kWh/m2/yr. 

Zero carbon is essentially achieved with a mixture of on-site renewables and carbon offsetting, whereas the PHS doesn’t use any on-site renewables and is more focused on saving energy than generating it.

Many who design and promote Passive House Design also promote other green policies such as when choosing insulation materials that do not involve the use of fossil fuel raw materials or excessive embodied energy. Other ecological targets can work comfortably in parallel with the passive house standard.

The PHS will not align with all aspirations or designs. Operating a house deigned using \the PHS may require empathy with the intention and thus knowing when to open windows and when to boost the ventilation system and when to change air filters etc… all these factors will be crucial to closing the performance gap in use. It is reported that smokers, who open windows or boost the ventilation frequently, will double their energy use in a Passive House.

Before committing to the PHS you should ensure you have a sufficient enough knowledge of the concept to enable you to reap the benefits it can bring. If you still think you need a full underfloor heating system then you have perhaps not really got your head around the PHS.

The Passive House Planning Package software (PHPP) consists of approx. 30 spreadsheets.

Among the inputs are the

  • treated floor area
  • the orientation & type of windows (both glazing and frames)
    • Windows only*: ≤0.8 W/m2K (U-value)
    • Windows after installation*: ≤0.85 W/m2K (U-value)
  • walls – ≤0.15 W/m2K (U-value)
  • floor – ≤0.15 W/m2K (U-value)
  • roof  – ≤0.15 W/m2K (U-value)
  • ventilation system –  inc. length and insulation of the ductwork
    • MVHR: Ventilation efficiency*: ≥75%
    • MVHR: Acoustics of plant*: ≤ 25dB
  • heat distribution*
  • Domestic Hot Water system*
  • boilers data*
  • electricity data*
  • Air permeability: n50 ≤0.6 ac/h EnerPhit ≤ 1.0 ac/h

* this information may not be forthcoming from various manufacturers…

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Heating and Cooling Demand: ≤15 kWh/m2/year  Specific Heat Load ≤ 10 kWh/m2/year (EnerPHit ≤25 kWh/m2/year)

Overheating (temperature>25degC) ≤10%

Primary Energy Demand: ≤120 kWh/m2/year

Performance Gap in practice Example 2.

By chance my own dwelling with a SAP/EPC 79/C rating has a calculated delivered energy demand of 160 kWh/m2/year, but an actual delivered energy demand of ≤120 kWh/m2/year is what I pay for on gas and electric bills. It could be seen as being as efficient as a passive house despite having very little floor insulation, poor pipe insulation and leaky construction as well as cold bridges at all the wall openings. This negative (SAP) performance gap is the result of occupier behaviour, a wood burning stove, condensing tumble dryer and no mechanical extract (We open windows for ventilation when needed.)
NB As energy is lost in conversion and transmission to the property; there is a difference between Primary Energy Demand and Delivered Energy Demand – the former is always higher than the latter.