Introduction

Rating systems and tools: domestic

Provided by Ewan Willars: Head of Policy, RIBA

In addition to the mandatory standards enforced through the building regulations and the semi-regulatory standards of the Code for Sustainable Homes, there are a range of independent standards, which are set out below. These include standards and assessment methods suitable for domestic buildings:

Standards and Assessment Methods for Domestic Buildings

  • The Passivhaus Standard
  • The AECB Carbon Lite Standards
  • BREDEM
  • The National Home Energy Rating
  • Assessment against Local Planning Standards
  • The Standard Assessment Procedure
  • Passive House Planning Package
  • EcoHomes XB

The Passivhaus Standard

The Passivhaus standard was developed in Germany in the early 1990s and, supported by the European Commission, is rapidly becoming a pan-European standard for low carbon dwellings. More than 6000 dwellings have been built to the Passivhaus standard. The Passivhaus standard has been incorporated into the AECB’s Carbon Lite standards (see below).

The standard is performance based and at its heart are requirements that annual space heating demand does not exceed 15 kWh/m2/yr and that primary energy use (for all purposes) does not exceed 120 kWh/m2/yr.

The standard also requires:

  • Fabric U values not exceeding 0.15 W/m2K.
  • Window U values not exceeding 0.8 W/m2K.
  • Air permeability not exceeding 0.6 air changes per hour at 50 Pa (demonstrated by a pressure test of the completed building).
  • Advanced whole-house mechanical ventilation with heat recovery with at least 75% heat recovery efficiency and electricity use no greater than 0.4 W/m3 of supply air.

The Passivhaus standard has recently been extended to apply to some non-domestic buildings.

The AECB Carbon Lite Standards

The Association for Environment Conscious Building (AECB), with support from the Carbon Trust, has a Carbon Lite programme aimed at improving the ‘carbon literacy’ of the construction industry.

At the heart of the programme is a set of three energy standards:

  • The Silver Standard is roughly equivalent to the EST Best Practice standard. It limits useful space heating energy to 40 kWh/m2/yr, primary energy use to 120 kWh/m2/yr and carbon dioxide emissions to 22 kg/m2/yr (with some permissible variation). Maximum U-values are in the range 0.15 to 0.25 W/m2K (1.0 to 2.0 W/m2K for windows and doors) and maximum air permeability of 3 m3/m2h at 50 Pa. Whole-house MEV or MVHR is required. Smart metering must be used for monitoring performance.
  • The middle standard is the Passivhaus standard (see above).
  • The Gold Standard attains the performance levels required by the Passivhaus standard, with the addition of renewable energy to reduce fossil fuel use for water heating, lighting, appliances and ventilation. The standard limits useful space heating energy to 15 kWh/m2/yr, primary energy use to 58 kWh/m2/yr and carbon dioxide emissions to 4 kg/m2/yr (with some permissible variation). Maximum U-values for the main fabric elements are 0.15 W/m2K (0.9 to 1.2 W/m2K for windows and doors) and maximum air permeability of 0.75 m3/m2h at 50 Pa. Efficient whole-house MVHR is required (with at least 75% heat recovery efficiency and electricity use no greater than 0.4 W/m3 of supply air). Smart metering must be used for monitoring performance.

 

BREDEM

The assessment methods used for most of the UK’s energy standards for domestic buildings are based on BREDEM, the Building Research Establishment Domestic Energy Model.

This predicts annual fuel use, fuel costs and carbon dioxide emissions under a standard occupancy pattern (a typical, floorspace-dependent pattern incorporating a standard heating regime). Standard occupancy is used for most assessments because the rating refers to the dwelling, not the way it is used by a particular household.

Some versions of BREDEM will also (in parallel) predict fuel use, fuel costs and carbon dioxide emissions under specified occupancy. Specified occupancy is useful for predicting households’ actual costs and ensuring the availability of affordable warmth.

 

Assessment against Local Planning Standards

For domestic developments, assessment against local planning standards (i.e. ‘Merton Rule’ standards) is best carried out by means of BREDEM-based performance simulation. The first step is to establish the energy demand and carbon dioxide emissions of the building as specified to meet the minimum requirements of Part L of the Building Regulations. This establishes the ‘base case’ to which the performance standard for the proposed building is related. The same simulation tool is then used to identify design and specification options, and to demonstrate how the standard will be met. 

The National Home Energy Rating

The National Home Energy Rating (NHER) is the leading domestic energy rating scheme in the UK. The NHER of a dwelling is based on the estimated total annual fuel use (for space heating, water heating, cooking, lighting and the use of appliances), per square metre of floorspace, under standard occupancy. It is expressed on a scale of 0 (very inefficient) to 20 (very low carbon), to an accuracy of one decimal place.

NHER assessments are location-dependent, so for example three identical dwellings in Cornwall, Cheshire and Caithness will all have different NHERs, depending on the regional climate and the exposure of the site

The Standard Assessment Procedure

The Standard Assessment Procedure (SAP) energy rating is the government’s preferred domestic energy rating. It provides the basis of assessment against the energy standards in the Building Regulations (in England and Wales, and Northern Ireland), the Energy Saving Trust’s Energy Best Practice Standards and the Code for Sustainable Homes. It is also the basis of assessments for the production of Energy Performance Certificates (EPCs) for new dwellings. SAP was introduced in 1995 and is updated on a regular basis to incorporate improved understanding of domestic energy use and reflect changes in the technologies used in dwellings.

The SAP of a dwelling is based on the annual fuel use for space heating, water heating and fixed internal lighting only, per square metre of floorspace, under standard occupancy. It is expressed on a scale of 1 (very inefficient) to 100+ (very efficient). Dwellings with SAP energy ratings greater than 100 are net energy exporters (due to local microgeneration).

SAP energy ratings are independent of location - all dwellings are assumed to be located in the East Midlands. This means that three identical dwellings built in Cornwall, Cheshire and Caithness will all have the same SAP. Existing dwellings may be assessed with a version of SAP known as Reduced Data SAP (RDSAP). This procedure uses ‘least unlikely’ default data for inputs such as external wall and ground floor insulation, window areas and ventilation, in order to reduce the cost and complexity of energy surveys. It is therefore slightly less accurate than the ‘full SAP’ procedure that is used for assessments of new dwellings. RDSAP is the basis of assessments for the production of Energy Performance Certificates (EPCs) for existing dwellings. 

Passive House Planning Package

The Passive House Planning Package (PHPP) is the means of assessing compliance with the PassivHaus standard and with the AECB Carbon Lite standards. The package has several thousand users throughout Europe, and it has proven accurate and reliable; it is however more complex than the SAP, demanding more input from a more experienced user.

The PHPP includes tools for:

  • Calculating the U-values of components with high thermal insulation.
  • Calculating energy balances.
  • Designing comfort ventilation.
  • Calculating the heat load.
  • Summer comfort calculations.

PHPP is a clearly structured Excel-workbook based design tool that can be used directly by architects and designers. At its core are worksheets for establishing heating energy balances (annual demand or monthly method), heat distribution and supply, electricity demand and primary energy demand.  

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