SAP Calculations Explained
Construction Energy Performance Certificate (EPC). Building Regulations require that a SAP calculation and a PEA are
submitted for new dwellings prior to the commencement of work.
A SAP calculation indicates a score from 1 to 100+ for the annual energy cost based on:
The elements of structure;
The heating and hot water system;
The internal lighting;
The renewable technologies used in the home.
The higher the score the lower the running costs, with 100 representing zero energy cost. Dwellings with a rating in excess
of 100 are net exporters of energy.
SAP calculations allow comparison to be made of the energy running costs of dwellings anywhere in the UK and are based
upon a notional standard occupancy that overcomes variations associated with physical location and the differing ways in
which people utilise their homes.
SAP ignores appliances’ use of energy (actual running costs can be 25% higher). A SAP calculation for a new dwelling is a
desktop exercise, the client or their designer submits drawings, plans and specifications of the development to the assessor.
It is recommended that the SAP calculation is undertaken early in the design process, this will help to prevent any costly
re-design of the dwelling. This should be submitted to Building Control with the full plan approval submission or a minimum
of 10 days before commencement on site.
SAP calculations usually involve 3 main stages:
1. As Designed Stage
From the suitably scaled or fully dimensioned plans and drawings provided by the agent, the accredited assessor prepares
summary information which includes the total floor area of the dwelling; the floor area of the warmer living area; the areas of
the heat loss floors, heat loss walls and heat loss roofs; dimensions of external windows and doors; storey heights, etc.
From the specification provided, the assessor calculates the performance of the thermal elements. These are expressed as
'U' values (the rate at which heat passes through the fabric of the building), the higher the 'U' value the greater the rate of
heat loss. The assessor then inputs this data into the SAP calculation. Data is entered relating to:
Type of dwelling;
Floors;
Walls;
Roofs;
Openings (windows, doors, roof lights);
Ventilation;
Main and secondary space heating;
Hot water generation;
Renewable technologies, including photovoltaic panels and solar water heating;
Energy efficient lighting.
The software determines whether the proposed dwelling will comply with the requirements of the Building Regulations with
regards to the conservation of fuel and power.
The assessor is able to use the software to model different variations of
the design if the initial specification doesn't show compliance. The assessor can then advise the designer of the
shortfalls and recommend solutions as required.
The client, designer and the assessor agree the final version of the design. This may involve amendments to the initial
design in order to achieve SAP compliance. Data from the final design is input into the computer program.
From this the assessor produces reports that the client or designer need to submit to Building Control, this will include a
Predicted Energy Assessment (this provides a rating of energy performance based upon the specified design and is
required to market the property).
2. Build Stage
Any changes on site can have a huge impact on the SAP so we suggest these amendments checked using the
appropriate software prior to making any changes on site.
SAP 2014 uses an air test figure of 5 m³/h·m² at 50 Pa in the model house, so this is what you are being measured against.
If you propose to reduce the number of air tests on a site then another of the same house type can use this but you have to
add a figure of 2; this can make it very difficult to pass with new technologies added to the specification. Most house builders
now test all plots.
The assessor will edit the SAP calculation to reflect the results of the air pressure test and any variations to the specification.
The software is used to check that the completed dwelling still meets the requirements of the Building Regulations with
regards to the conservation of fuel and power. If it does not, the assessor recommends remedial action.
For new build dwellings the assessor checks to ensure that the dwelling is registered on the Government's central database
register of national property addresses. If it is not, the assessor arranges for the address record to be created.
The assessor will require written confirmation that the as built dwelling has followed the as designed dwelling exactly;
including for example, thermal bridge details, party wall details or a list of any changes made. Any changes at this stage
can mean a fail and costly remedies, as at this stage renewables are often the only way out, hence our advice is to
follow the design fully, or check first!
3. As Built Stage
The assessor finalises the SAP calculation and creates the Energy Performance Certificate (this provides a rating of energy
performance based upon the dwelling as built). The EPC is required for a new dwelling prior to being sold or let.
In addition there are other documents that are required by Building Control such as the SAP worksheet report and the SAP
data input report.
The assessor provides the client with the EPC.
General
Since SAP calculations are based upon a desktop exercise, and not a site survey, it is critical that the correct information is
submitted. The following is a minimum suggested list of the information sources and data items required. Information
required to do a SAP/EPC.
1. Site address and postcode;
2. Site plan to include orientation of the dwelling(s);
3. Plans of each storey, normally to a recognised scale or fully dimensioned;
4. Elevations drawings for each elevation to a recognised scale or fully dimensioned;
5. Sectional drawings of the dwelling to a recognised scale or fully dimensioned;
6. A written specification which must include the following:
a) Details of the principal heating and hot water system to include make and model of boiler, details of heating
emitters, hot water cylinder size (if applicable) and the system controls;
b) Details of any secondary heating system present;
c) Details of ventilation systems;
d) Details of the internal and external lighting;
e) Details of the construction of all different floors to the property to include type and thicknesses of insulation and
any other building products used;
f) Details of the construction of all different external walls to the property to include type and thicknesses of
insulation and other building products used;
g) Details of the construction of all different roofs to the property to include type and thicknesses of insulation and
other building products used;
h) Details of thermal bridge details proposed. Part L accredited details to be followed at all times.
i) Details of the doors and windows to include sizes, type of frame, type of glazing, thickness of glazing, any low-e
films. If a lower than the Building Regulations U value is specified, a manufacturer certificate such as BFRC rating
is required; http://www.bfrc.org/ConsumerMain/find-a-supplier online search facility for doors and windows with
BFRC ratings;
j) Details of any renewable technologies utilised such as ground source heat pumps, solar water heating, waste
water heat recovery or photovoltaic.
As an alternative to specification on plans for walls, floors and other elements, a U Value calculation from the insulation
manufacturer is acceptable.