We provide SAP calculations for:
- New build properties
- Refurbishments and conversions
- Extensions and highly glazed areas.
It is a requirement to complete and submit the SAP Calculations and building regulations compliance report before you start to build. The earlier you consider the SAP calculation and its implications the more likely you are to comply with the building regulations.
Our aim is to offer you a quality service and provide you with the best solutions. We will discuss your requirements before making our recommendations on how to comply – saving you money and ensuring you have the best solution.
For free advice and a quote call 01634 799115.
We also provide Energy Performance Certificates and air pressure testing for new and existing dwellings. Most new properties will require an air-leakage test on completion. We can also provide this service – discounted if we provide your SAP calculation as well.
SAP Calculations explained
The SAP calculation is a mathematical model. It is the official method used in the UK to calculate the energy consumption in dwellings:
- To assess compliance to UK building regulations
- To generate Energy Performance Certificates
Due to the complexity of the model it is nearly always accessed via a computer program. Official licensed software is approved by BRE on behalf of the government. The model is upgraded from time to time to reflect changes in the building regulations and conventions. Training is required to understand and use SAP correctly.
On-construction-Domestic Energy-Assessors who are members of an approved scheme are authorised to issue EPC’s and prepare SAP calculations for Building Control.
What’s behind the legislation?
The driving force behind the regulations is basically energy saving legislation, either UK or European. The UK building regulations are tightened up every 4 years 2010 /2014, to reduce the carbon emissions of new homes ( zero being the ultimate target). The Energy Performance Certificate, despite getting unfortunately tangled up in the aborted HIPS pack, is in fact an EEC requirement and arose from the Energy Performance of Buildings Directive.
The model is essentially an energy balance designed to calculate the heating, hot water, lighting requirement, in order to determine the fuel use, cost and carbon footprint. The latter two are additionally expressed on SAP and Environmental ratings scales in bands A to G.
Note that this is only an estimate of the heating lighting and hot water requirements for the home – it does not include cooking or the running of any other electrical appliances. Also note that it is is based upon a standardized heating pattern – individuals being individual will use their heating more or less according to comfort.
SAP the Bones..
If the home is maintained at a temperature greater than the outside the heat loss roughly proportional to the difference between the inside and outside temperature. This heat loss from various routes can be modeled.
For walls, roofs and floors heat loss is modeled thus:
Q = UA(T-T0) where Q is the heat loss Watts in SI units
A is the area m2 and (T-T0) is the temperature difference in Kelvin
The heat loss increases with temperature difference, area and U value.
The U value for constructions is calculated using approved British Standard method (usually a U value calculator) considering the thickness and thermal properties of the construction elements. The better and thicker the insulation – the lower the U value.
The dimensions of the dwelling are used to accurately assess the areas of each heat loss element – this is input into the model along with the U value.
Adjustments are made for openings, i.e. doors, skylights and windows, by inputting the U values, dimensions and other details of these. The U values are supplied:
- Either calculated using an approved simulation model.
- Or from a certificate of testing by an approved accredited test
- Or using a default from the SAP table for specific types of windows.
The U values are for the window as a whole (frame and glass).
Other heat losses are taken into account…
Heat loss from ventilation from planned openings – Chimneys flues passive vents, kitchen and bathroom fans etc.
Heat loss from unplanned air leakage from construction joints gaps is required:
- Either assumed at design stage
- Or tested in the dwelling on completion ( air leakage fan test )
- Or based upon the test result of another completed dwelling with an additional penalty applied.
The air ventilation either planned or leakage – from the above – is converted into air changes per hour. Air that is replaced by cold air from outside – either planned ventilation or a draught will create an additional heating requirement.
Heat loss also created at linear edges where two surfaces meet e.g. floor to wall junctions, wall corners and wall to roof junctions. This heat loss is assessed and taken into account. The latest version of SAP requires a calculation of this linear heat loss for each length of individual junctions where surfaces such as floors walls and roofs meet along edges. Alternatively a default value for the whole dwelling can be assumed, However the default is a high value which acts as a penalty making it more difficult to achieve compliance as a whole. Some types of junction have a greater heat loss than others so this can influence the design. Also included is the heat loss from metal lintels above doors and windows.
Thus the heating requirement to maintain the inside temperature one degree above the outside temperature is calculated by summing the various components of the heat loss – floors, walls, roofs, openings, ventilation and linear-junctions.
As individuals we have our heating on at different times and temperatures so for the purposes of SAP the the occupancy is standardized i.e. a standard heating pattern is assumed (heated at certain times in the morning and evening / all day at weekends to a standard temperature.
Thus the heating load per degree calculated above has to be multiplied up per the difference between the outside weather and the standard heating pattern temperature. Here the concept of “Degree Days” is used: this is the difference between the mean outside temperature and a reference temperature for each day – added up for a whole period. It is a set of SAP data used to multiply the heat loss due to the difference between the inside and outside temperatures over a period. The UK is split up into several different regions according to the weather pattern and the degree data is different for each region – colder climates have higher values. The SAP program that we use, automatically looks up the correct regional data according to the post code. The latest version of SAP creates 12 monthly heating calculations using Degree Day data for the specific regional area.
A standardized hot water usage is also added to the heat requirement. There are also various tweaks to the calculation as a whole to compensate for things such as shelter from the wind.
Thus the calculation model works out the amount of heat energy that needs to be supplied to maintain the standard occupancy on an annual basis. Next the model examines the sources of heat into the home and calculates the amounts of fuel required to maintain this heating requirement. Not all the heat sources are useful e.g. heat from lighting in summer would provide heat when it is not required. Heat from the sun via windows is useful in the winter but not the summer.
Next the Heat and Energy inputs from each source are calculated as below. Various details of these individual systems are required.
- Main heating system (can be more than one)
- A secondary heating system if fitted.
- The hot water system
- Mechanical ventilation heat recovery systems (MVHR)
- Solar PV or solar hot water and windmills
- Any other energy saving technologies
- Other incidental gain/loss, heat gain from windows / pumps / and loss from hot water tanks piping etc are also taken into account.
The load on the main and secondary heating is calculated by subtracting the other sources of useful heat from the overall heating requirement. This is split between the main and secondary heating if latter is present ( about 20 % is assumed to be secondary if fitted ) An additional requirement for hot water is also taken into consideration.
The efficiency of each heating appliance is input in order to attain the gross energy input. This is obtained from either:
- Approved databases of individual appliances e.g SEDBUG
- Or from standard reference values published in SAP tables.
There are also minimum efficiencies required for new appliances.
Thus the annual fuel energy requirement to maintain heating hot water and lighting is calculated for each fuel used. kW.hours per annum. This could be converted into weight if the calorific value of the fuel was known but this is not required by SAP – interested in only cost and Carbon emissions per unit of energy consumed.
In order to estimate the annual cost – a unit cost for a unit of energy from each fuel used is required.
In order to estimate the carbon footprint, the emissions of C02 – a CO2 emissions factor for each fuel is required:
- CO2 emissions in kg per kW hrs – of energy used.
This is either by direct consumption of from fossil fuels or an indirect factor is assumed for electrical consumption. The constant is calculated for mains gas and then an additional of ratio is applied if using fuels other than mains gas – as obtained from SAP fuel factor tables.
Both the fuel costs and emissions factors are based on published tables.
Finally the calculated annual carbon emissions and fuel cost are divided by the floor area of the dwelling ( ground plus upper floors) to obtain
- Cost per square metre per annum
- CO2 emissions per square metre per annum.
A formula is used to convert these into a SAP rating and an EI rating which is split up into bands A to G for the EPC. “A” being the most efficient.
The nominal scale is 1 – 100 but can be greater than 100 if dwellings are net exporters of energy.
DER / TER for new dwellings
For new dwellings the Dwelling Emission Rate (DER) is the CO2 emissions per square metre per annum as calculated above but with a few additional tweaks / assumptions made.
The Target Emissions Rate TER is designed to set a target for which the DER should be below.
We are experts in getting the DER below the TER!
This is a parallel standard calculation for a dwelling with the same floor area as the proposed dwelling and DER calc, but is calculated using a set of standard reference inputs contained in SAP instead of the actual inputs. The resulting carbon emissions from this reference dwelling are then further reduced by various factors to reflect the reduction in CO2 emissions as required by the current set of regulations – the factor is updated to reduce the target by about 25 % every 4 years. The factors are quite stringent and are designed to stretch building design and technology and drive down fossil fuel consumption. Thus the best materials and techniques are often required to get the DER below the TER. There are minimal standards for materials and services as required in L1A for new dwellings – but it is normally the DER requirement that drives the dwelling as a whole towards the required TER standard – without restricting individual choice concerning the method of conformity. It also in theory allows individual fuels to compete for compliance on the basis of efficiency of use and carbon footprint. Note that the best DER compliant solution may not be the cheapest to run.
For example compared to a mains gas boiler – a heat pump has a much higher efficiency. Despite higher carbon emissions for electricity than mains gas the DER will be considerably lower. However because electricity has a much higher price the running costs may not be so reduced.
Thus switching from mains gas to a heat pump may reduce the EI rating but perhaps not the SAP rating.
In the UK, it is carbon footprint, and not the heating cost, that drives standards regulating the energy performance of new buildings.
Whatever your priorities we will find the best solution for you!
The above is not designed to be an exhaustive guide to SAP but rather provide an insight into the workings and logic under the “hood”.
Therefore get draft SAP calculations and Building regulations reports as soon as you can to avoid failure and disappointment later. The regulations have changed so don’t rely upon previous experience
For existing buildings being altered or extended or any other requirement under L1B – the assessment process is slightly different but equally as complicated. The Same SAP calculation methodology is used – but the DER and TER calculation sub-sets are not appropriate in this case because compliance is based upon alternative criteria. However if an EPC is required – for example flat conversions -this is produced using the full SAP calculation.
At Tophouse Assessments all our assessors are fully qualified and members of approved schemes.
If you have a planning requirement under the code for sustainable homes – please mention this when making enquiries.
Typical route for new dwellings
Planning permission /detailed building plans and specifications drawn up.
Detailed Plans and specifications sent to us these are input into SAP program.
DESIGN DRAFT SAP calculations are produced
- (consult with client and modify plans if necessary to comply)
DESIGN FINAL SAP Calculations are produced
- Building regulations compliance report sent to client to give to building control
- A list of SAP model input details – those used to create the compliance report
These documents are required by building control before you start to build. This is a new requirement
- A Predicted energy assessment can also be produced – if required to market off plan.
After construction is complete the details are revised and compliance rechecked
- Air pressure test result / certificate replaces the assumed design leakage. Sometimes a penalty is added if the test result is from a different dwelling in the same development in accordance with the regulations.
Client informs Assessor of any changes to plans during the build
- That any assumptions adopted in the design have been achieved.
- The boiler / heating details are checked and updated.
AS BUILT DRAFT calcs are produced that incorporate these changes (final chance to make amendments) after checking the AS BUILT FINAL calculations are locked into the SYSTEM
- Building Regs reports are generated an EPC is produced and uploaded into the official Register.
- Details and notes are retained – occasionally these are externally audited to ensure standards are maintained within the scheme.
See also our page on Part L Building Regulations – these have changed Oct 2010!
RdSAP versus SAP EPCs
Rd stands for reduced Data – RdSAP is a method for assessing existing pre-occupied dwellings for energy performance and are legally required when existing dwellings are sold or rented out – providing energy consumption data to the prospective occupier. This method is only approved to create EPC’s for existing dwellings and should not be used for newly built dwellings.
Although the calculation is broadly the same, various standardizing assumptions are made that greatly reduce the amount of data required – to that which can be collected by a short site visit by a member of an approved RdSAP scheme. This is different to the procedure for new dwellings where more information is available from plans. An EPC should state the method used to produce it. Either SAP or RdSAP. To produce compliance documents for new dwellings and EPC’s – the assessor must be a member of an approved “on Construction” Scheme. If a new dwelling is being marketed off plan before completion then a predictive energy performance certificate can be produced as an interim indication of likely performance until the dwelling is complete. Qualified assessors for either new or existing dwellings can be checked out on the Landmark Web site.