Certification | Air Tightness | Thermography




SAP Calculations


SAP Calculations are a mandatory requirement of all new build domestic properties in the UK. They are also required when substantially renovating a property, when converting a property (i.e. a barn conversion) and are commonly required for heavily glazed extensions.

The calculation takes into account all of the building elements which affect the thermal performance of the property such as the wall, roof and floor build-ups, insulation levels, window U-Values, and boiler details. These are then used to calculate the energy efficiency. The outputs from these calculations are the Energy Performance Certificate (EPC) and a set of Building Regulation Compliance Reports.  

 01  Do I need a SAP Calculation?

99% of the time the answer is YES. All new build homes require one as do all conversions and anything that constitutes a 'material change of use' (such as offices being turned into flats). Major renovations will also require one. For residential extensions, a SAP calculation is required when the glazing levels exceed 25% of the new floor area. In our experience, this is the majority of extensions since a large majority are now being designed to utilise daylight and will now have french doors, bifold doors or a glazed wall of some sort.



 02  When should I get my SAP Calculation?


​The sooner the better. We deal with new clients on a regular basis who haven't had their SAP calculation done until they are nearing completion. If the calculation fails at this stage there is a much more limited range of changes that can be made to improve the building specification. Improving wall and floor insulation is not a realistic option so it usually ends up with adding solar PV panels to the property after exhausting all other possible improvements. It is stressful and more expensive for the client than it would have been getting the calculation done before building begins, thus knowing that as long as you stick to the planned specification then you will be passing compliance at the end of it all.  




 03  What is Design Stage and As-Built Stage?

Before building commences we will review your architects' drawings and planned specification. From these details, we will model a 'Design-Stage SAP'. This allows us to calculate the energy performance of the property and ensure that all areas of the SAP are achieving Building Regulations compliance. At this stage, we will issue the Design-Stage Compliance Reports, a specification document and a Predicted EPC. This stage will also let you know the air tightness test target that you need to aim for. You can contact us at any time throughout the build for advice on how to achieve this target. See our air tightness page for more information.

Once you are nearing completion of the build get back in touch with us and we will arrange to carry out your air tightness (and ventilation) testing. At this stage, you will be issued an air tightness certificate and be asked to confirm that you have built the property to the same specification as the Design-Stage SAP. Once this is all complete we can issue you the As-Built Compliance Reports and the final EPC for the property. 




 04  How do I pass my SAP Calculation?

There are two parts to achieving compliance for a New Build SAP Calculation: 


The Dwelling Emissions Rating (DER)

This has to outperform the Target Emissions Rating (TER). The TER is set by reference to a notional dwelling (see below) within the SAP Calculation. This notional dwelling is of the same orientation, size and shape but uses a set of baseline values and a maximum level of glazing.

Dwelling Fabric Energy Efficiency (DFEE)

Again this figure needs to outperform the Target Fabric Energy Efficiency (TFEE). This is measured in energy demand i.e. how well a home retains its heat. This will also have a knock-on effect on the DER.


The Notional Dwelling

The figures below are those used in the notional dwelling within the SAP Calculation. These are much better figures than the minimum U-Value standards within Approved Document Part L.

Windows & Doors:                   The same as the actual dwelling up to 25% of the total floor area

External Walls:                         0.18 W/m2K

Party Walls:                              0.0 W/m2K

Roof:                                        0.13 W/m2K

Floor:                                       0.13 W/m2K


Glazing:                                   1.40 W/m2K (whole window U-Value)

                                                 0.63 G-Value

Opaque Doors:                        1.00 W/m2K

Air Tightness:                           5.00 m3/h/m2

Ventilation:                                Natural with intermittent extract fans

Heating:                                    Mains Gas boiler with 89.5% efficiency and radiators

There is flexibility to change these in your specification i.e. you can use a specification for the walls that has a U-Value of 0.24 W/m2K (for example) but the specification will need to be improved upon elsewhere i.e the floor or roof insulation. 

For conversions and extensions then the minimum standards in Approved Document Part L1B must be met.


 05  What happens if my current building specification fails?

We won't leave you with a fail. Our fee includes a recommended specification from us if you end up in this situation. It also covers any changes to your specification throughout the build such as if you're struggling to get a certain make of blockwork or a specific boiler is out of stock with the suppliers. A full design change (new drawings etc) will incur a fee from us but we've never had to do that yet. We like to work with our clients and not against them. It's why our regular clients keep coming back to us.

 06  How much do SAP Calculations cost?

This depends on the size and complexity of the property as well as how many you are building. A single 5 bedroom eco house with lots of dormers and sloping ceilings will cost more than a 2 bedroom terraced house on an estate of 100 plots. It all comes down to the time it will take us to model the property. We are highly competitive and think our repeat clients would agree. The feedback we generally receive is how great it is to have one person doing everything (SAP calculation, air tightness testing, ventilation testing & water calculations) and how helpful we have been throughout the build. 

To get a quote for a specific property or development please get in touch with us. 

Tips to Pass

  • Start your SAP calculations before you start to build

          Don't end up fighting for a pass at completion, know from the start that you're going to pass.

  • Fabric first... save energy before creating more

          Compliance can be achieved without the need for renewable technology to be installed. ​Focus on insulating the dwelling                when designing your specification. Achieving the Dwelling Fabric Energy Efficiency (DFEE)  figure in the SAP Calculation                will ensure you focus on saving energy before creating more.   

  • Air tightness is key... from the start!

          Heat loss through air leakage is huge and the SAP calculation knows it. Minimum standards in Part L for air leakage is                      10​.00 m3/h/m2, however, the leakier your dwelling is, the more money you will have to spend on insulation to improve the                SAP calculation. We generally aim for 5.00 m3/h/m2 - if you are confident and want to take on the challenge then we                        recommend going down to 4.00 m3/h/m2. If you are installing mechanical ventilation with heat recovery then you're going                to want to go as low as you can.


          A Passivhaus standard is 0.6 Air Changes per House (ACH) - this standard is based on volume but the figures roughly                    equate to the same on a typical house).

          Ask us throughout the build and we can help with air tightness tips and advice.

  • Don't forget about heating controls 

          Zoned heating layouts and weather compensators make a bigger difference in the SAP Calculation​ than you might expect.

  • Thermal bridging can turn a good property into a bad one

          Using the government's Approved Construction Details package (ACDs) will allow us to avoid using default figures. There              are other schemes that give an even better improvement​ and we will recommend them where we feel they will help. 

  • If you are a developer use bespoke PSI values

          When building multiple dwellings to the same specification calculate PSI values instead of using the accredited values -

          this will save you spending money elsewhere in the building specification. See our thermal modelling page for more                        information

Excessive Glazing Calculations


Domestic extensions commonly have what Building Regulations class as 'excessive glazing'. The allowance for glazing in a domestic extension is 25% of the new floor area + the area of openings no longer in existence due to the new extension (i.e. windows which have been removed or covered). This is because windows and doors have a significantly worse thermal performance than an insulated wall does so the Building Regulations Part L1B limit their existence to improve the general thermal performance of the extension.

Anything above the allowed amount is classed as excessive and triggers a requirement for a SAP calculation to be carried out. The purpose of this is to ensure that the dwelling with the actual extension outperforms the dwelling if it were to have an extension with the recommended glazing levels and minimum standards U-Values for the building fabric. This allows some flexibility in design where extra glazing can be used as long as other areas of the extension are improved upon to balance out the thermal performance. Further flexibility is granted when taking into account the existing part of the dwelling. i.e. were you to install a new boiler to heat the entire dwelling to replace an old and inefficient one, then this is taken into account in improving the thermal performance.

Tip: Ensure these calculations are carried out before purchasing materials and finalising designs to ensure that everything runs smoothly with your build.