If you have an SAP assessment in progress with Focus 360 Energy, you may have questions about your reports and the next steps for your project. Gathering the information already available will help you establish what has been completed and arrange any remaining work.
What has happened to Focus 360 Energy?
The London Gazette published a notice on 29 October 2026 confirming that Focus 360 Energy Limited resolved to wind up voluntarily on 23 October and appointed Kevin Weir of Hudson Weir Ltd as liquidator. Read the official notice in The Gazette (opens in a new tab).
The notice confirms the company’s status. It does not provide much detail or disclose what will happen to any payment you may have made. Those questions need to be checked against your own project and contract.
If your SAP work is unfinished
Gather your quote or booking confirmation, invoices, payment records and emails about the project. Then contact the liquidator using the details in the official notice. Ask whether any reports, calculation files or project documents relating to your job are held and can be released, and how to raise any claim connected with the work.
Keep your own copies of the drawings, specification and any SAP documents you have received. If building work is due to start or a Building Control deadline is approaching, note the date so a new assessor can understand the urgency.
Continuing your SAP assessment
A replacement assessor can review the existing reports alongside your latest drawings and specification to establish what is needed to complete the assessment. If the original calculation files are unavailable, a new model can be prepared from your plans and project details. Share any approaching deadlines early so a delivery date can be agreed.
If the project has changed since the last assessment, make a note of those changes. Windows, insulation, heating, ventilation and other details can affect the calculation. For an as-built assessment, the assessor will also need information about what was actually installed and the required test or commissioning evidence.
Information to have ready
Any SAP reports, PEA or BREL documents already produced
The latest plans, elevations and sections, plus a plot schedule for multiple homes
Your current construction specification, including insulation, windows, heating and ventilation
A short list of changes made or still being considered
Any Building Control comments and your required timescale
You do not need to have everything ready before speaking to an assessor. The documents you already have will help them identify any gaps and explain what is needed next.
If you need help with the remaining work
We can take over unfinished SAP assessments for new builds, conversions and extensions. If you need help, send us what you have and let us know your deadline. We’ll confirm the remaining work, fee and delivery date with you. Hemcalcs is independent of Focus 360 Energy.
If your extension has failed an Area Weighted U-value calculation (AKA Heat Loss Calculation), you might be facing a choice between less glazing and more insulation. Before changing the plans, there’s another option worth considering: a whole house SAP comparison.
The same extension can fail the Area Weighted check and pass SAP without any changes to the design or the existing house. One important reason is the heat that comes in through the glass.
Why has the heat loss check failed?
An Area Weighted U-value calculation compares the fabric of your proposed extension with a reference extension of the same size and shape. It considers the areas and U-values of the walls, roof, floor and openings together.
Glazing generally loses more heat than an insulated wall. As the glazed area increases beyond the reference allowance, the proposed extension can lose more heat through its fabric than the reference design. Better insulation or lower glazing U-values can help balance that difference.
But this comparison doesn’t account for solar heat entering through the glass. That’s where SAP can give a different result.
Your glazing brings heat in too
Think about a room warming up when the sun shines through the windows. That is solar gain. During the heating season, some of that warmth can reduce the work your heating system needs to do.
SAP accounts for useful solar gains when calculating heating demand. The window area, direction it faces, shading and the glass’s g-value affect the contribution. The g-value describes how much solar energy passes through the glazing, whereas its U-value describes heat loss. SAP 10.2 explains this in sections 6 and 9.
Less glazing in the reference extension
For a whole house SAP comparison, we model your home with the proposed extension and compare it with the same home plus a reference extension. That reference extension has the standard fabric specification and its openings are limited to the permitted allowance, including qualifying existing openings covered by the extension.
Your proposed extension may have more glazing, so it can receive more solar heat as well as lose more heat through the fabric. SAP accounts for both effects. The Area Weighted method only considers the fabric side of that balance.
This is why a design can fail the simpler check but still work in SAP. In our experience, extensions with reasonable insulation and glazing specifications often pass this comparison without changes to the existing house.
For projects in England using the 2021 Part L standards, including the 2023 amendments, this comparison is described in Approved Document L, paragraph 10.10.
Does that mean more glass is always better?
No. Additional glazing brings additional heat loss, and the useful solar gains won’t always make up for it. South-facing glazing generally receives more solar heat than north-facing glazing. SAP also limits how much of the incoming heat counts as useful, so doubling the glazing doesn’t simply double the benefit.
The calculation checks the balance for your particular design. A SAP comparison needs to satisfy the primary energy, carbon emissions and fabric energy efficiency measures; solar gains alone aren’t a guaranteed pass.
Which calculation should you choose?
The Area Weighted method remains a useful, simpler way to demonstrate compliance where the extension’s fabric performs well enough. If it passes, a whole house SAP comparison isn’t needed just to account for the extra glazing. You can check your design with our free Area Weighted U-value calculator using your measurements and calculated U-values.
If it fails, whole house SAP calculations for your extension can establish whether the design already works once useful solar gains are included. Assessing the whole house doesn’t automatically mean upgrading it.
Your walls, roof and floor meet the notional U-values. Your air tightness specification matches too. Yet the SAP calculation still fails on fabric energy efficiency. Where do you go next?
We ran a comparison using a new build assessment to see how much difference the lintels could make. Changing that one detail took the fabric result from a fail to a pass, without adding any insulation.
The insulation wasn’t the problem
For this SAP 10.2 comparison under England’s Part L 2021 requirements, we used wall, roof and floor U-values of 0.18, 0.11 and 0.13 W/m²K respectively, with air permeability at 5 m³/(h·m²) at 50 Pa. These matched the notional specification for those elements.
The original result was a Dwelling Fabric Energy Efficiency (DFEE) of 39.9 against a Target Fabric Energy Efficiency (TFEE) of 39.6 kWh/m² per year. The dwelling needed to be at or below its target, so it failed the fabric check.
U-values describe heat loss through elements such as walls and roofs. Thermal bridging accounts for additional heat loss where those elements meet, including around window openings and at floor edges. Junction performance is described by a Psi-value (ψ), measured in W/mK.
The notional dwelling uses reference Psi-values from Table R2 of the SAP 10.2 manual. Your assessment uses values supported by your construction details. Meeting the notional U-values doesn’t automatically mean the junctions perform as well.
Our original specification used RCD junction details, one of the very few sets OCDEAs are allowed to use. Several performed better than the notional values, including the ground floor edge, sills and jambs. The lintels went the other way: an insulated open back steel lintel at 0.310 W/mK, against the notional lintel value of 0.050.
Each junction’s contribution depends on its Psi-value multiplied by its length. We checked all the lengths in this assessment: the lintels were the biggest adverse contribution. The improvements elsewhere almost balanced them out, but left the junction heat loss slightly above the notional total.
One lintel change, a different result
We replaced the E2-04 insulated open back steel lintel detail with E2-02 independent insulated steel box lintels in the model. For the wall specification being tested, that reduced the lintel Psi-value from 0.310 to 0.023 W/mK. Everything else stayed the same.
Fabric check
Original lintel
Revised lintel
Lintel Psi-value (W/mK)
0.310
0.023
DFEE (kWh/m² per year)
39.86
37.78
TFEE (kWh/m² per year)
39.56
39.56
Fabric result
Fail: 0.78% above target
Pass: 4.48% below target
The walls, roof and floor still had the same insulation. We hadn’t improved the air tightness or changed the windows. The difference came from the lintel detail. This resolved the fabric check; carbon emissions and primary energy have their own targets in SAP.
Another option would have been to calculate a bespoke Psi-value for the original lintel and surrounding wall construction. That might have produced a lower value than the RCD detail, enough to close this small fabric gap without changing the lintel. We’d need to model the junction and rerun SAP to find out. We offer bespoke thermal bridging calculations through our sister site, Thermal-Bridging.com.
Not all insulated lintels perform alike
Both of the details above describe insulated steel lintels, but their calculated performance is very different. A specification that just says ‘insulated lintel’ leaves an important question unanswered.
These were the available RCD lintel options for the particular blockwork, insulation thickness and conductivity in our test. They show the range even within one wall specification.
These values belong to the wall build-up tested. Different blockwork, insulation or junction arrangements can change them.
E1 covers steel lintels with a perforated steel base plate; E2 covers other lintels. The individual detail matters more than the broad category. Your assessor needs the detail that matches what you intend to build.
Check the detail before adding more insulation
A fabric failure can lead straight to a request for thicker insulation. Sometimes that’s the right answer. In this comparison, the lintels deserved attention first.
Send your assessor the proposed lintel details alongside the wall construction, rather than leaving them to work from a generic description. Reviewing them during design gives you a chance to compare suitable options before anything is ordered or built.
Our new build SAP calculations include advice and reasonable revisions. If your design is struggling to pass, we can look at what’s driving the result and help identify which changes are worth making.
Maximum U-values for new elements under England’s Part L 2021 guidance, including the 2023 amendments. Lower values mean less heat loss.
These are the standard extension figures used for the reference method. They are also the fabric starting point for the notional extension used in a whole house SAP comparison. They are different from the notional U-values for a new home.
The figures describe the complete wall, roof, floor or opening, including the window frame. They are not the performance of the insulation or glass alone. Some windows and doors can also comply using an energy rating.
Meet these U-values and you might not need SAP
If your extension meets the reference method requirements, you do not need an Area Weighted U-value check or whole house SAP calculation. The U-values are one part of that: your openings must also stay within the allowance, and the heating and other building services must meet the relevant standards.
Any existing construction becoming part of the heated building envelope needs checking too. Meeting the energy requirements does not replace the other Building Regulations checks for your extension.
Check the opening allowance
Permitted opening area = 25% of the extension floor area + existing windows and doors covered or removed by the extension.
How do you know your construction meets the U-values?
A thickness of insulation is not a U-value. The materials, their arrangement and the construction details all matter. For windows and doors, ask your supplier for the whole unit figure.
We can calculate the U-values for your proposed wall, roof and floor for building control. Book your U-value calculations.
When we ask clients for their heating and ventilation specification, we often receive something like “Heating: ASHP. Ventilation: MVHR.”
That tells us what type of systems they’re planning, but very little about the equipment itself. We can run a SAP calculation using generic information. The problem is that the result may look different once we know exactly which products will be installed.
If your design is struggling to pass, it’s worth checking those details before spending money on extra insulation or solar panels.
ASHP and MVHR are descriptions, not specifications
Two air source heat pumps can have different efficiencies. Two MVHR units can recover different amounts of heat and use different amounts of electricity to move the air. Simply selecting the system type does not capture those differences.
SAP has a Product Characteristics Database containing performance data for specific equipment. Where suitable product data is available, we can use the relevant entry rather than relying on generic assumptions.
The make and model are the starting point. We also need to make sure the entry and installation details match your proposal. Picking the best looking efficiency figure from a brochure isn’t the same as modelling the system correctly.
To make a fair comparison, we used the same building fabric throughout, matching the notional dwelling specification, with air permeability set at 5 m³/(h·m²) at 50 Pa. Its fabric energy efficiency exactly met the target in every run. We then changed the heating, hot water and ventilation selections to see how they affected the results.
What difference does the heat pump make?
An air source heat pump uses electricity to provide heat. Its efficiency affects the energy SAP calculates the home will need for heating and, where applicable, hot water.
If your installer has already selected a heat pump, give your assessor the exact model. That lets us check the appropriate product data and the proposed system, rather than treating it as an unspecified ASHP.
For this comparison we used a Vaillant aroTHERM plus 5 kW heat pump paired with a Vaillant uniTOWER hot water cylinder. The generic heat pump was paired with a generic cylinder, so this compares the heating and hot water package, rather than the heat pump alone.
Keeping generic MVHR in the calculation, the specified heat pump and cylinder reduced the dwelling primary energy rate (DPER) from 78.69 to 67.49 kWh/m² per year. That is an improvement of around 14%, but it still exceeded its target of 55.90. Identifying this package helped, but did not resolve the primary energy failure on its own.
What difference does the MVHR make?
Mechanical ventilation with heat recovery uses heat from the outgoing air to warm the incoming fresh air. Its fans also use electricity. SAP accounts for both, so the unit’s heat recovery performance and fan power matter.
A generic MVHR selection cannot describe every unit on the market. The specific product data helps us represent the system you actually intend to install.
We selected a Nuaire MRXBOXAB-ECO3B with five wet rooms. Keeping the generic heat pump and cylinder, this reduced DPER from 78.69 to 56.33 kWh/m² per year, an improvement of around 28%. That put it close to the 56.05 target, but still just above it.
What happens when we specify both?
With both the Nuaire MVHR and Vaillant heat pump/cylinder package selected, DPER fell to 44.91 kWh/m² per year against a target of 55.90. The primary energy result now passed, coming in around 20% below the target. Compared with the generic combination, calculated primary energy use was around 43% lower.
Ventilation
Heating and hot water
DPER
Target (TPER)
Primary energy check
Generic MVHR
Generic heat pump and cylinder
78.69
56.05
Fail
Generic MVHR
Vaillant heat pump and uniTOWER
67.49
55.90
Fail
Nuaire MVHR
Generic heat pump and cylinder
56.33
56.05
Fail
Nuaire MVHR
Vaillant heat pump and uniTOWER
44.91
55.90
Pass
No MVHR
Generic heat pump and cylinder
56.61
56.05
Fail
No MVHR
Vaillant heat pump and uniTOWER
45.30
55.90
Pass
DPER and TPER are in kWh/m² per year. The specified heating package includes the hot water cylinder. Targets are shown separately for each run.
The dwelling emission rate (DER) was below its target in all six runs, ranging from 7.41 to 4.27 kg CO₂/m² per year. Dwelling fabric energy efficiency (DFEE) stayed at 39.56 kWh/m² per year, equal to its target in every run. The issue exposed by the generic selections here was primary energy, not a fabric shortfall.
We also ran the assessment without MVHR. With the specified Vaillant package, DPER was 45.30, compared with 44.91 using the Nuaire MVHR. With the generic heat pump and cylinder, the corresponding figures were 56.61 and 56.33. The generic MVHR runs performed considerably worse in this example. That is a reason to check the product data, not a reason to assume every MVHR system makes a design better or worse.
These figures describe calculated performance in this assessment, not measured energy use or a forecast of the household bills. The target also changed slightly when the heating and cylinder package changed; the table keeps that difference visible.
The useful question is whether the design needs changing, or whether the assessment needs better information. Identifying equipment already in your specification doesn’t mean you’ve upgraded the building. Choosing a different product to improve the result does.
Specific products are not an automatic pass
The actual product data may improve the result, make little difference or reveal that the original assumptions were too optimistic. Either way, you have a better basis for deciding what to do next.
A more efficient heating system also does not resolve every type of SAP failure. If the problem is the fabric energy efficiency result, you need to look at the building itself. Our guide to fabric energy efficiency explains that distinction.
What should you send your SAP assessor?
You don’t need to interpret the product database yourself. Send us:
The heat pump manufacturer and exact model, including the proposed indoor and outdoor units where relevant.
The MVHR manufacturer and exact model.
Your installer’s quotation or system specification, if available. Product links and technical sheets are useful too.
If you haven’t chosen the equipment yet, tell us. We can explain which assumptions we’re using and which details need confirming as the design develops.
Our new build SAP service includes advice and reasonable revisions. We check the specification and help you understand which details are affecting the result. If you would like us to assess your project, send us your plans for a quote.
Getting close to finishing your new home? Your new build EPC is produced from the As Built SAP assessment, using the details of what has actually been installed. The more complete that information is, the sooner we can check the calculation and prepare your certificate.
Start with the following information for each home:
The property address. Include the postcode and plot reference so we can match the documents to the right dwelling.
Final drawings. Floor plans, elevations and sections showing the home as built, including any changes to the layout, windows or doors.
Construction details. The wall, roof and floor build ups, including insulation products and thicknesses, plus any thermal bridging details used.
Windows and doors. The supplier’s specification, including whole window and door U-values and the glazing g-values. Include roof windows and rooflights too.
Heating, hot water and ventilation. The exact makes and models installed, including controls and hot water cylinders, with commissioning records where applicable. Include details of solar panels or other renewable systems.
Test results and photographs. Your air tightness test certificate where required, relevant ventilation test results and photos taken during construction.
You don’t need to rewrite manufacturers’ information. Product sheets, installation records and clear photographs of model labels are useful. If you’re unsure about a document, send it over and we’ll check.
Take photographs before the work is covered
For England’s Part L 2021, organise photos for each dwelling using the Appendix B references below. Show completed insulation and sealing before they’re covered.
Ground floor:1a perimeter edge insulation; 1b external door thresholds; 1c insulation below the damp proof course.
Walls, each main type:2a floor/wall junction; 2b structural penetrations.
Roofs, each main type:3a insulation at joist/rafter level; 3b eaves and gable edges.
Openings:4a windows; 4b external doors. Show each opening type’s position against the cavity closer or insulation line.
Air tightness:5 sealing details not already visible in photos 1–4.
Services:6a make, model and serial labels for heating, hot water, ventilation and renewables; 6b primary pipe insulation; 6c ventilation duct insulation outside the thermal envelope.
Keep original files with location, date and time recorded. Name them by plot and reference, for example P1-3b-eaves.jpg. Add close ups where needed.
We recommend using an app like Solocator, available for iPhone and Android, to stamp GPS location, date and time onto your photos. Enable location permissions and keep the original files with their location metadata.
A different heat pump, a larger window or an insulation substitution can change the result. Send us the final product details rather than the original specification if something has changed. We can then assess the home you’ve built and explain whether anything needs attention.
Ready to arrange your new build EPC?
Send us your plans and the information you have. We’ll check what’s there, identify anything missing and help you get the final assessment organised.
Planning bifold doors, a large window or a couple of rooflights? You may have heard that glazing cannot exceed 25% of your extension’s floor area. That is only part of the calculation.
For extensions in England, the 25% glazing rule also allows for existing windows and doors covered or removed because of the extension. You could have more room for glass than you think.
How does the 25% rule work?
Start with 25% of your extension’s internal floor area. Then add the area of existing windows and doors that will no longer face outside because of the extension.
Opening allowance = (extension floor area × 0.25) + qualifying existing openings
This is the reference method opening allowance in Approved Document L, paragraph 10.7(d). It is an opening area check: the insulation and building services need to meet their requirements too.
Calculating your extension’s glazing limit
Imagine an extension measuring 6m x 4m. It covers an existing window measuring 2m x 1m and a door measuring 1m x 2m.
Calculation
Area
Extension floor area: 6m x 4m
24 m²
25% of the floor area: 24 × 0.25
6 m²
Existing window: 2m x 1m
2 m²
Existing door: 1m x 2m
2 m²
Total opening allowance
10 m²
Now add up the proposed openings. Bifolds measuring 3m x 2.1m use 6.3 m². A 1.2m x 1m window adds 1.2 m², and two 1m x 1m rooflights add another 2 m².
That gives 9.5 m² of proposed openings against a 10 m² allowance. The design passes this area check with 0.5 m² to spare. Without including the covered window and door, the allowance would be just 6 m² and the same design would appear to exceed it by 3.5 m².
What should you include in the measurements?
Count the extension’s windows, external doors, roof windows and rooflights. Measure the complete opening, including the frame, rather than just the visible glass. An opaque external door counts too, despite the shorthand name “glazing allowance”.
For the covered openings, count the area of all openings that are being covered by the new extension, even if the wall they were on is being knocked through.
Can you exceed the 25% glazing rule?
Yes. Exceeding the opening allowance does not mean you have to shrink the bifolds or remove a rooflight.
Saving £100 on your SAP calculations means little if the resulting specification costs you thousands more to build.
A cheap SAP calculation for your new build can be good value. But comparing the fee alone tells you very little about the work behind it. The useful question is whether your assessor has allowed enough time to understand the project, check the details and help you make sensible decisions.
Are you buying a calculation or help with the design?
SAP software produces the result. The assessor decides how to model your home, checks the information and explains what the result means.
That involves more than measuring drawings and selecting a heating system. It means investigating specifications that are unclear, finding the right product data and checking that the assumptions reflect what you intend to build.
A very low fee leaves less room for that work. If the next assessment is booked immediately afterwards, how much time is available to investigate your junction details or compare alternative specifications?
You do not necessarily need the most expensive assessor. You need to know what attention your project will receive.
The details can change the result
‘Air source heat pump’ is not a complete specification. Neither is ‘MVHR’. The selected equipment, its performance data and the way it will be installed can affect the calculation.
A thorough assessor will make reasonable efforts to identify the actual models and use the correct product data. That might mean checking the manufacturer’s information, searching the product database or asking your installer for clarification.
The Product Characteristics Database includes heat pump records with specific model and installation conditions. Those details matter when choosing the entry for an assessment.
Defaults have a place where the relevant information is unavailable. They should not become a shortcut when the product has already been selected and suitable data can be found.
The same attention belongs in the fabric assessment:
Does the insulation match the performance and thickness you plan to install?
Do the thermal bridging values apply to your junction details?
Is the airtightness target realistic for the proposed construction and ventilation?
Are the window and door figures based on the products you are buying?
A model built around vague assumptions can send the design team looking for improvements the project may not need.
A pass is not always the end of the conversation
If the design passes with a particularly expensive specification, it is worth asking whether all of it is needed to achieve the intended result.
There may be scope to change an insulation product, adjust a thickness or reconsider a system without losing compliance. Equally, a feature might be there for comfort, acoustics or another design reason. A useful assessment helps you understand the choices.
That does not mean reducing everything to the bare minimum. It means knowing where your money is making a difference.
If the SAP fails, what is the best use of your budget?
‘Add more insulation’ is not much help on its own.
Your assessor should explain which target is failing and investigate practical ways to address it. A fabric energy efficiency shortfall needs a different response from an emissions shortfall. Solar panels, for example, do not solve a failing fabric energy efficiency result.
Comparing options is where the assessment becomes useful. Better junction details, changes to the glazing, improved airtightness, heating choices or additional renewables can have very different costs and effects.
The aim should be to identify improvements that offer good value for your particular design, rather than handing over a failure report and leaving you to guess. Our guide to fabric energy efficiency failures explains that distinction in more detail.
Check the whole quote, not just the headline fee
Before comparing two SAP quotes, check:
The stage: Design SAP, As Built SAP, or both?
The documents: which reports, certificates and U-value calculations are included for your project?
Advice: will the assessor explain the result and discuss improvements?
Revisions: what changes are included, and when would another fee apply?
Completion: what will the later assessment and EPC cost?
Our SAP calculation prices show the assessment fees and the reduced As Built follow-up fee when we completed your Design SAP. Comparing the full scope is more useful than comparing two starting prices.
Good value comes from a useful assessment
At Hemcalcs, advice and reasonable revisions are included. We review the details, explain the result and help you find practical changes where they are needed.
The Home Energy Model (HEM) is being developed to replace SAP. For architects, builders and developers, the interesting part is what it will calculate differently, and whether that changes how you approach a design.
These are three differences worth understanding. Our full HEM guide covers the wider background and the transition from SAP.
1. Energy use calculated every half hour
SAP calculations use a monthly energy balance. HEM can follow a home through the year in 30 minute steps, carrying information from one step into the next. That makes it possible to model the difference in how a design uses energy throughout the day.
Why does that matter? Think about a sunny spring afternoon. The solar panels are generating electricity, but the heating is off. Later, the heating comes on when the panels are producing very little. The daily generation total does not tell you how much electricity was available when the house needed it.
SAP broadly accounts for solar generation and estimates how much is used in the home. HEM’s shorter time steps are a more detailed way to represent that overlap between supply and demand.
What to consider: look beyond annual generation figures when choosing systems. How will the proposed design use the electricity it generates, and when will it still need energy from the grid?
2. A different way to model the building’s temperature
Insulation slows heat loss. Thermal mass describes the ability of materials to absorb and release heat. Both affect how a room warms up and cools down, but they do different jobs.
SAP accounts for thermal mass, while HEM represents the temperature response of building elements in more detail. This allows the simulation to follow heat being stored and released over time.
HEM also controls heating using operative temperature, which accounts for the surrounding surfaces as well as the air. That is relevant to comfort: sitting beside a cold surface can feel different from sitting in a room with warmer surfaces, even when the air temperature is the same.
Two homes with similar U-values can respond differently to sunshine and heating schedules. The location of insulation and the materials exposed to the inside are therefore worth considering alongside the headline U-values.
What to consider: keep the actual wall, floor and roof specifications available. Discuss glazing, shading and heating controls together, particularly in rooms with large areas of glass. More thermal mass is not automatically better for every design.
3. Heating, generation and storage working together
The first change is about the time steps used. This one is about how the systems interact.
In SAP 10.2, heat pump performance is assessed using a separate simulation outside the main monthly calculation. HEM brings the heat pump into the home’s changing conditions, alongside other systems. The government’s HEM overview identifies this as a reason for changing the methodology.
That gives the model more scope to examine combinations of heating, solar panels, batteries and controls. For example, electricity could charge a battery for later use or heat a cylinder to store energy as hot water. Those choices affect where the energy goes and what is available afterwards.
A battery is not a guarantee of lower costs, and a larger solar array does not automatically make the whole design work better. Equipment, capacity and controls need to suit the building.
What to consider: agree how the systems are intended to operate before finalising their specifications. A collection of individually efficient products still needs to work together.
For now, confirm which requirements apply to your project and involve your assessor while the design can still be adjusted. Our new build SAP service includes advice and reasonable revisions as the specification develops.
If your SAP calculation fails on fabric energy efficiency, don’t start ordering thicker insulation just yet. First, find out which parts of the design are causing the result.
There are lots of areas of a building that affect its fabric energy efficiency. It’s not just the amount of insulation your home has. Things like thermal bridging, air tightness and glazing specification affect it too.
Solar panels or a heat pump will not help here. To understand what will, start with the Dwelling Fabric Energy Efficiency (DFEE) metric on your SAP calculation.
This guide covers new homes in England assessed under SAP 10.2 and the 2021 Part L standards, including the 2023 amendments.
What do DFEE and TFEE mean?
DFEE stands for Dwelling Fabric Energy Efficiency. It is the calculated heating and cooling demand per square metre of floor area, expressed in kWh/m² per year. The lower this number is, the better.
In SAP, every model of a home has a notional dwelling created alongside. The notional dwelling is the same size and shape as the designed home, but this one meets the targets set out in Part L for new dwellings exactly.
From this notional dwelling, the target metrics are set and the TFEE is the Target Fabric Energy Efficiency. Your DFEE must be no higher than this target. The TFEE varies from home to home and is affected by size, shape and orientation.
If the TFEE for your design is 40, for example, your home needs to match or be below this number.
Why can the fabric fail when the insulation meets Part L?
This is a common frustration that occurs. U-values for the walls, roofs and floors are better than the Part L standards but the DFEE is failing.
There are two sets of U-values in Part L, the limiting standards and those that the notional dwelling uses. You can build a wall at 0.25W/m2K, as it’s within the 0.26W/m2K limiting U-value, but the notional dwelling, the one you need to beat, uses 0.18W/m2K. You’d need to make up that additional heat loss elsewhere.
Even if you build to the notional dwelling U-values, it’s the same story in other areas of the build. For example:
Air tightness has a limiting cap of 8m3/hr/m2 and a notional value of 5m3/hr/m2, both at 50 Pa
Thermal bridging values differ
Windows have a maximum U-value of 1.6W/m2K but the notional dwelling will use 1.2W/m2K
Another common issue is glazing area. The notional dwelling has a maximum total opening area of 25% of the floor area, including doors. If your design has total openings at 35%, the difference is an area equal to 10% of the floor area. Where additional glazing at 1.2W/m2K replaces wall at 0.18W/m2K, there is over six times the heat loss through that area under the same conditions. That isn’t the overall DFEE effect, which also takes solar gains into account.
Start with the information in the assessment. Ask your assessor to confirm that the drawings, construction details and product specifications match what you intend to build. Then compare a short list of improvements.
Understanding your dwelling compared to the notional dwelling is key. These are the parts of the notional dwelling that relate to the DFEE metric.
Total opening area, including doors – maximum 25% of the floor area
Walls – 0.18W/m2K
Roofs – 0.11W/m2K
Floors – 0.13W/m2K
Opaque and semi-glazed doors (up to 60% glazed) – 1.0W/m2K
Windows, roof windows and doors with more than 60% glazing – 1.2W/m2K
Rooflights – 1.7W/m2K
Air tightness – 5m3/hr/m2 at 50 Pa
Most DFEE failures are likely resolved with adaptations to the above, but SAP calculations can go even deeper.
What else affects fabric energy efficiency?
Insulation and calculated U-values
Heat loss through an element depends on both its area and its U-value. A lower U-value means less heat passing through it. The effect of upgrading a large external wall can therefore be different from upgrading a small section of roof.
Compare the complete construction, not just the insulation thickness. Send your assessor the proposed layers, materials and insulation product, then ask which alternatives are worth pricing.
Windows, doors and the amount of glazing
Ask the supplier for the whole window U-value (Uw-value), including the frame, rather than the centre pane figure. The glass alone does not describe the performance of the complete window. The BFRC explanation of window performance also distinguishes heat loss, solar gain and air leakage.
The g-value tells you how much solar energy passes through the glazing. A g-value of 0.50 means 50% passes through. A higher value can reduce winter heating demand, but it can also increase cooling demand. The effect on DFEE depends on the glazing area, orientation and shading as well as its U-value.
This is where fabric efficiency and Part O’s overheating requirements can pull in different directions. Solar control glass with a lower g-value can help limit summer heat gains, but it also reduces useful winter gains and can increase heating demand. Check the proposed glass in both assessments before ordering. The aim is to meet both requirements, rather than improve one result at the expense of the other.
If the windows are already chosen, share the actual specification before assuming that the only answer is to replace them with a more expensive product.
Thermal bridging at junctions
Thermal bridging is often ignored until it’s needed. These are the additional heat losses at junctions, such as window surrounds or where a wall meets a floor. The old Accredited Construction Details (ACDs) are no longer an option in SAP 10.2. Assessors can use suitable independently assessed or specifically calculated Psi-values instead, with default allowances where these aren’t available.
The notional dwelling uses your building’s junction lengths with the reference Psi-values in Table R2 of the SAP manual. If your junction details lose less heat than these reference values, they can help bring your DFEE below the target.
For example, the notional window jamb uses 0.050 W/mK. One Recognised Construction Detail for a fully filled masonry wall achieves 0.019 W/mK, reducing heat loss at that junction by 62%. That’s the junction saving, not the overall DFEE improvement, and the construction must match the detail.
Air tightness
Uncontrolled air leakage affects the heating demand calculated in SAP. At design stage, the assessment uses an assumed air permeability. The completed assessment uses the measured result.
A tighter assumption may improve the design calculation, but the builder needs to know what is expected. Agree a realistic target while there is still time to plan the work.
If the final test is worse than assumed, send the certificate to your SAP assessor before deciding what to do next. Exceeding the design assumption does not automatically establish that the final SAP fails: the assessment must be checked with the actual result, and the applicable air permeability limit still needs to be met.
Check that any figures being compared use the same units and reference pressure. CIBSE explains how air test results are used in SAP. Hemcalcs handles the SAP assessment; an air tester carries out the site test.
Will solar panels, a heat pump or MVHR fix a DFEE failure?
No, these are not direct fixes for the DFEE metric. Solar panels and heating efficiency affect other parts of SAP. Fabric energy efficiency is calculated separately.
MVHR can recover heat from ventilation air in the home’s overall energy assessment, but the DFEE calculation uses standardised natural ventilation assumptions. It does not credit your chosen MVHR system’s heat recovery. This is set out in Section 11 of the SAP 10.2 methodology.
These systems may still be right for your home. Just establish which result needs improving before treating them as the solution.
How to choose the improvements that make sense
Confirm the starting point. Make sure everyone is working from the same drawings and specification.
Identify what you can still change. Tell your assessor what is already built, ordered or fixed by the design.
Price the options with your builder. Include any effect on detailing, space and work already completed.
Record the agreed specification. Give the builder the details behind the passing calculation and check proposed substitutions before ordering.
If construction is complete, start with the final specification and air test certificate. There is little value in discussing an upgrade that would require opening up finished work before checking the alternatives.
Need help with a failing fabric result?
Send us your drawings, specification and existing SAP report if you have one. Tell us what stage the project has reached and what you can still change. We can assess the design and help you compare practical improvements.
