Why SAP Calculations Fail: The Most Common Issues and How to Fix Them

SAP calculations rarely fail because of one dramatic mistake. More often, it’s a mix of optimistic assumptions, specification drift and design decisions that don’t quite add up under SAP 10’s stricter rules.

Builders, architects and homeowners are often surprised when a SAP report comes back as a fail, especially if the design feels straightforward. SAP 10 is far less forgiving than SAP 2012, and small details now carry more weight.

The sections below cover the issues I see most frequently as a SAP Assessor and how they can be resolved without redesigning the entire project.

If you’re new to the process, here’s a clearer breakdown of what a SAP calculation involves and how compliance is measured.

Fabric Insulation and Thermal Performance

A large share of SAP failures come down to the basics. If the thermal performance of the walls, roof or floor lands slightly worse than expected, the DFEE target can be exceeded surprisingly quickly. For example, walls coming in at 0.20 instead of the assumed 0.18, or glazing at 1.4 instead of 1.2, can be enough to tip the model into failure. SAP 10 is sensitive to small weaknesses because the notional building performs extremely well.

Thankfully, fabric issues are usually the easiest to correct. Improving one weaker element, upgrading glazing or increasing insulation depth is often enough to bring the DFEE back into line without major design changes.

Setting Realistic Airtightness Targets

Airtightness assumptions are another major cause of SAP issues, especially at as-built stage. Design SAP often assumes an air permeability of 5, but many builds test at 7 or 8 unless airtightness is planned and checked throughout the project. A higher result increases heat loss, reduces ventilation efficiency and raises DER, tightening the compliance margin.

A simple way to avoid this is to model a slightly more conservative target such as 6. This builds in a buffer for real-world variation. A pre-test blower door check during second fix also helps identify leakage points before the final test.

Thermal Bridging and Junction Details

Thermal bridging is one of the most commonly overlooked areas of SAP and one of the most damaging when handled incorrectly. If accredited details or psi-values aren’t provided, SAP applies a default Y-value that is intentionally harsh. This alone can make compliance impossible even when the insulation and airtightness are solid.

Providing accredited details (LABC, RCD, manufacturer sets) almost always improves the Y-value enough to restore compliance. For many projects, addressing thermal bridging has more impact than improving fabric U-values.

Heating and Hot Water Specifications

Heating systems don’t need to be complex, but they do need to be specified accurately. SAP isn’t concerned with whether a system is “good”, only with how it performs relative to the notional building. Problems usually arise when the Design SAP assumes one system and the build installs another. An architect might assume a heat pump and MVHR, but the homeowner later opts for a boiler and trickle vents. Or a boiler may be fitted without the controls required to meet notional performance. Any of these can push DER up enough to fail.

Specifying the actual system earlier makes the design far more predictable. If changes happen mid-project, updating SAP at that point avoids a last-minute as-built failure. Most heating changes are quick for an assessor to update when they are flagged early.

Glazing Design and Solar Gain Balance

Glazing isn’t inherently a problem in SAP, but the balance matters. Windows lose more heat than walls, and if a build has large openings on cooler elevations, the SAP model may end up carrying more heat loss than it can offset. Orientation plays a part too. North and east facing glazing offer little useful solar gain under SAP 10, so their heat loss tends to dominate unless other elements compensate.

Rooflights introduce another layer. Their performance and solar gain characteristics differ from vertical glazing, and because they sit at an angle, they can influence SAP more strongly than expected.

Most glazing issues can be resolved without changing the architecture. Reducing the size or repositioning colder elevation glazing, improving the worst performing units or adjusting the ratio of rooflights to windows usually brings the model back into balance.

Ventilation Strategy and Airtightness Working Together

Ventilation rarely causes a SAP fail on its own, but it can tighten margins when it doesn’t align with the airtightness level. If the design assumes a certain level of airtightness and the build ends up more or less airtight than planned, the ventilation efficiency in the model can shift. MVHR tends to work best in tighter homes, while leakier envelopes rely more on background infiltration and mechanical extract. Using default fan values or missing performance data can raise heat loss slightly.

The simplest approach is to choose a ventilation system that suits the likely airtightness of the build and keep the SAP model updated if those assumptions change.

When Design SAP Doesn’t Match the As-Built Build

One of the most frustrating situations is when a design passes comfortably but the as-built version fails. This doesn’t usually happen because of one big change, but because several small ones accumulate during construction. Glazing might come from a different supplier, insulation depth might be reduced for practicality, a different heating system might be chosen or the air test might fall short of the target. PV is sometimes removed for budget reasons.

Keeping a simple record of changes and sending datasheets to your assessor as the build progresses makes a huge difference. Updating SAP during the project avoids expensive fixes at the end and makes the final BREL far more predictable.

For most projects, understanding your SAP calculation requirements early makes issues like glazing balance or U-value shortfall much easier to prevent.

Conclusion

Most SAP failures aren’t disasters. They’re usually the result of assumptions, communication gaps or specification drift that build up over time. Identifying the issue early and making one or two targeted improvements is almost always enough to bring a design back into the Part L requirements.

Fixing SAP at design stage is far easier than trying to correct it at the end of the build. If your project is struggling to pass, or you want to avoid these problems altogether, I can review your plans and identify exactly where the shortfall is coming from and how to resolve it in the most cost-effective way.