Common reasons a SAP assessment can fail

Poor fabric performance

Walls, floors or roofs with modest insulation levels raise heat loss across the whole dwelling. This is common where a specification has been carried over from an older project or where constructions are not fully defined on the drawings. Checking the build-ups and their U-values is usually the first step.

High heat loss through glazing

Large areas of glazing, or windows and doors with higher U-values than assumed, can have a significant effect. Rooflights and glazed doors are easy to overlook. Accurate sizes and performance figures from the window supplier help the calculation reflect the real design.

Airtightness assumptions

An ambitious design air permeability may help the calculation on paper but becomes a risk if the completed building does not achieve it on test. Equally, a cautious figure can make compliance harder than it needs to be. The target should be realistic for the construction method and the builder's experience.

Thermal bridging

Where junction details are not provided, default values are applied and these are deliberately conservative. Specifying recognised details for lintels, sills, eaves and floor junctions, or providing calculated values, can noticeably improve the outcome.

Heating system performance

The choice of heating system has a large effect on primary energy and carbon results. Designs relying on direct electric heating or less efficient systems often struggle without strong fabric or renewables. Controls and emitter types also matter, so full product details are helpful.

Hot water efficiency

Hot water can be a large share of a modern home's energy use. Cylinder size and heat loss, primary pipework insulation and the way water is heated all influence the result. Missing cylinder details often mean less favourable defaults are used.

Ventilation choices

The ventilation system affects both heat loss and fan energy. A system that does not suit the dwelling's airtightness, or one specified without product data, can reduce the result. Choosing the strategy alongside the airtightness target usually works best.

Missing or inconsistent project information

Drawings that disagree with each other, missing dimensions or unclear specifications force the assessor to make assumptions. Those assumptions are normally cautious, which can push a borderline design into non-compliance. Clear, consistent drawings and a complete specification make a real difference.

Changes made during construction

Substituting a different insulation product, window, boiler or heat pump during the build can change the as-built result. Changes are not necessarily a problem, but they should be checked against the SAP model before they are made rather than discovered at completion.

What to do if a design is not compliant

Start by identifying which target is not being met and by how much. An assessor can then test options, such as improving the fabric, revising glazing, upgrading the heating or adding solar PV, and show which combination closes the gap most efficiently. Making these changes at design stage is almost always simpler than making them on site.

If you have a question about a specific project, you can also contact us.