How do I prove my new build is underperforming?

What this usually means

A new build is designed and certified to specific performance standards — an airtightness target, insulation U-values, ventilation rates and an energy assessment — and those design figures are the benchmark against which underperformance is judged. The problem for homeowners is that the symptoms they notice are subjective and easily deflected: a developer can attribute 'cold' to the weather or the heating, and 'damp' to lifestyle. The way to make the complaint stick is to convert the symptoms into measurements and set them against the design, so the question becomes simply whether the as-built home meets the figures it was supposed to.

Each test targets a likely failure. A blower door test measures the actual air leakage and compares it with the design air-permeability figure used in the energy assessment — new builds frequently leak more than the figure claimed, which proves the draughts and heat loss. Thermal imaging reveals whether the insulation is continuous and the thermal bridges corrected as specified, exposing missing, gapped or compressed insulation. Ventilation flow measurement checks whether the installed system delivers its design airflow, which is often where damp originates. Humidity and temperature logging documents the conditions the occupants actually experience. None of these rely on opinion.

Assembled into an independent report, these measurements form an evidence pack that compares actual performance with the design and identifies specific defects. That is what gives a homeowner leverage: a developer or warranty provider can argue with a feeling, but not with an airtightness result that fails the design figure, or a thermal image showing a roof with missing insulation. The pack supports a snagging claim during the defects period or a warranty claim after, and frames the remedy in terms of bringing the home up to the standard it was sold and certified to meet.

Common causes

Signs and symptoms

A new build's performance is the product of several measurable quantities — air permeability, fabric U-values and continuity, thermal-bridge correction, and ventilation flow rates — each of which has a design value recorded in the energy and compliance assessments. Underperformance is, precisely, the as-built values falling short of those design values, and because each can be measured directly, the gap is provable rather than a matter of opinion. The blower door test quantifies air permeability; thermal imaging under a temperature difference reveals fabric continuity and bridging; flow measurement quantifies ventilation; and logging captures the resulting internal conditions. The comparison of measured-to-design is the proof.

This matters because the symptoms a homeowner feels are downstream of these quantities. Excess air leakage and fabric shortfalls produce the cold, draughts and high bills; ventilation below design produces the damp; and all of them trace back to the home not being built as the assessment assumed. An independent investigation that runs the tests and presents the results against the design figures therefore does two things: it identifies the specific defects to be remedied, and it establishes that the home does not meet the standard it was certified and sold to. That evidence base is what converts a dismissed complaint into a defensible snagging or warranty claim with a clear, measurable remedy.

How to build the evidence that a new build underperforms

Measure the home against its design — airtightness, insulation, ventilation and conditions — and assemble an independent report comparing actual to designed performance to support a claim.

1. 01

   Obtain the design figures

   Get the certified airtightness, U-values and ventilation rates.

2. 02

   Test the airtightness

   Run a blower door test and compare it with the design figure.

3. 03

   Image the fabric

   Use thermal imaging to expose missing insulation and bridges.

4. 04

   Measure the ventilation

   Check the installed system against its design airflow.

5. 05

   Log the conditions

   Record humidity and temperature the occupants experience.

6. 06

   Compile the evidence pack

   Present actual versus designed performance with the defects identified.

How to prevent it coming back

• Commission tests within the defects/snagging period where possible.
• Keep the design and certification documents for comparison.
• Use independent measurement, not subjective complaints.
• Frame the remedy as meeting the certified standard.