Air tightness targets claimed at planning stage

An air permeability figure written into an energy statement is a promise that someone will have to test on site, and the test happens long after the planning decision. That gap is where most air tightness planning claim problems start. A design value of 3.0 m3/(h.m2) at 50 Pa is easy to type and hard to build, and councils increasingly condition evidence that the built house matched the number the consent was granted on.

What the figure actually measures

Air permeability in England is expressed as cubic metres of air per hour per square metre of envelope area, at a pressure difference of 50 pascals, written as m3/(h.m2) at 50 Pa. It is a leakage rate normalised by the surface area of the thing being tested.

Approved Document L sets a limiting value that no new dwelling may exceed, and the calculation methodology then rewards anything better. Designs that simply sit at the backstop rarely satisfy the target emission rate, so real submissions carry a tighter figure, and that tighter figure is the commitment.

Passivhaus uses a different metric, air changes per hour at 50 Pa, and its 0.6 limit is not comparable with an m3/(h.m2) figure without knowing the envelope area and volume of the specific building. Mixing the two units in one document is a reliable way to lose an officer's confidence. Pick one, state it, and say which standard it belongs to.

Design claim against tested result

The claim in the Energy Statement is a modelling input. The tested result is a measurement of what was built. Three things drive them apart.

Detailing that was never drawn. Air tightness lives at junctions: wall to floor, wall to roof, around joists built into masonry, at service penetrations, at the loft hatch. If the drawings do not show a continuous air barrier through those junctions, the number is aspirational.

Sequencing on site. The air barrier gets breached by trades who arrive after it is finished. Electricians, plumbers and satellite installers all cut through it, and nobody owns the repair unless someone is named.

Substitution. A wet plaster finish behaves differently from dry lining on dabs, and swapping one for the other during value engineering moves the result by more than most people expect.

Detailing that decides the outcome

The measures that matter are unglamorous and inexpensive relative to the cost of a failed test:

• A continuous, drawn air barrier line, marked on every section, with the material named at each junction.
• Taped and sealed membrane laps, with the tape specified rather than left to site choice.
• Service penetrations through grommets or collars, not expanding foam.
• An insulated, gasketed loft hatch, or no hatch in the heated envelope at all.
• Sealed and gasketed downlighters, or surface mounted fittings instead.
• Parge coat or wet plaster on the inner leaf of masonry walls, carried behind dry lining where dabs are used.
• Attention to the sole plate and the wall to floor junction in timber frame, which is the most common single source of leakage.

One person with authority for the air barrier, named in the build programme, is worth more than any of the above on its own.

The performance gap and why councils care

The gap between predicted and measured performance is the reason authorities have stopped accepting design stage modelling as the end of the matter. A scheme approved on the basis of a fabric-first strategy delivers the carbon saving it was granted permission for only if the fabric performs.

Air tightness also interacts with two other planning issues. Tighter fabric shifts the ventilation strategy: a dwelling at a low air permeability needs designed mechanical ventilation, and the Overheating Risk Assessment assumptions about window opening and purge ventilation have to be consistent with it. And a tight, well insulated dwelling with large south facing glazing is a candidate for summer overheating even when its winter performance is excellent.

Where a scheme is phased, the tested results from phase one become the evidence base for phase two, which is a good reason to programme testing early rather than at practical completion. The wider sequencing question is covered in our note on energy masterplanning across a phased site.

Testing and who does it

Pressure testing follows the industry measurement standards and is carried out by testers registered with a recognised competent persons scheme. Approved Document L moved away from the older sampling approach towards testing of new dwellings, with a penalty value applied to any dwelling that is not tested, so check the current edition of the Approved Document for the regime that applies to your build.

Practical points that decide whether a test passes:

1. Test when the envelope is complete and before decoration covers the evidence, so failures can be found and fixed.
2. Carry out a preliminary test on the first plot of a repeated house type. Finding a systematic detailing fault on plot 1 costs little. Finding it on plot 40 is not.
3. Use smoke or thermal imaging during the depressurisation to locate leaks rather than only recording the result.
4. Keep the tester's certificate with the envelope area calculation attached, because the area assumption is where disputes start.

Conditions requiring test evidence

Authorities word these in several ways. The ones that work name the document and the trigger:

• Submission of as-built calculation outputs and air pressure test certificates before first occupation of each dwelling or phase.
• A requirement that the tested figure is no worse than the design figure relied on in the energy statement, with a stated tolerance.
• A remedial mechanism where the figure is not met, usually additional measures agreed in writing rather than an open obligation.

Read the tolerance carefully. A condition requiring the design value to be achieved exactly, with no allowance, is a condition that will be breached on a proportion of plots, and breach of condition is not something you want sitting on a completed development. If the wording is absolute, ask for a tolerance at the drafting stage rather than applying to vary the condition afterwards.

Where modern methods of construction are used, factory tolerances usually make the target easier to hit and the evidence easier to produce, which is one of the arguments set out in our note on modern methods of construction and the carbon case.

Recording the claim so it can be met

Write the figure once, in one place, and reference it everywhere else. An energy statement that says 3.0 in the summary, 3.5 in the appendix table and "better than Building Regulations" in the conclusions gives an officer three numbers to choose between and gives your contractor an excuse.

Then carry it forward:

State the figure and the metric in the energy statement, with the envelope area assumption. Repeat it in the specification. Name it in the preliminaries as a contractual requirement with a testing obligation. And put it on the drawings, as a note against the air barrier line.

An honest caveat: a design value chosen to make the carbon calculation work, rather than chosen because the detailing supports it, is the single most common cause of a failed test. If nobody on the team can say which junction detail delivers the number, the number is too low.

If you are preparing the supporting documents for a self-build or small site, PlanScope can draft the energy and sustainability statements, and you can enter your postcode to see which reports the site is likely to need.

This is general guidance, not legal advice, and the relevant Approved Documents and calculation methodologies change, so check the current versions before relying on any figure. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.