Approved Document L1A, BREL / SAP as-built · 23 checks · proplot.co.uk
Part L compliance on site — insulation continuity, thermal bridging details, airtightness sealing, service penetrations and the BREL photographic evidence set.
23 checks · Reviewed by the ProPlot site team · How we write these guides
In short
The insulation and airtightness inspection checks that the build matches the SAP specification before anything is covered. The inspector looks for continuity of insulation at every junction, correct product and thickness, sealed membranes and service penetrations, and the BREL photographic evidence required for the Part L as-built submission.
Since the 2021 changes to Part L, thermal performance is not just designed, it is evidenced. The as-built SAP calculation has to reflect what was actually installed, and the Building Regulations England Part L (BREL) report requires geo-located photographs of specific elements at the point they can still be seen. That has turned insulation and airtightness into a documented inspection stage in its own right, running alongside superstructure, roof and pre-plaster rather than sitting inside them.
The physical checks are about continuity. Insulation is only as good as its worst junction, so the inspector looks at reveals, eaves, wall-to-floor and wall-to-roof junctions, service penetrations and the corners where boards get cut in a hurry. Airtightness is the same idea in a different layer: membranes and tapes lapped and continuous, sealing at floor perimeters, around windows and doors, at loft hatches and at every cable, pipe and duct that passes through the envelope.
The paperwork side is where most sites lose time. Each BREL photograph needs to identify the plot and the location, be taken before the element is covered, and correspond to the items the assessor lists. Build the photo set as you go, plot by plot, and the as-built SAP and EPC follow without drama.
When it happens
Continuously through the fabric stages, with a formal check before insulation is covered — behind the outer leaf, in the roof, and at pre-plaster. The airtightness test itself is carried out once the envelope is complete.
Who inspects
Building control and the warranty inspector for compliance, the SAP assessor for the BREL evidence set, and an accredited tester for the air permeability test.
If it fails
Missing evidence means the as-built SAP cannot be signed off, which blocks the EPC and therefore the sale. A failed air test means finding and sealing leaks in a finished house, then re-testing.
Concealed work cannot be re-photographed later. Take these before the element is covered and file them against the plot.
A Reportable Item (RI) is a defect the inspector records against the plot. NHBC weigh each one by its Damage Potential, and the score feeds your site's Construction Quality Review. These are the ones raised most often here:
The measurable values most often argued about at the insulation & airtightness visit, each traced to an openly published source. Take a tape to the ones that apply before you book.
| What | Value | Type |
|---|---|---|
| AVCL jointing requirementsJoints in air and vapour control membranes must be adequately lapped, typically with a minimum overlap of 100mm, to ensure continuity of the membraneCheck that all AVCL joints have the specified minimum lap and are properly supported where possible. | 100 mm | Minimum |
| Clearance for walkways above insulationBoarded walkways and working platforms within roof spaces must be securely fixed and maintain a minimum 50mm clearance above the insulationCheck that all walkways in the roof space are securely fixed and maintain the specified clearance above the insulation. | 50 mm | Minimum |
| Limiting air permeability for a new dwellingNew dwelling air pressure test at completionWalk the airtightness line before plaster: service penetrations, loft hatch, and the joist ends at separating walls are where the result is won or lost. | 8 m³/(h·m²) at 50 Pa | Maximum |
| Limiting fabric U-values for a new dwellingNew dwelling, individual element limiting standardCheck the constructions actually built against the compliance report element by element — a substituted block or a thinner board changes the figure. | Roof 0.16; wall 0.26; floor 0.18; party wall 0.20; window, roof window and door 1.60 W/(m²·K) | Maximum |
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Confirm insulation type, thickness and manufacturer match the SAP specification (AD L1A)
AD L1ACheck boards are tight-butted with no gaps, and joints staggered where specified (AD L1A)
AD L1AVerify full fill or partial fill cavity insulation is retained hard against the inner leaf (NHBC 6.1)
NHBC 6.1Check loft insulation depth is achieved and maintained across the whole ceiling area (AD L1A)
AD L1AConfirm insulation is not compressed by services, ducting or storage boarding (AD L1A)
AD L1AVerify window and door reveals have insulated returns and correct cavity closers (AD L1A)
AD L1ACheck the wall-to-floor junction detail matches the approved construction detail (AD L1A)
AD L1ACheck the wall-to-roof junction and eaves detail maintain insulation continuity (AD L1A)
AD L1AConfirm party wall cavity is fully filled and sealed where the SAP assumes zero bypass (AD L1A)
AD L1AVerify lintels, cills and jambs use the insulated products assumed in the psi values (AD L1A)
AD L1ACheck the airtightness layer is continuous and identified on the drawings before work starts (AD L1A)
AD L1AConfirm membrane laps and tapes are complete, adhered and undamaged (BS 5250)
BS 5250Verify sealing at floor perimeters, skirting lines and around window and door frames (AD L1A)
AD L1ACheck loft hatches are insulated, sealed and draught-stripped (AD L1A)
AD L1AConfirm the air permeability test is booked and the result meets the SAP design figure (AD L1A)
AD L1ACheck every pipe, cable and duct through the envelope is sealed with the specified product (AD L1A)
AD L1AVerify soil vent pipes, extract terminals and flues are sealed at the wall or ceiling line (AD F)
AD FConfirm downlighters and recessed fittings do not breach the ceiling airtightness layer (AD L1A)
AD L1ACheck ventilation ductwork in unheated spaces is insulated to prevent condensation (AD F)
AD FCapture geo-located photographs of each required element before it is covered (BREL)
BRELEnsure every photo identifies the plot and the location within the dwelling (BREL)
BRELConfirm the photo set matches the element list the SAP assessor requires (BREL)
BRELFile the as-built SAP, BREL report and air test certificate against the plot (AD L1A)
AD L1AThe difference between a passing test and a comfortable, low-bill house is entirely in the care taken here. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.
Protect insulation from rain and mechanical damage on site
Wet mineral wool and compressed boards do not deliver the design U-value and the SAP calculation no longer describes the house.
When: From delivery to installation.
Tape and seal service penetrations as they are made, not afterwards
Going back to find every penetration once the room is full of trades is how leakage paths get missed.
When: Immediately as each penetration is formed.
Do a smoke or pre-test walkround before the formal air test
A failed test costs a re-test fee, a delay and often plasterboard being opened. An informal check finds the obvious leaks first.
When: A few days before the booked test.
Photograph every junction detail with the plot and room identifiable
This is your BREL evidence and it cannot be recreated after boarding.
When: Room by room as you complete.
Brief the trades that follow on what not to cut through
Most airtightness damage is done after the airtightness work is signed off, by someone who did not know it was there.
When: At the point handover between trades.
Keep offcuts and packaging out of the cavity and the service voids
Debris in a service void is a fire and vermin issue and it disrupts insulation continuity.
When: Daily clean as you go.
9 inspection points that apply at the insulation & airtightness stage, each written in our own words from openly published guidance, with how to check it on site and what counts as acceptable. Use the checklist above on the day and this register when you need to settle a detail.
Photographs taken too late
The element is already boarded, so the evidence set has a hole in it. Capture at the moment of installation, not at the end of the week.
Medium — but it directly moves the as-built SAP and the EPC band.
Insulation compressed by cables
A loft run of cable pressed through the quilt destroys the assumed U-value locally. Clip services above the insulation line.
Medium to high — a failed air test holds up completion.
Unsealed service penetrations
Air leakage concentrates at a handful of holes. Seal each one as it is formed rather than hunting them after a failed test.
High administratively — missing BREL photos cannot be recreated.
As-built spec differs from SAP
A substituted board or a different lintel changes the calculation. Report any substitution to the assessor before it is installed.
Medium — usually fixable before boarding if caught.
ProPlot's audit tool includes these insulation & airtightness checks as interactive helpers — tick items off, attach GPS-stamped photos, and share the report with your inspector in one tap. Free while in beta.
The key stage inspection sequence on a typical UK housing plot.
Excavation & foundations
21 checks →
Drainage
20 checks →
Basement / lower ground
28 checks →
Ground Floor & Slab
19 checks →
Superstructure
22 checks →
Timber Frame Erection
21 checks →
Roof
22 checks →
Windows and External Doors
24 checks →
First Fix
28 checks →
Fire Stopping
22 checks →
Pre-Plaster
21 checks →
Second Fix
31 checks →
External Works & Pre-Handover
23 checks →
Pre-handover
22 checks →
General Quality
20 checks →
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