Insulation, airtightness and the air test: what to check before it is covered

Insulation and airtightness decide how warm the house is and what it costs to heat. They are also the work most often hidden and most often done badly, because they happen when many trades are working in a crowded shell and everything is about to be covered. Below: what the installers do, what the test measures, and what to check before plasterboard goes on.

Insulation: what goes where

The design sets the thickness and type for each part of the building envelope, and an energy calculation shows that the whole house meets Part L. Typical locations are:

• Walls: in the cavity, between studs, as external board, or as part of ICF blocks.
• Roof: between and below the rafters, or above them in a warm roof, or on the attic floor.
• Floor: under or over the slab. See substructure and floor slab.
• Around openings: reveals and cills, where heat is easily lost.

Check that the product, thickness and fixing method match the specification. Insulation that is squashed, wet, gapped or left out around pipes performs far worse than the label. In a timber frame, insulation must fill the cavities with no voids. In a cavity wall, partial-fill boards must be fitted tight and held by retaining clips, and the cavity kept clear.

Thermal bridges

Most of the heat-loss calculation for a modern house is about junctions rather than flat walls, because the flat parts are already well insulated. That is why a good drawing and a photograph of the junction before it is covered matter more than the headline thickness of insulation.

A thermal bridge is a route where heat bypasses the insulation: a concrete lintel, a steel beam, a wall-floor junction, a poorly insulated window reveal. They cause cold spots, condensation and mould as well as heat loss. The detailing drawings should show how each junction is insulated. Ask your designer to point out the three most important junctions on the drawings, then check those on site.

Airtightness: what it is and where it is made

Airtightness is a continuous barrier that stops air moving in and out through the structure. In a masonry house it is often the internal plaster (wet plaster) or a membrane, while in a timber frame it is usually a membrane taped at every joint. It is only as good as its weakest gap.

Typical leakage paths are:

• Around windows and doors, where frame meets wall.
• Where pipes and cables pass through the membrane.
• At the junction of walls and floors, and of walls and ceilings.
• Around the roof hatch, rooflight and chimney.
• At sockets and switch boxes in external walls.
• Recessed lights in ceilings below an attic.

Ask your builder to use airtight grommets and tapes for penetrations and to seal back boxes. Decide this before first fix. See first fix.

The air test

A blower-door test fits a calibrated fan in an external door opening. The house is depressurised and the fan measures how much air flows through the leaks. A result is given as air permeability (cubic metres per hour per square metre of envelope at 50 pascals). A lower number means a tighter house.

• Target. The target comes from your energy calculation and the Part L requirements. Ask your designer what number the calculation assumes and what happens if the test fails it.
• Who tests. A competent tester, independent of the builder, following a recognised method. Ask for their qualifications and for the test to follow the standard your designer names.
• When. Many designers recommend a first test when the airtightness layer is finished but still accessible. At that point leaks can be found with smoke or a hand and fixed cheaply. A final test at completion gives the value your BER assessor uses.
• Result. Keep the report. It becomes part of the BER evidence and may be requested by the certifier or a future buyer.

If the result misses the target, the designer should recommend where to look and a retest. The cost of leak-hunting, repair and retesting depends on your contract, so agree who pays before the test.

Ventilation: tight house, planned air

A leaky house breathes by accident. A tight house must breathe by design. Part F of the Building Regulations requires adequate ventilation, and the design should say whether you use background vents with extract fans, demand-controlled systems, or a whole-house heat recovery ventilation system. Check:

• The unit is installed where the design shows it, with ducts of the specified size and with minimum bends.
• Ducts that run through cold areas are insulated.
• Supply and extract points are where the design needs them.
• The system is commissioned, with flows measured and a record left with you.
• Filters can be reached for changing.

See building regulations for self-builds and nZEB and Part L.

What to check and photograph before plasterboard

• Insulation in each wall, roof and floor area, showing thickness and no gaps. Take close-ups at corners and around openings.
• The airtightness membrane with taped joints and sealed edges.
• Window and door seals to the wall.
• Penetrations through the airtight layer, with grommets or tape.
• Ventilation ducts and their insulation.
• Fire stopping and cavity barriers, which also affect airtightness.

Do this before the plasterboard and before the next trade covers the layer. Ask the certifier to inspect the same elements and note the date.

Who is responsible

Put the test in the contract. Say who arranges it, who pays for the first test and for retests, what number counts as a pass and what the builder must do if the house misses it. Without this the discussion starts after the failure, when everyone has already moved on.

The designer specifies the target and details. The builder or the specialist installs the insulation and membranes, and is responsible for workmanship. The tester measures the result. The BER assessor uses the test result in the final rating. The certifier checks the stage inspection and the completion paperwork. In a self-managed build, you are the one who coordinates the trades. See assigned certifier explained.

What goes wrong, and what it costs

• Rushed insulation. Gaps, missed corners and compressed boards in the busiest week of the build.
• Membrane punctured by later trades. Electricians and plumbers cut through it for cables and pipes, then do not seal the hole.
• Failing the test. Retesting and repairs cost time and money, and a failed final test can affect the BER.
• Insulation left wet. Timber frame and insulation exposed to rain before the roof is on.
• No ventilation plan. A tight house with condensation and mould.
• Changing windows late. The new frames do not match the airtightness details.

Payments, site risks and cover

This stage is rarely a separate lender payment, but your builder may link a payment to it. The pre-plaster test is a useful condition: make it a hold point in your contract. See choosing a builder and contracts.

Insulation and membranes are flammable or easily damaged, and fire during the build is among the most expensive losses. Check smoking rules, hot-work procedures and temporary heating on site. Make sure cover includes fire damage to the works and materials, and the cover checker at /cover/ shows the types to ask about.

Before plastering

• Pre-plaster airtightness test passed or leaks fixed and retested.
• Insulation and membranes photographed and inspected.
• Ventilation ducts installed and labelled.
• The next stage: plastering, screed and drying out.