Airtight grommets in new build: getting penetrations right for Part L and the Future Homes Standard

On a new build, every penetration through the airtight layer is a decision someone made. Sometimes it was made on a drawing. More often it was made by whichever trade had a drill in their hand that morning. The difference between a home that passes its air test first time and one that needs a second test usually comes down to who made those decisions, and when.

What the regulations now expect

• Every home is tested. Since Part L 2021 came in, each new dwelling in England needs its own air test. The old practice of testing a sample of plots is gone, so you cannot rely on one good plot to carry a leaky one.
• The details are photographed. The as-built BREL report has to be backed by photographs taken during construction, airtightness details included. A penetration buried behind plasterboard with no photo is a compliance problem even if it happens to be sealed.
• Targets are tightening. The Future Homes Standard was published on 24 March 2026 and comes into force on 24 March 2027, with a transition period ending in March 2028. The notional dwelling's air permeability falls from 5 to 4 m³/(h·m²). Heat pumps and on-site solar become standard, which means more pipes and cables through the envelope, not fewer.
• Airtightness now drives the ventilation design. Under Approved Document F 2026, a home with a design air permeability below 5, or an as-built result below 3, counts as highly airtight, and the ventilation strategy has to match. Miss your design figure because of leaky penetrations and the ventilation design may no longer fit the house.

Step 1: design the penetrations out

The cheapest penetration is the one that does not exist. On the drawings, mark the airtight line and count every service that has to cross it. Then try to reduce the count.

• Use a service void. Battening the plasterboard off the airtight layer gives you a zone where electrics and plumbing can run freely without puncturing it. Sockets, switches and downlights in that zone are not penetrations at all.
• Group the incoming services. Bring water, power and data in at one plant-room location, ideally through ducts below the slab, rather than scattering them across the envelope.
• Allow spare ducts. A sealed spare duct is far easier to use later than a fresh hole drilled after handover.

Step 2: schedule the ones you cannot avoid

List each remaining penetration with its diameter and location. A typical house might have:

• soil and vent pipe through the ground floor and the top-floor ceiling
• MVHR intake and exhaust ducts
• heat pump flow and return, plus control cable
• incoming water main and electricity supply
• kitchen extract, tumble dryer vent and condensate
• PV cable from the roof

Give each one a named seal, whether a sized grommet, a liquid grommet or a top hat, and a named trade responsible for fitting it. That schedule becomes your photo checklist for the BREL report.

Step 3: fit the seal before the pipe is finished

A pre-formed grommet slides over the end of a pipe. Once both ends are connected, that is no longer possible. Sequencing is everything:

1. Form or coat the airtight layer around the opening.
2. Feed the pipe through and fit the grommet before the pipe is connected at both ends.
3. Bond the flange to the airtight layer: bedded into a liquid membrane, or taped to a board or sheet membrane.
4. Photograph it, with the plot and location visible.
5. Only then let the next trade close it in.

Construction types

Timber frame. The airtight layer is usually the inner sheathing or a membrane on the warm side. Grommets are bonded to the sheathing face, so keep fixings and battens clear of the flange. See our timber frame page.

Masonry. Bare blockwork is not airtight, so the airtight layer is a coating such as Passive Purple or a parge coat. The grommet has to bond into that layer, not to raw block. See the masonry revolution and our guide to fitting grommets into a liquid membrane.

Light gauge steel. Services often pass through sheathing boards at frame level, so the same rules apply as timber frame. See light gauge steel.

Do a mid-build check

The best moment to find a leaking penetration is while you can still reach it. A pre-test once the airtight layer and first fix are complete, with a smoke stick run round every penetration, costs far less than a failed final test with the plasterboard up. Our airtightness testing guide explains the process, and the pre-test checklist covers the penetrations themselves.

Products for new build penetrations

• Single round pipes and ducts: a sized airtight grommet. Our own, TED (Tight Entry Detail), arrives in December 2026. Ask us to include it in your specification.
• Cable bundles, awkward angles and penetrations near corners: The GOAT liquid grommet.
• The airtight layer itself: Passive Purple, with IM Fleece Tape on board joints and junctions.

Specifying a development? Send us your drawings and we will mark up the penetrations for you. New to grommets? Start with airtight pipe grommets explained.