Sealing service penetrations in retrofit: what to do when the pipes are already in

New build gets to plan its penetrations. Retrofit inherits them: decades of plumbers, electricians, satellite installers and kitchen fitters, each drilling a hole and moving on. Many of those holes are hidden behind kitchen units, under baths, in the floor void or in the loft. All of them leak.

The good news is that sealing them is one of the most cost-effective airtightness measures in an existing home. The catch is that the pipes are already connected, which changes how you can seal them.

What PAS 2035 asks for

PAS 2035 is the standard for publicly funded domestic retrofit, including ECO work. Under the 2023 revision, which took effect on 30 March 2025:

• Wherever energy efficiency measures improve the building fabric, the project needs an airtightness or leakage strategy.
• If that strategy sets an airtightness target, compliance has to be proved by a test.
• Ventilation has to keep pace. If the home ends up tighter than 5 m³/(h·m²), continuous ventilation such as MVHR or MEV is recommended.

Sealing holes around service pipes through external walls and suspended timber floors is named in the guidance as a typical airtightness measure. In practice, the penetrations are often where most of the cheap gains are.

Where old houses leak around services

• Under the kitchen sink and behind units: waste pipes and water supplies through the external wall or floor.
• Behind and under baths and showers: waste pipes into a suspended floor void that is open to outside air.
• Boiler and heat pump pipework: flow, return and condensate through the wall.
• Soil stacks through the ceiling into the loft, and through the ground floor.
• Incoming services: gas, water and electricity meters, and the data cable.
• Redundant holes left by old flues, removed vents and abandoned cables.

A smoke test or a depressurised walk-round with a smoke stick will find them quickly. Do it before work starts and record each one.

Why a standard grommet will not go on

A pre-formed grommet is slid over the free end of a pipe. Once the pipe is connected at both ends, there is no free end. Cutting a standard grommet open to wrap it round the pipe defeats its purpose: the cut becomes the leak. Manufacturers of standard grommets say not to do it.

So retrofit gives you three choices.

1. Use the plumbing works

If the pipe is being disconnected anyway, for a new boiler, a heat pump or a kitchen refit, fit a proper grommet while the end is free. Coordinate with the installer so it goes on before the joint is remade. This gives the most robust result and costs almost nothing extra.

2. A split retrofit grommet

Purpose-made split grommets are designed to wrap round an existing pipe and seal at the split. They suit small pipes and cables where the substrate around them is flat and sound.

3. A liquid grommet

Where the pipe is at an angle, tight to a corner, surrounded by crumbling plaster, or one of a cluster, a liquid grommet formed on site is usually the best answer. The GOAT is gunned round the pipe and tooled onto the surrounding surface. It cures flexible, so it moves with the pipe, and it bonds to timber, brick and concrete. Over-coating with Passive Purple carries the film out onto the wall.

Prepare the substrate first

Old walls are dusty, friable and often damp. No seal bonds to dust. Before any seal goes on:

• Cut back loose plaster and mortar around the pipe to sound material.
• Fill large voids with a suitable backing material first. A grommet or sealant bridges a gap; it does not fill a cavity.
• Brush down and prime where required. On porous masonry, a primer stops the substrate drinking the seal.
• Sort out the cause of any damp before sealing over it.

Do not forget the inside of the pipe

Air does not only leak round a pipe. It can travel through ducts and conduits, and between pipe insulation and the pipe, then out wherever the other end finishes. Seal the space inside cable ducts at both ends, and seal the end of pipe insulation where it passes through the airtight layer.

Fire and ventilation

An airtight seal is not a fire stop. Where services pass through party walls or compartment floors, a tested fire-stopping system is needed as well. And every airtightness gain in a retrofit has to be matched by adequate ventilation. Agree the strategy with the retrofit coordinator before you seal.

Going further

For whole-house retrofit airtightness, including liquid membranes over existing masonry, see our retrofit and social housing page. For the principles behind grommets, read airtight pipe grommets explained.

Our own airtight pipe grommet, TED (Tight Entry Detail), arrives in December 2026 for new pipework and replacements. For existing pipework today, send us a photo and we will tell you how to seal it.