Airtight pipe grommets explained: how to seal service penetrations properly

Every building has an airtight layer, and every building has pipes going through it: soil stacks, MVHR ducts, water and heating pipes, cable ducts, condensate lines, heat pump flow and return. Each one is a hole in the layer you are trying to keep continuous. When an air test comes back worse than expected, those holes are near the top of the list.

An airtight pipe grommet is the simplest reliable fix. This guide explains what one is, why it works, and where it sits alongside tape, sealant and liquid membranes. It is the first in a series. The rest cover new build, retrofit, MVHR and heat pumps, fitting grommets into a liquid membrane, ground floors and radon, and a pre-test checklist.

Why pipe penetrations leak

Air testers report service penetrations as one of the most common leakage paths. A Leeds Beckett University study of new-build homes named them first among the leakage paths it found. Testing firms routinely flag pipework, waste pipes and cable trays through walls and ceilings as repeat offenders.

The problem is geometry. The airtight layer is flat. A pipe is round. Joining the two needs a seal that wraps 360° around the pipe and lies flat on the layer, and most site methods only manage one of those:

  • Tape is flat. Wrapped around a pipe it creases, and every crease is a channel for air.
  • Expanding foam fills the void but is not an air seal. It shrinks, cracks when the pipe moves, and often never touches the airtight layer at all.
  • Rigid fillers and mortar crack the first time the pipe expands, gets pulled, or is clipped back.

Then there is movement. Heating pipes expand and contract every day. Soil stacks get knocked during second fix. Ducts get adjusted when the ceiling goes up. A seal that is perfect on day one can be split before the test.

What an airtight grommet actually is

An airtight grommet is a pre-formed, flexible collar with a flat flange. The collar grips the pipe; the flange bonds to the airtight layer around it. Because the collar is elastic rather than glued to the pipe, the pipe can move, expand and even slide through without breaking the seal.

That gives you three things site-formed seals struggle to deliver:

  1. Repeatability. The same detail every time, whoever fits it.
  2. Movement tolerance. The seal flexes with the pipe instead of fighting it.
  3. Inspectability. A fitted grommet is easy to check by eye and to photograph for building control evidence before it is covered up.

How a grommet joins the airtight layer

A grommet is only as good as its bond to the layer around it.

When a grommet is not the right answer

A pre-formed grommet suits a single round pipe or duct where you can reach one end to slide it on. It is not the best tool for:

  • Bundles of cables or several services through one hole.
  • Pipes at a steep angle or tight to a corner, where the flange cannot lie flat.
  • Existing pipework with no free end, which is common in retrofit. A standard grommet should not be cut to fit round finished pipework.
  • Openings formed before the service exists.

For those, a liquid grommet formed on site, The GOAT, is usually the better detail. The key-shaped grommet shows a real one in 360°.

Two safety points. An airtight grommet is not a fire stop. Where a pipe passes through a fire-separating element, the penetration needs a tested fire-stopping system as well. And flues and hot exhaust pipes need a detail rated for their temperature. Follow the flue manufacturer's instructions.

Why this matters more every year

Since Part L 2021, every new home in England has to be air tested, not a sample, and airtightness details have to be photographed for the BREL report. The Future Homes Standard, published in March 2026 and in force from March 2027, tightens the notional dwelling's air permeability from 5 to 4 m³/(h·m²). In retrofit, PAS 2035:2023 asks for an airtightness strategy whenever the fabric is being improved. More testing, tighter targets and photo evidence all point at the same weak spot: the penetrations. Our Building Regulations guide has the detail.

Our own grommet is on the way

We are launching TED, the Tight Entry Detail, an airtight pipe grommet range designed to work with Passive Purple, Airtight White and The GOAT. It arrives in December 2026, with sizes from small plumbing pipes to large soil and ventilation ducts. Want to know the day it lands? Drop us a line and we will tell you.

Got a penetration you are not sure how to detail? Send us a photo and tell us what is going through it.

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