How to fit an airtight grommet into a liquid membrane on masonry and timber frame

With a sheet membrane, a grommet is taped on. With a liquid membrane it can do better: the flange is bedded into the coating, so the membrane, the grommet and the pipe collar form one continuous airtight film with no lap to peel. This guide shows how, on the substrates we see most often.

Before you start

• Size the grommet to the pipe. Measure the outside diameter of the pipe, including any insulation if the seal is going onto the insulation jacket. Pick the grommet whose range covers it.
• Check the sequence. The grommet goes on over a free pipe end, so fit it before the pipe is connected at both ends. If that moment has passed, see sealing existing pipes.
• Check the clearances. The flange needs to lie flat all the way round. If a pipe is tight to a corner, a joist or another pipe, a liquid grommet such as The GOAT is usually the better detail.
• Read the datasheets. The steps below are the general method. The membrane's and grommet's own installation guides, including coverage, coats and curing times, take precedence.

Step by step

1. Make the opening neat. Core or cut the hole as close to the pipe size as practical. A ragged, oversized hole in blockwork leaves too little flat surface for the flange.
2. Fill large voids. Fill the gap between pipe and wall back to a flat face with a suitable filler or mortar, and let it cure. The grommet should bridge a small gap, not span a cavity.
3. Prepare the surface. Remove dust, loose mortar and snots. On porous or dusty masonry, prime first. See our primers.
4. Apply a first coat round the opening. Brush Passive Purple Brush onto the area the flange will cover, plus a margin beyond it, working it into the pores and the joint between pipe filler and wall.
5. Fit the grommet. Slide it over the pipe end, push it back to the wall, and press the flange flat into the wet coat. Work from the collar outwards to push out air bubbles and avoid creases.
6. Coat over the flange. Brush a second coat over the whole flange and well out onto the wall, so the edge of the flange is fully encapsulated. The collar itself grips the pipe, so keep the coating off the pipe unless the grommet's guide says otherwise.
7. Bring the field coat up to it. When the main wall area is coated, by brush or spray, take it over the detailing so there is no step or gap between the detail and the field.
8. Inspect and photograph. Look for pinholes, thin spots at the flange edge, and creases. Photograph each penetration with its location for the BREL report and your own records.

Substrate notes

Blockwork and brick. The main risk is dust and porosity. Blocks can be very absorbent and will pull the coating thin if not primed. Mortar joints near the pipe are often recessed. Make sure the coating fills them rather than bridging over a hollow. See blockwork is not an air barrier for what a properly sealed masonry flat looks like.

Timber sheathing (OSB, plywood). Board joints near the penetration need treating as part of the same detail. Coat the joint, or tape it with IM Fleece Tape, before the grommet goes on. Keep battens and fixings clear of the flange.

Concrete. Remove laitance and release agent, and let new concrete dry out enough for the coating to bond.

Plasterboard. If plasterboard is your airtight layer, coat it before the next board goes on. See airtight vapour control you can leave on show for related details.

Common mistakes

• Grommet stuck to dust. It looks fine and fails at test. Prepare the surface every time.
• Flange edge left proud. If the edge is not coated over, air finds the gap underneath it.
• Creased flange. Every crease is a channel. Press it flat from the centre outwards.
• Fitted after the pipe was finished, then cut to make it fit. Use a liquid grommet instead.
• Foam behind it. Foam is a filler, not an air seal, and it can push the flange off the wall as it expands.

Products

• Passive Purple: the airtight layer and vapour control, brush, roller or spray.
• Passive Purple Brush: for detailing round penetrations and junctions.
• The GOAT: a liquid grommet for clusters, corners and existing pipes.
• TED (Tight Entry Detail): our own airtight pipe grommet, arriving December 2026, made to bed into Passive Purple and Airtight White.

Not sure about a detail? Send us a photo. For the background, read airtight pipe grommets explained.