The key-shaped grommet: sealing a cable penetration before the cable exists

September 23, 2026

Service penetrations are where airtightness strategies go to die, and the reason is almost always sequencing. The membrane goes on. The seal is formed. Then, three weeks later, somebody needs a cable through that hole — and the neatest seal on site gets cut, pushed through, and taped back up by whoever is holding the cable.

Here is a penetration detailed by somebody who knew that was coming.

Click and drag to look around. The GOAT Liquid Grommet around a service penetration, applied in a deliberate key shape.

Why it is shaped like a key

The round part seals the opening. The extended section running below it is not decoration and it is not overspill — it follows the planned route of the cable. When the cable is eventually installed, it drops out of the opening, turns, and runs down inside that tail. The cable emerges through sealant rather than through a gap, and nobody has to cut the seal to install it.

In other words, the detail was formed around a service that did not exist yet. That is the whole idea: the airtight layer anticipates the sequence instead of being a victim of it.

What makes a liquid grommet work

A mechanical grommet is sized for one cable diameter, arrives in a box, and is either right or wrong. A liquid grommet is formed on site around whatever is actually there — a bundle rather than a single cable, a pipe at an awkward angle, a penetration that ended up 40mm from a corner.

It cures flexible. The seal has to survive the cable being pulled, dressed and clipped after it is installed. A rigid filler cracks at exactly that moment.
It bonds to the membrane. The seal and the surrounding airtight layer act as one detail, not as a patch stuck onto a layer.
It is waterproof within minutes. On a live site that matters, because the detail is often formed in an opening that is still exposed.

Where it goes wrong without one

Three failures account for most of the leakage found around services at test:

Foam. Expanding foam fills a void; it does not form an air seal, and it shrinks.
Tape over a round object. Tape is a flat product. Wrapping it around a pipe and onto a wall produces creases, and every crease is a channel.
Sealing before the service is in. The seal gets broken by the trade that follows. This is the one the key shape is designed to defeat.

The related details

A penetration seal is only as good as the layer it connects to. On a blockwork or timber-frame envelope that is Passive Purple, with Passive Purple Brush carrying the film into the junction around the opening before the field coat goes on.

For flat joints and linear junctions rather than round penetrations, see sealing gaps and penetrations with a Passivhaus sealant and sealing joist ends.