Why Does Silicone Sealant Turn Mouldy?

silicone sealant

Why does silicone sealant turn mouldy? Well, there are a number of reasons why this happens.

Nothing is ever perfect. Problems can and do occur. So you need to go through this checklist to see what has caused the problem and then come up with a solution.

The main cause of mould growth in silicone is permanent or semi-permanent moisture.

If you have ever tried painting over silicone sealant with a water-based paint you will see that the paint just runs off. This is usually the case with water too.

So, for mould to grow, there must be almost continuous moisture in the area.

Bath or shower water should run off your silicone leaving a surface that will dry out reasonably quickly in a modern home.

bath rim

What can prevent this from happening?

There are a few things that can trap water and keep it in contact with the sealant.

A deck bath shower mixer has a large flat area at the back of it. The body of the mixer tap can prevent water running off the bath rim and into the bath.

Sponges and flannels that are kept at the end of the bath will stay wet and can be a source of moisture in permanent contact with the sealant.

Shampoo and soap bottles can also have an effect in trapping water where it shouldn’t be.

A badly levelled bath can prevent water from running back off the rim. Or the design of the bath might include a raised lip as part of the design, usually found at the head end.

So unless there is something preventing the water running off your sealant there should be no permanent moisture. And if there is no permanent moisture your silicone should remain clean and mould-free.

Why does silicone sealant turn mouldy if none of the above issues affect you? There is one other reason.

Water is getting behind your silicone. And this is actually quite common.

silicone sealant mouldy


How did Water Get Behind The Sealant?

There are three possible reasons:

  • lack of adhesion
  • movement
  • grout failure

We will address each one in detail here.



The sealant needs to make a strong bond with the surfaces to which it is applied.

If the sealant did not adhere to the tiles and bath when first applied it will fail. Grease, soap residue and dust will prevent the sealant from forming an effective bond.

In an older bathroom where the sealant may have been replaced previously there could well be soap residue on the bath or tile surfaces.

New baths could have a thin film of grease from the manufacturing process. This is not common –  but do not assume that because a surface is brand new and  looks clean that it is clean.

When grout is applied it covers everything with a thin film of dust and takes a lot of cleaning to remove it fully.

All of these issues can prevent your silicone sealant forming a strong bond.



The bath or shower tray has moved or dropped when in use. This, in turn, pulls the seal away from the surfaces and allows water to get behind.

There are some mechanical seals available on the market that compensate for this movement. They attach to the bath and slide up behind the tiles or wall panels.  But our advice would be to eliminate the movement rather than try and accommodate it.

Baths and shower tray should be rock solid and not move deferentially to the walls they are attached to. A batten underneath the bath rim will prevent downward movement. Or you can chase out a shallow slot in the wall to rest the bath or tray on. See this article for more information on these techniques.

Movement of the bath will also cause grout to crack, leading to….


Grout Failure

Most grout is porous to some extent. It therefore retains moisture. Remember – permanent moisture allows mould to grow.

Even if a waterproof grout is used this can still crack under load. And the application process is not 100% effective and can lead to tiny gaps and pin holes. Water seeps through these gaps and holes and makes the whole substrate wet.

If the tiles are fitted onto plasterboard this exacerbates the problem. The board acts like a sponge, drawing more and more moisture in and eventually the tiles will start to work loose.

If you look carefully at silicone failure it usually starts in front of a grout line.



First of all make sure all surfaces are spotless when applying silicone sealant. Buy good quality, sanitary grade silicone sealant. Do not buy cheap sealant – it is a false economy.

Secondly, ensure the bath or shower tray does not move in relation to the walls.

For new installations: fit a batten solidly to the wall under the rim.

For existing installations: cut battens as props and place them under the rims around the back of the bath.

Rake out grout, re-apply a new layer and seal it with a grout sealer (a critical step most people forget about). If the grout turns mouldy again then moisture will be coming through from behind indicating that the whole wall may need to be re-covered.

Or install bathroom wall panels rather than tiles. They have no grout and are more tolerant of movement than tiles. One great advantage of this system is that it can be installed over your existing tiles without the need to remove them. This makes for a quick, clean installation. And it will leave you with a wipe-over, maintenance-free surface that will give you years of service.

There is a huge range of styles and designs. Many of the patterns look just like tiles but with none of the drawbacks.