Why Does Silicone Sealant Turn Mouldy?

silicone - Why Does Silicone Sealant Turn Mouldy?

There are a number of reasons why this happens.

The first thing to understand is that if mould spots start appearing in your sealant they will have grown through from the back. Mould needs permanent moisture to grow and water just rolls right off the surface of silicone.


if your silicone sealant is turning mouldy – water has got behind it


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



The bath or shower tray has moved or dropped when in use, pulling the seal away from the surfaces and allowing water to get behind. There are some mechanical seals available on the market that compensate for this movement 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 chase out a shallow slot in the wall to rest the bath or tray on.

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


Grout Failure

Most grout is porous. It therefore retains moisture. Remember – permanent moisture allows mould to grow. If the tiles are fitted onto plasterboard this exacerbates the problem as 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.

geocel 1 - Why Does Silicone Sealant Turn Mouldy?


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 will leave you with a wipe-over, maintenance-free surface that will give you years of service.

We have a large range of styles and designs including many panels that look just like tiles – but with none of the drawbacks.

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