Reasons Your Silicone Sealant is Growing Mould

Black mould in the silicone caulk joints on the edges of bathtubs, showers and sinks is not exactly inviting. And while bleach-based mould killer sprays do a good job of getting rid of it for a moment, it always seems to creep back. So why is your silicone sealant growing mould and what can you do to reduce the chances of it happening?

Quick answer: Silicone sealant usually goes mouldy because of how it’s finished or where water is able to sit, not because the silicone itself has failed. For example, a silicone joint with a dip in it, or one smoothed with a bare finger, creates a trough where water and bacteria collect. Sealant lifting away from the tile gives mould a way in behind the seal, and a warm, humid, poorly ventilated bathroom makes all of this worse. Once mould has taken hold, cleaning it off is only ever temporary, and the joint needs redoing rather than repeatedly wiped down. Good ventilation, drying surfaces after use, and finishing the joint hygienically with finishing tools the first time go a long way towards preventing mould, and sealant that’s cracked, lifting, or growing mould is best replaced rather than left.

1. Water is gathering in the joint

If the surface of the silicone joint has a dent or curve in it, it can create a “trough” where water and bacteria pool instead of running off the surface – perfect conditions for a bit of black mould to start growing! Instead, you’d want the surface of the caulk joint to be a smooth, sloping surface so water drains away rather than sitting in the joint.

Finishing tools like Silicone Block Pro or the Cramer Fugi kit create that straight, sloping profile in one pass so you’re not left with a trough for water to gather in.

Also, if the silicone caulk has already started to grow mould, then it’s likely to return because the contact between the silicone and the surface is already compromised. Cleaning it off is only ever a temporary fix. In that case you’re best off removing the old silicone and redoing the joint.

2. Bare finger finish

One way to make a silicone joint extra inviting for black mould is to finish smoothing it with a bare finger. Not only does this create that dented surface we mentioned earlier, where water can then pool, it also transfers bacteria from your finger straight into the silicone, especially if you licked your finger first! (Seriously, please, please don’t do that.)

Actual silicone finishing tools (or even a flat stick if you’re skilled) will give you a more hygienic finish to the silicone caulk – we’ve covered different finishing methods in more details in another guide (link).

3. Sealant coming loose from the wall

If silicone starts peeling away from the tile behind it, that gap gives mould somewhere to take hold, even if the sealant itself is still sound. If you notice the joint cracking or peeling, it’s time to get rid of the old silicone and reseal the joint.

4. Humid environment

Bathrooms are warm and steamy by nature, especially straight after a hot shower or bath, and that combination of heat and moisture is exactly what mould spores need to grow. Warm and wet environments that aren’t properly ventilated are perfect conditions for mould to grow https://www.screwfix.com/guides/sealants-adhesives/sealants/how-to-remove-mould-from-silicone-sealant , and condensation forming on tiles, grout and silicone as warm and cold air meet only adds to it. Without decent airflow, that moisture lingers rather than drying off, giving mould far more time to take hold.

Hang on, isn’t silicone waterproof? So why’s it growing mould in the first place?

Silicone itself doesn’t feed mould, but spores can settle on soap scum, skin cells and condensation sitting on the surface and then thrive in a warm, damp bathroom. And once a joint starts to lift or crack, moisture gets trapped behind the sealant too, which is often where the real problem starts.

How to prevent mould in silicone joints

The bad news is that your bathroom silicone joints aren’t going to stay pristine forever, no matter what you do. But there are a few ways to keep them in good condition for years:

• Good ventilation – so keep windows open or trickle vents on where you can.
• Dry the shower and surfaces after use with a squeegee or towel.
• Run the extractor fan during and after showers.
• Wipe down glass and tiles regularly to stop soap scum and condensation building up on the sealant.

How often should I replace silicone sealant?

Eventually, you will have to replace the old silicone sealant. There’s no fixed lifespan, but ROSEAL, a UK sealant manufacturer, recommends checking it regularly and replacing it once you see mould that won’t shift, cracking, or silicone lifting from the surface.

An easy way to remove the old silicone is to cut it off with a sealant removal tool like the Metex Silicone Cutter. A good old Stanley knife will also do the trick, but can accidentally scrape the tiles in the process if you’re not careful. You can remove any remaining traces of old silicone either by scraping them off, or with a solvent cleaner like Multisolve. Just make sure the solvent is fully dried and cleaned off before you apply new silicone, as it can dissolve your new sealant too!

If you’re after a smoother finish or an easier way to strip out old sealant, take a look at Silicone Block Pro and the Silicone Cutter. Both are available now on Amazon.

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