Why the paint on your bathroom ceiling keeps bubbling

Bubbling paint on a bathroom ceiling means moisture is getting behind the paint film and pushing it away from the surface. In most Wellington bathrooms that moisture is shower steam with nowhere to escape, and until the ventilation is fixed, any repaint will bubble again within a season or two.

The paint is the symptom, not the cause. Repainting a bubbling ceiling without dealing with the water is the most common waste of money in interior decorating, and the difference is worth understanding before you buy a tin of anything.

What is happening underneath the bubble?

Paint holds onto a ceiling through a mix of mechanical grip and chemical bond. Water breaks both. Warm saturated air rises off a shower, meets the coldest surface in the room, and condenses. If the paint film has any porosity, or a pinhole around a downlight or a fan surround, that condensed water works its way underneath. Once there it has nowhere to go. It sits between the paint and the plasterboard, softens the bond, and lifts the film into a dome.

When the room dries out the water evaporates and the bubble deflates into a wrinkled patch. The next shower reinflates it. That cycle is why bathroom bubbles get bigger every month rather than staying the same size.

Is it condensation or a leak from above?

Establish this first, because the two need completely different work. A leak needs a roofer or a plumber before a painter sets foot in the room.

What you see	Points to condensation	Points to a leak
Location	spread across the ceiling, worst above and around the shower	one defined patch, often nowhere near the shower
Staining	little colour, or black and grey mould specks	brown or tea-coloured ring with a darker edge
Timing	worse through winter, better in summer	worse after heavy rain or a southerly, whatever the season
Shape	many small blisters, sometimes one wide soft area	a single blister or sagging patch, occasionally heavy enough to drip
Company it keeps	mould in the corners, on the window reveal and on the silicone	damp smell, a stain on the wall below, marks in the roof space

If you can get into the roof space, look directly above the patch on a wet day. Wellington gets wind-driven rain that travels sideways under flashings and into places a straight downpour never reaches, so water can enter a metre or two away from where it finally appears.

Why does the extractor fan matter more than the paint?

A bathroom fan has one job: put the moisture outside the building. Two failures make it useless. The first is a fan that discharges into the roof space rather than through the roof or the soffit to open air, so the steam simply returns through the ceiling. The second is a fan that is undersized, or ducted with flexible pipe that has been squashed flat or run with tight bends.

The rental standard is a useful benchmark even if you own the house. Under the Healthy Homes ventilation standard, a fan installed in a room with a bath or shower after 1 July 2019 must vent to the outdoors, and the fan and all its ducting must either have a diameter of at least 120mm maintained the whole way through, or an exhaust capacity of at least 25 litres per second. If yours is a small unit on crushed flexible duct that stops in the ceiling cavity, that is your bubbling problem, and no paint system will overcome it.

Habits count too. Run the fan during the shower and keep it running well after you are out, then leave the door open. A fan wired to the light switch does about half the job it could.

Could the wrong paint have been used?

Sometimes, and it makes an existing problem visible faster. Standard flat ceiling paint is built to be cheap, matt and quick. It is porous and it carries no mould inhibitor. In a ventilated bedroom that is fine. Over a shower it absorbs moisture, holds it, and grows mould inside the film.

A kitchen and bathroom grade paint has a tighter film and a fungicide built in. It costs more per litre and it earns that back in this one room. The other frequent mistake is coating a ceiling that already carried solvent-based enamel, common in older Wellington houses, without sanding or priming. The new coat never bonds properly and steam finishes it off.

Third mistake: painting over a bubble. The trapped water is still under there.

How do you fix it properly?

1. Deal with the moisture source. Ventilation, or the leak. Nothing else matters until this is done.
2. Let the ceiling dry out completely. In a Wellington winter that can take a week or more with the room heated and aired. Painting a damp ceiling recreates the problem on day one.
3. Scrape every bubble back to sound, well-stuck paint. Do not stop at the visible edge of the blister. Keep going until the scraper stops lifting, because the failed area is always larger than it looks.
4. Treat mould with a proper fungicidal wash and give it time to work. Bleach on its own kills the surface growth and leaves the staining behind.
5. Sand the edges flush so the repair does not telegraph through, and fill where the plasterboard paper has torn.
6. Spot-prime bare plasterboard and any stained area with a stain-blocking sealer. Water stains bleed straight through waterborne topcoats otherwise.
7. Two full coats of a kitchen and bathroom grade ceiling paint.

If the plasterboard has gone soft or the paper face is delaminating, it needs replacing. You cannot paint a spongy ceiling into a sound one, and a fair amount of our plastering and GIB stopping work starts in exactly this position.

Is this a job you can do yourself?

A small bathroom ceiling with light bubbling and no leak is a reasonable weekend job. The area is small, you are working off a stepladder rather than staging, and the materials are inexpensive. Scrape properly, be patient about drying, use the right paint.

Get someone in when there is a leak, when the plasterboard is damaged, when the house was built in the 1980s or earlier and you plan to sand, since older coatings here can contain lead and want testing first, or when you have already repainted it once and it came back. A repeat failure means the diagnosis was wrong, not the brushwork.

What if it keeps returning?

Then the moisture source has not been found. The usual suspects at that stage are a fan discharging into the roof space, a shower dome or badly sealed downlight letting warm air straight into the ceiling cavity, an unflashed roof penetration, or a bathroom with no opening window and a fan nobody leaves running. Work through those before buying more paint.

If you want a second opinion on whether you are looking at condensation or a leak, or you want the ceiling made good once the cause is sorted, get in touch for a quote. We cover Wellington, Lower Hutt, Upper Hutt, Porirua and the Kapiti Coast.