Concrete moisture vapour is the single biggest cause of coating failure on the Central Coast, and it accounts for a large share of the epoxy, spray-on and sealed-surface jobs that blister, bubble or delaminate within the first year or two. Fixing it typically means specifying a moisture-tolerant primer or vapour barrier system, which usually adds somewhere around $15 to $30 per square metre to a job, well below the cost of stripping and redoing a failed coating from scratch. This guide covers what moisture vapour actually is, why it hits this region harder than inland areas, and what stops a coating failing because of it.
What is concrete moisture vapour, and why does it blister coatings?
Concrete is not waterproof. It’s a porous matrix of cement paste and aggregate laced with capillary pores, and water moves through that network as both liquid, under hydrostatic pressure from a wet water table or saturated soil, and as vapour, driven by temperature and humidity differentials between the ground and the surface.
When a coating (epoxy, a spray-on overlay, or even a film-forming sealer) is applied over a slab that’s still transmitting significant moisture vapour, that moisture has nowhere left to go. It gets trapped under the film as the coating cures, and two things happen: the bond between coating and concrete weakens, because proper adhesion relies on a dry, chemically receptive surface; and the trapped moisture keeps migrating, building vapour pressure underneath the coating until it pushes the film upward. The result is blistering, bubbling and eventually delamination, sometimes within weeks and almost always within the first wet-weather or humid-summer cycle after installation.
Slabs poured without an effective vapour barrier underneath are particularly vulnerable, and a great many older Central Coast homes fall into that category: vapour barriers under residential slabs were far less consistently used, or specified at all, before the 2000s.
How to tell blistering from other kinds of coating failure
Not every coating problem is moisture. It helps to know the difference before assuming the worst:
- Moisture vapour blistering typically shows as domed bubbles scattered across the coated area, often worse over damp patches, garden beds outside a wall, or low points where water pools nearby. It tends to appear gradually, sometimes months after application, and gets worse after rain or in humid weather.
- Poor surface preparation failure (inadequate grinding, contamination left on the slab) usually shows as delamination in sheets or patches that appeared almost immediately after application, rather than domed bubbles that develop over time.
- UV degradation shows as chalking, fading and a dull, powdery surface rather than bubbling, and develops slowly over years rather than weeks.
An honest inspection can usually tell these apart, but where there’s doubt, a proper moisture test settles it.
Why the Central Coast sees more of this than inland areas
Three regional factors compound here:
- Groundwater and soil. Clay soils and proximity to lakes, creeks and the water table across much of the Coast keep ground moisture higher for longer than drier inland conditions.
- Ambient humidity. Warm, humid summer air combined with a cooler slab creates the temperature differential that drives moisture upward through concrete by capillary action, and this cycle runs for a good part of the year here rather than just a few weeks.
- Older housing stock. A large share of Coast homes were built between the 1960s and 1990s, well before vapour barriers under residential slabs were routine. Combined with decades of garden irrigation, poor drainage falls or planting beds hard against a wall, a lot of older slabs are continuously fed moisture from below.
None of this makes a slab uncoatable. It means the moisture has to be measured and managed rather than assumed away, which is exactly the step a cut-price job is most likely to skip.
How is it tested, and what do the readings mean?
Two methods are standard in professional practice:
- Calcium chloride test: a sealed chamber left on the slab for 60-72 hours measures the moisture vapour emission rate (MVER) in grams per square metre per 24 hours.
- Relative humidity probe: a small hole drilled into the slab, fitted with a humidity probe, gives a reading of moisture inside the concrete at a specified depth. This method is generally considered more predictive of long-term behaviour under a coating.
As a rough guide only, MVER readings below 17 g/m²/24h are generally suitable for a standard coating, 17-34 calls for a moisture-tolerant primer, and above 34 needs a full vapour barrier or moisture-mitigation system. Our guide on moisture testing before epoxy goes through both test methods and the full reading thresholds in detail, and the same principles apply whether the coating going down is epoxy, a decorative overlay, or a premium sealer.
Which jobs are most at risk?
Interior, roofed slabs, garage floors especially, are the classic moisture vapour failure because they get a hard, continuous film coating and rarely have any drainage relief once the coating is down. Outdoor overlays and pool surrounds are less prone to trapped vapour pressure since they’re exposed to the weather and can breathe more, but they aren’t immune: a spray-on overlay or a heavy film-forming sealer applied over a permanently damp low spot, a shaded pool surround that never fully dries, or a slab hard against a garden bed with poor drainage falls can still blister for the same underlying reason.
What actually stops it
Once elevated moisture is confirmed, there are two practical fixes, and neither involves waiting for the slab to “dry out” on its own, which for a slab fed continuously from below or from poor drainage, it generally won’t:
- Moisture-tolerant primers. Some epoxy and polyaspartic systems use a primer chemistry specifically formulated to bond to damp concrete. These extend the workable moisture range but still have their own upper limit, specified by the manufacturer.
- Vapour barrier or moisture-mitigation coatings. A specialist product, typically epoxy or polyurethane based with very low water vapour permeability, applied as a dedicated layer before the topcoat. It doesn’t reduce the moisture in the slab; it contains it, allowing the finish coat to be applied over a genuinely wet slab without failing.
Outside these coating-side fixes, correcting the site cause where possible, improving drainage falls away from a wall, moving an irrigation line away from a slab edge, fixing a downpipe that’s been dumping water against a foundation, reduces the ongoing moisture load and gives any coating an easier job. Proper surface preparation matters just as much alongside moisture management: our guide on preparing concrete before resurfacing covers the full sequence, of which moisture testing is one non-negotiable step among several.
Indicative cost of moisture mitigation
| Item | Indicative cost |
|---|---|
| Moisture-tolerant primer upgrade | Typically itemised per m², a modest addition over standard primer |
| Full vapour barrier / moisture-mitigation coat | Roughly $15-$30/m², e.g. $750-$1,500 on a 50 m² garage |
| Drainage correction (downpipe, grading, irrigation adjustment) | Priced individually at inspection, depends on the fix required |
| Stripping and redoing a coating that has already blistered | Substantially more than doing the moisture check correctly the first time |
All figures are indicative Australian guide ranges only, confirmed after a site inspection and formal written quote.
FAQs
My coating blistered within a few months of being applied. Is that definitely moisture?
Domed bubbles that appeared gradually and worsen after rain or humid weather are the classic sign, but poor preparation can produce something similar. An inspection, and ideally a moisture test on the exposed slab where the coating has already lifted, will confirm which it was.
Can a slab with high moisture readings still be coated?
Usually yes, with the right system. Elevated readings call for a moisture-tolerant primer or a dedicated vapour barrier coat rather than the job being declined outright. What changes is the product specification and the cost, not whether the job can proceed.
Does this only affect epoxy garage floors?
No. It’s most commonly discussed in relation to epoxy because interior coatings trap vapour most completely, but any hard film coating, spray-on overlays, premium sealers, decorative toppings, can blister over a persistently damp slab if moisture isn’t checked and managed first.
Will fixing drainage around the slab stop it happening again?
It helps reduce the ongoing moisture load, but it isn’t a substitute for the coating-side fix if the slab itself lacks a vapour barrier. Both are worth doing where relevant: drainage correction reduces the problem at its source, and the right primer or barrier coat handles what’s already moving through the concrete.
How long does moisture testing add to a job timeline?
A calcium chloride test needs the chamber to sit sealed for 60-72 hours, so allow at least three days between test placement and a result that can inform the coating decision. It’s time well spent against the alternative of a coating failing within a year.
Is a vapour barrier a permanent fix?
It’s a durable one rather than a one-off patch: a properly installed vapour-mitigation coat is designed to last the life of the finish coating over it, provided it’s specified correctly for the moisture level identified in testing.
Worried your slab might blister a new coating? Send photos and a few details through our Get a free quote form, and we’ll flag whether a moisture check should be part of your inspection before anyone quotes the job.