Bathroom Cabinet Humidity Test: MDF vs Plywood vs PVC After 12 Months of Steam

Bathroom cabinets do not usually fail because of splashes. They fail because of condensation, and condensation behaves differently: it arrives as warm vapour, settles on the coolest surfaces in the room, and stays there long enough to penetrate anything that is not sealed. That is why the damage shows up on the underside of a wall cabinet, on the back panel against an external wall, and at the plinth, rather than on the door fronts that get wiped every day.

We have not run a twelve month humidity chamber test, and we will not present any figures as if we had. This is a construction and materials assessment comparing how moisture resistant MDF, plywood and PVC behave in a real British bathroom over an extended period, based on the physical properties of each board and on the failure patterns we see. The conclusion is that all three can work, and all three fail in the same place: at unsealed cut edges and drilled holes, in a room without adequate extraction.

Key findings at a glance

• Ventilation does more to protect bathroom furniture than the choice between MDF, plywood and PVC. Approved Document F of the Building Regulations sets a minimum intermittent extract rate of 15 litres per second for bathrooms.
• Standard MDF absorbs water irreversibly and swells permanently, while moisture resistant MDF is treated to slow that process but is not waterproof.
• Plywood resists swelling far better because its cross laminated structure holds dimensional stability, but the bond depends on the glue specification and its edges still wick water.
• PVC and other polymer boards do not absorb water at all, which makes them the safest choice around basins and in rooms with poor extraction.
• Every one of the three fails first at cut edges, screw holes, hinge cups and tap or waste cut outs, because these are where the protective face is interrupted.
• Wall hung cabinets outperform floor standing units in damp bathrooms simply because air can move underneath them and water cannot pool at a plinth.
• The visual warning signs differ: MDF thickens and lifts, plywood delaminates in layers, and PVC shows no change at all until a fixing pulls through.

What a bathroom actually does to furniture

A hot shower in a small unventilated bathroom saturates the air quickly. That vapour then condenses on whatever is coldest, which in most British bathrooms means tiles, mirrors, external wall surfaces and the undersides of wall mounted units. In an older solid wall property without insulation, the external wall is very cold indeed, and a cabinet fixed against it sits in the wettest position in the room.

Two features of British bathrooms make this worse than it needs to be. The first is that many bathrooms are internal or have a window that is never opened in winter, so the extract fan is the only route out for moisture. The second is that bathrooms are frequently the smallest room in the house, so the volume of air available to absorb the vapour is small and the surfaces are close to the source.

Approved Document F of the Building Regulations sets a minimum intermittent extract rate of 15 litres per second for bathrooms, and an extract fan meeting that rate, running long enough after use, changes the environment substantially. A fan with an overrun timer is worth more to the furniture in a bathroom than any upgrade in board specification. Where the room has no extraction and no opened window, we would specify polymer board and wall hung units regardless of budget, and that thinking applies across bathroom cabinets UK of every format.

How the three materials behave over a year of steam

Standard MDF is a fine fibre board bound with resin. It machines beautifully, takes paint evenly and produces the crisp edges that make bathroom furniture look built in. Its weakness is that the fibres swell when they take on water and do not return to their original dimension when they dry. The characteristic result after months of condensation is a thickened, slightly furred bottom edge on a door, often visible before it is felt.

Moisture resistant MDF, generally supplied with a green tint, uses a modified resin system that slows water uptake considerably. It is a genuine improvement and it is the right specification for bathroom furniture, but the name is misleading if it is read as waterproof. It resists humidity well and standing water poorly.

Plywood is built from thin veneers laid at right angles to each other and bonded. That cross grain structure is what gives it dimensional stability: the layers restrain each other, so the panel does not swell in the way a fibre board does. Its performance depends on the adhesive, and boards intended for humid environments use a more water resistant bond. Plywood’s failure mode is delamination, usually starting at an exposed edge where water has worked between two veneers, and it tends to take longer to develop than MDF swelling.

PVC and other polymer boards do not absorb water in any meaningful sense. They can sit in a wet environment indefinitely without dimensional change, which is why they appear in bathroom furniture designed for heavy use. The trade offs are mechanical rather than moisture related: polymer board is less rigid than plywood, it holds screws less firmly, and it can sag over long unsupported spans. Fixings in polymer board need to be specified properly or they will work loose over time, particularly on a wall hung unit carrying the weight of a basin.

The table below summarises the comparison. The ratings are our editorial assessment based on material properties, not measured test results.

The details that decide the outcome

Edge sealing is the whole game. A factory finished panel with all six faces coated will outlast a better board that has been cut on site and left raw. The places to inspect on any bathroom unit are the underside of the base panel, the rear edge of shelves, the hinge cup recesses, the waste pipe cut out in a vanity back panel and the tap hole in the top. Every one of these interrupts the protective face.

Fixing height matters next. A floor standing vanity has a plinth that sits in the zone where water collects on a bathroom floor, and that plinth is usually the least protected component on the unit. A wall hung unit removes the problem entirely, lets air circulate underneath and makes the floor easier to clean, which is why it is the format we recommend for any bathroom with condensation problems. The same logic applies to bathroom storage units UK of any height.

Hardware is the component people forget. Hinges, runners and handles in a humid room need to be corrosion resistant, and a cabinet with an excellent board specification and untreated steel hinges will show rust staining down the door long before the board itself has a problem. Soft close mechanisms also tolerate damp poorly if they are not specified for it.

What this means for UK homes

Fix the ventilation before you replace the furniture. An extract fan meeting the minimum rate in Approved Document F, with an overrun timer so it continues after the room is vacated, addresses the cause rather than the symptom. Leaving the door open after a shower helps, and wiping the mirror and surfaces removes water that would otherwise evaporate back into the room.

Specify wall hung wherever the wall construction allows it. It lifts the unit out of the wet zone, improves airflow and removes the plinth, which is the single most vulnerable component on a floor standing cabinet. Check the fixing substrate carefully: a plasterboard wall needs appropriate heavy duty fixings or a timber noggin behind, particularly where a basin adds weight.

Choose the material for the room’s worst month, not its best. In a well ventilated bathroom with a window that gets opened, moisture resistant MDF with properly sealed edges performs perfectly well and gives the crispest finish. In an internal bathroom with a weak fan and a cold external wall, polymer board or a plywood carcass is the more sensible specification, and the visual difference is minimal on bathroom vanity units UK with a good door finish.

Finally, maintain the seals. Silicone around a basin or a worktop is a consumable, not a permanent fixture, and a failed seal delivers water directly into the one place a cabinet cannot cope with it. Checking it annually takes minutes. Our full modern bathroom furniture UK range at Furniture in Fashion lists carcass materials and mounting formats so these choices can be made before purchase rather than after.

Methodology

This 2026 report is an editorial materials assessment compiled by our team as a UK furniture retailer. It is based on the established physical properties of MDF, moisture resistant MDF, plywood, laminate faced board and polymer board in humid conditions, on standard bathroom furniture construction practice covering edge sealing, fixings and mounting, and on the failure patterns we observe in bathroom furniture in service. Where regulation is referenced, it is the minimum intermittent extract rate for bathrooms set out in Approved Document F of the Building Regulations. No humidity chamber testing, controlled twelve month exposure trial or timed measurement was carried out for this report, and the comparison table is qualitative. The report contains no proprietary sales data, customer data or survey fieldwork.

Citing this report

Journalists and websites are welcome to cite this analysis, published by Furniture in Fashion in 2026. Please credit Furniture in Fashion as the publisher and link back to the original report. We describe this as a qualitative materials assessment and ask that it is not reported as laboratory test data.

Frequently asked questions

Is moisture resistant MDF waterproof?

No. It uses a modified resin that slows water uptake and copes well with humid air, but it will still swell if water is allowed to stand against an unsealed edge. Treat it as humidity resistant rather than waterproof.

Is plywood better than MDF for bathroom furniture?

For dimensional stability, yes. Plywood’s cross laminated structure resists swelling, and it holds screws better. Its weak point is delamination at exposed edges, so edge sealing remains essential.

Why does my bathroom cabinet door have a swollen bottom edge?

Condensation runs down the door and collects at the lowest edge, which is often the least sealed part of the panel. Once a fibre board core absorbs water it expands permanently, so the swelling cannot be reversed.

Are wall hung bathroom cabinets really better?

In damp bathrooms, yes. They remove the plinth from the wet zone, allow air to circulate underneath and prevent water pooling against the base. They do require a wall and fixings capable of carrying the load.

Does an extractor fan protect bathroom furniture?

It protects it more than any board upgrade does. Removing moist air before it condenses reduces the exposure that causes swelling, delamination and corroded hardware. A fan with an overrun timer is more effective than one that stops with the light.