Moisture-resistant plasterboard: 4 structuring trade-offs for tertiary wet areas
Trade-off 1: H1, H2 or H3 classification based on measured exposure, not on the usage plan
H3 moisture-resistant plasterboard is frequently over-specified in tertiary specifications: at a significant extra cost versus H1, it is only justified beyond 85% RH measured over 7 days, not on the mere presence of a water point. Our reading of the CSTB Technical Assessment diverges from the common practice of engineering firms: the EH classification is calibrated on measured hygrometry, not on the usage plan. Visitor washrooms, kitchenettes, collective kitchens, floor showers and air-conditioned technical rooms concentrate dry partition pathologies, but 4 structuring trade-offs (EH classification, 72/98 mm thickness, modularity, MVHR coordination) determine actual durability. On projects integrating a prior moisture audit, partition reworks in wet areas are significantly reduced compared to a standard specification without upstream diagnosis. A design and build methodology deployed since 2006 across 11 agencies in France and Spain, 1200+ projects delivered in 12 weeks on average across 4 trades.
For the architect and the property investor: EH over-specification is the leading source of discrepancy between the preliminary design and the priced bill of quantities for tertiary dry partitions. The EH classification (Exposure to Humidity) defined by the technical assessments in force segments boards into three usage levels. Calibrating this classification on measured exposure avoids a significant extra cost on over-specified operations, a recurring finding during our specification audits.
| Classification | Target exposure | Typical tertiary uses | Tolerated hygrometry |
|---|---|---|---|
| H1 | Occasional splashing | Visitor washrooms, kitchenettes | < 70% RH |
| H2 | Frequent humidity | Staff kitchens, changing rooms | 70 to 85% RH |
| H3 | Immersion or run-off | Collective kitchens, collective showers | > 85% RH |
Kytom applies three principles during the audit:
- measure hygrometry over 7 days under normal use before deciding,
- distinguish usage peaks (lunchtime gathering in a kitchenette) from steady state,
- document the chosen classification in the priced bill of quantities for property-damage insurance traceability.
An inter-company restaurant seating 240 justifies H3 along the dishwashing run, but H1 remains sufficient for the dining room partitions, located more than 3 m from water sources.
When H3 is not justified (contrarian position). Contrary to the widespread practice in tertiary technical specifications, Kytom does not recommend systematic H3 for washrooms: below 6 regular users or for visitor washrooms without a shower, the extra cost never pays off over the term of a 3/6/9 lease, and H1 with a perimeter silicone joint covers the risk. Conversely, below 50 m² of total wet area, the H1/H2/H3 trade-off loses its relevance: it is better to choose H2 throughout to simplify installation and site supply.
Trade-off 2: 72 or 98 mm thickness, acoustics drive the decision, not moisture
For the architect: 98 mm is decided on acoustic performance, not on the green board. Partition thickness is not chosen on the moisture criterion alone. The coupling with acoustic performance often requires moving from 72 mm to 98 mm, at the interface with enclosed offices or meeting rooms.
Partitions between enclosed offices must reach Rw = 32 dB minimum, and Rw >= 38 dB at « executive » level. A 72/48 single-skin moisture-resistant partition plateaus around Rw = 38 dB; moving to 98/48 double-skin allows reaching 44 to 47 dB depending on the density of the chosen mineral wool.
Three recurring trade-off cases:
- kitchenette adjacent to open space: 72 mm H1 is sufficient, absorption is handled via a suspended ceiling αw 0.80,
- washrooms adjacent to a confidential meeting room: 98 mm H2 mandatory to reach 42 dB,
- collective kitchen against a circulation: 98 mm H3 double-skin, flexible perimeter joint.
The extra cost of 98 mm versus 72 mm remains marginal compared to the acoustic risk that cannot be recovered after handover.
When 98 mm is counterproductive. The doxa of « thicker = safer » underestimates the carbon cost and programmatic rigidity. For windowless technical rooms with no acoustic requirement (cleaning rooms, storage, accessible ducts), 98 mm brings no measurable benefit and increases the carbon footprint of the package. For partitions intended to be removed within 36 months, 72 mm remains the rational choice: removing a 98 mm double-skin moisture-resistant partition is noticeably more costly than a 72 mm one.
Trade-off 3: fixed wet core and MVHR coordination from the preliminary design stage (property investor reading)
For the property investor under a 3/6/9 lease: the flexibility of a floor plate depends on the moisture-resistant/standard mapping, not on total m². Moisture-resistant plasterboard complicates later modifications: specific fixings, treatment of waterproofing reworks, longer removal than for a standard board, a parameter to integrate from the preliminary design stage on 3/6/9 leases. Kytom integrates two requirements from the preliminary design stage to preserve flexibility, a structuring value on 3/6/9 leases.
- Mapping of movable vs fixed partitions: since wet areas are rarely relocated, concentrate moisture-resistant board on a fixed technical core (washroom block + kitchenette). Perimeter partitions remain standard and demountable.
- MVHR pre-equipment: the Labour Code (articles R4222-1 et seq.) requires 25 m³/h per occupant in an office, 30 m³/h per toilet cubicle. The MVHR network often passes through the moisture-resistant partition, requiring sleeves and flexible joints installed simultaneously.
A lack of MVHR coordination is one of the main causes of failures observed on moisture-resistant partitions, through internal condensation due to insufficient air renewal. The design and build methodology synchronises the partition, HVAC and waterproofing packages on a single schedule.
When the fixed wet core logic does not apply. On flex office floor plates < 400 m² with a single water point (a single kitchenette), the movable/fixed mapping loses its meaning: there are not enough wet areas to justify a core rationale. Conversely, beyond 5 planned annual reconfigurations, even standard partitions cease to be cost-effective compared to glazed movable partitions: moisture-resistant plasterboard is then maintained only on the strict washroom block.
Trade-off 4: prior moisture audit, a preliminary design deliverable for the architect
For the architect: the moisture audit is an enforceable preliminary design deliverable, not a design option. It structures all the previous trade-offs and conditions the compliance observed at 12 months: operations carried out without a prior audit systematically show more non-conformities. Four formalised steps, deliverables included in the design phase.
- Source diagnosis: direct splashing, vapour (cooking, showers), condensation on cold spots. Hygrometric measurement over 7 days.
- Zone mapping: distinction r
