Two boards carry most of the world's interior walls and ceilings: calcium silicate board and gypsum board. They look similar and are priced close enough to invite substitution at tender time — but they are not interchangeable. This guide compares them across eight performance dimensions and finishes with a decision matrix, so you match the board to the zone instead of the price list. Weighing the fiber cement family? Our fiber cement vs gypsum board comparison covers that line.
What Is a Calcium Silicate Board?
Calcium silicate board is an inorganic flat sheet made from siliceous and calcareous raw materials, reinforced with mineral or plant fibres, and autoclave-cured under high pressure and temperature. The hydrothermal process forms a stable crystalline matrix — which is why the finished board does not burn, feed mould, or release formaldehyde. In our calcium silicate board range, density spans roughly 0.65–1.75 g/cm³, with the low-density 0.85 series the flagship for interior walls, ceilings and partitions. Standard sizes are 1220×2440 mm and 1220×3050 mm, 4–25 mm thick.
Gypsum board is a different material: a gypsum core faced with paper, inexpensive, quick to finish, and code-accepted in dry interior zones. This comparison is not about declaring one board "better" — it is about knowing where the paper-faced product stops meeting the specification.
Eight-Point Head-to-Head
| Property | Calcium Silicate Board | Gypsum Board |
|---|---|---|
| Fire reaction | A1 non-combustible (GB 8624); no melt or drip at 1000°C; smoke toxicity AQ1 | Paper facing ignites; core calcines and loses integrity |
| Moisture behaviour | Wet-expansion ≤0.25%; 30-day immersion without mould or delamination | Absorbs moisture; sags, softens, powders |
| Formaldehyde | Not detected (ENF); French A+; zero asbestos | Core is low-emission but paper and joint compounds add risk |
| Density | 0.65–1.75 g/cm³ (0.85 flagship) | ~0.7–0.9 g/cm³ |
| Screw holding | ≥1200 N; one screw carries 15–20 kg | Weak pull-out; heavy fittings need backing or anchors |
| Surface prep | Flat, no skim coat — finish applied directly | Jointing, skim coat and sanding typical |
| Wet zones | Suitable — bathroom, kitchen, basement | Not recommended |
| Rated assemblies | 0.5–3 h fire-rated systems | 0.5–1 h typical with linings |
Fire Safety: The Divergence That Decides
Fire is where the two materials separate most sharply. Calcium silicate board is classified A1 non-combustible to GB 8624: at 1000°C it does not burn, melt or drip, and its smoke is AQ1 (safe level). A gypsum board's core is nominally non-combustible, but the facing paper ignites early and the calcined core loses integrity, so it is relied on only within its rated system build-up.
For escape routes, lift lobbies, kitchen extract shafts and building-services corridors, that difference is the difference between a compliant wall and a re-design. The short test clip below shows an A1 board under direct flame — no ignition, no dripping, no smoke plume.
Rated performance depends on thickness and build-up: 8 mm board on 75 mm steel studs with rock wool reaches about 1 hour, 9 mm about 2 hours and 12 mm up to 3 hours in our tested assemblies. Where the code demands a specific rating, our certification reading guide explains how to verify the paperwork.
Moisture & Humidity: Where Gypsum Gives Up
Gypsum board's well-documented weakness is water: persistent humidity makes it sag on ceilings, soften at wall bases, and delaminate at the paper-to-core bond. Calcium silicate board is different chemistry — wet expansion ≤0.25%, dry shrinkage ≤0.15%, and a fully inorganic matrix that gives mould nothing to feed on. A 30-day immersion test produces no mould and no delamination.
That is why the board is a routine specification for bathroom and kitchen wet areas, ceiling systems in humid zones, and coastal or basement projects where seasonal condensation is a fact of life. For splashbacks and cooker walls, our waterproof kitchen wall panel guide shows how the same chemistry handles daily grease, steam and cleaning chemicals.
Screw Holding & Impact Resistance
Fastener pull-out is the hidden gap. A single screw in 12 mm calcium silicate board holds at least 1200 N — enough to carry 15–20 kg directly on the surface. Gypsum's friable core gives screws little to grip, so heavy cabinets, mirrors and handrails need anchors or double-layer construction. Denser calcium silicate grades also resist trolley and door impacts that dent paper-faced sheets in corridors and lobbies.
Installation & Labour: Fewer Steps Per Square Metre
Gypsum board's advantage is that every tradesperson knows it. Calcium silicate board's advantage is finishing in fewer steps: the surface is flat off the rack, so decoration goes on directly — no skim coat, no joint-compound sanding, no drying wait. Thin sheets cut with a utility knife, 6 mm material bends for curved reveals, and a 6 mm sheet weighs about 18 kg.
For dry, sequenced fit-outs — especially prefabricated and modular construction, where panels are cut off-site — the eliminated wet trade compresses the schedule. Our installation guide walks through the five-step method used on our tested systems.
Indoor Air Quality & Emissions
Fully inorganic boards bring a documentation trail that matters in schools, hospitals and homes: formaldehyde not detected at ENF grade, TVOC ≤0.06 mg/m³, zero asbestos, benzene and heavy metals. The range holds French A+ indoor-air certification, CE marking, China green building material certification, and REACH registration for Europe.
None of this means gypsum board is "dirty" — modern gypsum cores are low-emission. It means the paper, joint compounds and adhesives in a gypsum system each add their own emissions budget, while an all-inorganic system removes them from the conversation. Projects chasing green-building points can see the full audit trail in our fire safety and green certification guide.
Lifecycle Cost, Not Sticker Price
It would be dishonest to pretend the sheets cost the same: calcium silicate carries a higher per-sheet price than standard gypsum, and in a dry, code-simple interior gypsum remains an economical choice. The lifecycle question is what the wall costs over 20–30 years:
- Labour: no skim coat and no sanding trade removes a day from a typical floor.
- Rework: mould, sag, and fire-upgrade callbacks in wet or rated zones disappear.
- Durability: the inorganic matrix does not rot, off-gas, or degrade in humidity.
- Compliance: A1 + CE + French A+ de-risk approvals in fire- and air-quality-regulated markets.
Where a project carries fire ratings, moisture exposure, or an air-quality specification, the installed-cost gap narrows and often reverses. A full board selection guide across our product lines shows how to price the trade-offs properly.
When Each Board Makes Sense
Specify calcium silicate board for: fire-rated walls, shafts and escape routes; bathrooms, kitchens, basements and any persistent-humidity zone; schools, hospitals and homes with air-quality limits; high-traffic areas needing impact resistance; and prefab builds wanting a light, dry-installed substrate.
Gypsum board remains the practical choice for: standard dry interiors, low-occupancy spaces, cost-sensitive fit-outs, and curved or heavily-serviced ceilings where its ease of cutting and finishing is genuinely hard to beat. Many projects are a hybrid — gypsum in the dry core, calcium silicate at the wet edge and the fire boundary. For facade-grade alternatives, our fiber cement vs aluminium composite panel comparison covers exterior cladding.
Quick Decision Matrix
- Fire-rated / escape route → Calcium silicate (A1)
- Bathroom, kitchen, basement → Calcium silicate
- Schools, hospitals, residential → Calcium silicate (ENF, French A+)
- Dry standard office or bedroom → Gypsum board
- Impact-heavy lobby or corridor → Denser calcium silicate grade
FAQ
Is calcium silicate board waterproof?
Not fully waterproof, but highly moisture-resistant: wet expansion ≤0.25% and 30-day immersion without mould or delamination — why it is specified in bathrooms, kitchens and basements where gypsum fails.
What fire rating does calcium silicate board have?
A1 non-combustible to GB 8624: no burning, melting or dripping at 1000°C, with AQ1 smoke. Assemblies rate from 0.5 to 3 hours by thickness and build-up.
Can calcium silicate board replace gypsum board in wet areas?
Yes, for interior wet and humid zones. The inorganic matrix gives mould nothing to feed on, while gypsum softens and sags under persistent moisture.
Does calcium silicate board contain formaldehyde or asbestos?
No. Fully inorganic, zero asbestos, benzene and heavy metals; formaldehyde not detected at ENF grade; French A+, CE and green building certifications.
Is calcium silicate board more expensive than gypsum board?
Per sheet, usually yes — but installed and lifecycle cost can reverse the gap once skim coats, moisture rework and fire-upgrade callbacks are priced in.
