MgO Board vs Gypsum Board vs Fiber Cement Board

MgO board sits between gypsum board and fiber cement board. It is a cement-like board that keeps most of its strength when wet and is lighter than fiber cement, and all three can be used in fire-rated walls. Its weak point is moisture: chloride-based MgO boards have sweated and corroded fixings in humid service. Gypsum board is the light, high-volume interior lining; fiber cement is the proven exterior board.

This comparison is for producers and investors deciding which board to make, using standards and independent tests.

MgO Board vs Gypsum Board vs Fiber Cement at a Glance

What Each Board Is Made Of

MgO board. Light-burned magnesium oxide reacts with a magnesium salt solution to form the binder. With magnesium chloride it forms magnesium oxychloride cement; with magnesium sulfate, magnesium oxysulfate. Fillers and glass-fiber mesh complete the board. On our reference design, the line buys light-burned MgO with 60–80% active MgO, makes up magnesium sulfate as a 15–25% solution and uses wood chips, bamboo powder or calcium carbonate powder as fillers. The difference between the two binders is the subject of our article on magnesium oxysulfate vs oxychloride board.

Gypsum board. Calcined gypsum (stucco) is mixed with water and additives, cast between two sheets of paper and left to set back into gypsum, CaSO₄·2H₂O, then dried. By its formula, gypsum is about 21% chemically combined water by mass, which matters in a fire. On our reference gypsum board line, a 12 mm board uses about 7.0 kg of stucco and 0.45 kg of facing paper per m².

Fiber cement board. Cement, fine silica or limestone and cellulose pulp are formed into thin layers on a sheet machine, stacked, often pressed, and cured either in air or in a steam autoclave. EN 12467 describes the sheets as consisting “essentially of cement or a calcium silicate formed by a chemical reaction of a siliceous and a calcareous material, reinforced by fibres”. Our fiber cement board manufacturing process article covers the steps.

Fire Performance Compared

All three boards are mineral-based and appear in fire-rated wall systems. They differ in what the facing, the fillers and the standard allow you to claim.

Gypsum board. The paper faces are combustible, but the core is not. Under EN 520, paper-faced plasterboard can be classified without further testing as A2-s1,d0 or B-s1,d0, depending on its thickness, core density, paper grammage and substrate (EN 520 Annex B, as reproduced in Rwanda’s DRS 632). In a fire, the chemically combined water is driven off slowly, which holds the temperature on the far side of the wall down while it lasts. For fire-resistance ratings, Type X board is formulated with special core additives; ASTM C1396 defines 5/8 in. Type X as board giving at least a one-hour rating when installed on both sides of load-bearing 2×4 wood studs at 16 in. centers (Gypsum Association). Our article on fire-resistant gypsum board explains how Type X and Type F are made.

Fiber cement board. The binder is inorganic. Facade sheets can declare reaction to fire A2-s1,d0; an Eternit facade sheet declaration to EN 12467 is one example. EN 12467 also contains a route for sheets that satisfy Class A1 without testing; check the current edition for its conditions.

MgO board. The magnesia binder is inorganic, but the board’s fire classification depends on the whole formula, including organic fillers such as wood chips, and on the test the market asks for. ICC 1125 includes surface-burning requirements among its criteria for MgO boards (International Code Council). Base any claim on a test report for your own formula; our article on MgO board fire ratings explains the tests.

In short, gypsum board has the most codified fire record; MgO board has to prove its claims formula by formula.

Moisture and Water Resistance

This is where the three boards really differ.

Gypsum board. Standard board is an interior product for dry conditions. EN 520 defines boards with additives that reduce water absorption, graded by total water absorption of no more than 5% (H1), 10% (H2) or 25% (H3), with surface absorption limits of 180, 220 and 300 g/m² (EN 520, Table 5, as reproduced in DRS 632). Type E boards are made for sheathing external walls. Even H1 board is designed to resist moisture, not to stand in water; see our article on moisture-resistant gypsum board.

Fiber cement board. EN 12467 sorts sheets into four categories by exposure. Category A sheets are “intended for applications where they may be subjected to heat, high moisture and severe frost”, which in practice means exposed facades; Category C sheets are for internal use where they may meet heat and moisture but not frost (EN 12467, clauses 5.2.2–5.2.5). It has the longest record in exterior exposure.

MgO board. MgO boards absorb water, and the binder and formula decide how much and what happens next. BRANZ tested four boards, two high-chloride and two low-chloride with a sulfate-type binder: all four gained 27–39% in mass in a soak of just over 48 hours and none met the warm-water soaking criterion, yet all four met the PAS 670:2021 criterion for bending strength retention after 90 days at 30 °C and 90% RH (BRANZ, 2023). In a Technical University of Denmark study, a chloride board showed its first droplets at 89% relative humidity and two sulfate boards at 95%, by which point they had gained about 60% in mass (Nielsen et al., MATEC Web of Conferences, 2019). On our reference design, sulfate boards reach a softening coefficient of up to 0.9, keeping about 90% of their strength after long immersion.

The Canadian Construction Materials Centre advises that MgO products it has not evaluated should not be used where humidity exceeds 84% RH (CCMC advisory, 2018). Read together, the evidence makes MgO board a candidate for damp interiors and substrates where its formula has passed humidity testing, not a drop-in replacement for fiber cement on exposed facades.

Strength, Weight and Handling

The three standards do not measure strength the same way, so compare classes, not raw numbers.

• Gypsum board. EN 520 specifies breaking loads for each board. For 12.5 mm Type A board, at least 550 N longitudinally and 210 N transversely; for the higher-strength Type R, 725 N and 300 N (EN 520, Tables 2 and 3, as reproduced in DRS 632).
• Fiber cement board. EN 12467 sorts sheets into five classes by bending strength, measured as modulus of rupture; facade sheets are usually in the higher classes.
• MgO board. PAS 670 and ICC 1125 both include flexural strength requirements. High-density oxychloride board is somewhat tougher in bending, which keeps it in use for heavy hanging loads; sulfate board keeps more strength in water.

Weight per m² follows density and thickness. As an illustrative calculation at 12 mm:

Gypsum board is by far the lightest, a large part of why it dominates interior partitions and ceilings. MgO board is lighter than fiber cement at the same thickness.

The Risks Each Board Carries

MgO board: sweating and corrosion

The best documented MgO failures involve oxychloride boards used as wind barriers in humid climates: excess magnesium chloride draws water from the air until the board weeps salty water, which attacks steel studs and screws. In 2015 the Danish housing ministry concluded that the boards “are unsuitable for use as wind barriers in the Danish climate”. Estimates of the cost range from roughly DKK 800 million to over DKK 1 billion, depending on the source: the ministry put replacement in social housing at about DKK 800 million, with large uncertainty, and The Copenhagen Post reported possible damage of over a billion kroner (Danish housing ministry memo, 2015; The Copenhagen Post, 2015).

In BRANZ’s 90-day humidity exposure, plated screws corroded in the two higher-chloride boards, while the low-chloride boards caused no corrosion and stainless steel 304 and 316 screws were unaffected in all four. Sulfate boards have much lower chloride than oxychloride boards; check chloride content and humidity test results for the specific board. Our article on MgO board problems covers the causes and the fixes.

In the UK, the housing warranty providers NHBC, Premier Guarantee and LABC stopped accepting MgO boards in 2020, and BSI published PAS 670 in 2021 (RICS, 2021). For an MgO producer, approvals and test reports come before market access in many export markets.

Gypsum board: water

Standard gypsum board loses strength when wet. The remedy is H-type or glass-mat board in damp areas, and correct detailing.

Fiber cement board: weight and dust

Fiber cement is heavy to handle, and cutting it releases respirable crystalline silica. The US OSHA construction standard sets specific dust controls for handheld power saws cutting fiber-cement board and a permissible exposure limit of 50 μg/m³ as an 8-hour time-weighted average (OSHA 29 CFR 1926.1153). Producers usually supply cutting and dust guidance with the board.

How the Production Lines Differ

For an investor, the three boards lead to three very different plants.

Three points stand out:

1. MgO board is the least thermal plant. There is no calcination, autoclave or gas-fired dryer on a standard line. The trade-off is curing space: output is limited by how many boards can cure at the same time, which our MgO board manufacturing process article explains.
2. Gypsum board is a volume business. A continuous line, a calciner and a large dryer only pay at millions of square meters a year. Our comparison of gypsum board vs cement board looks at that choice from the gypsum side.
3. MgO raw materials need the most attention. China mined 12.9 million of the world’s 21.1 million tonnes of magnesite in 2024, and Liaoning, its main producing province, cut the number of magnesite-mining enterprises from 114 in 2020 to 63 in 2024 as it consolidates the sector (USGS, 2026). Test the MgO you can actually buy, and qualify a second source.

Which to produce? If your market buys interior partitions and ceilings by the million square meters, and you have gypsum, gypsum board is hard to beat. If it buys exterior cladding and facades, fiber cement has the record. MgO board fits a market that wants a water-tolerant, fire-resistant interior board or substrate at moderate volume, provided you make sulfate board, test it to your buyers’ standard and sell it only for the uses it has passed.

FAQ

Is MgO board better than gypsum board?

For damp interiors and as a substrate it can be: on our reference design, sulfate MgO board keeps up to about 90% of its strength after long immersion, while standard gypsum board is made for dry conditions. Gypsum board is much lighter, is made at higher volumes and has a long codified fire record. MgO performance depends on binder and formula, so it needs test reports to PAS 670, ICC 1125 or the buyer’s standard.

Is MgO board better than fiber cement board?

They overlap as cement-like boards, but fiber cement has the longer record in exposed exterior use, with EN 12467 Category A sheets made for heat, high moisture and severe frost. MgO board is lighter at the same thickness, but independent tests found MgO boards absorbing a lot of water in soaking and humid-air tests. Use MgO outdoors only where the formula has been tested for that exposure.

Which board is the most fire-resistant?

All three are used in fire-rated walls, and fire resistance is a property of the tested wall system, not the board alone. Gypsum board has the most established route, with Type X board defined in ASTM C1396 and EN 520 boards classified without further testing. Fiber cement often declares A2-s1,d0. MgO board claims must be backed by a test report for that formula.

Does MgO board corrode screws?

High-chloride boards can. BRANZ found plated screws corroded in boards with about 10% chlorine after 90 days at 30 °C and 90% RH, while low-chloride boards caused no significant corrosion and stainless steel screws were unaffected. Sulfate boards have much lower chloride than oxychloride boards; still check chloride content and humidity test results, and specify stainless fasteners where boards meet humidity.

Which board plant needs the least process equipment?

An MgO board line: it buys its binder in, forms boards on templates and cures them at ambient temperature, with no calcination, autoclave or steam boiler. A gypsum board plant needs a calciner, a continuous line and a large gas-fired dryer; a fiber cement plant needs a sheet machine, usually a press, and an autoclave and boiler if it autoclaves. MgO plants need large curing areas instead.

Planning an MgO Board Plant?

If the comparison points to MgO board, the next questions are which magnesium oxide you can buy and which standard your buyers test to. Our MgO board production line page covers the equipment, and the MgO board plant setup guide walks through planning. If your market points to the other boards, see our gypsum board production line and calcium silicate and fiber cement board production lines.

Sources

• Rwanda Standards Board, DRS 632:2026 (draft), Gypsum plasterboards — Definitions, requirements and test methods, prepared with reference to BS EN 520:2004+A1:2009: https://www.rsb.gov.rw/fileadmin/user_upload/DRS_632_2026.pdf
• Gypsum Association, “What is a Type X gypsum panel?”: https://gypsum.org/what-is-a-type-x-gypsum-panel/
• Knauf UAE, Regular (RG) 12.5 mm gypsum board product data: https://knauf.com/en-AE/p/product/regular-rg-12.5mm-gypsum-board-enbs-10003_0606
• iTeh Standards, SIST EN 12467:2013+A2:2018 (EN 12467:2012+A2:2018), catalog entry and sample pages: https://iteh.es/catalog/standards/sist/e6248861-5e94-4617-a934-1a3c85c01195/sist-en-12467-2013a2-2018
• Eternit, declaration of conformity ET0206 (2011), facade sheets to DIN EN 12467:2006-12: https://www.specifiedby.com/equitone-by-etex/equitone-natura-fibre-cement-cladding-decorative-rainscreen-facade-material/declaration-of-performance_6eaf1744.pdf
• BSI, PAS 670:2021, Magnesium oxide-based boards for use in buildings. Specification: https://knowledge.bsigroup.com/products/magnesium-oxide-based-boards-for-use-in-buildings-specification
• International Code Council, ICC 1125-2026: https://shop.iccsafe.org/icc-1125-2026-standard-for-classification-of-magnesium-oxide-board-and-construction.html
• BRANZ, Research Now: Materials #3 — MgO boards (April 2023): https://www.branz.co.nz/documents/4290/BRANZ_RESEARCH_now_Materials_3_MGO_final.pdf
• S. W. Nielsen, C. Rode, T. Bunch-Nielsen et al., “Properties of magnesium oxide boards used as sheathing in exterior walls,” MATEC Web of Conferences 282, 02091 (2019): https://www.matec-conferences.org/articles/matecconf/pdf/2019/31/matecconf_cesbp2019_02091.pdf
• Canadian Construction Materials Centre, MgO Board Advisory Update (1 March 2018), as published by the Saskatchewan Building Officials Association: https://sboa.sk.ca/ccmc-mgo-board-advisory-update/
• Danish Ministry of Immigration, Integration and Housing, Orientering vedr. MgO-pladesagen i det almene byggeri (memo, 1 October 2015), Folketinget UUI Alm.del Bilag 11: https://ft.dk/samling/20151/almdel/UUI/bilag/11/1554986.pdf
• The Copenhagen Post, “Building sector fears fallout from fungus-forming facade panels” (14 August 2015): https://cphpost.dk/2015-08-14/news/building-sector-fears-fallout-from-fungus-forming-facade-panels
• RICS Built Environment Journal, “Addressing concerns with magnesium oxide boards” (6 July 2021): https://ww3.rics.org/uk/en/journals/built-environment-journal/addressing-concerns-with-magnesium-oxide-boards.html
• US Occupational Safety and Health Administration, 29 CFR 1926.1153, Respirable crystalline silica (construction): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.1153
• US Geological Survey, Mineral Commodity Summaries 2026: Magnesium Compounds: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-magnesium-compounds.pdf