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Why Type X gypsum works
Type X works because the gypsum core is full of water that is part of the mineral, not a damp sponge. National Gypsum’s public explainer puts that chemically combined water at 21 percent of the board’s weight. Heat drives it off as steam. That delay is why codes use Type X behind a covering. It is not an ASTM E136 pass for the siding you see, and it is not a permit.
You are looking at a board, not a finished house. Steam leaving the core is the delay. Glass fibers in Type X keep that cooked core from falling apart. The building official still wants 5/8 inch exterior Type X behind the covering, not this diagram.
Why it matters
Where Oregon R327 is adopted, Type X behind the covering is how many drawings keep wood siding or a boxed soffit. If you think “drywall is fireproof,” you will spec interior 1/2 inch board and skip the inspection. The chemistry explains the delay. The Type X path is where the gypsum goes.
How calcination works
Gypsum is calcium sulfate dihydrate. Two water molecules sit on each calcium sulfate unit. National Gypsum:
- Chemically combined water is 21 percent of the weight.
- Heat releases that water as steam. The process is calcination.
- What remains is calcium sulfate hemihydrate, commonly called plaster of paris.
- Walls of plaster or gypsum board will not transmit heat in excess of 212 °F (the boiling point of water) until the gypsum in the board is completely calcined. Their explainer contrasts that with a fire-test furnace that can exceed 1,800 °F.
After the water is gone, the delay is over. This page does not invent a later chemistry.
Type X. The same explainer says Type X has special additives for increased fire resistance. Glass fibers, for example, are added to the slurry to reinforce the gypsum core as it calcines. That is why Type X is not generic 1/2 inch board.
ASTM C1396’s public abstract says gypsum board has a noncombustible core, essentially gypsum, with paper on the faces. The public page does not expose the Type X definition. Do not invent one from the abstract.
The Gypsum Association’s “Why Gypsum” page is the natural industry citation. It was geo-blocked from this session. R327 already names the Association’s Fire Resistance and Sound Control Design Manual for the one-hour path. Do not invent a GA file number.
The paths this chemistry does not replace
| Job | What decides it | Not this page |
|---|---|---|
| Where the gypsum goes | R327 Type X-behind-covering, or the one-hour manual path | A steam diagram |
| Visible siding class | Ignition-resistant versus noncombustible | 21 percent water |
| One-hour claim on an eave | Listed assembly, or FEMA’s labeled “typically” line | This exhibit |
| Interior drywall | Not R327’s exterior Type X sentence | A 1/2 inch bag |
FEMA P-737 says enclosing overhangs with 5/8-inch Type X typically meets a one-hour rating. Labeled best practice.
What people mix up
- Treating Type X as noncombustible siding.
- Specing interior drywall for the R327 delay.
- Reading 212 °F as a forever rating.
- Inventing a Gypsum Association design number from this explainer.
- Putting this chemistry on a Craftsman tail as a fifth numbered eave item. It is not.
How to check
- If you claim Type X under R327, you still need 5/8 inch exterior gypsum sheathing behind the covering. See Type X behind the covering.
- If you claim a one-hour assembly, name the manual design the project uses. Do not point at this diagram.
- Do not write “gypsum is fireproof” on a drawing.
Limits
This page does not host ASTM C1396. It does not rate an assembly. It does not replace the Type X detailing guide. Gypsum Association gypsum.org was not reachable this session. Manufacturer explainers are not a listing.
FireApproved is not a listing body and not the AHJ.
Related records
Questions
Does the 21 percent water make the siding noncombustible?
No. The chemically combined water is in the gypsum core. Type X behind the covering is a deemed-to-comply assembly path under Oregon R327. It is not an ASTM E136 pass for the board you see.
Will the wall stay at 212 °F forever?
No. National Gypsum’s public explainer says walls of plaster or gypsum board will not transmit heat in excess of 212 °F until the gypsum in the board is completely calcined. After that, the delay is over. The 1,800 °F furnace figure in that explainer is a fire-test furnace, not your wildfire.
Is Type X the same as a one-hour assembly?
Not by this chemistry page. R327 names 5/8 inch Type X exterior gypsum sheathing behind the covering as one path, and a one-hour exterior-wall assembly from the Gypsum Association manual as another. FEMA P-737 says enclosing overhangs with 5/8-inch Type X typically meets a one-hour rating. That is FEMA best practice, not the Oregon wording.
Do I spec interior 1/2 inch drywall for this delay?
No. Where R327 uses Type X, it names 5/8 inch Type X exterior gypsum sheathing behind the covering. Interior drywall is not that product. See Type X behind the covering.