wood · process
Moisture content is the PUR adhesive process variable
Updated 2026-11-10
PUR cures by reacting with water. That makes moisture content the single most important process variable in a glulam plant — more important than clamp pressure, more important than spread rate, and much more important than the brand on the drum.
The working window
Structural timber is normally bonded at 8–12% moisture content. Inside that window, the reaction proceeds at a rate the glue line can handle. Outside it:
- Above ~15%: CO₂ is produced faster than the line can release it; the bond foams and weakens.
- Frozen timber: water is unavailable; cure stalls and the joint opens when pressure is released.
- Below ~6%: too little water for a clean cure; bond strength drops and clamp times stretch.
Measuring it
A pin-type moisture meter reads the surface layer, which is where the planer and the kiln leave a gradient. Take readings after planing, on freshly exposed faces, and average several points per board. A meter calibrated for the species is worth the cost.
What a bad bond looks like
| Symptom | Likely cause |
|---|---|
| Thick, crumbly, foamed glue line | Wet or frozen timber |
| Thin, starved line | Insufficient spread or open time exceeded |
| Joint opens after clamping | Cure incomplete; moisture or temperature |
| Good line, fails in shear | Contaminated surface or wrong species preparation |
Moisture is not uniform, even after kilning
A kiln-dried board is drier at the surface than in the core. After planing, the exposed face can read 8% while the centre is at 12%, and the meter reads the face.
Three rules follow:
- Measure after planing, on a freshly exposed face, and let the board equilibrate for a few minutes first.
- Take several readings per board and use the highest, not the average — the wettest part of the section drives the foaming risk.
- Re-check after machining to final thickness, because the moisture gradient moves as material is removed.
| Species group | Typical equilibrium MC, heated building | Note |
|---|---|---|
| Softwood (spruce, pine) | 8–10% | The glulam standard case |
| Dense hardwood (oak) | 9–11% | Slower to equilibrate |
| Tropical hardwood | 10–12% | Higher EMC at the same RH |
The window in the specification is a service-condition window, not a manufacturing convenience. Bonding at 14% because the stock arrived wet means the joint dries in service and shrinks away from the glue line — a failure that appears months later and is blamed on the adhesive.
Two percent of moisture is a dimensional change
Wood moves as it dries: roughly 0.25–0.3% linear change per 1% moisture change in the tangential direction. On a 45 mm lamella, a 2% MC difference between the joint face and the core is about 0.2 mm of movement — the same order as the glue line itself.
That is why the moisture window is narrow and why the failure is delayed. A joint bonded at 14% and dried in service does not fail at the press; it fails six months later when the timber has shrunk away from a glue line that was sound when it was made.
What this means for buyers
- Write the moisture window into the bonding procedure, not just the purchase spec.
- Measure after planing; surface readings on rough stock mislead.
- Do not bond frozen timber, even with extra clamp time.
- Break a sample joint weekly — it is the only direct check on the glue line.