# Moisture content is the PUR adhesive process variable

> Why 8–12% is the working window, how to measure moisture on the shop floor, and what wet or frozen timber does to a glue line that looks fine.

Updated: 2026-11-10 · Source: https://karmindadhesive.com/resources/pur-adhesive-moisture-content/

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

1. Write the moisture window into the bonding procedure, not just the purchase spec.
2. Measure after planing; surface readings on rough stock mislead.
3. Do not bond frozen timber, even with extra clamp time.
4. Break a sample joint weekly — it is the only direct check on the glue line.