Clear Timber Intumescent Coatings – Things to consider

26thJuly 2021
By Ying'An
Clear Timber Intumescent Coatings – Things to consider

The use of prefabricated engineered timber in mid-rise, large-scale, and complex construction continues to gain momentum worldwide due to its sustainability/environmental credentials and cost efficiencies, including reduced construction timeline, reduced dead load, and improved safety overall.
Australia, a country that generally speaking, keeps track with the rest of the world, has similarly seen an increase in the use of Cross Laminated Timber (CLT) and Glue Laminated Timber (glulam) products that offer many superior options to traditional building materials like steel and concrete.
But, don’t take our word for it; even though CLT and Glulam are both used for structural timber design, there are material and characteristic differences. In this blog we investigate.

Cross-laminated timber

Cross-laminated timber (CLT) is an exclusive engineered wood product prefabricated utilizing several layers of kiln-dried timber (lamellas), with each layer glued together at 90-degree points on their broad faces. CLT Boards are regularly comprised of three, five, seven, or nine alternating layers of dimension lumber.
The alternating direction of the layers actually provides higher dimensional stability than stacking them in one direction, ensuring CLT has a high strength-to-weight ratio which has advantages for structural, fire, thermal, and acoustic performance. Like steel, CLT can be designed to different dimensions: Board thicknesses generally range between 100 to 300 mm (4 to 12 in), however, boards as thick as 500 mm (20 in) can be created. Board sizes range from 1.2 to 3 m (4 to 10 ft) in width and 5 to 19.5 m (16 to 64 ft) long. Most of the CLT panels are pre-fabricated and made easy for transportation based on regulations and guidelines

Glue-laminated timber

Glue-laminated timber(glulam) is made with different layers of solid wood lumber bonded with high-strength glue forming a solitary structural unit – and unlike CLT the layers are not cross-layed. Glulam usually has a high member capacity and can be utilized broadly in all types of structures.
To create a strong, dimensionally steady building material, glulam is machined and built to exact specifications. Sold in several standard widths and lengths, glulam columns can be tailored to meet almost any design requirements. And similar to steel, the construction design allows it to be “shaped” without losing its structural integrity, making glulam a material which is regularly utilized for large curved or arching members required in the construction of vaulted roofs, domes, and even bridges.
Glulam is robust and provides more strength and stiffness than sole lumbers. Steel dowels and plates bolted together are commonly specified for structural timber connections.

Key differences between CLT and Glulam

The direction of the Layers

As mentioned above, the largest difference lies in the fact that in Cross Laminated Timber the layers are cross-matched at 90 degrees, whereas Glulam timbers are made of separate layers with the grain lining up. But both are engineered timbers and are manufactured based on the design.

So why is this significant?

There are some key contrasts and reasons behind the two different products. Since CLT is made with alternating layers at 90 degrees, it has multi-directional strength, thus creates two-way traversing attributes similar to concrete slabs creating primary direction and secondary direction.
Since glulam is made with layers all orientated in the same direction, it is utilized most commonly for one-way traversing requirements like columns, beams and trusses. Regularly most timber projects use both glulam and CLT, with each employed to take advantage of their novel characteristics. Thus, glulam is generally utilised for columns and beams while, CLT is used for structural core, wall panels and floors.

Charring and protection

Timber burns – no surprises there, right! But before it burns completely, it develops a black outer layer known as Char. This char acts as an insulation in the first instance and can, slow down the rate of burn – thus the term fire resistance. Fire resistance is measured by the time elapsed from the conception of the fire up until the time where the material fails to work, commonly expressed in minutes, e.g. FRL 30, 45, 60 or 120.
The catch with char though is, despite protecting the timber from burning as fast as it may otherwise, the timber is still losing strength with the reduced cross-section area. There must be adequate virgin solid wood remaining behind the char layer to support the loads applied to attain the designed fire resistance period. To increase the life of the timbers under fire, each Glulam or CLT panel within the building can be treated with an intumescent coating which works to protect the timber by by slowing down the timber burning rate under the fire to a certain amount of time, thus leaving enough effective cross-section in place.

Considerations in using and protecting timber

The behavior in which the spread of flames is prevented and that in which penetration of fire is resisted relies upon the nature, form, and arrangement of the materials involved in the fire and on the property of the igniting fire.
In a fire test where the character of the igniting fire is controlled, the qualification between the two phases of fire performance is somewhat determinable. The words “fire-retarding” or “fire retardant” may be used for treatments that limit flaming performance.
Understanding this, there are two general methods are available to reduce the flaring attributes of wood through the use of fire-resistant protections.

Impregnation treatment

One technique comprises an impregnation treatment that applies water-borne synthetic compounds within the wood. Numerous synthetic compounds display fire-retarding properties, but because of cost or other objectionable characteristics, comparatively, few are considered generally practical.
The penetration of the treatment into the wood is typically achieved through vacuum-pressure techniques. Significant considerations include the depth of penetration and the quantity of chemicals deposited in the wood.

Use of intumescent paints

Another technique for controlling the flaming attributes of wood is to use appropriate coatings on the wood surfaces. All work however as a thin-film coating which preserves the natural beauty of the timber, whilst adding significant fire rating performance.
The advantage of these treatments is the decrease in the immediate flammability of the wood, minimizing its fuel contribution to a burning fire. The fundamental purposes of the coating will be accomplished if the spread of flame from a growing fire can be significantly slowed, if flaming can be diminished and even cease after the removal of the igniting source, and if the progress of the char into the wood can be delayed or kept under control.


Under sustained, severe fire exposure, there is no impregnation treatment or intumescent paint that will provide fire protection.


Structures that have already been erected are obviously unsuitable for pressure impregnation treatments. For best outcomes in new-build constructions, the lumbers should be cut to finished measurements before treatment with little to no mechanical interference after treatment.
As with specialist materials, whilst it may look easy to apply, ensuring the right thickness, adequate coverage and use of the right product are exceedingly important and should require the use of a qualified applicator.
The Aithon product is a waterborne intumescent and is not designed for external use. Certain maintenance schedules are required if exposed to water damage in interior environments.
If you would like any further information on the use of intumescent coatings for timber fire protection, please get in touch with us and we will provide you with solid solutions.

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