Preservative treated battens vs thermally modified battens: a comparison guide
Russwood have launched a new cladding battening system featuring performance-enhancing, thermally treated Thermopine® battens as an environmentally-friendly, durable and dimensionally stable alternative to traditional preservative impregnated battens. In this blog Architectural Liaison Technician Dominykas Maurusaitis explores the benefits of the thermally modified battens compared to preservative treated battens.
Preservative impregnation vs thermal modification
Timber battens are used in the support structure of most timber clad buildings. Most timber battens tend to be made of low cost, low quality, and low durability softwoods. These are typically generic whitewood or redwood species which have been industrially treated under high pressure with preservative chemicals. The development of modern timber technologies now offers alternative methods to improve performance and durability of timber without sacrificing environmental benefits.
Pressure impregnated preservative treatments (such as ACQ and MCA) can be used to raise the durability performance of a low durability species to the desired level, and also allows for sapwood (the non-durable portion of the new-growth wood in the outer portion of a tree) to be treated and included in the timber section. Chemical impregnation by this means is effective, but there are a number of issues surrounding the use of pressure treated timber which we’ll take a look at in more detail in this blog.
Thermal modification of timber is a modern and environmentally friendly way of enhancing stability and durability of timber. The process involves heating the timber to over 200°C in the presence of water steam and high pressure. No chemicals are used in this process, avoiding some of the downsides associated with traditional preservative treatments.
Protective layer vs durability throughout the whole cross-section
Pressure impregnation does not penetrate and treat an entire piece of timber throughout its cross-section. Depending on various factors such as the species and its density, the protective layer (also known as the ‘cordon sanitaire’ layer) may only be a few millimetres thick around the surface of the timber. As a result, any post treatment work carried out on site (eg cutting, machining, boring holes) may compromise the protective envelope and leave the unprotected timber exposed to attack from insect or fungal invasion. Any such areas should then be re-treated, adding additional costs and labour time.
In contrast, the thermal modification process treats the whole thickness of the timber, not just the external envelope. Species such as pine (durability class 4 – slightly durable) can be raised to durability class 2 (durable) through Thermal Modification. The otherwise non-durable sapwood can also be included, as the process improves it to the same durability standard as the heartwood. This durability performance exceeds the service class requirements for support battens, and has a higher durability rating than a standard preservative treated battens. As the process affects the timber throughout its full thickness, any piece which is cut, machined or drilled on site is not subject to any increased risk of decay through insect or fungal attack.
Health and environment
Preservative treatments present significant health and environmental concerns.
- Chemical byproducts, liquid waste and airborne particulates from the initial treatment are potentially harmful.
- Sawdust from working with treated wood is hazardous (can lead to various health issues including cancers), and therefore respirators should be worn, and contact with skin and eyes should be minimized. Sawdust, cuttings and waste must be collected and disposed of as hazardous material.
- Most preservative treated timber products should not be used indoors, or where increased risk of toxicity exposure due to contact is of concern.
- When the timber reaches the end of its service life it must be disposed of as hazardous material. It should not be burned, as the smoke and ash are known to have high concentrations of toxic chemicals. Pressure treated waste can contaminate the soil and groundwater if buried.
Thermal Modification does not use any harmful chemicals during the treatment process. The modification makes changes in the cellular structure and chemical composition of the wood itself, and results in improved performance characteristics. This means that working with thermally modified Thermopine® poses no danger to people or the environment.
Movement vs stability
In some cases, pressure treated timber is supplied at moisture contents above that of the intended service conditions. Consequently, the timber will be susceptible to shrinkage, warping, and movement as it dries. This can lead to higher wastage on site, or undesirable movement in the cladding supported by the pressure treated battens.
Conversely, thermal modification dramatically reduces the ability of timber to absorb and retain water. Thermally modified Thermopine® is supplied at a low moisture content (approx. 8% +/- 2%), which it is the equilibrium moisture content of this product for exterior use in the UK. Consequently, thermally modified timber is very stable, and will not warp, shrink or swell when in service.
Fixings
Pressure treated timber is very corrosive to standard steel fixings, as the high presence of copper in the preservative chemicals leads to galvanic corrosion. Stainless steel can be used provided that it is a minimum grade of A2 (316), alternatively galvanized coatings can be used to give protection for the desired service.
Thermopine® does not contain any copper-based chemicals and therefore is less corrosive on the fixings. Nevertheless, we would recommend using screws with a corrosion-resistant coating, like our GoFix® MS II screws with SC 7®coating, and stainless-steel F-BohrFix® ZB Screws, both of which are included in the Russwood Battening System.