How VPI Treatment Works on Different Timber Species

Why Species Matters for VPI Treatment

VPI treatment forces preservative solution into timber under pressure — but how deeply and how uniformly that preservative penetrates depends on the anatomical structure of the wood. Different species have different vessel sizes, different pit structures, and different ratios of permeable sapwood to less permeable heartwood. These differences directly affect treatment outcome.

Understanding timber treatability is important for two reasons: it tells you whether VPI treatment will achieve adequate penetration for your application, and it tells you what moisture content and preparation conditions are needed to get the best result from the treatment cycle.

Sapwood vs Heartwood: The Most Important Distinction

In almost all species, the sapwood — the outer, younger rings of the log — is significantly more permeable to preservative solution than the heartwood. Sapwood cells are living or recently dead, their pits are still open, and they absorb liquids readily. Heartwood cells are older, their pit membranes are aspirated or encrusted with extractives, and they resist liquid penetration.

This means that the treatability of a piece of timber depends heavily on the proportion of sapwood to heartwood in the cross-section. A board cut from the outer part of a log, predominantly sapwood, will be highly treatable. A board cut from the centre of a large log, predominantly heartwood, may show good penetration at the edges (where sapwood remains) and very limited penetration in the core.

For structural timber requiring full cross-section protection, specifying sapwood-rich material or using smaller cross-sections that are more easily penetrated all the way through is important. The treatment record should show penetration depth and retention at both the surface and the core to confirm that adequate treatment has been achieved throughout.

• Sapwood: highly permeable — absorbs preservative readily
• Heartwood: much lower permeability — resists penetration in many species
• Full penetration is easier to achieve in smaller cross-sections
• Species with high heartwood-to-sapwood ratio need larger cross-sections treated in shorter lengths
• Treatment records should confirm penetration at the core, not just the surface

How Common Sri Lankan Species Respond to VPI Treatment

Rubberwood is one of the most treatable timber species. Its wide, open vessels and high sapwood content allow preservative to penetrate rapidly and uniformly throughout the cross-section. Treatment times are shorter than for most other species, and penetration to the core is consistently achieved under standard VPI conditions. This treatability, combined with its susceptibility to beetles, makes rubberwood the species for which VPI treatment delivers the clearest return.

Mahogany (Swietenia macrophylla and Khaya species) is moderately treatable. It has smaller vessels than rubberwood and a more pronounced heartwood zone in mature logs. Penetration into sapwood is good; heartwood penetration is variable and depends on the log age and the individual piece. For large-section mahogany structural members, confirming core penetration through sample cross-sections is advisable.

Teak is the most difficult common species to treat. Its heartwood contains natural oils (tectoquinone and other extractives) that fill the vessel walls and actively repel water-borne preservatives. Surface penetration in teak heartwood under standard VPI conditions may be only a few millimetres. However, teak heartwood has high natural durability — it does not need VPI treatment for most applications. The sapwood of teak is far less durable and should be treated; fortunately, teak sapwood is also more permeable.

Pine (imported softwood species) is generally highly treatable. Softwood tracheids provide relatively open pathways for preservative solution, and pine absorbs Boron Borax readily under standard treatment conditions. The main variable in pine treatability is resin content — highly resinous heartwood may resist penetration, but this is less relevant for the common construction pine species used in Sri Lanka.

How Moisture Content Affects Treatability

The moisture content of the timber at the time of VPI treatment directly affects how much preservative can be absorbed. Wood that is already saturated with water has no space for additional liquid — the preservative solution cannot displace the water already occupying the cell lumens. This is why kiln drying before VPI treatment is not just good practice but a technical requirement for effective treatment.

Timber kiln-dried to 12–18% MC before VPI treatment has most of its free water removed. The vacuum stage of the VPI cycle removes residual air and any remaining surface moisture, and the pressure stage forces preservative into the now-empty cell lumens. Treatment on kiln-dried timber achieves higher retention levels and more uniform penetration than treatment on green or partially dried timber — significantly so for species with inherently lower treatability.

Reading a Treatment Record

A treatment record from a VPI facility should specify the species treated, the cross-section dimensions, the moisture content before treatment, the vacuum duration and level, the preservative concentration and volume absorbed, the pressure level and duration, and the final retention figure. The retention figure — expressed in kg of preservative per cubic metre of timber — is the key number: it tells you how much Boron Borax is now in the wood.

Minimum retention requirements vary by application and are specified in the relevant standards. Your treatment provider should be able to confirm that the batch meets the retention level required for your specific application and exposure class.