Cornwall’s rich history is reflected in its heritage buildings, many of which stand as a testament to the region’s architectural and cultural legacy. Unfortunately, these venerable structures are not immune to the threat of woodworm infestations, which can compromise the integrity of the woodwork. In this guide, we’ll explore specialised methods for treating woodworm in Cornwall’s heritage buildings, emphasising preservation and respect for the unique character of these historical treasures.

Before delving into treatment methods, we need to discus a trending myth within the heritage conservation sector, and that is, that woodworm can be eradicated by simply reducing the moisture content within the affected timbers to below 18%. However, I find active Common Furniture Beetle (Anobium punctatum) infestations within timbers as low as 9-12% (moisture content) on a weekly basis. And when we consider that on average, well ventilate loft space timbers will typically range from 10-12%, it is impossible to achieve a level of dryness that would lead to the eradication of these wood boring beetles in a holistic manner. As an example to somewhat evidence this statement, kiln dried firewood typically has a moisture content of 10-15%!

As much I want this myth of a chemical free process to be true, sadly, this story was started by a group who dislike damp proofers and the Property Care Association, which in part, I truly understand. Reducing the amount of moisture within the timber will slow the infestation or make it less prone to attack, it will not however completely eradicate a wood boring beetle infestation.

A timber rafter with active Common Furniture Beetle (Anobium punctatum), with the timber moisture level reading just 9%, which is extremely dry!

The first question to ask: is the infestation active?

Before diving into treatment methods, it’s crucial to identify whether you’re dealing with an active or historic infestation?

Small round exit (flight) holes in the wood can evidence both active and historic wood boring beetle activity, where as holes accompanied by fine, powdery frass (wood dust) around the affected area can evidence an active infestation.

Measuring timber moisture

Reducing the moisture levels within timber will undoubtably reduce the impact of an established infestation and indeed, make a timber less prone to attack to not just wood boring beetles but also other timber predators, such as wet and dry rot.

Using a pinned moisture meter (as pictured) we typically expect timber moisture levels to be:

Skirting boards on inner or partition walls – 9-10%
Skirting boards on outer walls – 10-12%
Roof rafters in loft spaces – 10-12%

Measures of 13-16% evidence increased moisture levels and suggest the requirement for low level / non invasive investigation and monitoring.

Measurements of 17%+ require immediate invasive investigation as this level of moisture signals a building defect of some sort and greatly increases the risk of fungus or wood boring beetle attack.

Historic England’s guide to measuring moisture in building materials LINK

A timber rafter with extensive Common Furniture Beetle (Anobium punctatum) activity, with the timber moisture level reading just 11%.

This images is an extreme example, it shows a bathroom vent plumbed into a PIV unit, blowing steam half into the loft space and half down onto the landing, which was then working its way back up through a poorly sealed loft hatch. This sadly led to a very wet loft space with both wood boring beetle and black mould infestations.

Identifying building defects that cause wood boring beetle outbreaks within loft spaces

It is essential that you address the cause of any outbreak as ultimately, any infestation is evidence of a building defect that will also be presenting a threat to structural timbers and internal finishes. By far the most common area within a Cornish home to develop a wood boring beetle outbreak is the loft space, and this can be for a number of reasons.

1: High humidity within the rooms below due to the lack of or faulty kitchen and bathroom fans.

2: Incorrectly installed bathroom fans venting into the loft space, unbelievably common!

3: A poorly sealed loft hatch will allow warm air into the attic space, which can condensate on cold roof timbers.

4: A lack of ventilation within the roof structure combined with the installation of non-breathable bitumen felt. The insertion of roof tiles vents can in part resolve this issue.

5: A roof leak from a broken tile or slate, or perhaps a defective lead valley.

Note 1: Any moisture ingress or humidity issues should be dealt with at their source, once rectified, other measures can be taken to support air flow etc.

Note 2: The installation of a breathable (vapour open) roofing membrane does improve vapour permeability through the roof build-up, however, it does not improve air flow, which is an essential component in the process of keeping the moisture levels of your roof timbers within a healthy range.

Identifying building defects that cause wood boring beetle outbreaks in suspended timber floors

Building defects below suspended timber floors that can support woodworm present in a number of ways, which are usually related to some form of humidity or water ingress issue. A client will typically report a rippled effect across their timber floor, some ‘bounciness’ under foot or will be able to identify areas of the floor that have visibly sunk. An invasive inspection via the removal of floor boards is almost always required to assess the sub-floor void and evidence the cause of an outbreak.

The most common culprits for high humidity or water ingress are blocked or missing sub-floor ventilation ducts, escapes of water from plumbing, heating or drainage systems, downpipes discharging to the ground and high external ground levels.

A rippled or swelling timber floor strongly suggests that there is high humidity within the sub-floor void.

Cornwall’s top 5 wood boring beetles

1: Common Furniture Beetle (Anobium Punctatum):
This is the most common of the woodworm beetles found in both Cornwall and the UK as a whole. Despite its name it is just as likely to attack building timbers as it is to attack furniture. The Common Furniture Beetles leaves small round holes approximately 1.5 to 2mm diameter and the bore-dust under a microscope shows up as lemon shaped pellets that are slightly gritty to touch. The adult beetle is between 3 to 5mm long and it looks like it has a monks type hood over its head.

Each female Common Furniture Beetle on average lays between 20 to 60 eggs which are laid in cracks or within the flight holes. Within 2 to 3 weeks the larva hatch and immediate start to tunnel through the timber. The Larval stage can last from between 3 and 4 years and is spent entirely in the wood. The larvae are greyish white in colour with a narrow but dark band over their mouth Bore holes of 1-2mm in diameter with gritty, cream coloured bore dust.

2: Wood-Boring Weevil (Euophryum confine):
Wood Boring Weevils can be found in hardwoods, plywoods and softwoods all of which have had past or current fungal activity. The flight holes are generally irregularly oval to slit-shaped with jagged edges. Tunnels formed by the larvae and adults are circular about 1/16” in diameter generally straight and run along the grain of the wood near to the surface. The Wood Boring Weevils feed on damp and decaying wood but will also continue to eat dry wood that has previously been attacked by fungus.

The female lays eggs just below the surface of the wood in either existing cracks or in the flight holes. The eggs take approximately 16 days to hatch and the lava pupates in 6 to 8 months after hatching. The pupal stage lasts for 16 days and usually occurs between the months of June and October. The adult will continue to tunnel throughout its lifetime. The Wood Boring Weevil will not normally colonize dry wood.

3: Deathwatch Beetle (Xestobium rufovillosum):
The Death Watch Beetle (Xestobium Rufovillosum) is an indigenous British Insect that infects large hardwood timbers such as oak, ash and also horse chestnut. This Beetle is also attracted to timbers which have suffered some kind of fungal decay such as Wet Rot (Coniophora Puteana).

The Death Watch Beetle can be quite hard to both detect and treat as the larvae can live in the timber for up to 13 years boring holes of 2 to 3mm in the timbers, they leave behind a coarse frass/dust. It is more prevalent in older buildings and is known for producing a tapping or ticking sound, especially during the mating season (April to June).

4: Powderpost Beetle (Lyctus brunneus):
This beetle predominantly infests hardwoods, such as oak, beech, and ash. The larvae create tunnels in the wood, resulting in fine, powdery frass. Infestations are more common in newly processed timber. The powder-post beetle is active year-round, and loves buildings that are centrally heated. Tell-tale signs of a powder-post infestations are bore holes that are around 1.5mm in diameter, and a very fine bore dust that resembles powder.

5: House Longhorn Beetle (Hylotrupes bajulus):
Although less common than other woodworm species, the house longhorn beetle primarily infests softwood timbers, such as pine. It is more prevalent in the southern parts of the UK (has now been identified in Cornwall) and can cause significant structural damage. It has very large bore/flight holes at 3 to 7mm!

This image shows a historic infestation of Common Furniture Beetle (Anobium Punctatum) and an active infestation of Wood-Boring Weevil (Euophryum confine), the two most commonly found wood bring insects in Cornwall.

This image shows historic wood boring beetle activity. A full survey of the roof timbers was carried out, there were no building defects to report. Therefore, no treatments were required, which is always the best possible outcome for us, the homeowner and this beautifully old heritage building.

It is important to survey and potentially treat every timber in the effected area as the Common Furniture Beetle (Anobium punctatum) can fly once it has passed through it’s larvae stage, leaving the timber via its flight hole to lay its eggs within other suitable timber.

Treating woodworm and other wood boring beetle infestations

Wood boring beetle treatment is a broad and complex subject, this is because we need to consider the type of beetle infestation, the size of the infected area as well as the location, the structural integrity and importance of the affected timber and then consider the varying treatments that are available and their suitability in this circumstance.

It would be inappropriate to offer any form of guidance for timber of structural importance that may be compromised by wood boring beetles. In this instance, it is essential that you employ the services of a timber and damp specialist.

For small outbreaks effecting less than .25m2 (a square foot) that have been diagnosed as Common Furniture Beetle (Anobium punctatum), you may wish to treat the area yourself with an off-the-shelf product that contains either Permethrin or Boron. Please do ensure that you wear sufficient PPE (Personal Protective Equipment) to include goggles (EN374), gloves, a disposable chemical proof suit (Category III, Type 5-B and 6-B / EN14126 barrier to infective agents), waterproof footwear and a face mask with P3 (EN14387) filters.

Note 1: There is a very high risk of self contamination. Proceed with extreme caution when using high strength insecticides. Low levels of exposure to these chemicals can pose serious health risks to both humans, animals and aquatic life.

Note 2: For outbreaks coving an area greater than .25m2 we recommended you having any insecticide applications carried out by a trained professional who will conduct a more thorough treatment and minimise the risk of exposure to these chemicals within your home.

Please note: all advice given by Truscott Building Preservation is specific to granite, slate and lime constructed buildings located in Cornwall only. It is important to note that buildings constructed in areas with less severe weather patterns and softer masonry types WILL require the use of very different techniques and materials. Our advice is not generic to the whole of the UK, and we suggest that you find a local expert who can best advise on the correct techniques and materials required to preserve your heritage building.