Sunnybrae Cottage, Pitlochry

Context

This small vernacular highland cottage in Pitlochry has only recently been taken into care by Historic Scotland. It is a are survival of cruck truss construction and a heather thatch under a more recent corrugated iron roof.

Issue

The cottage has come with a fair amount of problems associated with dampness (high relative humidity). The most serious of these is a woodworm colonisation of the smaller pieces of timber in the roof.

Investigation

Three relative humidity and temperature monitors were installed over a period of 12 months. One in the roofspace where the woodworm is most active, one on the ground floor to compare conditions there and one outside to record just how cold the outside temperature is in this highland village.

The monitoring confirmed that the conditions in the roofspace were ideal for woodworm. Cold and damp conditions for most of the year allow the woodworm larvae enough moisture to survive. Sunny days in the summer warm the tin roof enough to give ideal conditions for the woodworm when they emerge as beetles.

The monitoring also showed that condensation was common in the building and that sometimes the temperature fell below zero – both issues cause significant damage.

Action Taken

Consideration is being given as to how to interpret the building in the future so any intervention had to be reversible. Presently the building remains something of a building site. Two Mitsubishi dehumidifiers were installed, both running of a single thermostat-humidistat (by Meaco) sited in the roofspace. The dehumidifiers are supplemented by two oil-free electric radiators that come on when the temperature is below 10°C. This increases the effective power of the dehumidifiers and protects the building against frost.

A chemical treatment for the woodworm would be justified here but the instruction from Historic Scotland was to not disturb even a cobweb in the roofspace so this treatment was put on hold.

Outcome

Relative humidity is being kept much lower than before with condensation eliminated. Frost is being kept out of the building, even when the outside temperature is very low. Relative humidity cannot be kept reliably below 65% (wood moisture equivalent of 15%, the threshold at which woodworm will thrive) at all times. This is because there are still small gaps in the roof awaiting repair. Sticky traps have been deployed and window cills monitored for the presence of adult beetles.