Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

be found in brick or stone walls, with

correspondingly complex, variable and

unpredictable physical properties and

interactions. Moreover, the original

wide palette of materials is often further

complicated by subsequent alterations

and additions, such as changes from

wattle and daub to brick infill, from lime

to cement render and from permeable

to impermeable finishes. Many of these

variations can occur within a single

elevation. By comparison, brick and stone

walls are relatively homogeneous and

predictable. As a result, desktop heat-loss

calculations using standard formulae

and computer modelling are less reliable

where walls are timber-framed. On-site

detailed physical investigation is required.

Sensitive infrared thermographic cameras

may be used to locate concealed timbers,

identify the make-up of infill panels and

assess heat loss and damp penetration.

Decay-detecting micro-drills are also very

useful for these investigations.

For such assessments to be of value,

the assessor must have a good working

knowledge of how the building was

constructed, what changes might have

occurred since, and the causes and extent

of any degradation.

Typically, timber-framed buildings

were built using freshly felled timber

that shrank and moved significantly,

particularly over the first 30 years.

The timbers were usually left exposed

externally and internally and the spaces

between the frame were filled with clay-

based daub, often finished with limewash.

Gaps that formed between the frame and

infill as the materials settled and shrank

were regularly filled and additional coats

This timber-framed house has retained its original eaves and verges, both

beautiful and practical. The house must always have been tiled.

The depth of thatch shelters the wall below. If the thatch were replaced with tile

or slate (as so much was) the wall becomes very vulnerable.

These rafters have been restored to their original

overhang, greatly improving protection of the wall below.

Timber frames are full of joints

and cracks through which air (and

water) can penetrate. The most

effective improvement that can be

made to the overall hygrothermal

performance is to fill these gaps. A

sensitive thermographic camera is the

best way of locating them, provided

there is a reasonable temperature

difference (5–10

C) between the inside

and outside, and preferably when the

wind is blowing. It is as important to

survey the outside of the building to

identify where heat is escaping, as it is

internally to identify where cold air is

entering. The survey should be repeated

when the remedial work has been

completed, but is almost meaningless

unless carried out in the same weather

conditions. Most timber-framed

buildings are too air-porous for standard

air pressure tests to be meaningful.

Optimum methods and materials

for gap filling will vary depending on the

size and location of the gaps, but should

always be flexible and breathable: sheep’s

wool pushed into the gap with a thin blade

and finished with haired lime plaster can

be very effective. Proprietary sealants,

mastics and cementitious mortars should

not be used.

There is a much greater variety of

constructional materials and details

in timber-framed walls than might