Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

impermeable materials are generally

damaging, if permeable materials are

reintroduced without reinstating the

original protection (such as overhangs),

their exposure to extensive and

persistent wetting will lead to fungal

degradation, loss of cohesion and frost

damage. Furthermore, heat loss through

persistently wet daub, render or brick

is much greater. Recent changes in

weather patterns may also create greater

problems for poorly protected buildings.

It is therefore an essential element of

any building upgrade (particularly for

timber-framed buildings) that adequate

overhangs and other protective measures

are re-introduced, even where the

evidence for them is inconclusive.

Another important issue is that

moisture content is critical and often

finely balanced. Typical ambient moisture

content of timber in a well maintained

building is around 16 per cent (lower

if heated). This tends to rise to around

18–20 per cent in well-maintained

external walls. Many fungi will germinate

at around 27 per cent, but can survive

down to 23–24 per cent. Deathwatch

beetle thrive where there is or has been

fungal activity and can survive in timber

down to 16 per cent moisture content

or lower. Controlling water penetration,

condensation and evaporation are

therefore critically important, and using

the wrong materials or details might

raise the moisture content by just a few

per cent and risk starting or re-starting

degradation. Equally, reintroducing the

right materials and detailing should

lower the moisture content by just

a few per cent into the safe zone.

SUMMARY

1 The decision whether to retrofit

insulation, and if so, which approach

to adopt, cannot be taken in

isolation. A detailed appraisal of

the building, including the historic

significance of the timber frame,

infill panels and other features,

as well as an accurate condition

assessment, must be carried out.

2 Most traditional timber-framed

buildings will be listed. There

should be discussion at an early

stage with the local conservation

officer about the problems identified

and proposed remedies.

3 Upgrading the hygrothermal

performance of timber-framed walls

by retrofitting insulation is very

difficult and can rarely be achieved

without significantly compromising

the historic significance and/or

appearance of the building. Any

potential benefits in terms of cost

saving, comfort and reduced carbon

emissions need to be weighed against

the initial cost, loss of historic

fabric and potential for further

degradation of historic fabric.

4 Where timber-framed walls retain

a high proportion of original or

historically significant fabric,

retrofitting insulation should

be considered a last resort and

only used when other potential

improvements have been explored.

5 Heat loss through the various

materials that make up a relatively

thin timber-framed wall is often

compounded by air leakage around

the edges of panels and through

joints in the frame. Minimising

uncontrolled air movement is

critical and will often prove more

effective and less damaging.

6 Alternative measures to upgrade the

overall performance of the complete

building should be considered. These

might include reinstatement of roof

overhangs and fitting of pentice

boards, removal of impermeable

materials and finishes, and measures

to reduce wind exposure.

7 Timber-framed walls generally

have low thermal mass and high

uncontrolled air penetration. Heating

systems that make use of large

internal masonry stacks or stone

floors as heat stores are often more

effective than systems that heat

the air via conventional radiators.

Radiators should never be placed

against external timber-framed walls.

ROBERT DEMAUS

BEng MSc (Timber

Conservation) specialises in the location,

assessment and conservation of

structural timber in historic buildings,

combining advanced technologies

with historic knowledge and practical

experience, working for national

institutions and private clients

throughout Britain and abroad.

The severely degraded timber framed end wall has been strengthened and protected behind weatherboarding,

with a layer of breathable multi-layer insulation included.