Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

RADICALRETROFIT

at Trinity College, Cambridge

JONATHAN TAYLOR

HE COLLEGE estates of

Cambridge University include

a high proportion of nationally

important listed buildings by leading

architects from every period since the

15th century, and the colleges have a

continuing interest in showcasing the

achievements of the architectural avant-

garde. So, it is not surprising to find they

have taken an interest in the greatest

architectural challenge of the modern

era – sustainability. Nevertheless, the

scope of the interventions at New Court

at Trinity College is astonishing. Despite

being Grade I listed (the grade includes

the top two per cent of all listed buildings

in England), its retrofit is expected to

achieve an 88 per cent reduction in

carbon emissions.

New Court was designed by William

Wilkins to provide accommodation for

students and has been in continuous use

for this purpose since its completion in

1825. The building’s construction around

a central court is relatively conventional,

with pitched roofs of slate with dormer

windows behind parapets, solid masonry

walls, and single-glazed casement

windows. The façades facing onto the

court are of brick, originally rendered

with Roman cement incorporating fine

mouldings, much of which had been

repaired or replaced with a cementitious

render. Others are of bare-faced brick and,

facing the river Cam, ashlar limestone.

Refurbishment was required to meet

fire officer requirements, to remove

asbestos, to repair the fabric and to

bring the existing accommodation

up to a standard that would meet

the needs of the college for the next

30 years. The accommodation includes

160 student rooms, some with en suite

bathrooms, and a few teaching rooms

and offices. Its listed status means that

there was no requirement for an EPC,

and that there was some flexibility

under the requirements of the Building

Regulations. But it was expensive to heat

and its interior environment was poor.

Penetrating damp meant that a high

level of heating was required to keep the

ground floor warm, while students on the

floors above regularly had to keep their

windows open to avoid over-heating.

Fabric repairs and improvements were

therefore essential, both for students’

comfort and to reduce heating bills,

but the desire to improve the thermal

performance of the building went further

than this, driven by an ethical interest in

reducing carbon emissions.

As many buildings across the college’s

estate face similar conflicts between

heritage requirements and economic/

ethical requirements for conserving

energy, the opportunity was taken to

explore a radical approach which would

challenge the flexibility of current heritage

protection policy. This would provide

a model for further improvements to

buildings in other colleges.

The sustainability measures included:

• the repair and improvement of the

external envelope to conserve historic

fabric and to reduce uncontrolled

heat loss through damp and drafts

(windows, walls, doors and ceilings)

• the addition of 60mm vapour

permeable insulation to the inner

face of external walls, accepting some

limited thermal bridging through

cross walls

• reglazing the existing windows with

10mm thin double-glazed units

• the introduction of underfloor heating

beneath the original Georgian floor

boards, to be warmed by ground

source heat pumps at 36°C, and

controlled by occupancy sensors in

each room

• mechanical ventilation using the

existing chimney flues for air supply

and venting stale air, with heat

exchangers to pre-heat the intake

• the installation of PV solar cells on

south-facing roofs, accepting that they

will be visible from other buildings.

PRELIMINARY INVESTIGATION

AND MONITORING

A thorough understanding of the

construction of the buildings and its

defects was necessary in order to deal

with its principal problems appropriately

– damp at ground floor level and

uncontrolled heat loss. Additionally,

retrofitting to insulate the fabric and

to control the air permeability of the

external envelope has implications for

moisture levels. In particular, insulating

the interior faces of external walls

and the underside of roof spaces leads

to parts of the structure becoming

Detail of the fine riverside elevation of New Court, Trinity College, completed in 1825 and now listed Grade I (Photo: Tim Soar)