Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

walls were then lined using a vapour-

permeable Pavatex fibreboard insulation

system, 60 mm thick, and plastered

to give a U-value of 0.25W/m

/°C.

WUFI and other static thermal

modelling indicated that leaving the cross

walls and even cornices exposed without

insulation would have surprisingly

little impact on heat loss, due to the

relatively low thermal conductivity

of the bricks. It was concluded that

cold-bridging would be insufficient

to allow condensation to reach a level

where mould growth could occur.

Where rooms still retained original

cornices, it was decided to terminate

the insulation (and the replastering) just

below the cornice. This would result in

a rather curious detail, with the cornice

running around three sides of a room as

normal, before diving into a recess on

the fourth side. However, the scheme

developed by the architects, 5th Studio,

demonstrated that the impact of this

detail on the character of the interior

would be significantly reduced when

combined with modern furnishings and

fittings, and it was seen as an ‘honest’

approach to the retrofit.

WINDOWS AND SHUTTERS

The original window shutters and their

housing had to be removed for several

reasons; first, repairs were best carried

out in the workshop; second, it enabled

their position to be modified so their

relationship with the face of the wall,

which was now insulated, could be

maintained; and third, it allowed the

insulation to be run behind them into the

reveal. After conservation and repair, the

joinery was reinstated in its new position.

This created a slight gap between the

shutters and the face of the windows,

providing the opportunity to improve

security with window locks.

Although the original windows

had been replaced in the late 19th

century, upgrading the glazing remains a

controversial decision. In listed building

terms, all alterations are considered

to be part of a building’s history and

the conservation authorities are rarely

in favour of the replacement of single

glazing with sealed units. However, many

factors may be taken into consideration

when assessing the significance of a

later alteration, and in this case it was

concluded that the replacement was

acceptable in principle. The original

glass was saved for use in the repair of

windows on the estate, and modern sealed

units with a thickness of just 10mm were

chosen from the Holloseal range. For the

outer pane machine-drawn cylinder glass

was used to produce an uneven reflection

similar to that of early glass.

The timber casements were

also draft stripped, and contacts

were added to the casements to

detect when they were opened,

automatically turning off the heating.

ROOF INSULATION

Rooms on the top floor were partially

within the roof, with no insulation in the

sections of the ceilings formed against

the rafters, nor in the dormer windows.

All the original lath and plaster had

been replaced in the past with gypsum

and metal lath, so from a listed building

perspective these ceilings had little

significance, allowing them to be remade

with rigid insulation bats between the

rafters and below, retaining a generous

ventilation channel between the top and

the underside of the roof covering.

The roof slates had been relayed in

the past over an impervious roofing felt.

A vapour permeable insulation system

at ceiling level would allow moisture to

enter a cooler space, increasing the risk

of condensation on the underside of the

felt. The usual solution to this problem is

to introduce additional ventilators, but

Computer modelling indicated that the cooling effect of a 60mm-lining of vapour permeable insulation would

not cause an unacceptable increase in moisture in the masonry, but long-term monitoring was essential. The

diagram shows an external weather station and probes installed at different depths in the masonry to monitor

changes in temperature and relative humidity. (Image: 5th Studio)

Thermal image showing the cooling effect of

condensation and poor insulation in one of the attic

bedrooms: the ceilings (modern gypsum plaster on

metal lath) were stripped out and replaced with

insulation below and between the rafters to achieve

a U-value of 0.15w/m

/°C and air-tightness of

3.0m

/h/m

@50Pa. (Image: ArchiMetrics)