Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

THE LIME CYCLE

as effectively as hydraulic lime but then

delivers the flexibility that is essential

for the conservation and repair of old

buildings. In one experiment, a piece

of carpet was skimmed with a plaster

made from this lime and allowed to set.

It did so overnight. It was possible to

dent the plaster with a thumb without

cracking it and even to bend the piece of

carpet without damaging the plaster. The

remaining mix from the experiment was

left in a tub. This same batch is now a hard

lump and the carpet plaster is still intact –

2½ years of poking and prodding, later.

DEVELOPING A LIME-BASED

INSULATION PRODUCT

In 2014 development of an insulation

system based on a non-hydraulic setting

mortar was awarded funding under

a Small Business Research Initiative

(SBRI) established by The City of

Cardiff Council, Cadw, Innovate UK

(formerly Technology Strategy Board)

and Low Carbon Trust with principal

funding from the Welsh Government.

The aim of the scheme was to assist the

development of ‘innovative measures

that will improve the energy performance

of traditional and historic buildings’.

The first step was to prepare sample

insulation panels (left) with a thickness

of 62mm (2½"). These contained Vivus

lime, mineralised wood chips and

various permeable aggregates selected

to ensure a very high degree of vapour

transfer, ideally suited to traditional

solid wall construction. Independent

testing confirmed thermal conductivity

of 0.1 W/mK in the least efficient sample

of Vivus render, in order to establish

a base-line, compared to 0.5 W/mK

typically found in conventional sand/

cement renders. (Thermal conductivity

is measured in watts per square metre of

surface area for a temperature gradient

of one kelvin for every metre thickness –

W/mK.) The tester also confirmed that

in his view even better results could be

expected if different aggregate materials

and thicknesses were used.

Currently the insulation systems in

common use are all impermeable and are

not readily compatible with older building

walls. The research confirmed that a

suitable insulation panel would be a useful

tool in the retrofit armoury.

Testing and experimentation is on-

going to determine exactly how thick the

panel or how deep the insulation needs

to be, to provide adequate insulation and

to buffer humidity, but without being too

deep to apply to older walls with existing

architectural features. Findings are

expected during the course of 2017.

Other materials such as plasters and

renders were also developed using the

same quick setting non-hydraulic lime

and a similar range of aggregates. During

workshop trials these were shown to be

successful in their ability to set and, once

dry, in their ability to absorb and readily

release humidity. Conventional lime

sand mortars tend to have a much higher

degree of capillarity due to the impervious

nature of the mineral aggregate, drawing

moisture in and retaining it for longer, off-

setting some of the benefits of the lime.

The advantage of a premixed product

using carefully selected aggregates is that

the resulting render, plaster or insulation

panel is able to work in a diffusive manner,

without capillarity.

The plasters and renders also work in

conjunction with the insulation material

to create a holistic approach to insulating

and finishing historic buildings. The

materials are all compatible with those

found in older buildings. The panels

are best fitted to either internal or

external faces of exterior walls by being

solid bedded onto the surface using

the non-hydraulic setting lime mortar.

The reasoning is that the panel will

then become an integral part of the

wall, thus ensuring the original design

is maintained, promoting seamless

humidity extraction through the

structure. This simple approach contrasts

with many modern retrofit solutions

which include air-gaps, capillarity and

impervious layers.

MANUFACTURING AND TESTING

Following successful completion of the

insulation tests, the product was approved

for a second phase of SBRI funding.

£142,000 was awarded for developing

commercial production, for developing

variations in the setting time, and for

demonstrating the products in a ‘whole

house project’.

The facilities of a manufacturing

company in Derbyshire were used

to test production of the material

in normal commercial mixing and

blending apparatus, and to benchmark a

manufacturing process and ability.

Due to the high temperature of the

chemical reaction (approaching 200°C

during slaking), it very quickly became

obvious that specialist machinery

would need to be developed in order to

manufacture the binders if they were

to ever reach the market. Nevertheless,

enough materials were produced for

demonstrating the product. The first

successful prototype machine is now in

operation, with basic materials being

produced in autumn 2016.

The house chosen for the ‘whole

house’ demonstration was Mill Cottage, in

Pontcanna, Cardiff, which was saturated

and rotting before the works began.

The house is of 18th-century origin with

19th-century rebuilds. As with many

One of the pre-production insulation panels tested,

62mm (2½

) thick

FIGURE 1

CARBONATION

Carbon dioxide (CO

)

slowly absorbed

from the air causing

the lime to set

FIRING

At high temperature, crushed

chalk or limestone (calcium

carbonate) turns to quicklime

and carbon dioxide

SLAKING

Water is added to the

quicklime producing a

violent reaction generating

heat and steam (if the

quicklime is mixed with

aggregate before slaking,

this is said to be a ‘hot

mixed’ mortar)

Calcium hydroxide (CaOH)

‘Lime’ – often referred to as

air lime, non-hydraulic lime,

lime putty, or if dried, as

‘hydrated’ lime

Calcium

carbonate

(CaCo

)

eg limestone

or chalk

Calcium oxide

(CaO)

‘Quicklime’