Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

INDOOR AIR QUALITY

AND VENTILATION

in traditional building retrofit

NICHOLAS HEATH

HEN PEOPLE think of

retrofit, the first thing

that springs to mind is

often insulation. This is perhaps

unsurprising, given the increasing

and logical preference for a fabric-

first approach to improving energy

efficiency. However, for a retrofit

to be thorough and low risk in the

long term, insulation is just one

part of the equation. To maintain

the health of both buildings and

occupants, insulation must be

part of a wider, whole-building

approach that also considers

indoor air quality and ventilation,

among other things.

Much has been written about the

energy efficiency of traditional

buildings and construction

materials. Rather than focussing

purely on thermal performance,

this article is concerned with the

movement of air and moisture

both in and around building

fabric. This issue can be split

into two broad areas: first, the

moisture characteristics of the

fabric elements themselves;

and second, the junctions and

spaces between the various

building fabric elements.

BUILDING FABRIC AND

MOISTURE

Many traditional building materials

allow some degree of moisture movement

within the fabric. Different terms are

used to describe this, with ‘permeability’

or ‘breathability’ being among the most

common, but these do not present the full

picture. ‘Moisture open’ is a more holistic

term, covering the following key areas:

• vapour permeability – a material’s

ability to allow water vapour to pass

through it

• capillarity – a material’s ability to draw

up or transfer liquid water

• hygroscopicity – a material’s ability

to absorb, temporarily ‘store’ and

then release water molecules from

the surrounding environment as

relative humidity changes (often called

moisture buffering).

When adding insulation to traditional

building fabric, it is essential that these

characteristics are considered and

compatible systems are used in order to

minimise the risk of excessive moisture

building up in or on materials.

Insulation and airtightness

systems then need to be applied

coherently to avoid similar

moisture build-up issues as a

result of either a) cold bridges at

uninsulated areas or b) moisture

building up within or on fabric.

The unintended consequences

of excess moisture build-up as

a result of imperfect retrofit are

becoming more and more widely

recognised, and these include

not only cosmetic and fabric

damage but also decreased air

quality and its potential negative

health impacts. In extreme cases,

the moisture balance between

ingress (rainfall on the outer

face of a wall, for example) and

evaporation may be tipped, so a

structure becomes progressively

more cold and damp, which in

turn attracts more condensation

until saturation occurs.

VENTILATION AND

AIRTIGHTNESS

Ventilation

is defined in the

current Building Regulations as

‘the removal of “stale” indoor air

from a building and its replacement

with fresh outside air’. It is needed

not only to control internal

moisture levels but also to get rid

of pollutants and to maintain an

indoor environment that is healthy

for both the occupants and the fabric.

In uninsulated older buildings

ventilation is often largely uncontrolled

and relies on a combination of gaps in the

building fabric, window and door opening

and more deliberate measures like

chimneys and other vents. In many cases

this ventilation can be excessive, particularly

at colder and windier times of year, resulting

in substantial and unnecessary heat

loss. However, it is essential to maintain

controlled, intended ventilation paths.

Solid brick walls, bay windows and tiled roofs in a typical Edwardian

terrace. Venting top and bottom, the original vertical sliding sashes

(right) offer controlled and highly efficient air circulation when retrofitted

with draught seals.