Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

configuration of the building in question –

not all older buildings can accommodate

the ducting or fan units required for some

ventilation systems. Other factors such

as complexity and occupant type must

also be considered, as they can be key

determinants of success.

TABLE 2 A simple overview of each of the principal

ventilation options, and their main pros and cons

INTERMITTENT EXTRACT (suitable band B–E)

Pros

• low cost

• easy to install

• easy to use

Cons

• fan noise

• user can choose not

to use

PASSIVE STACK VENTILATION (suitable band B–C)

Pros

• low cost

• easy to install

(in top-floor wet rooms)

• silent

• continuous

Cons

• hard to accommodate

vertical ducting (in

ground-floor wet rooms)

• summer ventilation may

be insufficient

CENTRALISED MECHANICAL EXTRACT (suitable band A–C)

Pros

• medium cost

• potentially easy to

install

• easy to use

• continuous

• maintains background

ventilation

Cons

• requires ducting,

which may be hard to

accommodate

• uses electricity

• potential fan noise

DECENTRALISED MECHANICAL EXTRACT (suitable band A–C)

Pros

• low cost

• easy to install

• easy to use

• continuous

• maintains background

ventilation

• less ducting than

centralised system

Cons

• uses electricity

• room-side fan: increased

potential for fan noise

WHOLE-HOUSE MECHANICAL HEAT RECOVERY (suitable band A–B)

Pros

• air quality: intake air

is filtered

• comfort: air movement

and exchange

throughout home

• efficiency: heat

recovery reduces heat

demand and tempers

incoming air

Cons

• most expensive system

• requires ducting to most

rooms

• uses electricity

• potential fan noise

• correct commissioning

can be complex

The suitable band information refers to Table 1 (Source:

A Bristolian’s Guide to Solid Wall Insulation

membranes) the greater the potential

for application errors or future failure,

in contrast to a simpler system (a wet

lime plaster finish and/or backing coat

for a wood-fibre insulation system, for

example) which could be more robust

in the longer term.

• End-use and end-users

– how is

the building going to be used post-

retrofit and who will be using it?

How engaged are they? What, if any,

behavioural changes may be needed?

Are they tech-savvy or would a

simple, passive ventilation system suit

them better? All these questions need

to be answered to inform the design of

a suitable ventilation strategy.

Considering all these aspects before

embarking on building work is more likely

to result in a successful retrofit.

VENTILATION OPTIONS

Once a thorough assessment of insulation,

airtightness and ventilation has been

made, an appropriate ventilation system

needs to be identified. There are many

different types of ventilation system, all

with different pros and cons (see Table 2).

Broadly these divide into passive and

mechanical systems. Passive ventilation

systems work on the basis that warm air

rises and avoids the need for extractor

fans. Ducts rise through the building

to the roof, extracting moist air from

bathrooms and kitchens. The extracted

air is replaced by air drawn in from

outside through adjacent rooms, so the

whole house is naturally ventilated. These

systems rely on changing air pressures

and particularly on wind movement over

roof vents, so the design of the system

must be carefully considered and they are

not always suitable or feasible.

The simplest mechanical ventilation

systems use the same system of air

displacement but use fans to extract moist

air from those rooms with the highest

moisture level. These may be intermittent

(a simple extractor fan in a bathroom

for example, activated by a humidistat)

or constantly operating at a low speed

(a ‘decentralised mechanical extract

ventilation system’ or DMEV). More

complex systems have centralised fans

and can be either continuous or demand

controlled. In theory this makes the

systems more reliable and the fans can be

less noisy as they are located in loft spaces

or cupboards.

Mechanical ventilation and heat-

recovery (MVHR) systems are the most

complex mechanical systems, having a

heat exchanger and two sets of ducts, one

to extract moist air and the other to supply

fresh air back to the adjacent rooms,

preheated using the heat from the waste

air. MVHR systems are generally best

suited to comprehensive retrofits where a

high level of airtightness is achieved.

The type of system suitable for

a building depends largely on its

post-retrofit airtightness and on the