Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

When a building is insulated, it is

likely to ‘behave’ differently as a result.

In particular, its internal conditions are

likely to change. Relative humidity may

be more prone to increases, for example,

particularly where occupants are unaware

of the change in building conditions

and do not adjust their ventilation or

other habits accordingly. The more

comprehensive the retrofit, the more

likely it is that such changes will occur.

A common contributor to such

changes is an increase in airtightness,

often as an unintentional by-product

of adding insulation which blocks up

previous air leakage routes. Without

adequate ventilation, moisture in the

building is now less able to escape, and

this can cause problems even where

moisture-open insulation systems are

used (although such systems should

considerably ease the moisture transfer

process). This is exacerbated where

insulation is partial (leaving some cold

surfaces) and where intended as well as

unintended ventilation routes are blocked

up, leading to problems such as moisture

build-up in cold voids (roof spaces and

cellars for example) or deterioration of air

quality in the occupied spaces.

More holistic retrofit projects often

deliberately target improved airtightness,

with the aim of sealing up unintended

ventilation routes and thereby reducing

unwanted heat loss. This is a sensible

strategy, but must include consequential

measures such as additional controlled

ventilation to ensure that indoor air

quality remains good.

Deliberately making a building more

airtight and then having to add more

ventilation may seem like a paradox, but

the key issue here is one of controllability.

Uncontrolled ventilation can cause

excessive heat loss, uncomfortable

draughts and locally poor air quality;

controlled ventilation keeps indoor air

quality good while minimising heat loss

and increasing comfort levels.

A COHERENT APPROACH

The aim, then, is to:

• reduce heat loss via insulation and

airtightness

• retain a moisture balance in the

building fabric via a coherent,

thorough application of appropriate

systems and through additional

intentional ventilation where

necessary

• retain good indoor air quality

via an adequate, fool-proof

ventilation strategy.

A successful retrofit considers insulation,

airtightness and ventilation as integrated

parts of a whole-building approach.

ASSESSING VENTILATION

NEEDS

If considering a retrofit project on an

older building, particularly a deep retrofit

that aims to insulate all parts of the

building and increase its airtightness, it

is essential to consider the ventilation

requirements at the outset. Perhaps

the best piece of advice is to seek the

services of a reputable, independent

ventilation expert with experience of

retrofitting traditional buildings and an

understanding of the issues covered in

this publication.

As part of the assessment process, it

is helpful to establish current airtightness

levels and ventilation provision in the

building, as well as any residual moisture

or likely future moisture load. Informal

initial checks should include intended

ventilation routes (such as gaps below

doors, wall and window vents, chimneys,

extractor fans, roof and sub-floor vents)

and unintended ventilation routes (such

as structural cracks, poorly-fitting

windows and doors, gaps between

floorboards and at floor perimeters), and

can be simply and effectively informed

by occupant experience. For a more

formal measurement, airtightness may

be measured by a fan pressurisation

test, a fairly simple measurement of the

building’s air permeability (AP, measured

in m3/hr/m2@50Pa).

It is then necessary to identify the

airtightness level being targeted by the

retrofit project. This is also commonly

measured in terms of AP. For context,

current Building Regulations require

new-build homes to achieve an AP of

5, while the default assumption for an

older home is likely to be much worse.

The table above provides an example of

different ratings and what they mean.

(N.B. This table is taken from the recent

retrofit publication

A Bristolian’s Guide

to Wall Insulation

, which provides

detailed guidance on many of the

principles outlined in this article.)

Identifying the baseline performance

will help identify air leakage routes that

should be targeted for improvement

and intended ventilation paths that

must be maintained, while identifying

the target AP will help inform the

amount and type of ventilation

provision likely to be needed.

Once a retrofit project starts, repeat

fan pressurisation tests can be very

helpful, both during the retrofit (to

check that planned airtightness works

have been effective) and afterwards

(to identify the actual airtightness

of the retrofitted building).

As well as understanding baseline and

post-retrofit airtightness and ventilation

performance, there are a number of other

issues which require consideration at the

planning stage to ensure that a healthy

indoor environment is maintained:

• Moisture buffering

– the use of a

fully moisture-open insulation system

will support the performance of the

ventilation system, providing a greater

‘buffer’ for moisture management

when needed.

• Airtightness method

– as well as

coherent design, the manner in which

airtightness is to be achieved merits

consideration. The more complex the

system (a moisture-closed insulation

system, for example, or one that relies

on extensive use of tapes and/or

TABLE 1 Air permeability ratings for existing homes

Band Air permeability (m³/hr/m²@50Pa)

Described condition

Less than 3

Very airtight

Between 3 and 5

Fairly airtight

Between 5 and 10

Acceptably airtight

Between 10 and 20

Not airtight – a leaky building

Above 20

Very leaky

(Source:

A Bristolian’s Guide to Solid Wall Insulation,

see Further Information)

Shutters on the ground floor of a Georgian terrace in

Spitalfields, London: commonly used on the continent

to keep interiors shaded and ventilated during the

day, here the focus was on privacy and security.