Heritage Retrofit

BCD SPECIAL REPORT ON

HERITAGE RETROFIT

FIRST ANNUAL EDITION

these U-values have been measured

rather than derived from the standard

calculating method, which has been

shown to have limitations when used

to estimate heat loss for solid walls (see

Further Information: Baker and BRE).

The wall at Drewsteignton is quite

different, being a north-west-facing,

600mm thick granite construction. In

this wall we find higher moisture levels

(in terms of both %RH and AH g/m

) and

narrower saturation margins. We also find,

over the past three years, a trend of rising

RH in the centre of the wall which, year

on year, moves this part of the wall closer

to saturation conditions. As this trend

has continued over a number of years, we

conclude that the high RH within the wall

is not solely a response to atmospheric

conditions but is also a function of certain

qualities of the construction that might

limit or inhibit drying.

This may be, in part, down to the

heavyweight nature of the wall and its

aspect, but vapour profiles have climbed

since the wall was insulated and have

not returned to pre-insulation levels.

This suggests that the insulation itself

may be having some impact on the

wall’s performance, although it is not

clear whether this is primarily due to its

thickness or its impermeability.

The wall at Drewsteignton has been

insulated with a greater quantity of more

thermally resistive insulation and this

reduced the measured in situ U-value

from 1.20 W/m2K to 0.16 W/m2K. This

ensures that less heat passes into the cold

side of the masonry during the winter

period, thus saturation margins are

lower and air is more likely to become

saturated and remain saturated for

longer periods, limiting the wall’s ability

to dry. Furthermore, the foil-facing of

the PIR board acts as a barrier to the

movement of moisture from the core

of the wall, which can no longer access

the potential evaporative surface of the

interior wall face.

In conclusion, we find that the

performance of these walls is in part

conditioned by their individual material

components, including changes made to

the fabric to improve energy efficiency.

Interstitial condensation has been a

particular concern, yet the internally

insulated brick wall at Shrewsbury, which

uses a limited quantity of insulation and

does not incorporate a VCL, has stable

vapour responses that operate within

safe limits. In contrast, at Drewsteignton,

where insulation has reduced the U-value

of the wall to a fraction of its previous

heat loss and a VCL limits the movement

of vapour towards the internal side of the

wall, vapour conditions are deteriorating.

The measurements from the BPS

tell us that, rather than internally

generated moisture, the influence of the

external environment in combination

with the individual circumstances of

the walls – their materials, aspect and

condition – has the greatest impact on

their moisture performance. These walls

are solid, there is no capillary break

in the form of a cavity or damp-proof

course to prevent moisture, particularly

wind-driven rain, penetrating

deep into the core of the wall.

Many solid walls are thick, built with

heavyweight materials and they can be

shaded and/or sheltered. This means that

the ability of heat and air movement to

dry these walls may be limited. While

this may not, prior to insulation, create a

moisture problem in the wall, the method

by which a wall is retrofitted must take

into account all the factors which might

impinge upon its performance.

The decision as to what type and

what thickness of wall insulation might

be suitable for a solid wall cannot be

answered by looking at heat loss reduction

alone. Those charged with improving the

energy profiles of these buildings must

view the building as a whole, looking at

how it may perform in its specific context

including individual wall aspects and what

the effect of its constituent materials,

condition and finishes may be.

The wall at Drewsteignton shows that

the use of a relatively large quantity of

higher performing close-cell insulation,

incorporating an impermeable VCL,

can result in a risky vapour profile.

This is not to say that the application of

similar material to the internal wall at

Shrewsbury would have produced the

same results. Indeed, at this location the

wall’s performance may have been more

satisfactory as this wall is able to dry