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T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 6
T W E N T Y T H I R D E D I T I O N
3.1
STRUCTURE & FABR I C :
ROOF ING
it as it lowers, buffering the environment and
reducing the likelihood of condensation.
Ongoing research by Historic England
(formerly English Heritage) is shedding more
light on the humidity-buffering capacity of
timber in roof spaces. Brian Ridout, writing
in
Context
in 2014 (see Further Information)
demonstrated how, in a traditional roof, the
large surface area of wood can absorb excess
moisture vapour rising from the rooms below
without risk of decay. However, if high levels
of insulation are introduced at ceiling level, as
is common practice, the temperature of the
roof space drops and the relative humidity
(RH) increases because cooler air can hold less
moisture. Historic England’s research revealed
that buffering is effective at lower RH levels
because moisture vapour is absorbed into the
chemical structure of cellulose fibres at the
surface of the timber and is readily released
again as the RH levels fall. However, there is
only so much moisture that can be absorbed
in this way. If RH continues to rise to 80 or
90 percent, Ridout found that the moisture
content of the timber begins to climb at a
faster rate, from 16 to 21 per cent. It is now
understood that this is likely to be caused by
moisture accumulating as liquid within the
pores and spaces, enabling the moisture to
penetrate deeper into the timber by capillary
attraction. A point may be reached where the
timbers no longer dry out readily, and damp
levels may then become critical.
INSULATING A PITCHED ROOF
In a pitched roof, insulation is usually
introduced in one of three areas: above the
rafters so the whole roof structure is warm,
below and between the rafters so the roof
space itself is warm, or between and above the
ceiling joists – the ‘cold roof’ option.
Where a disused building requires total
refurbishment, it may be possible to achieve
a degree of environmental control that
excludes moist air from spaces cooled by the
insulation. However, where retrofitting an
existing building this is an unrealistic option
as an impermeable vapour barrier must be
integral, separating the whole of the insulated
space from the space below, from eave to
eave, in an unbroken plane. In particular, the
use of vapour barriers or ‘air vapour control
layers’ (AVCLs) on the warm side of the
insulation prevent evaporation from the side
where evaporation is most likely, trapping
condensation.
Current conservation advice from
Historic England and Historic Environment
Scotland favours the use of vapour permeable
insulation materials such as sheep’s wool,
hemp, and blown cellulose (made from
recycled paper) to avoid trapping moisture
in cool spaces. Most lime centres also stock
a range of wood fibre boards with good
insulation values, some of which are suitable
for a lime plaster finish. This approach also
enhances the buffering properties of the
room. All these natural materials need to be
chemically treated to minimise the risk of fire
and insect infestation (clothes moths thrive
in poorly treated wool, for example). These
materials should not be used in conjunction
with AVCLs since this would defeat the
object of using vapour permeable insulation.
In a ceiling, for example, a well-constructed
system of insulation batts, boards and a
plaster finish would wick any moisture that
condenses in the insulation to the warm
surface where it evaporates.
Cold roofs
Where a cold roof is acceptable and the space
is accessible, the simplest place to install
insulation is between the ceiling joists.
Particular attention needs to be paid to the
insulation of access hatches and other gaps
or thermal bridges, as large uninsulated areas
act like an open window, emptying heat from
the interior. Problems arise where sections of
the ceiling are not accessible, such as dormers
and raised ceilings framed by the rafters as
a continuous ventilation gap must be left
between the insulation and the roof covering.
Lights set into the ceiling pose another
problem, presenting a potential fire hazard
when covered by insulation. LED fittings with
a low heat output may be the answer. High
voltage cables will need to be re-run above the
insulation for the same reason.
Closed-cell insulation batts should never
be used between joists or rafters as they trap
moisture at the interface with timber.
Warm roofs
Where a warm roof is required, it is usually
possible to install all the insulation required
within the depth of the rafters while leaving
a clear ventilation gap of at least 50mm below
their upper edge. However, for rafters of less
than 200mm (dormer roofs for example) it
may be necessary to fix insulation boards to
the lower face of the rafters.
It is important to maintain a permanent
ventilation channel of at least 50mm between
the insulation and the underside of the battens
or sarking. If the roof has been lined with
a breather membrane, the insulation could
be soaked if it comes into contact with the
material during a storm. If the ventilation
channels are separated from the roof tiles by
impermeable sarking (closed sarking boards
or bituminous felt), through ventilation from
the eaves to the ridges may be achieved with
minimal visual impact by raising the ridge
capping slightly.
To ensure that the insulation does not
impinge on the ventilation channel, semi rigid
batts may be used, held in place by battens
fixed to the sides of the rafters 50mm or so
below their upper edge. The insulation batts
need to be cut to form a snug fit, and then
lined with plaster board or wood fibre boards.
If the roof covering is being relayed, it may
be possible to introduce some insulation above
the rafters, with the existing roof covering
reinstated on new battens fixed though the
insulation into the rafters. From a structural
perspective this is the safest place as all the
structural elements are kept warm, but it is
difficult to protect a breathable insulation batt
in this position from wind-driven rain, and the
increased height of the tiles or slates may be
unacceptable visually. One option is to use a
breathable multi-layer quilt of perforated foil with
fibrous insulation between the rafters below.
BEST PRACTICE
Historic buildings vary widely in their form
and construction, and a solution which is
suitable in one situation may be unsuitable in
another. It is therefore important to consider
all possible consequences and proceed with
caution. Key issues to bear in mind when
developing a solution include:
Reversibility
Any solution should be
removable without damaging the original
fabric in the process. Spray-on insulation
foams, for example, are not reversible,
preventing the reuse of the roof slates or tiles.
(They also trap moisture and can be highly
damaging.)
Effects of cooling
The design of the
insulation measures needs to take into
account the risks of condensation on the
cool side. Some heat loss may be necessary to
prevent condensation and decay, and it may
not be possible to achieve insulation levels
comparable with that of a modern building.
Buffering
Traditional materials such as
plaster, masonry and timber help to moderate
humidity levels provided that they are not
coated with an impervious paint. Buffering
potential can be increased by the introduction
of appropriate fabric where needed.
Moisture ingress
Maintenance of the
roof covering, flashings and gutters becomes
even more important when a roof has been
insulated, as moisture tolerances may be
stretched. Maintenance lower down the
building is also critical, such as plumbing
leaks and poor pointing, and bathrooms and
kitchens require extractor fans to expel moist
air, preferably with a both a humidity sensor
and a timer.
Further Information
English Heritage,
Practical Building
Conservation: Building Environment
, Ashgate,
Farnham, 2014
English Heritage,
Practical Building
Conservation: Roofing
, Ashgate, Farnham, 2013
Historic Scotland,
Fabric Improvements for
Energy Efficiency in Traditional Buildings
,
Edinburgh, 2013
B Ridout, ‘Ventilation and Timber Decay’,
Context
134, IHBC, May 2014
JONATHAN TAYLOR
MSc IHBC is the editor of
The Building Conservation Directory
. A former
conservation officer, he studied conservation
at Heriot-Watt University, Edinburgh.
Foam sprayed on the underside of the roof tiles
blocked all ventilation, while the insulation at ceiling
level has created a cold roof, so the timbers now
suffer from damp problems. (Photo: Robert Hill,
Historic Building Advisory Service)