Interior Decoration and Permeability
Mark Parsons
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Lime-washed permeable walls of St Nicholas Church, Compton, Surrey with a simple medieval decorative scheme |
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Most churches and chapels
constructed before the first
world war have solid masonry
walls. When first constructed, these
walls were breathable, inside and out,
because they were built using permeable
lime mortars and their interiors were
usually lime plastered and finished with
limewash. These simple lime-based
materials helped the walls to dry quickly.
Following the development of fast-setting
cements in the 19th century and
of cheaper more durable synthetic paint
systems in the 20th, routine repairs led
to the interior surfaces of many churches
being effectively sealed. As the long term
consequences of this were not widely
understood until late in the 20th century,
few churches have escaped.
Typical results include areas of
blistering and peeling paint, black
mildewed north easterly and westerly
corners, drip stains beneath wall-heads
and valley gutters, and mildewed and
stained stone window surrounds. Add to
this the glossy sheen of the walls and the
rusted-up window vents, and the church
surveyor is faced with a virtual Rubik’s
cube of related issues.
Today, when a churchwarden or
PCC asks for advice on redecorating
the church interior walls, the answer
to this apparently simple question
suddenly becomes extraordinarily
complex. You cannot ‘simply’ restore a
vapour permeable painted surface either
‘technically’ or in the case of more ancient
structures ‘conservatively’. The advice
given has to be based on an understanding
of the church’s interior environment,
whether damp, dry, airy or humid. The
external condition of the church would
require assessment before good advice
could be given or the specification process
begun. If this is a ‘one-off’ commission
or you are a newly appointed church
architect, an initial inspection of the
fabric is best followed by a study of past
quinquennial inspection reports or
possibly the church’s log book. These
documents should provide information
on any ‘progressive’ repairs that have
occurred and will help to identify longer
term problems.
The level of ‘dampness’ in a church
building is largely a result of the actions
(and inaction) of its managers and
occupants over time. This level is best
expressed in terms of relative humidity
(at 100% relative humidity or ‘RH’, air of a
given temperature cannot hold any more
moisture): churches typically average
80 to 100 per cent RH in the UK, with
average internal RH readings of around
90 per cent being considered high. Drying
rates are mainly affected by the amount
of sunlight entering into the building,
the movement of warmed air and the
permeability of its fabric.
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Solid concrete floors following a 20th-century rebuild;
note how the moisture travels up through the renewed
stonework and forms a salty crust on the original fabric. |
If the floor is impermeable to water
vapour due to ground conditions or
modern interventions (such as a plastic
DPM and/or concrete slab, or impervious
coverings such as rubber backed carpets)
the result will, no doubt, be rising damp
at the base of the walls and columns and
the crystallisation of salts (efflorescence).
Poorly detailed and badly maintained roof
abutment and drainage, together with
general orientation and exposure, are
often the primary sources of the rainwater
penetration which causes internal peeling
paints, salting, and ‘black spot’ mildew
and plaster surface degradation.
Poor ventilation can lead to
condensation on painted wall surfaces,
usually at high level in northerly corners
and typically indicated by black spot
mildew growth. White mildew is a
powdery fungus which is more apparent
on damp timber surfaces and appears
on furnishings and ceiling boards. You
may also see evidence of high humidity
on window panes with wet surfaces
and extensive green algal growth. Stone
window surrounds will also be prone to
black spot mildew. Ventilation of church
interior spaces and suspended timber
floor structures must be understood
and action taken to maintain through-ventilation.
Salting and surface paint loss can
also be caused by an interaction between
modern plasterwork and either open
external mortar joints or those closed
with hard cement mortars. Normally at
high or low levels and around windows
and door openings some re-plastering will
have taken place using modern moisture
absorbing plasters, which are generally
termed ‘Carlite’ or ‘pink’ or ‘browning’.
These have been used as a quick low-cost
solution to failing plasterwork at higher
level (particularly over the chancel arch)
or degrading and loosening plasters
at low level. They are quick and easy
to apply.
Where wall plaster must be replaced, it is best to use fibre-reinforced
natural lime plasters. However, in some
circumstances an alternative proprietary
lime-rich renovating plaster mix may be
used. Limelite for example contains both
lime and fibrous reinforcement, albeit
with a small amount of cement. When
set, these plasters have a light and open
structure that allows water vapour to
pass through. There are mixes suitable
for upper and lower wall conditions, and
the skim coat adheres particularly well to
all types of surface, even those which are
impermeable. The manufacturers offer
good technical guidance and support.
Proprietary lime-rich renovating
plasters such as Limelite are best used in
the following circumstances:
- where working at high level – wall and
ceiling-renovating plasters are lighter
weight and will penetrate crevices and
timber lath more readily
- where the underlying masonry is
particularly hard, impermeable and
smooth
- as a skim coat where the surface surrounding the repair is inconsistent,
particularly in terms of permeability,
and where it is important to provide a
smooth overall appearance
- where it is necessary to work at cold
temperatures (around 10°C) and/or
with high humidity and/or in poorly
ventilated spaces.
One other cause of poor paint adhesion
is the use of inappropriate paints to cover
traditional finishes such as limewash,
casein-based paints and whiting. Where
this is identified as a problem and the
church is ancient or from a period
when complex decorative schemes were
popular, care must be taken when offering
advice on recoating. In these cases you
are required to assume the possibility of
underlying paint schemes that might be
restored at a future date.
Wet penetration caused by a poorly
maintained surface water drainage system
or a system that does not have sufficient
capacity to control the level of water
runoff in heavy rainfall requires careful
consideration. Checks should be made
and action taken where necessary on
basic maintenance and ‘jobbing’ repairs.
Ground conditions at the base of a wall
must also be assessed. Is the area drained
(a French drain or ‘dry area’)? Are there
gullies at the base of the rainwater
downpipes? Soil and debris may have built
up. Sheds and oil tanks may have been
placed against a north wall restricting
evaporation. However, the more
fundamental difficulties often relate to the
general condition of roofs, wall head and
valley gutters.
CHARACTERISTICS |
One-coat
(eg Classidur) |
Contract matt emulsion |
Mineral paint
(eg Keim & Beeck) |
Limewash |
Vapour permeability |
Very good |
Varies† |
Very good |
Excellent |
Reversibility |
Varies‡ |
Varies‡ |
Very poor |
Excellent |
Cover |
Excellent |
Good |
Good |
Poor |
Adherence on moist ground |
Good |
Poor |
Poor |
Poor |
Durability |
Very good |
Poor |
Very good |
Poor |
Finish |
Flat matt |
Matt |
Flat matt |
Flat matt |
Colour retention |
Excellent |
Varies |
Excellent |
Excellent |
Dilution/cleaning |
Proprietary solvent |
Water |
Water |
Water |
Cost |
High |
Low |
High |
Low |
NOTES
† Contract matt emulsions with high chalk content have good vapour permeability
‡ Film-forming paints are generally easier to remove where underlying layers are of limewash |
PAINT ‘SOLUTIONS’
Selecting an appropriate paint system
is thus complicated by the need for
the coating to adhere to the substrate,
which may vary from one area to the
next, and if the substrate remains
breathable, the new paint system must
not trap moisture. Other issues which
also need to be considered include cost
(materials and labour), resistance to
staining from the substrate, resistance
to abrasion, and appearance.
One-coat paints
Specially formulated one-coat paints such
as Classidur Tradition are designed to
adhere to a variety of backgrounds and
substrates without sealants or undercoats,
and have a flat matt finish. These paints
have good vapour permeability, resist salt
formation and retain elasticity over time.
To avoid black spot mildew, one-coat
paints should not be of the ‘plant-based
oil’ (soya) variety. Cover depends on
substrate – further coats may be required
to provide even cover.
POSITIVES
The paint is designed to adhere well to
all kinds of substrate, including moist
backgrounds. It is non-yellowing with
excellent stain-covering properties,
hardens rapidly and has very good
mechanical resistance. As it does not
penetrate into the underlying paints, it
may be used where there is evidence of
underlying limewash and other traditional
paints. Due to its chemical properties
it might be removed as a single layer at
some future date.
NEGATIVES
The paint is relatively expensive. It can be
difficult to apply in some circumstances.
It is only available in white.
Basic (low cost) contract matt emulsions
Emulsion paints are water-based paints
in which the paint material is dispersed
in a liquid that consists mainly of water.
Where suitable these have the advantage
of being fast-drying with low toxicity, low
cost, easy application, and easy cleaning
of equipment, among other factors. On
the basis that low cost means minimal use
of expensive oil-based compounds such as
vinyl, a basic trade product will have high
mineral (packer) content such as chalk.
As a result, the paint will be permeable
to water vapour. (If you are uncertain
whether the paint has a high mineral
content, try using it outside to see how rapidly it degrades.)
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Here, all the detail has been overpainted in gloss oil paints.
The floor is covered in rubber backed carpeting |
Use where the church does not have
problems with damp or paint adherence,
and where there are already many
overlying coats of emulsion. (Although not ideal, in some circumstances you
will have to accept that there is no other
sensible answer, but it can be a difficult
case to argue with the DAC.)
POSITIVES
The paint is low cost, readily available and
easy to apply.
NEGATIVES
It is prone to static and particulate greying
especially over heaters. It is also prone to
scratching and staining.
Mineral Paints
Also known as silicate paints, these highly
porous paint systems are based on mineral
silicates which bond with a lime or stone
substrate through the development of
an insoluble microcrystalline structure.
They are stocked by most specialist
conservation materials suppliers. Free
from organic solvents, plasticisers and
biocides, they are naturally resistant to
mould and fungal growth and are suitable
for allergy sufferers. Surfaces are cleanable
and resistant to disinfectants making
the paint suitable for application in food
preparation areas.
POSITIVES
Mineral paints adhere to all types of
surface (but often require an undercoat
in order to equalise absorbency of the
general wall surface) and they come in a
wide range of whites and other colours.
There is good technical support on
site and over the phone, and detailed
technical literature is provided. They
are not prone to static and particulate
greying, and do not degrade in sunlight.
They have a long life and performance
warranties are available as long as the
selling agents approve the specification
and attend site occasionally.
NEGATIVES
Mineral paints are relatively high cost,
particularly if sealants and undercoats are
required (this depends on the underlying
properties of the painted surface). They
are unsuitable for use where historic
paint schemes might exist as mineral
paint systems are designed to penetrate
the underlying painted surfaces and
are almost impossible to remove. All
unpainted surfaces including stone
detailing, monuments and furnishing
must be protected as this type of paint
dries quickly and is non-reversible.
Limewash
The paint used traditionally both
internally and externally, limewash is
the most vapour permeable option.
Although less durable than modern
alternatives, it is ideal for lime plastered
and rendered walls, or for bare masonry
pointed with lime mortars. It can be
cheaply made by diluting lime putty,
and earth pigments such as ochre may
be added if a colour wash is required.
A binder such as linseed oil or tallow is
sometimes added, particularly for external
use, and an increasing number of new
formulations are being developed to
broaden its application. Limewash has
a very attractive appearance internally
and externally, good shadowing and
sunlight reflection, and it can be
reapplied in small areas where paint
surfaces are prone to damage.
POSITIVES
Limewash is a highly permeable, low-cost
option which can be used on old
limewash and lime plaster. It can also
help to consolidate friable lime surfaces.
It retains an attractive appearance over
long periods of weathering and use. It is
also historically appropriate in many older
churches where it was the original finish.
NEGATIVES
Limewash cannot be used on
impermeable surfaces or over modern
non-porous paint systems (although
it is often possible to remove a nontraditional
paint, particularly where
there are underlying coats of limewash).
It requires a large number of colour
wash coats to build up opacity.
Decorators require hand and eye
protection due to its caustic effect.
In conclusion, no matter what paint is
specified it will quickly degrade if the
underlying painted surface has not had
sufficient time to dry out following
repairs, or if loose paint has not been
thoroughly removed. Make sure the
plasterwork is only repaired with lime-based
or renovating plasters no matter
how small the hole. Fillers of any sort
should not be specified or allowed. Open
joints, particularly where the walls abut
timberwork, must not be sealed in order
to allow the timber to ventilate. Open
joints in the masonry construction require
repointing using a natural lime mortar
and, finally, surface waterproofing agents
such as water soluble PVA (Polyvinyl
acetate) should never be used.
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Further Information
N Ashurst, Cleaning Historic Buildings,
Vol 1, Donhead, Shaftesbury, 1994: Ch 7
describes paint removal and how to
apply traditional alternatives
G Davies, ‘Vapour Permeable Paint’,
The Building Conservation Directory,
Cathedral Communications, Tisbury,
1996: article
on the use of traditional limewash and
contemporary alternatives
English Heritage, Practical Building
Conservation: Building Environment,
Ashgate, Farnham, 2014: large volume
with comprehensive coverage of
humidity and permeability issues
Internet search term: ‘traditional vapour
permeable interior paints’ provides links
to all of the current manufacturers and
the companies which market their products
Internet search term: ‘traditional lime and
renovating plaster’ provides links to all
current manufacturers of traditional and
renovating wall plasters and the companies
which market their products
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Historic Churches, 2015
Author
MARK PARSONS is an architect accredited
in building conservation (AABC) and a
partner in the practice of Anthony Short &
Partners LLP in Ashbourne, Derbyshire. He is responsible for the
quinquennial inspections of 130 churches in
the dioceses of Derby, Southwell and Lichfield.
Further
information
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