HomeMy WebLinkAboutItem 7 - Mural Discussion ITEM 7—MURAL ORDINANCE—EMAILS FROM MARK BUECHEL
You never paint unpainted historic masonry. if we had any review authority over the project it
would be denied. It seals the masonry, traps moisture, and results in accelerating of the demise
of the bricks. You simply do not ever paint historic masonry. we would require them to paint a
panel and mount the panel to the building through mortar joints with spacers so it breathes
behind. If they have already painted a primer and it is a latex or non-breathable product they
have destroyed the brick. a mural on historic brick will remove about 30 years from the life of the
wall.
Mark T Buechel, AIA
Senior Preservation Architect
State Historic Preservation Office
Wisconsin Historical Society
816 State Street, Madison, Wi. 53706
608-264-6491 (0)
mark.buechelp_wisconsinhistory.orq
Older buildings are fairly simple in construction,they usually comprise of thick and solid walls, with no
cavity.The result of this construction method means that moisture will always be, in some form, present
within the building fabric.The original material uses to create these buildings were relatively simple too;
they were softer than the host material (i.e. Stone) and they allowed moisture to escape and not
become trapped.The way I always explain it when I lecture is that for millenniums we always assumed
mother nature would beat us and we allowed for ways for water to get out. Now we think we can
conquer mother nature so we design and build things to be water proof and maintenance free. Neither
is possible, mother nature will eventually always win. With old masonry buildings the soft lime mortar
wicks moisture, moisture transfer is an inherent trait of lime so when water gets into the wall cavity it
wicks back out through the lime mortar. Look at an old building the day after a good rain,the bricks
have dried but the lime mortar is still damp because it is wicking the moisture out from within the wall.
Modern mortar is Portland cement based, Portland cement is a waterproofing material. On a side note
this is why we want pointing to match the historic mortar because if modern hard Portland cement is
used it prevents the moisture within the wall from getting out.
Damp is commonly found in older buildings. Up until recently it was (and unfortunately by some it still
is) believed that the only way to eradicate this issue was to waterproof the entire building with a
waterproof coating such as modern masonry paint or by injecting damp proof courses with the aim to
stop water penetrating the building. Whilst this offered a temporary solution it was and is still not the
answer, with the majority of these applications ultimately failing. Since there is no vapor barrier on the
inside the moisture that moves from inside to out gets stopped at the exterior waterproofing.
Another one of the main reasons for this failure is that a building can undergo significant movement,
both structurally and thermally. Once a crack appears water can penetrate the crack and be held within
the wall behind the non-breathable or waterproof coating, which can include cement and paint. A
secondary issue with cracking is during the winter or colder months, as water freezes it has an expansion
rate of roughly 9%per freeze. As this freeze and thaw cycle is repeated when we reach certain
temperatures,the cracks are able to increase and widen in size, which allows further water to ingress
with no means of escape.
A build-up of moisture can lead to damp within the walls which may cause 'blistering' and 'bubbling' of
the paint where the water is trying to escape,this is referred to as hydraulic pressure. I'm sure you have
ITEM 7—MURAL ORDINANCE—EMAILS FROM MARK BUECHEL
seen this on painted surfaces, the bubbles are from trapped water. In more serious cases the render
may be 'blown' or forced off by the pressure of the trapped water.You see this most commonly at
parapets where the front of the brick faces are blown off.This is either because they replaced the roof
and ran the new membrane all the way up the back side of the parapet wall trapping moisture or they
painted the outside.The trapped water is more suspect to freeze when it has weather on both sides like
a parapet so it freezes and blows the faces off the brick.
Water that is trapped within a wall can lead to serious deterioration of the building fabric. Any non-
breathable paint applied to the building will act like a film around its surface. If you imagine wrapping
your walls and ceilings in cling film,this will stop the moisture from going in and out but will trap it and
the water will build up within the surface.This is where the term "film-forming" paint comes from as it
seals the building and stops the building from breathing and allowing moisture out of the walls.
The reason that modern paints such as emulsions and masonry paints are classified as film forming is
due to the chemicals that are used,which create a plastic like layer, enabling them to sit on the surface
of a wall or ceiling.
The most appropriate paint for historic fabric is one that has a low SD Value.there are at least 8
different ways to measure the breathability of a material. Until the industry adopts a defined standard
this will always be an issue, however when it comes to paint the most accepted and adopted standard is
SD Value, it helps that it is probably the easiest to understand, interpret and compare. SD Value is a
German method, although it used throughout the world, and it stands for steam diffusion or air layer
equivalence. It is a measure of how much of a barrier a paint coating is to water vapor and how easily
the vapor can pass through this barrier, it is measured in meters.The lower the SD Value means that
more moisture is able to pass through,the higher the SD Value,the lower the moisture transfer.
Technically any SD below 1 can call itself breathable but a truly breathable paint is one with an SD
between 0.01—0.5.
Modern paints are not breathable like historic paints were. Historic paints were often a lime wash paint
which is highly breathable.There are some paints that are more breathable than others, clay paints like
tempra are breathable. And you can still buy lime wash paints. Oil can be and it cannot be depending on
the additives.The same with latex, latex is a film that is not breathable but if a minimal amount is in the
paint it hinders but does not destroy it.the historic paint has likely all washed off. If there was still paint
on the wall then painting over it is less likely to cause issues except if you use a non-breathable paint. If
the painted area is small enough the moisture can often still get out by traveling around the mural. Since
moisture flows down I would not let the mural go to the ground,the lower masonry should not be
painted even in a small mural.
Mark T Buechel,AIA
Senior Preservation Architect
State Historic Preservation Office
Wisconsin Historical Society
816 State Street, Madison,Wi. 53706
608-264-6491(0)
mark.buechel@wisconsinhistory.org