The impacts of water can devastate structures, so it’s no surprise that
humans have been managing water for as long as we’ve been building shelters.
Water can seem benign most of the time, but managing it properly is
critical to having lasting infrastructure. So, how exactly is water kept out of buildings?
My name is Hafeez, and welcome to the Engineering Hub.
Historically, waterproofing is just about as old as carpentry and masonry.
When the Egyptian pyramids of Giza were discovered, archeologists were baffled
to find so much in perfect condition despite the annual flooding of the nearby Nile River.
The Egyptians are estimated to have perfected bitumen emulsion around 3600 BC,
which is also a key ingredient of most modern waterproofing systems.
In order to understand how water is kept out of buildings, we need to be specific about which
types of water we’re referring to. Generally, water demands on buildings are in 3 categories:
1) Rain water, or any water that is generally in motion, including wind-driven rain. This kind
of water can flow into buildings through gaps in membranes, improper lapping, or capillary action.
This is usually addressed by having a steep enough drainage plane and positively lapped
waterproof or water resistant materials—we’ll share more secrets about this shortly.
2) Ground water, flood water, or any water that is generally stationary, and thus constantly applying
pressure to the water barrier material, can be viewed in its own category too. If the constant
pressure of the water is able to break down the water barrier, or seep in through laps or cracks,
then the water will flow through this path of least resistance into the building until the
pressure is relieved. Here it’s much more critical to avoid any small pin holes, and this water needs
to be held by a thicker and more durable membrane with much lower porosity than membranes only
resisting say rain water, since water constantly sitting or applying pressure puts much more load
on the membrane. In foundations this is commonly aided by adding gravel and drain pipes around
the foundation to reduce the water load, but if this is inadequate or missing the membrane can
break down much quicker than intended. 3) The last and most tricky kind of water
is humid air (or water vapor), which is water suspended in the air that
can condense inside walls and cause rot. This type of water needs its own special barrier,
often referred to as the air and vapor barrier, to make sure no air with water vapor passes
beyond a certain threshold to avoid condensation leading to rot inside framing. This kind of water
is tricky because it can come from both inside and outside—so where do you put the barrier?
The placement of (or even the requirement for) the air and vapour barrier depends on the climate zone
and construction materials used. A common rule of thumb is that the vapour barrier goes on the
warm side of the wall, since warm air holds more moisture, and when it cools down to the dew point
that moisture condenses like water on your car in the morning. The vapour barrier is often simply a
sheet of plastic, tape, or sometimes closed-cell sealants or spray foams are used too.
Okay, now that we have a baseline understanding of the types of water
that can get into buildings, let’s put this into context.
Over time, we humans have demanded more from our structures. Early structures were simple
single story huts, tents, or dwellings, only needing water protection from rain.
Once we evolved to using foundations to carry heavier loads and utilize underground storage,
we needed to protect from ground water too. And then there’s protection from water vapor,
which is a comparatively new design criteria. In many western countries water vapor plagues
buildings, particularly buildings from more recent decades too. We actually began a study of
this phenomenon to share with you in this video, but the problem seemed deeply region specific,
so we’ll share our overarching hypothesis here and you let us know in the comments if you
agree or have a different perspective based on the construction you’re familiar with!
Focusing on some key issues from Canada and the United States here, current water vapor causing
condensation in building framing began in large part due to changes in construction practices from
the 1970’s and 80’s. As heating costs increased from the 1970’s energy crises, there was a drive
to build homes more air tight and with much more insulation, but often without proper consideration
for how the framing would need to breathe to avoid condensation leading to rot. These
more tightly sealed homes could save on heating costs, but they unintentionally were trapping
moisture and causing rot! In comparison, even though some older structures may not have had
air or vapour barriers, they were unintentionally able to breathe which let wet materials dry out.
In Canada, the water ingress issues of this era were brought to the forefront with what’s referred
to as the Leaky Condo Crisis. In the late 70s and early 80s there was a big boom in construction on
the west coast of Canada (British Columbia). With that boom in construction came lots of
construction practices from other locations with different weather than the coastal Vancouver.
For those unfamiliar, Vancouver weather is basically a temperate rainforest (similar to
Seattle). It’s the 2nd warmest city in Canada, and it’s often called Raincouver too!
The construction practices used in the boom were brought from much dryer climates, and
as a result tonnes of water issues occurred. Walls got wet, from both condensation and water ingress,
and were never able to dry! It got to the point where half of all condos in the city
were impacted, and any building constructed in a 15 to 20 year range was likely defective.
Over a billion dollars of damages were estimated, law suites ensued, and government intervention
programs even came into place. In many ways, this colossal failure event sparked the birth
of building science as a profession, both for specialty contractors and engineers too.
Some of the specific issues were that buildings were not being built with continuous (or any)
membranes, vapor barriers were omitted or placed in the wrong locations, or overhangs
were omitted which kept walls and other elements perpetually wet.
Today, it’s much more common to have a special trade or maybe even engineer on
a project that is designated as responsible for the building enclosure, which means the water,
air, vapor, and thermal barriers. So how do these professionals properly keep water
out of buildings? We’ll highlight a few of the top design concepts here, and you let us know in the
comments if you’d like to see a deeper dive! 1. Rain Screening – This process involves creating
a separate layer of the wall façade which is designed to take the beating of Mother Nature,
from summer to winter, and this is the layer that the majority of rain flows on. Then,
behind the rain screen is the primary waterproofing membrane layer. In addition
to protecting the waterproofing, the rainscreen has a vented cavity to help keep things dry.
2. Slopes and Overhangs – The roof is obviously a very important part of keeping water out
of buildings, and keeping water away from the building is one of the best strategies
to reduce the likelihood of water ingress. This is why high slope roofs are so common,
as well as overhangs that extend past the walls to help keep walls dry.
On top of this, the attic spaces and often work together with overhangs that have vented
soffits to manage air, vapor, and heat flows—but that’s a topic for a whole separate video.
3. Belt and Suspenders – This is a general concept about assuming the first layer of protection will
always fail. Doing so can be expensive, but also creates very durable structures.
For example on roofs, several layers of membrane are installed assuming that the water may make
it past the first layer, but there’s still more layers to protect beyond that! Even more durable
is to add a high-slope, which significantly reduces the water load on the rooftop
before even challenging the first barrier. For water vapor this concept applies too—first
you assume incidental water could get somewhere it shouldn’t be (i.e. the vapor barrier fails), then
you check—is there a mechanism for water to dry in this case?
If so, there’s the belt to match your suspenders! 4. Completely Continuous Barriers.
This one seems basic in concept, making sure all the waterproofing is continuous everywhere,
but in practice it can be a real headache to sequence all the different sub-trades in
construction work to get all the layers to overlap in the right spots without missing any gaps!
These are just a few of the main concepts, but it’s certainly true that the devil is
in the details here so if you guys are interested let us know and we’d
be happy to share more on this topic! As society continues to demand more and
more from our buildings, the problems that need to be solved get more complicated—so it’s important
to be continually evaluating our knowledge and sharpening our problem solving skills.
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