Moisture Control in Homes: “A Watershed Moment”

Published by Luke Griess October 17th, 2018| for Scott Home Inspection


Moisture Control in Homes: ” A Watershed Moment”


What is the most basic protection that a home provides you?

Shelter. We need shelter to keep bugs, critters, and beasts at bay. We also need shelter to protect us from the burning noonday sun, as well as from the cold winter winds.  All of these things are easy enough to keep out; all you have to do is build a cube, with 4 walls, a ceiling, and a floor.  Maybe throw in a door so that you can come and go as you please, and perhaps a few windows for natural light!

Simple enough, build a cube and you have a home!  But wait, there’s still one threat we haven’t accounted for.  Something we can’t live without, but we’re often trying to avoid contact with.  Water!

Water seems like a harmless enough threat, until you get caught outside in a severe rainstorm, or even worse, if you fall out of your boat into the lake!  In these situations, water can be pretty threatening.  But the threat doesn’t end if we simply stay in our boat or put a roof over our heads.  We have to keep the water from permeating our cube entirely, otherwise, it can be destructive in some brand new and sinister ways. This is why moisture control is very important.

Moisture in building materials leads to rot, which can compromise the structural integrity of the home.  Moisture on organic materials that doesn’t dry out quickly can lead to mold, which can compromise the structural integrity of our lungs and impact our health.   There are countless other problems that moisture can cause, such as ruining your treasured possessions, watering down your beer, or simply dampening your spirits.  Seriously, this is why building science has been combating water as one of its key adversaries since the advent of the mud hut!

Moisture control does not seem like a difficult concept in theory….but, water can flow in multiple and unexpected ways.  In building science, there are 4 key ways that water can flow through a home.

1. Bulk Water 

The first way is through bulk water entry, such as rain or snow, or flooding from ground water.

A pitched roof over the home does most of the job, and if we put an impermeable roofing material on top, with each shingle overlapping the shingle below, all the way to the peak, we have a “watershed” moment!

But hold on! Now all that water is running off the roof, and is cascading down onto the ground right next to the house.  This is making a muddy mess, and we’re in danger of flooding.  This may also be causing damage and settling to the very foundation which our cube sits atop! However, do not fret; we have some moisture control solutions:

Gutters – to collect the roof runoff!

Downspouts – to drain that water down to the ground!

Downspout Extensions – to kick that moisture away from the house and into the yard!

Grading and drainage – to slope the dirt around the home to make sure that water runs away from the foundation!

Sump pumps – If moisture still comes in contact with our foundation, we can prevent our basement or crawlspace from flooding by installing a buried perimeter drain system, to collect that water, direct it to a sump pit, and install a sump pump to discharge the water!

This all works great. Is it a fool-proof way to keep the bulk-water out?  At least for our simple cube.  However, the problem is, nobody really wants to live in a cube these days.  We want unique homes that are aesthetically pleasing, with several different angles, shapes, and changes in materials.  This complicates things immensely.  Now we have to apply our “one shingle over the other” theory to all of these oddly shaped areas.

We need to install flashings! What are they? Flashings come in various shapes and sizes, depending on the application, but typically they are comprised of sheet metal which has been shaped for a particular application, and installed to direct the flow of water away at a transition of materials or change of direction in the structure. Roofs meet sidewalls, and need flashing under the siding but over the roof.  Kick-out flashing is needed at those tricky roof to sidewall to gutter junctions.  Flashing is needed where siding rests atop brick or concrete, and flashing details are required over windows, doors, and over deck connections to the house.

Building code has developed requirements and best practices for moisture management, but unfortunately not all homes were built to today’s code standards, and even on new homes these details are easy to overlook. Add to this the possibility of poor workmanship, and the fact that many weekend warriors tackle projects that are out of their league, and you can imagine there are numerous ways for our “watershed” moment to end in failure.

2. Capillary action 

So we’ve solved the bulk water issue, haven’t we?  Water runs downhill, and we’ve overlapped everything on our home to make sure it runs off of the building instead of into it.  But wait!  Water can change direction on us.  It can run uphill, through a principle called capillary action. This concept is demonstrated if you dip a paper towel into a spill on your counter top.  As soon as it makes contact with the liquid, the paper towel sucks that moisture upwards.

Now imagine your siding is somewhat porous.  Not as porous as a paper towel, one would hope, but all wood and masonry materials are permeable to a degree. So we keep these materials away from contact with potential standing water to aid in moisture control.  We keep the siding or brick off the ground, leaving a 6-8” gap between the siding and the soil where the foundation is exposed, to prevent water from soaking up into the wall. We install building paper or house wrap behind these siding materials, to provide a drainage plane in case moisture does penetrate.

At roof edges, we install drip edge flashing under the shingles. This provides a simple capillary break, so that the water can drip off of the bottom of the metal flashing material rather than wicking up under the shingles and causing potential damage to the roof decking and eaves.

3. Air-Transported Moisture 

So the first two flow paths were easy enough to understand, but this is where we start wading into deeper water, figuratively speaking, of course.

Moisture is present in the very air we breathe! And our houses are “breathing” this air too, in a sense.  Inevitable gaps or holes in the building envelope allow air to both enter and escape. With the stack effect, heated air rises through our home, and escapes out the top.  If air is leaving, it’s also entering, and that’s usually occurring near the bottom plane of the home.  In that air, depending on humidity level, gallons upon gallons of moisture can be hitching a ride through our house daily.  If that air travels across a surface where the temperature is below the dew point, condensation occurs.  Now those gallons of moisture can be beading up on our building materials.  And rot and mold come into play again.

Older homes were built in a manner that allowed them to breathe more easily, with very little insulation to cause resistance, and numerable open paths for air to travel, so that wetted building materials could easily dry out, preventing air-transported moisture from causing damage.  The problem is, these old mouth breather houses also exhale a lot of the air that we pay good money to heat or cool. And, energy costs today are far higher than they were back in the old days.

In the modern era, we’re building much tighter homes.  We’re sealing gaps and holes with caulking or spray foam.  We’re putting in tightly sealed windows and doors.  We’re installing sealed air barriers at the walls, and filling the wall cavities completely with densely packed or air- impermeable insulation such as closed cell foam.  The result of these efforts is fewer paths for air to enter or escape.  Basically, our cube now looks like a thermos.  So we’re retaining much more of that conditioned air.  And we can prove this with blower door testing. Where it was not uncommon for older homes to lose more than 100% of their conditioned air in the space of an hour, many new homes only lose 10-15% in an hour.  How’s that for progress!

But we have to be very careful now not to allow moisture to travel into these assemblies, or it can take a very long time to dry out.  We have also now created the potential that humidity cannot easily escape, and the air can become stale and stagnant, so we install mechanical ventilation systems to allow the home to breathe again, but this time in a controlled and more efficient way.  One example is the HRV (Heat Recovery Ventilator), which uses a power fan to simultaneously pull incoming fresh air and outgoing exhaust air through a heat exchanger, to help limit energy loss.

Good example of vapor sealing

Bad example of vapor sealing

4. Vapor Diffusion 

The moisture is not only traveling in the air that flows through larger gaps, but it can also move through building materials in the form of water vapor.  Moisture in the soil under the home can evaporate, and those vapors can travel through the permeable building materials.  This is why we install low permeability vapor barriers for moisture control, such as the plastic you sometimes see on the ground in crawlspaces, to prevent the vapors from the soil from permeating the floor and other areas of the home.

So as you can see, we’re fighting a battle on at least 4 major fronts, just to stay dry.  To compound the confusion, the methods that we use to combat these various and bold paths of moisture will vary from climate to climate.  In hot, humid climates, the general path of vapor travel is from the outside to the inside, so the vapor retarders are installed on the outside of the walls, behind the siding.  In colder climates, the path that vapor travels is generally from inside to out, so instead the vapor retarder is installed on the inside of the wall, behind the drywall.  And in climates like Colorado, where we actually get to experience 4 distinct seasons, we’ve argued back and forth for decades on where and how to install the vapor retarder, or we’ve eschewed it altogether, lest in solving a winter problem we cause a summer problem.  Today, modern house-wrap materials such as Tyvek are designed so that they can perform as both an air and moisture barrier, while still allowing water vapors from inside to pass through to the outside.

In humid climates, air conditioning and de-humidifiers can be absolutely essential to moisture control, not just for comfort, but to remove moisture from the indoor air. Conversely, in our dry Colorado climate, a device called an evaporative cooler or “swamp cooler” may be installed in lieu of air conditioning.  These systems pump water into pads which surround a large fan.  The fan pulls outside air through the wetted pads and then into the house, and as the water evaporates, the air is cooled.  And then, get this, in the dry winter months, we might use a whole house humidifier on our duct system to add humidity to the heated air and increase comfort.  Both of these devices could be a recipe for disaster in many humid climates, but you can get away with almost anything in the desert.

With all of these “watershed” moments and innovations in building science, we’ve helped push back the tides. However, moisture will undoubtedly continue to fight back, against homes both old and new.  A certified Home Inspector can help identify these potential issues, recommend solutions, and potentially help you to avoid investing in a home that is losing this war with water.  A home inspector is trained to identify those failing components or improper installations which can lead to hidden moisture damage, so you can fix any issues with a home’s moisture control.

If you believe you have a moisture issue in your home, you may benefit from a mold and moisture inspection or a mold air sample test. Give us a call today! 303-373-2424


About the Author: 

Luke Griess is an ASHI certified Home Inspector and Certified HERS rater, with over 20 years cumulative experience in the home inspection, residential energy services, and construction industries.




*This article has been published with permission of Scott Home Inspection. 

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