Posted by: SherlockHomesSY | 28 Sep 2011

Handling Basement Mold in the SouthEast

Basement-Crawlspace Mold

Help, Everything in My Basement has Mold On It!

I’ve received several phone calls since returning to Atlanta, GA in February 2011 essentially complaining about mold in unconditioned basements.  I’ve also found this condition in various kinds of inspections of new & existing homes.  The problem is quite simple really.  At a minimum, humidity levels in outdoor air are high enough to support mold growth inside the house with natural air exchanges due to holes in walls, floors, and ceilings.  The solution is equally as simple: seal up the holes and dehumidify the air.

There are a variety of unfinished basements—all with the same problem.  They include poured concrete basements, walk-in basements, and wholly framed basements.  Some of these basements have vented crawlspaces opening into them.  When a crawlspace is also present, the problem increases many fold.

Basement Moisture Physics

It really doesn’t matter whether the basement is considered air tight or leaky if a dehumidifying system isn’t present for the area.  Water vapor (humidity) is a gas that will go through almost all building materials.  It’ll come in through walls and concrete foundation walls and floor slabs.  However, an air tight basement has far less problems than an air leaky one.

The physics is rather simple.  Warm, humid air flows through holes and building materials into a cooler basement.  The basement temperature is often close to the dew point.  In any case, the relative humidity rises.  When it exceeds 60%, mold begins to growth.  After that, 50% relative humidity will sustain it.  When there are fewer and smaller holes, there’s less moisture vapor.  In all cases, some way to lower the relative humidity is needed.

Air conditioners help reduce humidity when they’re running.   However, they aren’t much good when they aren’t used to condition the basement.  They’re also useless when it’s not summer time.

Due to the way house air and vapor pressures work, it’s easier for the moisture vapor to get in than it is to get out.  Also, insulation, wood, wall board and personal items soak up humidity.  Don’t expect moisture that gets in to just as rapidly leave.

Basement Only Solution

Use these procedures to control humidity in a basement.  We’ll handle the complications of a crawlspace later in this blog post.

  1. Seal all holes in walls, floors, and ceilings
  2. Seal ductwork (if any)
  3. Weatherstrip windows & doors
  4. Add a dehumidifier

Seal Holes. Holes in basements are sneaky.  The largest air leaks are through the rimboards—the outer band of the floor overhead.  The best way to deal with area is with spray foam in most cases.  It goes on a whole lot faster than caulk.  Besides, doing so also insulates the area.

The next largest hole is between concrete and wood, whether at a sill plate or bottom plate.  Sometimes a foam gasket is placed between them when the house is built.  In theory, the gasket seals all the holes, but they don’t because the concrete is uneven.  They make a great capillary break to keep moisture from wicking out of the concrete in the wood.  What is needed is a bead of sealant where the concrete meets the wood.

The building trades don’t mind punching holes through the wall to install plumbing, gas pipes, air conditioner linesets, outlet boxes, ventilation fans and other utility penetrations.  In addition, the basement is unfinished right?  Therefore, rough openings around windows and doors are left unsealed.  Almost all of these holes can be filled using cans of spray foam available at your local building supply store.  When holes go through concrete, hydraulic cement or sealant may be appropriate.

There are holes in the overhead floor assembly over the basement too.  Plumbers leave the biggest ones to install tubs, showers, and toilets.  HVAC contractors typically don’t seal the joint between the floor and a register boot.  Electricians, plumbers, and HVAC contractors make holes through the floor to run pipes and cables.

Don’t forget to deal with any holes in the floor too.  There is supposed to be a vapor barrier between the ground and concrete.  If so, cracks aren’t usually a big deal, but they can be filled with concrete sealant.  However, the vapor barrier was punctured to install rough-in plumbing pipes for the future bathroom.  There may also be backwater valves.  Find and fix these holes too.

Seal Ductwork. Sometimes, there is ductwork running through the basement—usually insufficiently sealed.  When the blower fan runs, whatever is in the basement is drawn into the house: humidity, mold, and odors.  Leave duct sealing to an HVAC contractor, unless you plan to have one come out to retune your system immediately after you’re finished.  Sealing ductwork can make bad airflow problems worse, shortening the life of the equipment.

Weatherstrip Doors & Windows. After filling holes and sealing ductwork, check the weatherstripping around doors and windows.  Often, pets have destroyed it.  It’s also not uncommon for window frames and door slabs to warp.

When air sealing and weatherstripping is complete, for all practical purposes, the moisture vapor expressway is shutdown.  The path for moisture getting into the house now is through building materials, typically concrete, brick, OSB, and plywood.

Install a Dehumidifier. For the purposes of this article, the goal of air sealing is to reduce the amount off “easy” humidity getting into the basement so the smallest dehumidifier can be installed to do the job.  Dehumidifiers are power hungry, so you don’t want them running all the time.

When choosing a dehumidifier, the thing to watch for is dehumidification per Watt.  A 150 pint is probably more energy efficient than two 75-pint ones.  Look for the ENERGY STAR label.  Also, when choosing one, make sure it’s durable.  Most dehumidifiers available at local building supply stores are no longer functioning a year later.  A good place to look for basement dehumidifiers is at this website.

Mini Basements: Crawlspaces

When you think about it, crawlspaces are mini basements.  Ideally, in the Southeast, they should be air sealed and insulated and treated like any other basement.  Unfortunately, in most cases, they’re not.

Typical Crawlspace Construction. The usual crawlspace has big holes in the foundation for vents.  Sometimes, these vents are closed, even in the summertime. They also have dirt floors, and foundation water proofing is often neglected.  If there was any consideration for moisture coming through the foundation wall, most builders chose to dampproof, rather than water proof walls.  Dampproofing membranes break after several years.  To make matters worse, concrete blocks, instead of poured concrete, are often used.  Finally, footing drains may have been left out.

Crawlspace Moisture Vapor Sources. Moisture vapor typically comes into the crawlspace in three directions: through the dirt floor, foundation walls, and vents.  Water comes in through all three places in addition to spills and drains through the floor above.

The dirt floor is often uncovered or loosely covered with a vapor barrier.  Water may not come through it, but water vapor sneaks around edges and lap joints.  From moisture vapor’s point of view, the vapor barrier isn’t there.  For whatever reason, termite folks think loose sheets of plastic covering 70% or more of the crawlspace is enough.  I’ve seen enough termites to know better.  When the vapor barrier is present, the edges need to be sealed to walls, columns, and other foundation supports.  Any lap joints also need to be sealed.

Ductwork Condensation. Insulated ductwork in a crawlspace needs special handling too.  In addition to air sealing the ducts, the insulation jackets need to be sealed to each other.  When joints are open, humid air comes through them and condenses on cold ductwork—perhaps soaking the insulation.  In the Southeast, insulation jackets around ductwork need a vapor barrier around them.  Typically, it’s metal foil to act as a radiant barrier for attic heat.

Crawlspace Only Solution Summary. Handling a crawlspace then is everything mentioned for a basement, laying a sealed vapor barrier, and sealing the vapor barrier around any ductwork.  The problem with a dehumidifier in a vented crawlspace is that it is energy inefficient.  Dehumidifiers cannot dry outdoor air.  It’s better to use ducted duct fans instead.  One end is open to the crawlspace.  The other goes outdoors.  The downside is that this approach can increase condensation in the crawlspace, but may be better than nothing at all.

Crawlspaces Open to Basements

Crawlspaces open to basements is an indoor air quality disaster happening everyday in the Southeast for houses that have them–unless the crawlspaces are air sealed and insulated.  Moisture vapor levels are already high.  There are several problems to consider.

Crawlspace Ventilation. First, there are probably not enough vents.  People up North in the 1940s told the Southeast to put vents in around the perimeter of the foundation in the form of the building code–Nationwide.  Until a crawlspace study project in North Carolina was undertaken, people thought this was a good idea.

Crawlspace Moisture Vapor Physics. The law of physics for moisture is that it goes from areas of high concentration to low concentration.  The thought is that high moisture would push its way out the vents, and it does. Unfortunately, they forgot wood is sponge!  Tree cells are made to hold water.  In addition, when the humidity comes in, it converts to “rain” to condense all over everything.  Now the moisture is soaked up by the ground, ready to evaporate again another day.  When unsealed duct insulation is present, it’s likely to become damp or soaked.  It’s no wonder moisture levels in usual crawlspaces are high and stay that way.

Crawlspace Water Physics. To add insult to injury, water is often present.  Water running along slopes at the foundation or draining down them comes right through.  Downspouts often discharge water close the foundation.  Splashblocks are largely ineffective as the water needs to drain at least 5 feet away.  Soil wicks up water 16 feet.  How often do you see houses closer than this near lakes, ponds, rivers, and streams?   In our region, ground water also runs through aquifers, often close to ground level.  Sometimes, septic system drain fields are close to the house.  Before there is any hope of control moisture vapor, any water intrusion must be stopped!

Crawlspace-Basement Wall. The wall between the crawlspace and basement needs to be effectively isolated from each other to avoid the high moisture levels in the crawlspace from invading the basement, unless the crawlspace is air sealed and insulated.  To do that, any openings need to be closed and wall joints need to be air sealed, just as they are at exterior framed basement walls.

Closing the entrance to the crawlspace isn’t as easy as it sounds.  Often, large duct runs through the openings.  The way most of them are created, anyone wanting into the crawlspace has to crawl around them.  I suggest creating a separate door for people to crawl through—one that can be weather-stripped.  If it were my house, I’d cut the duct where it goes through the wall, install a wall panel, attach sealed opposing duct collars to both sides of the panel, then seal the cut ends of the duct to collar.  Some folks just spay foam around the duct.

Limited Crawlspace Ventilation. Typically, when a basement is connected to a crawlspace, ventilation is limited.  Someone erroneously thought big holes into the basement were the way to solve the ventilation issue.  Unfortunately, they cause indoor air quality problems for the basement instead.  There is no real moisture ventilation.  In this situation, ducted duct fans can be used to move air where there would normally be vents.

Covering the Cost

My heart goes out to clients.  Many homeowners either don’t know or live in denial until the buyer’s home inspector comes around.  The problem was usually there when the sellers bought the house.  Now, they’ve fallen in love with another house and don’t want to fix their current one.  The buyer, on the other hand, doesn’t want to inherit a problem.  The fault for this situation rests with the builder, but they’re long gone.  What to do now?!!

Facts. Let’s face these facts.  Mold should not be cleaned up until moisture issues are resolved.  The moisture issues are now known.  The buyer isn’t going to purchase the house unless the mold is removed.  The buyer doesn’t want a problem house.  The seller doesn’t want to spend money that doesn’t increase their equity in the house.  In addition, they don’t want to spend money for benefits they won’t enjoy. The solution is for the parties to acknowledge these facts and create a plan.

If there is only a basement, perhaps the seller can air seal and remove the mold in the basement.  The buyer could and should choose the dehumidifier because cheap ones don’t last long.

If there is only a mini-basement (crawlspace), perhaps the seller can air seal it and address any water issues.  If there are substantial mold colonies, they should be removed.  The buyer should consider encapsulating the crawlspace with insulation and a durable vapor barrier, and then install a good dehumidifier.

If there is a crawlspace connected to the basement, both options could be used.

Energy Efficient Mortgages. There is a better way!  Air sealing and insulating a crawlspace is an energy improvement.  Another way to handle it is with an energy efficient mortgage.  The cost of encapsulating the crawlspace is funded over and above the purchase price of the home and paid for with the monthly energy savings.  The energy savings should pay for the additional mortgage increment—essentially making the upgrade “costless”.  This option is available for current homeowners too.



  1. The easiest way to make your home more energy efficient is to seal any air leaks, and one that is often overlooked is the bathroom ventilation fan and exhaust vent. The back-draft flap these units come with do a very poor job of stopping leaks. To address this issue, I use a replacement insert fan from the Larson Fan Company (online). Their fans has a true damper built in, that does a great job in keeping warm air in during the winter and hot, humid air out in the summer. This product has reduced my annual energy bills by over ten percent. It saves the most when air conditioning is being used.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s


%d bloggers like this: