Excessive Moisture in the Crawlspace

Submitted by Ray Thornburg on Fri, 08/03/2012 - 10:33

Excessive moisture in the Crawlspace

Excessive moisture in the crawlspace can be a common problem especially in the low lying high humidity environment of the Charleston lowcountry. Here are some things to consider if your home inspector notes moisture issues in your crawlspace.


The signs of moisture problems in the crawlspace include

·       Loose or Falling Insulation

·       The dirt is wet

·       The insulation is wet

·       Visible puddles

·       Visible condensation

·       Efflorescence

·       Rust on metallic fasteners such as gang nails, piping supports, beams

·       Microbial growth

·       Corrosion on metallic piping or their metallic supports

The problems of excessive crawlspace moisture include

·        Microbial growth on structural members can cause decay, wood rot, and dry rot.

·        Rusting on metallic systems can cause failure.

·        Excessive moisture retention in the soil can affect the soil bearing capacity thus causing foundation or structural support issues.

·        Excessive moisture can attract termites or other pests

·        Excessive moisture can cause health problems

crawl space moisture  and ventilation dilemma At left you can see that the key to solving the excessive moisture in the crawl space dilemma is three fold.  First -  eliminate a major moisture source by putting a vapor barrier on the ground completely sealed and taped. Make it at least 6 mil thick and cover the ground completely with no gaps. Second -Adding ventilation allows air currents to carry moisture away and allows natural drying processes. Consider adding much more than the minimum ventilation. It's ok to be creative.  Third -  Eliminate the temperature difference between the floor and crawl space by adding insulation under the floor and around ductwork. Remember condensation occurs on the warm side of a cold surface so properly placed insulation is important.  It may be that your situation is not bad enough to merit using all three solutions. Maybe just adding the vapor barrier will be enough. Fourth - Did I say there was only three? The fourth option is a reversal as it converts the crawlspace to one which is conditioned. In this scenario the crawlspace vents are closed off,  the ground and walls are sealed and insulated, a dehumidifier is installed in the crawlspace and the under floor insulation is typically removed.

Some people think that closing the vents during certain times of the year will help. This is certainly not the case in the Charleston lowcountry. First this approach takes an uncanny psychic weather prediction sense which is impossible. Even if you could time the closing of the vents with weather events it would do nothing to address moisture rising from the ground or pressure driven moisture. Best to leave them open unless you choose a conditioned crawl space. In Charleston weather can change quickly.

There are also some humidity triggered ventilation systems on the market. Some of these compare the outside humidity to the crawl space humidity and activate fans only when needed. A system like this might be a good solution for a crawl space that has "dead spots" where air doesn't circulate well because of obstructions etc.

Crawlspaces and Moisture

Excessive humidity levels and moisture under the home is undesirable and should be minimized. This is a complex issue and there are several approaches to solving this problem. Getting expert advice on your particular situation is advised.

In general for every home steps must be taken to prevent ground water from accumulating in or around the foundation. Properly sloping the grade away from the exterior of the home and filling in low areas under and around the home where moisture can accumulate. In some cases a sump pump should be considered.

The first approach is to increase ventilation as much as possible under the home. This method depends on a lot of ventilationproper insulation of under floor area and installing a vapor barrier on the ground. A vapor barrier is very very important especially in the Charleston lowcountry area due to its high water table. Complete coverage is essential with no gaps and a 6 mil or greater product. In addition proper insulation around ductwork is essential. If there are moisture problems in the crawlspace the minimum ventilation standards ( 1 sq. ft per 150 sq. ft.) may not be enough esp. if the home is low to the ground. Much more should be considered with this method. Sources of moisture should be eliminated. These include plumbing leaks and appliances improperly using the crawl space as an exit point (like condensate lines and dryer vents). These "leaks" can be a major cause of excessive moisture in the crawlspace. A condensate line improperly exiting into the crawlspace can dump all the moisture removed from the interior into the relatively confined space of a crawl space causing moisture issues.  In low lying areas or areas with consistently high humidity levels you may want to consider the second approach.

The second approach is the exact opposite. In this method the crawlspace is sealed with a thick plastic membrane on the ground and up the sides of the foundation. The sides of the crawlspace are insulated and a dehumidifier is installed or the crawl is conditioned by the existing system. This essentially makes the area a simi-conditioned space. Proper insulation around ductwork is essential. There are some contractors out there who specialize in this method.

If condensation is noted on your report remember that condensation occurs as a function of temperature difference between two "spaces or objects" and humidity. When this happens dewpoint is invoked and condensation occurs. Eliminate the potential difference with proper insulation and the condensation will stop. Also lowering the humidity level will help.

It is also worth understanding how moisture can actually travel through solid surfaces like sheetrock and plywood and that good building science makes provisions for certain building systems to dry out to prevent moisture entrapment. An example of this is how your air conditioner actually pulls moisture from building materials. Preventing this natural drying process with moisture resistant finishes or incorrectly applied vapor barriers can exasperate the problem sometimes. An example of this would be the plastic foam of floating laminate flooring or the kraft paper facing on insulation. We recommend unfaced insulation on the under floor areas of the crawl space in this climate zone (1-4). This is because the kraft paper facing acts as a vapor barrier and interferes with the natural drying action of air conditioners and something known as the stack effect and natural vapor drive. reference irc 2006 318.1 exception 2,3.

Proper sizing of HVAC equipment is essential to good building moisture management. For example a too "powerful" of a system will cool the home off so quickly that it doesn't have time to remove the moisture from the air. Also a recently updated system might actually blow much cooler air than originally designed resulting in condensation around ducts. A quality HVAC contractor will take the necessary steps to properly calculate heating and cooling requirements based the science of his trade. Typically units should be no more than 15 % oversized except to satisfy the manufacturers next closest nominal size.

Each home is unique and these are just some of the factors an expert will consider when trying to evaluate and solve moisture related crawlspace problems. A home inspection may reveal moisture problems but the solution sometimes must be measured by many factors which should be considered. More information on this topic can be found on the Building Science website.

Below are some gruesome photos I took while doing home inspections around the Charleston lowcountry area and a brief discussion. Click the photos to enlarge. One symptom of excessive moisture in the crawlspace is mold. However the topic of mold is big enough to deserve its own page. Your can join our discussion about mold at our Apparent Mold Page.


puddle under charleston home

    That's right .... it's actually a mini lake under this home. I should have brought my fishing pole. Dirt was never knocked down after initial construction so low spots near the foundation are holding water causing all kinds of damage. It appears that dry cement was spread out onto the ground in a haphazard attempt to dry the soil out. In some cases sump pumps can be used to keep a crawlspace dry.




rusted foundation strap     

     So much moisture in this crawlspace the galvanized foundation straps are rusted clean through.






falling insulation

    Falling insulation is a common symptom of excessive moisture in the crawlspace. One reason for this is because condensation forming on it weighs it down causing it to fall.







corrosion on piping


     Green corrosion on copper piping is another symptom of excessive moisture in the crawlspace.  In some areas of the country the minimum number of foundation vents are not going to be enough. 





copper pipe leaking

     In this case the excessive moisture has caused leaking of the copper piping in the Charleston home.






Foundation strap rusted through on charleston home

     Here is another close up of a rusted foundation strap. See how the brick is lighter in color at the bottom. This is called efflorescence and is caused by the repeated evaporation of water which leaves dissolved mineral deposits on the brick. Note that it occurs at the same level the strap is rusted through.




rusted gangnail on charleston home

    Here is an example of a rusted gang nail found on this Charleston home during an inspection. How much strength do you think is left in such a gang nail that is so heavily rusted. This is a good reason to address excessive moisture in the crawlspace immediately.




Summerville home inspector discover lake under this Moncks Corner home.         One sign of excessive moisture in the crawlspace that I forgot to mention is if you can float a boat and go fishing then ..... you might have a problem.  In this picture on a new home you can see the puddles on top of the vapor barrier. What's not so obvious is that the vapor barrier is actually floating on about 6 inches of water throughout the crawlspace. Notice how wet the piers look and that the insulation is already starting to fall. This kind of problem is best solved by a civil engineer, hydraulic engineer or other qualified professional. Now I bet you're glad you got that home inspection!



high moisture reading crawlspace    So this is what happens when a un-insulated floor meets a modern air conditioned home. The condensation eventually raises the moisture content of the wood to the point that damage can occur. The arrow shows the plywood rippling somewhat (delaminating).  Sometimes you can even see or feel the condensation either on the framing or on the insulation.




ventilated crawlspace access cover

 Shown at left is an easy way to add additional ventilation. This guy replace his regular access panel with diamond grid for additional ventilation. Assuming the minimum size access of 18x24 that's an additional 432 square inches (3 sq. ft.) of ventilation! To meet code however a 1/4" grid hardware cloth should be attached to the back side to keep bugs out.




Steps to Dry Out a Crawl Space

I you're experiencing excessive moisture in the crawl space consider these steps. Which one you use depend on the severity of the problem.

  • Install a vapor barrier. Properly taped and lapped.
  • Increase ventilation. Be creative. Sometimes a block can be removed for this. Consider passive systems first and active systems (like fans) if passive designs are not sufficient. (i.e. Active systems require maintenance and sometimes fail leaving the home without protection.)
  • Improve grading around the home. Manage roof drainage better where needed. Make sure flower beds are not holding water.
  • Improve grading under the home. Fill in low spots.
  • Consider a sump pump for severe problems. These need to be installed by professionals typically.
  • Consider a enclosed crawl space system. These are typically installed by experienced professionals.



Example of an encapsulated crawlspace  

     At left is a diagram of an encapsulated crawlspace system. Click the image to enlarge. It is important to use a quality vapor barrier which is thick enough to last. Black is better than clear. Thick is better than thin. The thin clear plastic has a tendency to deteriorate over time so doesn't last. There are some quality vapor barriers that can be ordered over the internet some as thick as 10 or 12 mill. with reinforcement.







crawlspace conversion to non vented    Here is an example of a crawl space conversion. Originally the home had moisture issues and was traditionally vented. The contractor installed a sump pump, a dehumidifier and the tarp type material seen here. I don't know about the durability of that material but when I measured the wood moisture content it was actually lower then what we typically measure inside the home. Moisture issue solved!


encapsulated crawlspace example  Here is another encapsulated crawlspace with some premium vapor barrier installed. The dehumidifier is in the background. Notice that the floor is still insulated in these examples.  We feel that this is the best approach. The dehumidifier only lowers the humidity to an acceptable level, it doesn't remove all humidity. So a temperature difference between a cool house and a warm crawlspace (like on a warm southern summer day)  can still cause condensation on an uninsulated subfloor. We have seen problems with mold reoccurring on subfloor in these cases.

The other issue we find is not running the plastic to the bottom of the floor. The dehumidifier will struggle to keep up as more moisture than you think will be pulled through brick skirting and concrete block. It's easy to under estimate the importance of sealing (and insulating) vent opengs, etc and the total perimeter wall. It's best to do it right the first time.

Submitted by Ray Thornburg on Fri, 08/03/2012 - 10:33