Follow These 6 Steps to Determine the Correct Size Dehumidifier for Your Crawlspace

There are two sizes of Dri-CrawlSpace® Dehumidifier available.  Most crawlspaces can be guaranteed to maintain proper humidity using one or the other, or sometimes a combination of two units.

• Tips for Sizing and Choosing a Dehumidifier
• Explanations of Terms Used in Sizing a Dehumidifier

STEP 1.)  Determine Humidity Level

Before you chose a crawlspace dehumidifier, you must first assess the situation and determine the level of dampness in your crawlspace.  Since most people do not have a hygrometer, we have simplified humidity to 4 levels of dampness. These four levels of dampness are used to determine how much moisture is present, that must be removed:

To help you accurately determine your moisture or humidity level, consider the environment around your house.  Is it often rainy?  Do you live near water?  Here is a map showing humidity for the United States.  Click the humidity map to see current humidity conditions.

STEP 2.)  Determine Air Changes per Hour

Next, take your dampness or humidity level to find your required number of air changes per hour (ACH).  Remember this number, you will need it in the CFM formula further below.

STEP 3.)  Measure and Calculate Your Cubic Feet

Determine the cubic feet of your crawlspace.  Take the length multiplied by the width multiplied by the height.  If there are multiple areas or rooms, you need to consider if the air can freely flow between them.  Separate areas may need to be addressed separately with their own dehumidifier.  But, if the areas are open to each other and air can freely flow between them, simply add the areas together.

STEP 4.)  Calculate Your Required Airflow

You will need your Cubic Feet and Air Changes number to determine Airflow.  Airflow is measured in cubic feet per minute (CFM).  Simply use the formula below to find the CFM you need for your crawlspace.

Example:  Let’s do an example using a 30′ x 30′ pretend house.  Height of the crawlspace is 3 feet high.  Area of the crawlspace is 900 ft².  Multiplied by the 3 ft height of the crawlspace gives us 2700 ft³.  We will use a common moisture level of "moderately damp", which means we need 3 air changes per hour.  Do the math and we find a 135 CFM requirement.

STEP 5.)  Determine Minimum Dehumidification Capacity

The following chart provides a helpful reference for finding your minimum dehumidification capacity needed for your crawlspace.  You should remember that this chart is a guide for general purposes and that there are many variables in dehumidification such as climate, number of occupants, number of moisture producing appliances and the number of windows and doors you have.  To talk to a technician about your crawlspace issues, call 1-800-564-0667.

STEP 6.)  Choose Dehumidifier Based on Airflow and Capacity

The final step is finding equipment that matches your requirements.  We designed the Dri-CrawlSpace^® Dehumidifiers in two sizes.  One or the other, or a combination of the two units can accommodate any residential or commercial crawlspace application.  As you can see from the table below, the Dri-CrawlSpace^® COMPACT will handle the requirements of our example (which needs 14 Pints/Day and 135 CFM) with some additional capacity to handle situations when you may have an unusually humid or rainy stretch of weather.  

Model	CFM	Pints/Day	Price
Dri-CrawlSpace® COMPACT	200	35	$789 Purchase Crawlspace Dehumidifiers
Dri-CrawlSpace® PLUS	350	50	$1197 Purchase Crawlspace Dehumidifiers

Remember, the two reasons you should use a dehumidifier is to protect your home and your health.  A properly sized and installed crawlspace dehumidifier, along with a vapor barrier, effectively helps to prevent mold, metal rusting, and the swelling of doors, wall panels and floors.  Keeping the indoor air moisture level low also helps deter dust mites and other allergens that thrive in humid conditions.

To insure that your dehumidifier removes as many pints of moisture as it claims, the Association of Home Appliance Manufacturers (AHAM) licenses U.S. manufacturers and private-brand sellers to certify, under the AHAM program, the water removal capacity rating and energy factors of dehumidifiers sold in the United States.  AHAM requires the water removal capacity be verified through independent laboratories using the American National Standards Institute (ANSI) test method.

Explanations of Terms Used in Sizing a Dehumidifier

Dehumidifier Capacity - Pints Per Day (PPD) Dehumidifiers are rated by the maximum amount of moisture they will remove from the air in a twenty-four hour period.  This rating is measured in pints.  The capacity of a dehumidifier is expressed in pints of water condensed in 24 hours at 80 degrees and 60% relative humidity.  This capacity is often shown as "Pints/Day - 80°F@60%", or sometimes just "Pints".  The temperature, 80°F and RH level of 60% is a standard benchmark used by AHAM (Association of Home Appliance Manufacturers) and Energy Star. Airflow - Cubic Feet Per Minute (CFM) A lesser known, yet very important capacity for dehumidifiers and any air treatment equipment is Airflow Rate.  This is the speed the air moves through the unit, measured in CFM, cubic feet per minute.  It basically tells you how powerful the fan is.   Every dehumidifier has a fan that pulls the air through the unit.  Generally speaking, higher CFM means the air in your crawlspace will be dehumidified faster. Relative Humidity (% RH) When dealing with moisture issues, many people hear the terms relative humidity and dew point without fully grasping what they mean.  Relative humidity (RH) is the term most used to describe the percentage of water vapor that exists in a gaseous mixture of air and water.  A RH reading is a measure of the amount of water vapor present in the air compared with the amount of water the air can hold at the temperature at the time of measurement.  Relative humidity is expressed as a percentage and is calculated as follows: Relative Humidity = actual vapor density  x 100% saturation vapor density In plain English, the RH level tells you the percentage of water in the air.  When relative humidity is 100%, water vapor in the air will turn into rain, condensation, dew, snow or frost.  You can feel relative humidity on your skin because it is a measure of how much water vapor the air can hold before it turns to liquid. Relative humidity is affected by temperature.  As temperature becomes colder, relative humidity will raise.  Relative humidity in a room may be 50%, but when you introduce a cold glass of water, the air around the glass gets cold and condensation forms on the cold surfaces.  What happens here is that relative humidity of air contacting the cold surfaces raises from 50% to 100% as water vapor turns to liquid. Dew Point (Degrees) Dew point is the temperature at which water vapor condenses into liquid.  The dew point indirectly indicates a measure of how much water vapor there actually is in the air.

As we have learned from previous blogs, homes that feature crawl spaces often suffer from moisture damage related to the crawlspace environment.  Your crawl space is sucking up moisture from the ground and your home is sucking up moisture from the  crawl space.  As you know, bad things happen to your house when you mix moisture and wood.  Since indoor airflow naturally drifts upwards, we will show you how using a vapor barrier and a dehumidifier in your crawlspace will significantly reduce the amount of moisture in your home.  This slowly traveling moist air carries with it mold and other harmful particles.   One of our Dri-CrawlSpace^® dehumidifiers has more than enough power to dy out any mold and prevent more from infiltrating your home.  Further down this article, you can read other additional practices that anyone can do to help reduce humidity in the home.

There are four balancing factors that dictate a home’s moisture balance: source strength, temperature, moisture transfer rate and circulation-ventilation rate. If you have a major problem with any of these four factors, you are pretty much guaranteed to have moisture problem in your home.  Identifying a problem basically requires determining if there is a constant source of moisture such a roof leaks, gutter and downspout problems, damp foundations and flat property with inadequate natural drainage.  Source strength of moisture in a crawlspace is often as strong as leaking pipe or HVAC equipment or as mild as water vapor seaping up from underground.  Temperature differences also promote condensation.  Cool ground temperature always keeps your crawlspace signifigantly colder (and damper) than the living space above.  Using exhaust fans when cooking and showering helps reduce moisture and, all dryer vents should be vented outside.  Moisture transfer solutions include sealing all air leaks from vents, windows, doors, duct work and eliminating any puddles of water.  Doing this will ease the load for your crawl space dehumidifier.  The fourth and final factor is the circulation-ventilation rate.  This refers to the rate of fresh, dry air entering the home and flowing through it.  Any areas where air does not move freely can become moisture pockets.  In most geographic areas, fresh outside air is not dry and tends to worsen the humidity problems when it enters the home.  That’s where a crawlspace dehumidifier can save your home.

Optimally, you want to keep the relative humidity level in your home between 30% and 50%.  Ideal crawlspace humidity is 30% - 40%.  In the upper floors, it is ok if the humidity approaches 60%.  It is important to seal the ducts and vents because leakage in the crawlspace can cause pressure imbalances in the crawlspace and house.  Duct leakage allows air to be pulled through openings into the house due to the house pressure becoming negative with respect to the crawlspace.  This air brings with it mold spores and other contaminants that negatively affect your indoor air quality. Many times water gathers in crawl spaces from run-off from improperly graded soil.  The grade should be sloped away from the foundation at least 5%.  Inspect the gutters to make sure they are free of debris and that the downspouts empty 8 to 10 feet from the foundation.  If you’re using vents, trim back any dense shrubs that may be blocking natural ventilation.  Check for leaky plumbing; plumbing leaks can be a primary source of crawl space moisture.

Once you have properly secured your home from the sources of excessive moisture, building a vapor barrier, sometimes called a "vapor diffusion retarder" (VDR), in your crawl space is the next step.  Before you begin constructing your barrier, it is vital to remove all standing water and to clean up any mold that may be present in your crawlspace.  If you have standing water, a shop-vacuum may do the job.  If you have a large amount of water, you may need to place a dehumidifier in the crawl space for several days prior to clean-up.  Place one of our Dri-CrawlSpace^® brand Crawlspace Dehumidifiers on a piece of plywood or cinder block to keep it off the naked ground.  Run your drain hose out the vent and away from the house.  Make sure your hose is gradually sloping downward for the entire length.  The water drains by gravity and cannot go up, over an obstacle.  When the crawl space is sufficiently dry, you may begin cleaning. Depending on how much moisture there is and how dirty your crawlspace is, you may want to wear cheap, throw away painters jumpsuits to protect your clothing.  If a large amount of mold is present, you should at least wear a paper dust mask to prevent inhalation of mold particles that may make you sick.  We recommend using a dust respirator mask with replaceable filters.

When removing mold, you should also wear rubber gloves and eye protection because you will be handling caustic chemicals.  Respirator masks are available that filter dust and gaseous chemicals.  Remember, you will be in a confined space with little air circulation.  Take appropriate safety precautions.  Read all labels and instructions carefully when using chemical cleaners.  Never mix liquid chlorine bleach with ammonia or other household cleaning products.  This could result in the production of hazardous gases.  A good mold and mildew cleaning solution for wood, vinyl and aluminum siding is: one third cup of laundry detergent, one quart of chlorine bleach and three quarts of warm water.  Sponge vigorously all contaminated areas thoroughly and completely.  Let the area dry and use an air freshener if mildew smells persist.  When you are certain all mold is removed and your crawlspace is dry, tightly seal all spaces, cracks and openings with plastic, caulk, spray foam, flashing, etc.  Make sure all air ductwork and vents are properly sealed to eliminate humid air from entering the crawl space.  Mold can be found in the ductwork of cooling and heating systems.  Sometimes there are air leaks in the ductwork, allowing moisture to gather, where mold and mildew can grow.  Check your filters regularly because mold can also grow on dirty air conditioner and furnace filters.

Repairs to hardwood floors damaged by moisture can be very expensive.  EPA suggests spraying icynene insulation on the wood under toe space grilles in the crawlspace to help stop condensation and seal the wood.  Icynene foam insulation provides both insulation and an air barrier…

A vapor barrier is used to prevent the penetration or movement of moisture from one area to another.  It acts as a physical shield that repels moisture and keeps it from entering enclosed cavities of your home.  Vapor barriers must be continuous to be effective. Cover the entire crawl space floor with 6-8 mil polyethylene (plastic) sheeting and overlap all seams two feet and seal with duct tape.  Extend the outside edge of the plastic sheeting up the foundation wall at least six inches above the grade.  Seal the plastic to the foundation wall with concrete tape, construction adhesive or a pressure-treated nailing strip.  The vapor barrier on the insulation should be placed against the heated side or the sub-flooring. The polyethylene sheeting should be layed over the crawlspace soil with a minimum of joints.  It is recommended that you have your crawl space dimensions cut from a roll of plastic rather than using many pre-cut sheets that have to be joined together.  As an additional step, one or two inches of sand or gravel may be placed on top of the polyethylene.  The sand or gravel weighs down the vapor barrier, preventing condensation on the undersurface while absorbing the water droplets that may condense off the top surface.  This also protects the plastic from tearing when you walk on it in the crawl space.

Some contractors sell an encapsulation system that includes what we have described above, plus a mold inhibiting paint job, and an extra thick, 7 layer, 20 mil vapor barrier.  Unless you plan to entertain tea parties in your crawlspace, we feel this system is excessive and often an unnecessary expense when it comes to controlling humidity and indoor air quality in your home.  All joking aside, compare the average cost of an encapsulation system of $12,000 to installing a basic vapor barrier (using inexpensive supplies from a local home improvement store) and installing one of our heavy-duty Dri-CrawlSpace^® brand Crawlspace Dehumidifiers.

The EPA relative humidity guidelines for comfortable humidity levels in your home’s living space recommends maintaining 30%-50% RH.  Some EPA mold resources  recommend the higher end of acceptable relative humidity as high as 60%, but this is not true for your crawlspace.  And since this is a reputable government resource, many other industries and associations have based their standards and practices on these recommendations.  Unfortunately, this is very limited information and has been used as a guideline in more than what is applicable.  In fact, many industries have been misled and developed ineffective and sometimes harmful products and procedures that we all use in our homes.

For example.  AHAM, the Association of Home Appliance Manufacturers is the recognized trade association that sets standards to which most available home appliances will adhere.  They give a baseline for comparing performance of appliances.  For dehumidifiers, the baseline is Pints per day of water removal at 80°F and 60% Relative Humidity.  Yes, that does give a common point for comparing one unit to another, but who keeps their home at 80°F?  And 60% relative humidity is barely acceptable for healthy conditions.  Unfortunately for the consumer, manufacturers focus on optimizing their equipment to compare with competitors only at this level of operation.  So what ends up happening, we have a lot of dehumidifiers that look like this picture, which all compete to be energy efficient and effective at the AHAM level of 80°F and 60% relative humidity.  This relative humidity is just at the edge of guidelines suggested by the EPA government agency (Ever eat minimum quality government cheese?) and it’s an unrealistic temperature.  These are the same guidelines adapted by Energy Star as well as most building codes.

OscarAir is one of these forward thinking organizations.  We have performed our own testing and research.  We have developed an easy to follow FREE REPORT on Crawl Space Dehumidifier Selection that includes 7 things you should know before buying any crawlspace dehumidifer.  Furthermore, we have developed efficient and effective crawlspace dehumidifiers with a performance guarantee.  Sign up to receive the FREE REPORT and call OscarAir to have a specialist calculate and specify the correct size and number of dehumidifiers to use in your crawlspace.