Buying an underpowered dehumidifier is a costly mistake that many people make. You buy a 30-pint dehumidifier to dry out your soggy basement. You plug it in and run it day and night. Two weeks later, your basement still smells like mold and mildew. The problem isn’t the brand you bought. It’s that you picked the wrong size.
Your moisture load is how much water your dehumidifier needs to extract from the air every day. Also called PPD or pints per day, your moisture load is determined by factors both inside and outside your home. Once you have calculated your home’s moisture load, choosing the perfect sized dehumidifier is easy.
One square foot does not equal another when calculating moisture load. Different factors like sunlight exposure and use affect each room. Spaces below ground level will typically have higher moisture loads because of their exposure to ground dampness. The guest bedroom down the hall that sees little sunlight and use probably has a very low moisture load.
Moisture can enter your home in many ways. Outdoor air enters through walls, windows, crawl spaces and more. Indoor activities like cooking and showering release moisture into the air. Laundry rooms and bathrooms without fans trap humid air inside instead of venting it outdoors. Wet basement floors conduct moisture year-round.
Take a look at each room’s characteristics and consider these common moisture sources to determine moisture load before moving on to step two.
Once you know the square footage of the room you want to dry, you can start calculating which size unit you need. Square footage alone is not enough information to purchase the correct size unit. Instead, use it as your starting point.
Factor in what percent humidity you normally see inside that space. Purchase a hygrometer if you do not know this already. Hygrometers cost under $15 and remove the guesswork from what humidity level normally exists inside your home or office.
Square footage will get you part of the way there. The real moisture load is always higher than your square footage tells you. Each additional person adds approximately 5 pints of moisture to your daily load. For each bathroom or kitchen, add another 3-6 pints. Laundry rooms contribute about 4 pints, while a crawl space under the floor adds about 20 on its own. Hot tubs, pools, and even poor ventilation each add about 10 or more.
Let’s say you have a 1,200-square-foot home at around 70-80% humidity. Square footage brings you to a baseline of 55 pints. Add 4 occupants (+20), two bathrooms (+6), a kitchen (+6), and laundry room (+4).
Your final moisture load is 91 pints per day. A unit with a 50-pint capacity will run day and night without reducing your moisture load enough. You would need a 90 or 100-pint whole-home unit to achieve your desired humidity range.
Dehumidifier manufacturers test their units in ideal conditions. While your rooms may measure 90 percent humidity, they are likely even more humid than your meter tells you. Conditioned spaces cannot usually get above 70 percent humidity because our bodies give off moisture throughout the day. Add an additional 15 percent buffer on top of your moisture load estimate before you buy.
Going back to the example above, 91 x 1.15 = 104.65. Round up, and you will want a 105-pint dehumidifier. It’s always better to overshoot your estimated load slightly than risk having an underpowered unit run continuously.
When sizing dehumidifiers for larger spaces or whole homes, the math changes very slightly. The NHS told us that Once you get above 1,500 square feet, portable dehumidifiers become ineffective. You want to look at whole-home units or commercial dehumidifiers that tie into your HVAC system.
Also pay attention to climate. For homes in moderate climates (total annual rainfall less than 35 inches) between 1,500-2,500 sqft., you will need a unit rated at 70-90 pints per day. However, homes in very humid climates like Houston or Florida require larger units at 90-120 pints per day during the same square footage. Homes that are 3,500-5,000 square feet and in humid climates need even higher pints per day capacities at 150-200+.
Remember that whole house units also require you to size the airflow through your vents. The formula is simple: Divide your target PPD by 24, then multiply that number by 2.5. The resulting number is the CFM your ductwork and HVAC system will need to support. If your HVAC cannot handle it, your dehumidifier will never work at its fullest capacity.
Basements and crawl spaces need extra attention. If you have a basement, ground moisture will enter through concrete floors and cinder block walls every day of the year.
means you almost always want to size for higher moisture loads in basements than you do above ground.
Regardless if your air is too wet or too dry, you should shoot to keep humidity levels between 40 – 50% indoors. Less than 40 percent and your air is too dry. Dry air causes cracked paint and furniture, wood swallowing screws, and skin irritations. Above 50 percent and you will soon have problems with mold, mildew, dust mites and awful smells.
Tip: Get a good hygrometer. If your unit does not shut off after reaching your target humidity range, it’s too small for the space. If it shuts off after less than an hour and hardly ever turns back on, you probably got a unit that is slightly too big. While not ideal, having a dehumidifier that is oversized is far better than the alternative.
Follow these steps and you can avoid the headache of buying a new dehumidifier every year or two. Take down your measurements, calculate your moisture load, then add 15 percent capacity before buying. Follow these guidelines and you’ll know at first glance whether that capacity is sufficient for your space.
Dehumidifiers are not one-size-fits-all appliances. While two rooms may be the exact same size, their moisture loads can be completely different. Learn how to calculate moisture load and you will be well on your way to picking the perfect sized unit every time.
Background: This study was conducted by FEMA’s Mitigation Assessment Team (MAT). MAT flew out to disaster areas to determine cause and failure of typical drying processes.
What Happened: Occupied homes, which suffered flood damage were “dried-out” with conventional air movers and portable residential grade dehumidifiers for several days.
Failure: Several weeks later, mold was discovered growing inside wall cavities. The surface drywall was dry to the touch and the room relative humidity had been stabilized at 45%.
Scientific Reason: The conventional dehumidifiers were unable to reduce the Grains Per Pound (GPP) low enough to begin pulling moisture from inside the insulation. The air throughout the room was dry, but the air inside the wall was staying at 100% humidity!
Proposed Fix: MAT studied the building and concluded that LGR dehumidifiers, used with high-pressure ventilation fans (“inject dry” systems), would literally inject dry air into the walls.
Take Away: You can’t determine if a drying job is working by “feel.” You must hit the “Low Grain” sweet spot to ensure that your studs, insulation, and other structural materials are safe.
Imagine your basement concrete as a stiff sponge. Due to natural occurrences known as capillary action, moisture from the wet ground outside is being sucked through your floors and walls and into your home. Twenty-four hours a day, seven days a week your walls are “sweating” into your basement.
When your evaporation rate (speed air movers lift moisture from wet materials) is equal to your dehumidification rate (speed moisture is removed from the air). If you run too many air movers with a small dehumidifier, you’ll lift more water off your materials than your machine can handle, creating a swamp.
A unit that’s too large for the space will short-cycle or turn on and off too frequently. It will dry the air, but not run long enough to remove the moisture from your “softer” materials such as couches, carpet, drywall or wood floors. While it is better to slightly over-sized than under-sized for basement jobs, try to get as close to that 15% buffer as possible.
If your home suffered flood damage (wet drywall, carpets that can’t snap back, etc.) a standard unit will reach its limits at around 40% humidity and just shut off. LGR dehumidifiers are able to push the humidity much lower creating what’s called “vapor pressure” to extract water from your studs.
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