Tips on Reducing Humidity Levels in Your Home
Build your homes tight, then test to ensure tightness.
A blower door test is used to determine the air leakage rate of the home. A typical blower door test would de-pressurize the home to 50pa in pressure, then using a fan pressure gauge with fan airflow speed, be able to determine a quantifiable amount of air leakage. These results would be measured in CFM50. The amount of infiltration (CFM50) is calculated with the home's volume to determine the amount of “air changes per hour” (ACH). Florida Building Code requires homes to be less than 7 ACH. Anything less than 5 ACH is considered to be a tight building assembly. According to Florida Building code, any home with 3 ACH or less requires whole building ventilation. From a building science perspective dealing with hot humid climates, any home with 5 ACH or less should have a whole building ventilation system.
Whole building ventilation is crucial, but follow these rules:
● Ventilation must be mechanically controlled, based on minimum runtimes and measured airflow readings. Do not over ventilate. To determine how much ventilation is needed, ASHRAE 62.2 Standard provides a calculation:
Ventilation Rate in CFM= (floor area/100) + (number of bedrooms +1)*7.5
... an acceptable amount of ventilation would be anything within -10% range of this ASHRAE calculation. Example: 2150 sqft house w/ 4 bedrooms, (2150/100)+(4+1)7.5= 59CFM (10% of 59= 5.9) so an acceptable range of ventilation should be 53.1 - 59 CFM every hour.
● Ventilation air must be temperature and humidity controlled.
● Only use supply type ventilation systems. This provides a positive pressurized home.
This positive pressure provides an “air curtain” to any thermal boundary penetrations,
reducing outside uncontrolled hot humid air from entering the home.
● Do not install the outside intake vent on the roof deck. Install where the vent is pulling
the outside air from under a soffit, porch or lanai. But remember, if from under a porch or
lanai, do not allow outdoor cooking near the outside intake vent.
● Do not use Exhaust type ventilation. Repeat- DO NOT use Exhaust Type. These provide
a constant negative pressure on the building that pulls uncontrolled outside air into the building. That means hot humid air coming into the building anywhere it can come in from. Dryer vents, Kitchen exhaust vents, bath exhaust vents, just to name a few. Then you will see condensation in the home at these locations and high levels of humidity within the home.
● For Balanced Type of ventilation systems, DO NOT use HRVs or ERVs. Remember we want a positive pressurized building. These provide an even pressure. Then when the
HVAC is in operating mode, could cause the home to become under a negative pressure. Proper installation on ERVs is a consistent issue as well.
HVAC Airflow is king, and here is why:
● The home was designed with a specific airflow requirement to each room. This airflow provides the correct amount of thermal comfort to that room. All of the rooms calculated together is what determines the air handler sizing. Too much or too less airflow to a room means the whole home is not balanced per the design. This also creates an uncomfortable environment for the occupant.
● Rooms that have incorrect airflow are caused by incorrectly sized ducts, ducts installed sagging or kinked, and/ or duct leakage.
● Duct leakage from the ducts into the attic WILL cause moisture accumulation at the point of duct leakage. Sweating ducts!
● Duct leakage within the supply ducts into the attic will cause the home to be placed under a negative pressure. Not good!
● The most common area for duct leakage is at the duct boot-to-drywall connection. The duct boot should be caulked to the drywall.
● Other common areas for duct leakage are usually found at the return and/or supply plenums at the air handler. Ensure that this connection is sealed all around the air handler. Areas that visually appear hard to reach; Yep!, those areas were hard to reach for the installer to seal as well. You most likely have duct leakage at that point.
● Static pressure is one of the most important factors in HVAC design. Simply put, static pressure refers to the resistance to airflow in a HVAC system's components and duct work. The push of the air must be greater than the resistance to the flow or no air will circulate through the ducts. HVAC systems are designed to maintain a certain airflow and static pressure. Higher static pressures cause a reduction in airflow speed. This is typically caused by sagging and kinked flex duct.
● Low supply air temperatures reduce duct surface temperatures. Lowering the duct surface temperature will lead to condensation on the cold ducts. Sweating ducts!
● Ducts should be pulled tight and straight with minimal turns and radius. Sagging and kinked flex duct will reduce air flow reducing supply air temperatures.
● Flex duct should not turn 90°. Flex duct turns that are 45° or less are recommended. Sharp turns reduce air flow, raise static pressure, and reduce supply duct surface temperature, thus sweating ducts!
HVAC System install. It's all in the design, so consider these concepts:
● Always take into consideration the size of the attic, the mechanical equipment and ducts. A tight attic is tough to ventilate or condition which can increase attic humidity and temperatures.
● Install air handlers inside the home, within conditioned space. Best practice is to install in the central location of home. Centrally located air handler units will have less duct installed with shorter duct runs, thus able to maintain proper duct surface temperatures and maintain proper and consistent airflow temperatures to each vent throughout the house.
● Supply plenums should be 4 feet or longer from the air handler to the first branch of the duct system. The main function of the plenum box is equalizing the air pressure in order to even the air distribution. Even air distribution provides equal supply air flow temperature.
● R-8 Flex duct is recommended. R-6 duct is mostly what is used in our market. R-8 will
allow for more deficiencies in a duct install, while still maintaining the proper surface
temperature of the duct, lowering the risk of condensation on ducts.
● Longer duct runs, 30’ or more, should have a higher level of insulation to ensure supply
airflow temperature is not reduced.
● Duct connections are extremely important. If the duct connection is not sealed and
insulated properly, condensation will build up at the connections. When installing these connections, it is crucial to seal the duct collar to the connection, seal the duct inner liner to the collar, then wrap and seal the outside duct insulation around the connection without compression. If the outside duct insulation has been compressed during install, condensation will collect at this point. If the duct insulation was compressed, then it is recommended to add an additional insulation wrap around the connection.
● Do not adjust HVAC fan speed to aid in dehumidification. Fan speed adjustments are available to provide a more accurate amount of airflow to the home based on airflow calculations, duct design, and static pressure. Adjusting fan speed may aid in both the increase and decrease of humidity levels within a home, however, ensuring the home has the proper airflow requirements per design should be your primary concern. After that, any additional needs for dehumidification, should be performed by a dehumidifier.
● The air conditioner is for thermal comfort. By design, higher SEER rated HVAC systems reduce latent load, therefore limit the amount of dehumidification. Higher SEER rated units use less energy but provide less dehumidification. Lower SEER rating units use more energy, but provide higher levels of dehumidification. After all items on this list have been exhausted (ie. air sealing, ventilation, air flow, mechanical and building pressures), then if the home is still unable to reach the desired humidity level, a dehumidifier should be installed. Keep in mind, within our hot-humid climate, acceptable ranges of indoor humidity is as follows:
○ 46-50%, Perfect, but difficult to achieve year round.
○ 51-55%, Average and acceptable to see year round.
○ 56-60%, Slightly high, may fluctuate from season to season. If consistently over
58%, then consult a professional and after all recommendations on this list have
been completed, a dehumidifier may be required.
○ 61%-anything higher, If consistently over 61%, then consult a professional and
after all recommendations on this list have been completed, a dehumidifier may be required.
Do not install Zoned HVAC systems. Multiple reasons:
● When one zone is turned off, the supply temperature of the operating zone will be reduced.
● Return vents that are placed within the dedicated zones, if that zone is not operating, the return airflow is still being pulled from that zone. Limiting the comfort of one area to provide comfort to another area. If both zones are being occupied, then this would cause a fluctuation of comfort back and forth, never providing consistent comfort to multiple zones.
● Zone systems that use bypass ducts are worse. They recirculate cold air back across the AC coil reducing the supply air temperature. Colder supply air within a duct will reduce duct surface temperature, This will cause sweating ducts.
● Zone systems that utilize “dump ducts” will over cool an area where the airflow is being dumbed, usually causing a short cycle of HVAC operation, therefore not providing consistent cooling across the home.
● Remember, the home was designed to have a specific amount of airflow to various rooms. With zone systems, the fluctuation of zones and air flows due to zoning will never provide the correct airflow to the room per design. It will always be far less or far more, and we do not want either of these scenarios.
Attic Ventilation, two types, two completely different recommendations. Here you go: Vented Attic- typically blown in insulation
● A vented attic must be sealed between the living space and the attic.Test to ensure tightness with a blower door.
● All ceiling penetrations should be sealed/ caulked to ceiling drywall.
● Any canned/ recessed lights must be insulated/ sealed type units and sealed/
caulked to drywall.
● Interior attic accesses need to have a gasket in place and have continuous
insulation that equals the insulation value of the attic space. For homes that have attic stairs that are located within the home, they make insulated covers for these stair assemblies.
● Attic insulation should have blown-in type insulation at a minimum R30. R38 is recommended. If batt type insulation was used, then use blown-in insulation to cover/ fill the gaps between the batts to provide a consistent and level insulation coverage.
● Ensure the attic has plenty of ventilation, both at the roof deck and at the soffit. To increase attic ventilation, add additional roof vents and increase soffit venting.
● Do not use any type of electric fan in the attic to provide attic ventilation. This includes a Solar Attic Fan...Let me say this again... DO NOT USE A SOLAR ATTIC FAN! These fans add an unwanted negative pressure in the attic. This pressure will cause a negative pressure on the living space of the home. In return, pulls out the air conditioned air from the living space and brings in uncontrolled hot humid air into the living space. They just are not good, no way around it.
Sealed Attic- spray foam insulation
● A sealed attic, must be sealed. Test to ensure tightness with a blower door. Test with the attic access open to accurately assess attic air infiltration.
● Use a thermal imaging camera with the blower door running and inspect for thermal abnormalities within the attic that may show locations with attic air infiltrations.
● Install mechanical equipment and ducts after the spray foam insulation is installed. Once the ducts and equipment are installed, it may be difficult to install the correct amount from behind the mechanical equipment and ducts.
● A sealed attic is a conditioned attic. The thermal boundary of this type of attic is located at the roof deck. The attic is within the thermal boundary, same as the living room, kitchen, bedrooms, etc. This attic space must be conditioned.
● Check insulation to ensure that the attic has the minimum specified thickness of spray foam. Minimum 5 1⁄2 inch thickness. This would be a minimum R20 insulation value. For larger attics and larger roof surface areas, recommend R21-R24 insulation levels.
Bathrooms are important too. We call it spot ventilation:
● Test to ensure the bathroom exhaust fans are pulling the correct amount of air flow out of the bathroom.
● Bathroom exhaust ducts primary function is for the removal of moisture in the air. While assisting in removal of smell is a side benefit, remember the primary function. So anywhere there is a tub or shower, the bathroom exhaust fan should be tested and confirmed to be properly working.
● Ducts for bathroom exhaust fans should be directed to the outside of the house. Never install the exhaust duct into the attic.
● Ducts installed on a bathroom exhaust should never bend within 1’ of the exhaust housing unit. Similar to a HVAC plenum, the first foot of the duct will equalize the pressure of the airflow. Disrupting this equalization process within the first foot of the duct, will greatly affect the airflow movement and reduce the amount of airflow the exhaust fan will pull from the bathroom.
● Duct installed should be short runs with limited bends. Ducts greater than 20’ in length and/or more than one 90° bend, the duct diameter should be increased by 1”. In addition to an increase in the size of the exterior vent. So a typical 80CFM bath fan would have a 4” duct and 4’ vent. If the ducts were installed with a 90° bend, a few smaller 30-40° bends and the length is 20’, then increase the duct and vent size to a minimum 5”. A typical 110 CFM fan would have a 5” duct. Same scenario, increase to a 6” duct and vent, etc.
● Do not install a bathroom exhaust fan that pulls greater than 140CFM. Typical bath exhaust fans should pull between 80-110CFM. Any more than that will cause the home to become negatively pressurized. We do not want that.
● Recommend to install a bath exhaust fan with a built-in humidistat. These will run until the humidity level has been lowered due to showering & bathing.
● If the bath exhaust fan does not have a built-in humidistat, then install a timer switch that will run the fan for a set amount of time after the unit has been turned off. Just long enough to remove any humidity caused by bathing and showering. 5-10 minutes should suffice, if the exhaust fan has been installed correctly.
It is important to understand that with today’s building technology, the house is a complete system. All building components are designed to work together to provide a comfortable and safe living environment. Walls, windows, roofs, and all mechanical systems must be properly installed and operating correctly, as designed. With the above list, if all of the components are addressed, the home will not have any issues maintaining a comfortable living environment, nor will it have issues with sweating ducts or condensation build up. However, if one item has been left out of balance with the system, the building may experience issues.