The relative humidity within a residence should be kept within the range of 30% to 60%.
Relative humidity indicates the present humidity level relative to the maximum amount of moisture the air could hold (100% RH or saturation). The threshold of 100% RH, also known as the dew point temperature, marks the point at which the air is saturated and unable to retain additional moisture. By maintaining evaporator coils at a lower temperature, the air reaches the dew point, compelling it to release excess moisture as it cools. Consequently, this excess moisture transforms into liquid water and drains away.
Higher temperatures enable air to retain more moisture before reaching saturation. On colder days, the air's capacity to hold moisture decreases, meaning that even at 100% relative humidity (RH), the total amount of moisture in the air could be less compared to a warmer day.
Humidity significantly influences comfort and indoor air quality. Elevated humidity levels impede our ability to release heat through sweat, as the evaporation process becomes less efficient. For optimal comfort, the recommended relative humidity range is generally between 30% and 60%. In terms of indoor air quality (IAQ), maintaining a relative humidity level between 40% and 50% is typically considered beneficial. Excessive moisture in homes with high relative humidity can foster microbial growth, posing potential health risks.
Factors Contributing to Elevated Indoor Relative Humidity:
1-Â Â Â Â Â Low Heat Load / Short Equipment Run Time / System Oversizing:
 Inadequate demand for heating, limited equipment operation duration, or an oversized HVAC system can lead to high humidity levels.
2-Â Â Â Â Â High External Humidity Drivers / Humidity Entering the Home:
External factors, such as high humidity levels in the surrounding environment, can contribute to elevated indoor humidity.
3-Â Â Â Â Â High Internal Humidity Drivers / Humidity Being Generated Inside the Home:
Activities like cooking, bathing, or excessive use of appliances can generate internal humidity, contributing to elevated levels.
4-Â Â Â Â Â Poor or No Spot Ventilation in Kitchens and Bathrooms:
Inadequate or underutilized ventilation systems in key areas like kitchens and bathrooms can hinder moisture removal.
5-Â Â Â Â Â High Evaporator Coil Temperature:
Conditions like elevated evaporator coil temperature or high Sensible Heat Ratio (SHR) can affect the efficiency of de-humidification.
6-Â Â Â Â Â Insufficient Total De-humidification Capacity:
If the overall de-humidification capacity of the system is insufficient for the space, it may struggle to maintain optimal humidity levels.
7-Â Â Â Â Â Low Space Temperature:
Lower indoor temperatures can lead to higher relative humidity, especially if the cooling system is not appropriately balanced.
8-Â Â Â Â Â Relying on the A/C Alone to Dehumidify:
Depending solely on the air conditioning system for de-humidification without considering dedicated de-humidification methods can result in sub-optimal humidity control.
This comprises the factors leading to elevated relative humidity in a home or building. Overall humidity decreases when you extract more moisture than you introduce, while it rises when the space accumulates more moisture than is removed.
Before delving into what to investigate and how to resolve issues, let's initially dispel some common misconceptions that frequently arise.
Misguided suggestion: Simply cool the space more.
Reality: Lowering the temperature alone results in higher relative humidity.Â
The HVAC system plays a role in dehumidifying a space. This occurs when the evaporator coil operates below the dew point, causing water to leave the pan and drain away. Termed "latent heat removal," this process is beneficial when aiming to reduce the relative humidity (RH%) in a space.
Sensible cooling is essential for enhancing comfort in various seasons by lowering the air temperature. However, when the goal is to reduce indoor relative humidity (RH%), relying solely on sensible cooling can be counterproductive.
If air is cooled without undergoing dehumidification, the relative humidity within the space actually rises. This is because as the temperature decreases, the air's capacity to hold water vapor diminishes, leading to a higher likelihood of liquid water formation.
In the process of dehumidification using cooling equipment, what's significant is the water exiting through the drain (latent heat removal), rather than simply lowering the temperature of the space (sensible cooling).
In the context of dehumidification:
Extracting water (latent heat removal) is beneficial.
Lowering room temperature (sensible heat removal) is undesirable.
Misguided suggestion: Increase insulation to reduce humidity.
Reality: Adding insulation reduces the heat load and typically results in higher relative humidity.
To enable an air conditioner to extract humidity and channel it into the drain, the system must be operational. The ambient temperature in the space needs to be sufficiently warm for the air conditioner to engage.
By incorporating standard insulation into the ceiling, floors, and walls, you reduce the heat load without altering the humidity load. This typically leads to an increase in the relative humidity percentage.
Certain insulation materials, like closed-cell foam, can function as both air and vapor barriers, preventing moisture infiltration. While this aids in lowering humidity, it's the air and vapor barrier components, rather than the insulation itself, that contribute to this effect.
Misguided suggestion: Blame the air conditioner for high humidity.
Reality: Numerous humidity problems stem from unusually high moisture levels, not the air conditioner.
The air conditioner must be appropriately sized and chosen based on sensible and latent capacity aligned with the building design. Instances arise where homes deviate from the original design or do not conform precisely to it, and weather conditions may differ from the predictions made by models.
How to Address Elevated Humidity
Numerous approaches can be employed to tackle high humidity, considering the available tools, the severity of the situation, budget constraints, and client preferences. While not possible to cover every conceivable option, here are some recommendations:
Inspect for issues like roof leaks, appropriate grading around the home, and the presence of standing water.
Conduct humidity, temperature, and dew point measurements at different spots throughout the home. This approach frequently helps identify the root cause of an issue. It's important to note that the dew point tends to rise as you approach the ceiling due to the lower density of water vapor compared to air.
Thoroughly examine the HVAC equipment. When grappling with humidity issues, configuring the equipment for 350 CFM per ton is generally a recommended approach. Ensure proper wiring, especially in multi-stage or dehumidification-equipped systems. Verify the system's airflow, with the total system static and fan chart method being a typically straightforward technique for newer equipment. .
Examine the ductwork thoroughly, addressing and sealing any leaks. Leaky ducts create pressure imbalances in the home, leading to the potential for air movement either in or out.
Ensure that dryer vents, bath fans, or kitchen ventilation systems are not leaking or discharging into attics or crawlspaces. Verify a secure connection of the dryer vent to the dryer.
Inspect and measure incoming fresh air through devices like fresh air intakes, energy recovery ventilators (ERVs), or heat recovery ventilators (HRVs). If the influx is excessive, consider reduction, but accurate calculations, and likely blower door testing, should precede any adjustments to outdoor air intake.
Search for issues such as gaps around boot entries, holes in walls connecting attics or crawl spaces to living spaces, etc. Sealing these areas can significantly diminish moisture sources.
Evaluate the condition of seals, sweeps, and weather stripping around doors and windows.
Assess whether the HVAC equipment may be oversized, potentially contributing to the humidity issue.
If the issue is a big priority, consider supplemental whole-home dehumidification
The objectives of inspecting the home and equipment are to provide the following recommendations, as applicable, for reducing indoor humidity:
1-Â Â Â Â Â Implement or add point ventilation in the kitchen and bathrooms to eliminate excess moisture at the source during use.
2-Â Â Â Â Â Modify behaviors that contribute to moisture-related issues, such as leaving doors open.
3-Â Â Â Â Â Introduce new weatherstripping and door sweeps.
4-Â Â Â Â Â Seal or incorporate sealed can lights, secure areas around boots, and close gaps between the attic/crawlspace and the home or walls.
5-Â Â Â Â Â Adjust HVAC system settings to extend operation time with a cooler evaporator coil (reheat being an extreme example).
6-Â Â Â Â Â Propose appropriate equipment sizing or the installation of whole-home dehumidification systems where necessary.
It's conceivable to seal a building so tightly that it might compromise indoor air quality. When considering sealing measures, it's advisable to conduct both pre- and post-blower door tests on the space. This enables you to determine if mechanical outdoor air intake is necessary. In such instances, a ventilating dehumidifier (and in some cases, an energy recovery ventilator, or ERV) for humid climates is recommended.
Additionally, it's important to note that operating a colder coil may lead to increased condensation on the equipment, ducts, and vents, as they will also be colder. While a colder coil can effectively reduce space humidity, it may not be a viable option if it results in excessive sweating of equipment, ducts, and vents. This consideration is often influenced by the installation location of the equipment and ducts, with attics being particularly problematic.
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When condensation occurs, there are two primary approaches: either lower the dew point (humidity) of the surrounding air or raise the temperature of the sweating surface. In some cases, lowering the dew point of the air is challenging, as seen in ducts located in attics. In such situations, the only viable option is to increase the temperature of the duct by adding more insulation or allowing warmer air to pass through it.
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It's worth mentioning that variable-speed blowers and multi-stage compression, coupled with humidity controls, can significantly aid in managing coil temperature and run-time. However, it's crucial to recognize that they are not a universal solution for all issues. Over-promising can lead to customer dissatisfaction.
Again, for achieving reduced humidity in a home, aim for:
Extended run times
Colder evaporator coil
Minimized external moisture intake
Reduced internal moisture generation
Higher indoor temperatures
Supplementary dehumidification as needed
Targeted ventilation during cooking or bathing