With the advent of indoor air quality (IAQ) issues, humidification has become an important part of providing acceptable indoor environments, especially in northern climates. Humidification design has not received a lot of attention because for many years humidification systems have been “optional.” Successful humidification system design depends on understanding the science of humidification and developing the “art” of proper application.

The following includes a general discussion of design criteria, building envelopes, load calculations, system selection, equipment selection, and equipment installation.

Design Criteria

The first step for a successful humidification system design is to assist the owner in establishing obtainable design goals. Are humidity requirements based on process requirements, code requirements, or human comfort? Process and code requirements may dictate specific humidity levels. Humidification levels for human comfort are not normally mandated by codes. Guidelines for humidity levels for human comfort are available through such sources as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). Operating costs and limitations imposed by building envelopes also play an important part in determining humidification design criteria. Low humidity levels will minimize operating costs, but may not provide satisfactory human comfort. High humidity levels can cause extensive damage to building materials due to condensation and can result in high operating costs. So what is an appropriate goal for normal human occupancy? In most northern climates, 30% rh is a relatively common design target that generally satisfies most human comfort needs at reasonable operating costs.

Moisture levels can be measured not only by relative humidity but by specific humidity which is the moisture content expressed in weight of water vapor in pounds (or grains) per pound of dry air. A property associated with specific humidity is the dewpoint temperature or the temperature at which water vapor begins to condense to liquid water. Dewpoint temperature is important because any surface at or below the dewpoint temperature will result in unwanted liquid water. In selecting humidity levels, keep in mind that all building surfaces (e.g., window panes, mullions) inside the vapor barrier must be above the dewpoint temperature.

Dewpoint temperatures can be determined by using a psychrometric chart. Figure 1 shows the relationship of rh as a function of temperature and moisture. Note that the dewpoint temperatures occur along the saturation (100% rh) line.

In evaluating design criteria, the supply air temperature delivering the humidity to the space can be a limiting factor. Humidity of supply air should be limited to 85% to 90% rh to prevent condensation inside of the ductwork. This allows a margin of safety if the control system hunts or overshoots the design humidity setpoint. At 50˚F drybulb temperature and 90% rh, the supply air dewpoint temperature should be less than 47˚F. With no other space gains, this will result in a space humidity of about 40% at 72˚F. This indicates that if a humidity level higher than 40% rh is required, a higher air supply air temperature is required. Humidification capacity can be increased by raising the supply air temperature and increasing the air flow. The higher air flow is required in order to meet the sensible cooling load when higher supply air temperatures are used.

Building/Space Envelopes

The building envelope is defined as a system of building elements which separate conditioned space from the exterior. Space envelope is similar, but separates spaces with different conditions. The building envelope will include the exterior walls, roof, windows, doors, etc. The space envelope will include interior partitions separating a humidified space from a space at a lower humidity level.

Building envelope insulation must be sufficient to keep the surface temperature on the conditioned side of the vapor barrier above the dewpoint temperature. Methods for determining surface temperatures can be can be found in ASHRAE Handbook of Fundamentals. The generalized form of the equation is:

Tsurface = Tspace – (Tspace - Tambient) x Rinside/Rtotal (equation 1)

Where Rinside is the R-value inside the vapor barrier, and Rtotal is the total R-value of the building envelope assembly.

A sample problem using this equation illustrates how to determine the surface temperature at a single glazed window with a total R-value of 1.13 when the inside air temperature is 70˚F and an outdoor air temperature is 0˚F. The air film R-value for a non-reflective surface is 0.68. The surface temperature of the glass is calculated to be:

Tsurface = 70˚Fspace – (70˚Fspace - 0˚Fambient) x 0.68/1.13 = 27.9˚F (equation 2)

Using the chart in Figure 1, the corresponding maximum space relative humidity to avoid condensation at 70˚F space temperature is 20% rh. Note that the maximum humidity level is limited by the building envelope assembly with the lowest composite R-value. Many times this is a glazed element such as a window or door, but it could be other elements such as a window frame with either no thermal break or poor thermal break.

Spaces such as surgery rooms, computer rooms, and other rooms which require higher humidity levels due to process requirements are ideally located towards the building interior away from any exterior walls. If these type of spaces are located on exterior walls with windows, be sure to specify windows and doors with low u-factors, and frames and mullions with high performance thermal breaks.

Vapor barriers are required in the construction of humidified spaces. Not only do vapor barriers retard the flow of moisture to unconditioned spaces or the outdoors and reduce humidification loads, but it also protects building elements from damaging effects of condensation within the envelope. Do not attempt to humidify a building or space which does not have a good vapor barrier in the exterior wall. Retrofitting an existing building with humidification can have catastrophic results if the building does not have an adequate vapor barrier. Vapor barriers are also required in the envelope for interior spaces which have higher humidity requirements than adjacent spaces. Inclusion of an internal vapor barrier will reduce moisture migration, lower humidification loads, and associated operating costs. The vapor barrier should completely encase the conditioned space, floors, and walls including above ceilings, and roofs/ceilings. Constructing the ideal vapor barrier is difficult. Sheet materials with low permeability (perm) with all joints sealed with a low perm tape comes close in achieving this goal. Examples of good vapor barriers include aluminum foil or plastic sheeting with overlapped and taped joints. These materials have a perm rate of less than 0.10 grains/hour/sq ft/in. Hg vapor pressure. Table 1 lists perm rates of various building materials. These and more values can be found in Chapter 24 of the 1997 ASHRAE Handbook of Fundamentals or from manufacturer’s published data.

Location of the vapor barrier is just as important as the vapor barrier itself. The idea is to prevent moisture from coming into contact with building envelope surfaces below the dewpoint temperature. Vapor barriers should always be located on the warm side of the building envelope assembly and at a location above the dewpoint temperature.

Vapor barriers are not perfect and will allow some moisture to pass through. Exterior wall details should include measures to vent to the outside any moisture that may pass through the vapor barrier. Always avoid the use of more than one vapor barrier that will trap moisture inside the building envelope assembly.

Figure 1. To find the dewpoint temperature, enter the chart at the bottom using drybulb or direct measured air temperature. Draw a line vertically until you intersect the desired relative humidity line that represents the space control point. Then draw a line horizontally to the left until you intersect the 100% relative humidity curve (also called the dewoint curve). Then draw a line straight down to intersect the temperature line. This will be the dewpoint temperature. The opposite can be done to determine the maximum allowable space relative humidity if you know the dewpoint temperature and space temperature.

Load Calculations

Humidification loads include ventilation loads and moisture migration loads. If a proper vapor barrier is used, the ventilation loads will be the major portion of the total humidification load. The moisture migration load is a small portion of the total (less than 1%) and usually is ignored. When a good vapor barrier is not used or when ventilation loads are very minimal, the moisture migration load can become significant and should be calculated as described in Chapter 22 of the 1997 ASHRAE Handbook of Fundamentals.

The ventilation load equation is expressed in Table 2. To apply this equation properly, the ventilation air flow rate must be determined. In systems with no forced ventilation, the ventilation air equals the infiltration air quantity. In systems with a fixed percentage of outside air, the humidity load is based on the outside air flow rate and the design outside air conditions. If airside economizers are used, the peak humidity load occurs when the product of outside air flow rate and grain difference is at its maximum. This usually occurs at outside air temperatures between 30˚ and 55˚F.

Another issue with calculating humidification loads for systems with airside economizers is determining outside air humidity levels for various outside air temperatures. Outside specific humidity can vary with location, time of year, time of day, and local meteorology. Ideally, an analysis should include an hour by hour calculation or an analysis using representative bin weather data.

However, there are other, less complex methods to achieve acceptable results. If the humidity level is critical to a process, humidifiers can be sized with 100% outside air and with the outside air having zero grains moisture and leaving supply air humidity at its maximum based on 90% rh at the supply air temperature. This method will ensure a humidifier that is large enough, but may result in an oversized humidifier which can be difficult to control. Another method is to estimate outdoor air dewpoint temperatures as a function of outdoor air drybulb temperature minus the daily range temperature difference (20˚- 30˚F). This simulation can be easily set up in a spreadsheet format.

Table 1. Perm rates of various building materials (grains/hour/sq ft/in. Hg vapor pressure).

System Selection

Once load calculations are completed, a humidification delivery system needs to be selected. If the main air-handling system is to be used, one should determine if there is sufficient supply air to deliver the amount of moisture required. When humidification loads are based on ventilation rates and space dewpoint temperatures below the supply air temperature, calculations based on the sensible cooling supply air quantity and temperature should be sufficient to transport the required moisture to the space.

For spaces with high humidity levels and/or spaces with loads based on moisture migration, the supply air flow rate may be based on humidification requirements in lieu of temperature requirements. This may require that constant volume systems be used instead of variable volume systems or booster humidifiers be used downstream of reheat coils. Humidifiers located directly in the conditioned space, independent of air handling systems, may also have to be considered.

After the air delivery system is selected, the type of humidifier and water/steam source can be selected. There are two basic types of humidification processes, adiabatic (evaporative) and isothermal (direct steam injection). Adiabatic types evaporate liquid water into an air stream. As water is evaporated, 970 Btu per pound of water is taken from the air stream. This adiabatic process reduces the air temperature as moisture is absorbed. The evaporative cooling from adiabatic processes can reduce cooling loads, outside air quantities during economizer cycles, and reduce humidifier loads. Usually at some point when the outside air is low enough and the outside air is at its minimum, additional heat energy for an adiabatic process is required to keep supply air temperatures within design parameters.

Evaporative-type humidifiers come in various styles. One of the oldest systems is the spray coil. This system pumps water from a collection basin under the coil and sprays water over a coil in the air stream. A float valve opens to maintain water level in the collection sump. This system has fallen out of favor due to high maintenance costs and IAQ problems associated with standing water. Evaporation pads are similar to spray coils but use a replaceable evaporative pad. This reduces some of the maintenance costs, but still has the same IAQ problems as the spray coil.

Foggers/misters atomize the water into droplets 10 to 50 micron in size. Most foggers use spray nozzles with compressed air. Other types use ultrasonic vibration to create sub-micron water droplets. The effectiveness of these systems are determined by vapor pressure difference between the air and water, surface area of the water (size of the water droplets), and mixing of fog particles with air molecules. In general, most of adiabatic systems are usually limited to increasing air streams to 50% rh, which limits the space humidity to 28% rh at 72˚F space temperature when using 55˚F supply air.

The key to successfully applying foggers or misters in air-handling systems is designing good mixing systems and allowing for proper absorption distance. The best application for this type of humidifier is for direct spray into conditioned spaces with high ceilings or industrial applications such as greenhouses. Isothermal systems use pure water vapor or steam. Steam is usually not limited by the performance factors of the adiabatic systems. One hundred percent of moisture can be absorbed and relative humidity levels of 100% can be achieved, but are normally limited to 90% to allow for a margin of error of the control system. Steam humidifiers come in many types and configurations. If properly applied, each can perform effectively.

The main difference between different steam grid humidifiers is the amount of distance required to absorb the visible vapor trail which can easily condense on any surface that it comes in contact with. Absorption distance can vary from a few inches to many feet. Within this absorption distance, there should be no obstacles on which moisture can condense. The length of absorption distance is usually published by each manufacturer and is a function of humidity levels upstream and downstream of the humidifier, and performance of the steam distributor grid. In specifying steam humidifier grids, be sure to include the maximum absorption length allowed for in your design. Do not rely on manufacturers in reviewing your design for proper application of their equipment.

Steam/water source can greatly affect the indoor air quality and performance of the humidifier. Water used for adiabatic processes should be free of dissolved minerals which deposit on spray heads or destroy ultrasonic type humidifiers. Another concern with ultrasonic-type humidifiers is that any dissolved minerals can be atomized and become airborne. Steam systems are often treated with chemicals to reduce corrosion. Some of these chemicals, depending on the amount released to the air stream, have been found to adversely affect health or performance of copiers and printers which use toners.

Proper consideration must be given to the water source used for humidification. This issue should be explained in detail to owners so they can understand the issues which can affect equipment selection. The use of clean steam or steam-to-steam generators has become more popular in recent years. These systems will usually use high-quality treated water with chemicals and minerals removed which can adversely affect health and system performance.

Table 2. The ventilation load equation.

Equipment Installation

One of the big questions is: “Where is the best location for the humidifier?” Options include the supply duct downstream of the air-handling unit, in the air-handling unit upstream of the cooling coil, or downstream of the cooling coil before the supply fan. Each option has advantages and disadvantages. The two primary issues to be concerned with are allowing for proper absorption distance and providing good mixing before any obstacles are encountered in the air stream. The absorption distance for isothermal humidifiers is a function of dispersion mechanics (number and size of injection nozzles) and total humidification capacity. Adiabatic humidifiers must also consider supply air temperature and droplet size. Proper mixing is a very important factor in absorption of the vapor trail before it encounters any obstacles. Distribution tubes should be away from internal air-handling equipment and bulk heads which can result in uneven air flow within the unit. Examples of locations to avoid include immediately downstream of face and bypass dampers, coils with integral face and bypass dampers, sound attenuators, coil blank-offs, and upstream of filter banks. Elbows and duct fittings downstream of humidifiers should also be considered obstacles to be avoided.

Proper mixing must include even air flow across steam or mister outlets. Avoid elbows, duct fittings, bulk heads, dampers, or sound attenuators directly upstream of humidifiers. Humidifiers located immediately downstream of fan discharges, where velocity profiles have not had a chance to develop, should be avoided. Avoid locating humidifiers in zones of low air velocity where steam and air do not mix effectively. Air stream takeoffs before the vapor has mixed can result in some zones receiving more humidity than others. Be sure to allow for adequate mixing and absorption distance before any takeoffs. In evaluating humidifier locations in ductwork, be sure to have at least three duct diameters of straight duct upstream to have even air flow and at least the calculated absorption distance plus 12 in. of straight duct before any elbows or duct fittings. For primary humidifiers, this results in having at least six to 16 ft of straight duct which can be a problem in today’s tight mechanical rooms.

With proper allowance for absorption of the vapor trail, drain pans are technically not required. However, it is not bad design practice to provide some sort of drainage to remove condensed moisture that may form when less than ideal conditions are present (variable volume, start-up, etc.). Drain pans in air handling unit sections downstream of humidifiers are a good idea.

Steam quality should be considered when analyzing humidifier performance. If steam is from a local humidification steam generator or a local pressure reducing valve, the quality of the steam will be fairly high. In these cases humidifier grids can be used without separators. If steam is piped a considerable distance, heat losses from the piping system will result in mixed flow of steam and condensate. In these cases steam separators should be used to remove the condensate. Some manufacturers recommend the use of temperature switches to allow time for the separator to come up to temperature before allowing steam to enter the distribution manifolds. With cast iron units, this is probably a good idea. Considerable heat is consumed and condensate is formed in raising the temperature of cast iron to steam temperature. With stainless steel units, little heat is required and the expense of a temperature switch has marginal benefit. Steam piping should be properly trapped upstream of humidifiers and humidifier grids should also be trapped. Be careful to follow the manufacturer’s recommendations for trapping. Traps from humidifier grids are usually operating at atmospheric pressure with very little hydraulic head. These traps should not be piped to a condensate system with any back pressure. Consider piping these traps to a separate condensate pump/receiver unit or wasting the small amount of condensate to the drain.

Local steam generators or humidifier grids will sometimes discharge water to drain to reduce mineral concentrations. Steam will condense inside of large humidifier grids at points lower than the steam separator and will require additional condensate drains. This water is usually close to 212˚F and needs to be cooled to at least 140˚F to meet local code requirements. Be sure to specify drain coolers and provide nonpotable cold water to mix with this drain water before discharging to drain.


Successful humidification systems are dependent on many factors, establishing correct design criteria, providing an adequate vapor barrier, estimating loads correctly, proper system selection, equipment selection, steam source evaluation, and proper installation. Science and art must come together in understanding and applying humidification systems within the limits imposed by these systems.