Determining the Humidity Limits-
Many people believe that 25 percent relative humidity as a lower level is still too high. The debate breaks predictably into several camps with the engineers (the aircraft people being the most vocal) arguing for no lower limit for health and only a discussion on comfort. Whereas the lung researchers and some MD's argue that until there is definitive research, why not keep the level high from a prudent avoidance perspective. This of course terrifies the microbiologists and mold researchers since higher lower limits clearly lead to mold growth in buildings and are associated with microbial contamination in typical residential humidifiers.
So on the lower limit there is no real consensus, but only a current compromise recommendation. It is pretty clear that the lower limit will not go up. The only question is how low it will end up. At present, 25 percent relative humidity is the current compromise recommendation within ASHRAE.
On the upper end, there is an emerging consensus. Interior relative humidity should be maintained so that a 70 percent relative humidity at a building surface is avoided in order to control mold growth and should never rise above 60 percent in any event.
Relative Humidity, Surface Humidity, and Condensation
Consensus among microbiologists gives the critical relative humidity for adverse biological activity to occur on building envelope surfaces to be 70 percent. Where a relative humidity above 70 percent occurs at surfaces, mold growth, dust mite growth, decay, corrosion, etc. can occur. Therefore, conditions should be maintained within a building such that the critical 70 (or higher) percent relative humidity at a building envelope surface does not occur. Due to climate differences, interior conditions which must be maintained to avoid the critical relative humidity at a surface vary from region to region and time of year. They also vary based on the thermal resistance of the building envelope.
This means in winter months in cold climates interior relative humidity should be kept as low as possible but within the comfort and health range (i.e. above 25 percent if you believe ASHRAE Standard 62-2001).
In the summer months it means that interior relative humidity should never exceed 60 percent for both comfort and health reasons.
There is a fundamental difference between relative humidity measured in the middle of a conditioned space, and the relative humidity found at surfaces due to the significant difference in temperature typically found between surfaces and the air in the middle of a conditioned space.
For a given sample of air containing water, relative humidity goes up as the temperature goes down. If the air in the middle of a room is 70 degrees at a relative humidity of 40 percent, any surface below 45 degrees will be able to condense water. Any surface below 54 degrees will have air adjacent it at a relative humidity of 70 percent - the mold limit.
Whereas when air in the middle of the room is 70 degrees at a relative humidity of 25 percent, the temperature of a condensing surface drops to 32 degrees from 45 degrees. And a surface with a relative humidity adjacent to it of 70 percent drops to 40 degrees from 54 degrees.
In other words, for condensation to occur with air at 70 degrees and a relative humidity of 25 percent, surfaces need to be colder than 32 degrees. For mold to grow, surfaces need to be colder than 40 degrees. Of course, in a nice and happy coincidence, mold does not like to grow at surfaces below 40 degrees, but will happily grow at 54 degrees. What does this tell us? Well, if surfaces are likely to be cold - say like in the winter - you are better off having a lower relative humidity.
Where relative humidities near surfaces are maintained below 70 percent, mold and other biological growth can be controlled. Since relative humidities are dependant on both temperature and vapor pressure, mold control is dependant on controlling both the temperature and vapor pressure near surfaces.
Surface Humidity and Building Assemblies: Applications in Heating Climates
In heating climates, mold growth on interior surfaces occurs during the heating season because the interior surfaces of exterior walls are cool from heat loss and because moisture levels within the conditioned space are too high. Mold growth control is facilitated by preventing the interior surfaces of exterior wall and other building assemblies from becoming too cold and by limiting interior moisture levels. The key is to prevent relative humidities adjacent surfaces from rising above 70 percent. The thermal resistance of the building envelope and the local climate determine the interior surface temperatures of exterior walls and other building assemblies. Controlled ventilation and source control limit the interior moisture levels.
Experience has shown, that where interior moisture levels in very cold climates during the heating season are limited to the 25 percent relative humidity at 70 degrees, relative humidities adjacent to the interior surfaces of exterior walls (of typical code minimum thermal resistance) fall below 70 percent and mold growth is controlled. The colder the climate (for the thermal resistance of any given building envelope) the lower the interior relative humidity necessary to prevent 70 percent relative humidities occurring adjacent interior surfaces of exterior walls. Building enclosures of similar thermal resistance interior moisture levels during the heating season. A 25 percent interior relative humidity at 70 degrees would be appropriate for Minneapolis. Whereas interior relative humidities up to 35 percent at 70 degrees would be appropriate for Cincinnati - which is located in a cold climate rather than a very cold
climate . Correspondingly, the higher the desired interior relative humidity, the higher the
thermal resistance necessary to control relative humidities adjacent to interior surfaces.
In a mixed climate, during the heating season, interior moisture levels should be limited to 45 percent relative humidity at 70 degrees. This limits the relative humidity adjacent to the interior surface of exterior walls to below 70 percent for the typical thermal resistance found in most building assemblies in this climate zones.
In cooling climates, interior mold growth also occurs because interior surfaces are typically cold and then exposed to moisture levels that are too high. The cold surfaces in cooling climates arise from the air conditioning of enclosures. When exterior hot air is cooled, its relative humidity increases. If the exterior hot air is also humid, cooling this air will typically raise its relative humidity above the point at which mold growth can occur (70 percent).
Where air conditioned "cold" air is supplied to a room, and this air can be "blown" against an interior surface due to poor diffuser design, diffuser location, or diffuser performance, creating cold spots on the interior gypsum board surfaces. Although this cold air is typically dehumidified before it is supplied to the conditioned space, it can create a mold problem on room surfaces as a result of high levels of airborne moisture within the room contacting the cooled surface. This typically leads to a rise in relative humidity near the surface and a corresponding mold problem.
If exterior humid air comes in contact with the interstitial cavity side of cooled interior gypsum board mold and other biological growth can occur. Cooling this exterior hot, humid air by air conditioning or contact with cool surfaces will raise its relative humidity above 70 percent. When nutrients are present mold and other growth occurs. This is exacerbated with the use of impermeable wall coverings such as vinyl wallpaper that can trap moisture between the interior finish and the gypsum board. When these interior finishes are coupled with cold spots (from poor diffuser placement and/or overcooling) and exterior moisture, mold and other growth can occur.
Accordingly, one of the most practical solutions in controlling mold and other biological growth in cooling climates is the prevention of hot, humid exterior air, or other forms of moisture transport, from contacting the interior cold (air conditioned) gypsum board surfaces (controlling the vapor pressure at the surface). This is most commonly facilitated by maintaining the conditioned space at a positive air pressure to the exterior and the installation of an exterior vapor diffusion retarder. Pressurization of building assemblies
is expedited by airtight construction.
Accurate Home Inspection of Atlanta
In our state they usually suggest 30 to 50 RH. Up to 60 is considered okay by many of the guidelines too.