ASHRAE STD 62.1
Ventilation for Acceptable Indoor Air Quality
|Publication Date:||26 June 2019|
The 60°F (15°C) indoor-air dew-point limit avoids the microbial growth problems frequently observed when humid outdoor air infiltrates into buildings that are mechanically cooled. Microbial growth is common during cooling seasons and especially when cooling and occupancy are intermittent. Examples include in schools during summer vacations, apartments and condominiums that are intermittently occupied during summer months, college dormitories and military barracks that are unoccupied for long periods, and health care buildings and hotels in hot or humid climates that contain both naturally ventilated and mechanically cooled spaces
Humidity-related requirements of earlier versions of Standard 62.1 were intended to address both mold growth and comfort concerns by limiting indoor humidity to 65% rh. That requirement, however, did not explicitly extend to unoccupied hours when microbial growth often accelerates. More importantly, because it did not establish a coincident dry-bulb temperature, the 65% rh limit did not limit the mass of water vapor available for surface absorption during periods when cooling is intermittent to conserve energy.
Microbial growth is governed by the availability of moisture in the surfaces of building materials, coatings, furnishings and mechanical systems. The relative humidity (RH) of the air does not affect microbial growth until the water vapor is absorbed or condenses into the surface. Limiting the indoor-air dew point rather than the RH limits the total mass of water vapor available for condensation or absorption. Further, limiting the dew point to 60°F (15°C) prevents actual condensation until the air contacts a surface that is cooler than 60°F. Few surfaces are cooled that low in buildings, even allowing for typical cold-air leakage into interstitial spaces and the frequently less-than-perfect insulation of pipes, valves, and duct work.
This specific limit is a compromise between energy and microbial growth concerns. Lower indoor dew points would further reduce risk. For example, a 55°F (13°C) maximum dew point is the guidance contained in the 2001 and 2008 Humidity Control Design Guide for Commercial and Institutional Buildings and in the 2015 ASHRAE Handbook- Applications, Chapter 62, "Moisture Management in Buildings," and Chapter 23, "Museums, Galleries, Archives and Libraries." The 55°F dew-point limit is also required for all high-performance buildings as defined by the General Services Administration's 2017 PBS-P100, Facilities Standards for the Public Buildings Service. But a dew-point limit of 55°F (13°C), while certainly an improvement appropriate for reducing risks and improving comfort in high-quality buildings, could also increase energy consumption in unoccupied buildings in highly humid climates, especially when a building is not airtight. A dew-point limit of 60°F may provide a more affordable balance between the equally important concerns of reducing energy consumption and reducing risks to occupant health from microbial growth.
As noted by Section 5.9, Exception 1, buildings or spaces that are neither equipped with nor served by mechanical cooling equipment can be exempted from the dew-point limit, because their surfaces tend to stay warm during humid weather, which helps avoid moisture absorption and the risk of microbial amplification.
Note: In this addendum, changes to the current standard are indicated in the text by underlining (for additions) and (for deletions) unless the instructions specifically mention some other means of indicating the changes.