Moisture & Mold

Control building pressure to reduce moisture, mold and fungi

Mold/moisture relationships are now recognized as significant sources of indoor air problems. Pressure differentials between the building envelope interior/exterior result in the greatest path of water transport and moisture intrusion into buildings.

Moisture is wicked into building materials when humidity levels are high. The building materials act as a substrate to catalyze mold growth. Saturated conditions are not required for mold growth. Although most toxic molds thrive in saturated conditions, only a relative humidity of 70% or more is required for growth.

Pressurization is critical!

Use your HVAC system to create a pressurization flow, and then control that flow using airflow measurement devices. Your goal is to provide a net positive pressurization flow (during cooling) or net neutral pressurization flow (during heating). You cannot overcome wind driven infiltration. The best you can do is net control. You should control the pressurization flow directly and not try to control building static pressure.

Mold growth may occur in negatively pressurized buildings during periods of dehumidification

An outside dew point of 65° F or greater will result in conditions favoring mold growth in a negatively pressurized building with an indoor air temperature of 74° F. Conditions where the outside air dew point exceeds 65° F are quite common in most areas of the country.

Map of areas susceptible to mold growth potential during periods of dehumidification (cooling)

The map is based on actual hourly dry-bulb and dew point temperatures over a five year period at over 100 weather stations. The map shows the amount of time that the dew point exceeds 65° F, the temperature where the envelope humidity can exceed 70%. Florida experiences this condition 60-70% of the time. Even Boston experiences at least 36 days when the dew point is 65° or above – this is important because mold can develop within hours under these conditions.

Gallons of water transported across the building envelope…

This chart demonstrates how much water can be carried across the building envelope in a year per 1,000 cfm of negative pressure. It is based on water transported when the dew point temperature of the outside air exceeds 65° F.

In Miami for example, for every 1,000 CFM of negative airflow, nearly 1000 gallons of water can be brought through the walls of a building annually (not being filtered by the outside air intake system). In New York, this value is nearly 200 gallons.

Water can also condense in the exterior walls of a building as is shown by the ‘saturation’ component of the graph (when the dew point is greater than 75° F). In Miami, over 200 gallons can actually condense in the envelope from improper building pressure.

Net positive building pressure during periods of dehumidification (cooling) results in a net drying effect

During periods of dehumidification (cooling), net positive building pressurization tends to dry out the building envelope.

Maintain net neutral building pressure during periods of humidification (heating) to prevent mold growth

Your goal is to keep the building net neutral during periods of humidification. Net positive building pressure under these conditions can result in condensation in the winter that can lead to mold growth and damage to the envelope structure. Net negative building pressure under these conditions in winter will result in temperature control problems. You will also be using your walls as an outside air filter (which is not a good idea).

Maintain Net Neutral Building Pressure During Periods of Humidification (Heating) to Prevent Mold

Maintaining net neutral building pressurization requires precise instrumentation

Maintaining net neutral (and net positive) building pressurization requires precise instrumentation, and is an ideal application for Ebtron products.