Attic ventilation and mold
At this time of year, we are often called upon by mold growth in attics. The main reason for this to happen is the following:
- Inadequate attic ventilation
- Ventilate bathroom leaks in the attic
- Ventilate clothes dryer exhaust in the attic
- Inadequate insulation or missing insulation
- Ice dam
- Poor installation and / or aging roofing materials
I’m going to start with the first one, inadequate attic ventilation. Building science experts recognize that proper ventilation is a requirement to prevent microbial growth, shorten the life expectancy of roofing materials, and remove potentially harmful gases through the chimney effect.
All building codes (BOCA National Building Code, International Building Code, Standard Building Code, and Uniform Building Code) require the attic space to be ventilated. Generally, codes require that a minimum net free ventilation area for attic vents be a 1-150 ratio of the attic space. This rate can be adjusted to 1-300 if the attic vents are balanced. Some of these ventilation requirements are over 60 years old and are considered adequate by some.
The ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) position on ventilation is an attempt to prevent condensation from forming on the underside of the roof sheathing. The 1997 ASHRAE Foundations Manual cites several disadvantages of providing attic ventilation. Basically, in hot and humid climates, ventilation can increase vapor pressure (relative humidity) and condensation problems, so it must be analyzed on a case-by-case basis. For colder climates, the benefits generally outweigh the disadvantages.
In Pennsylvania, we are in what is called a mixed-use climate. Generally speaking, experts in building science and myself in the building science field agree that proper attic ventilation is the way to go. You must ventilate through the ceiling and to the outside to avoid potential problems with microbial growth.