Attic Ventilation and Mold
This time of year we are often called out for problem with mold growth in attics. The main reasons for this to occur are as follows:
* Improper attic ventilation
* Venting bathroom exhausts into attic space
* Venting clothes dryer exhaust into attic space
* Inadequate insulation or missing insulation
* Ice damming
* Poor installation and/or aging of roofing materials
I’m going to start with the first one, improper attic ventilation. Proper ventilation is recognized by building science experts as a requirement for preventing microbial growth, preventing shortening the life expectancy of roofing materials and removal of potentially harmful gases via the stack effect.
All of the building codes – BOCA National Building Code, International Building Code, Standard Building Code and Uniform Building Code require attic space to be ventilated.
Generally, codes require a minimum net free ventilating area for attic vents to be 1-150 ratio of the attic space. This rate can be adjusted to 1-300 if attic vents are balanced. Some of these ventilation requirements are more than 60 years old and considered inadequate by some. ASHRAE’s (American Society of Heating, Refrigerating and Air Conditioning Engineers) position on ventilation is an attempt to prevent condensation from occurring on the underside of roof sheathing. ASHRAE’s 1997 Fundamentals Handbook cites several disadvantages to providing attic ventilation. Basically, in warm humid climates venting can increase vapor pressure (relative humidity) and condensation issues, so, it has to be looked at on a case-by-case basis. For colder climates, the benefits generally outweigh the disadvantages. We are in what we call a mixed-use climate. It is generally agreed by building science experts and myself in the building science arena that proper attic ventilation is the way to go.
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