WHAT CAUSES MOLD
GROWTH IN THE ATTIC?
THE 6 MOST COMMON REASONS FOR ATTIC MOLD GROWTH
1. Insufficient Ventilation
2. Obstructed Soffit Vents
3. Exhaust Fans Improperly Installed
4. Inoperable Attic Fan
5. Roof Leaks
6. Improperly Installed Insulation
Mold growth observed during a Portland home inspection
WHY IS MOLD GROWING IN MY ATTIC?
An all too common problem with houses here in the Portland Oregon area: Mold growth in the attic. Mold in the home can lower indoor air quality in the home and lead to respiratory issues in some people. Building science has advanced a lot in the last few decades, making houses more efficient, and less prone to air leakage. Older homes are generally more “leaky”, allowing for more natural air infiltration and exfiltration. For these reasons we tend to see more mold in attics in houses built after the 1970’s
1. INSUFFICIENT VENTILATION
There are many types of roof or attic construction, but the most common type of construction in Portland houses is a vented attic. A vented attic will have ventilation both at the high points of the attic (near the ridge or gable end) and the low points (near the soffit or eaves). Ventilation should be split evenly between the ridge and the soffit.
Mold growth on the underside of the roof decking
2. OBSTRUCTED SOFFIT VENTS
Soffit vents can easily be blocked by attic insulation if proper baffling or blocking has not been securely installed. Blown insulation or batt insulation will inhibit the flow of intake air into the attic if it is not kept from blocking the soffit vents.
Blown-in fiberglass insulation that is blocking the soffit vents. Note the mold growth on roof decking
3. EHAUST FAN INSTALLATION
Bath fans, clothes dryer exhaust ducting, range hoods and laundry room fans should be vented to the exterior, and never vent into the attic. This brings especially most air into the attic and will greatly increase humidity levels.
Bathroom exhaust duct terminating in the attic
4. INOPERABLE ATTIC FAN
Some attics are equipped with an attic fan, that is operated by a thermostat to turn the fan on in warmer weather. Unfortunately, attics are often times out of site and out of mind.
Attic fan that was not functional at the time of the inspection
5. ROOF LEAKS
Some roof leaks can go unnoticed for years, slowly causing damage to the roof structure and introducing moisture to the attic. It is best to periodically check the attic for signs of moisture intrusion, especially during the wetter months of the year. Roof valleys, or flashing around chimneys or other roof penetrations can be more susceptible to leaks. Be sure to pay special attention to these areas.
High moisture levels indicated by pin-type moisture meter
6. IMPROPERLY INSTALLED INSULATION
Typical attic roof assemblies are a vented type. Although some roof assemblies are unvented, these require the use of closed cell foam, that cannot retain moisture. In a vented roof assembly or attic, it is essential that air be able to flow throughout the attic, to dry out moisture that may condense on the underside of the roof decking. Fiberglass batt insulation that is installed up against the underside of the roof decking does not allow the proper flow of ventilation air. This traps moisture between the roof decking and insulation. These conditions are conducive to mold or organic growth and can cause damage to building materials and lower indoor air quality in the home.
Fiberglass batt insulation installed up against roof decking