How to Prevent Mold in Buildings
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• Attic Condensation and Ice Dam Leaks: If roof leaks or attic moisture condensation due to a combination of inadequate attic ventilation and a building moisture source (wet basement, plumbing leaks, roof leaks) causes excessive moisture or actual wet conditions in an attic, conditions are ripe for extensive mold growth. Visible mold may appear on wood surfaces in an attic such as on rafters or roof sheathing.
  
  Hidden mold may be present and may be even more of a problem if it forms in insulation or in the ducts and air handler of an air conditioning or heating/air conditioning system. Typical building air convection currents tend to move air up and out from lower to upper building levels, so one would not think that much mold would move down from an attic into the living area. But important exceptions to this can quickly move problem mold from an attic into a living area. For more details on this topic see Attic Condensation & Ice Dams, Detection and Correcting Venting and Condensation Problems in Buildings.

• Building Exterior Leaks and Mold No mold cleanup project will be successful unless you correct the conditions that caused mold growth in the first place.
  
  An expert inspection and report should find and suggest remedies for site and building exterior conditions that produce mold or for building areas that serve as a mold reservoir or as amplifiers for allergens, mold, mildew, excessive pollen or pet dander.
  
  The basic steps: find all unwanted moisture sources, correct appropriate building, site, landscaping, & construction details. 90% of the wet basements and crawl spaces I see are caused by bad or missing roof gutters and downspouts. An IAQ investigator who has training and experience in building science, mycology (mold science), and IAQ, or in some cases an experienced ASHI-Certified home inspector or sick building investigator who is who has a similar in-depth understanding of construction failures can be helpful at this step.
  
  

The photograph shows a double problem with this wood shingle exterior wall: shingles are improperly installed and falling off of the home, inviting water to enter the wall structure. Because there is no roof overhang or eaves, and not even a gutter on the home, roof runoff runs right down this wall.

• Building Interior Leaks and Mold: address interior conditions that produce or serve as reservoir or amplifier for common allergens: mold, pollen, pet dander as well as other possible respiratory irritants such as latex, paints, product-outgassing furniture or carpets, and renovation hazards such as lead paint or chemicals or other environmental issues.
  
  Plumbing leaks that are unattended, water running under kitchen or bathroom cabinets when fixtures are in use, and air conditioning system condensate leaks into the building or into the HVAC duct work are examples of indoor leaks that can lead to a mold problem if not promptly attended.

The photograph shows extensive staining from protracted plumbing leaks in a house that was left unattended. The result was extensive basement mold requiring demolition and cleaning of large area of the home.

• Indoor Humidity Control: What indoor humidity should we maintain in order to avoid a mold problem?
  
  A maximum indoor relative humidity of 55 should be OK, 50 better, 45 for an attic knee wall space which is not vented to outdoors. The moisture level you detect varies depending on when, where, and how you measure it.
  
  For normal home use we like modern dehumidifiers such as this Sears (TM) unit which permit setting a specific humidity level and also permit connecting the dehumidifier to a drain so that it can operate unattended. The article cited at the end of this paragraph explains the need for maintaining an anti-mold low humidity level in a building, sets moisture level targets, and tells you how to get the indoor moisture level into the desired range.
  
  
  More detail about how to control indoor humidity is at Indoor Humidity Control:

• Duct Cleaning for mold or bacterial contamination: Leaks into duct work wet organic dust and debris deposited on the duct interior, inviting mold growth. Metal ducts and air handlers can often be cleaned.
  
  My experience is that fiberglass and fiberglass-lined ducts and air handlers cannot be cleaned effectively and worse, improper cleaning may cause dangerous release of fiberglass into the building air.
  
  Don't just spray ducts with disinfectants and sanitizers. The US EPA advises caution about using these chemicals, the AIHA also advises that occupants may suffer from respiratory irritation from these chemicals, and research on fungicidal treatment of fiberglass HVAC ducts suggests that their effectiveness is limited anyway.
  
  The photo shows stains indicating a history of leaks into this fiberglass lined HVAC duct. We recommended replacing the insulation, cleaning the duct interior (or replacing the duct work if that was less costly), and making sure that the leak source was repaired.

• Indoor Air Cleaning Products can help improve indoor air quality, but watch out for products that are ineffective.
  
  At least one study has shown that there at least up to 2005 there was not one portable free-standing home "air cleaner" or "air purifier" on the market that moved enough cubic feet of air per minute to actually clean a home much less a room of problem airborne particles.
  
  Portable air cleaners: The photo above shows a popular portable air cleaner in a room where we ran a time-lapse impaction sampler to collect particles at regular intervals to see what the machine seemed to be doing. We compared the particle levels in this room with the door shut, with the door open, and with other building areas.
  
  OPINION: Our clients' experience, confirmed by our own field testing suggest that in a room without severe dust and particle reservoirs (no pets, no wall to wall carpets), some air cleaners might reduce, but by no means eliminate, problematic airborne particles. The particle reservoir forms what is in a practical sense, an infinite particle source.
  
  Central HVAC systems, with a much larger air handling capacity, can be equipped with filtration that can substantially reduce the level of indoor airborne particles. Their effectiveness can be increased still further if the system permits the fan and filtration to run continuously.
  
  Air Cleaning Products are discussed and listed further \ here.