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This article discusses the initial response to building flooding, and in more depth, building methods and materials useful for improving the mold-resistance of new construction, renovated buildings, and repairs after mold remediation in buildings. Readers should also see Mold Action Guide after Flooding: How to Minimize Mold Damage After a Building Flood and Flood Response for Septic Systems what to do after a septic system has been exposed to flooding. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

PRIORITIES - Priorities for Preventing Indoor Mold Contamination in Buildings

All mold is everywhere, all the time, according to one of our mold mentors, an experienced mycologist. So how do we prevent a mold problem in a building? We avoid or quickly correct conditions that invite problem mold colonies from growing indoors.

In areas subject to high humidity, wet soils, rain, freezing weather, or other conditions that encourage high indoor humidity, building leaks, poor building ventilation, or other conditions that tend to produce indoor mold and its potential harmful effects and high costs, it is possible to reduce the chances of a future mold problem.

1. Respond immediately to building leaks and floods. This means within 24-48 hours all of the critical steps need to be taken if you want to maximize the chance of avoiding a costly mold cleanup project. This topic will be expanded in detail at this website (coming). Basically:
• Before entering a building that has been flooded review FIRST PRIORITIES: What to Do After a Building Has Been Flooded to address life, health, and safety concerns.
• Stop the water entry
• Remove standing water
• Remove wet carpets, furniture, and boxes of wet stored items
• Remove floor trim and lower portions of walls (such as drywall or paneling) (at least 12") and any wall insulation, in rooms where the floors were wet or flooded.
• Remove upper portions of wall coverings (drywall or paneling) higher than 12" if these areas are wet, or if water entered the wall cavity from above.
• Remove ceilings that have been flooded from above
• Remove wet materials (such as areas listed above) until you find a 12" or greater area of dry clean margin.
• Use dehumidifiers, fans, heaters, to dry the exposed building areas and surfaces.
• If mold is already visible or suspected, use containment to avoid air movement from the damaged (moldy) area to other building areas. Containment generally means negative air and poly plastic barriers.
• Read: Mold Action Guide after Flooding: How to Minimize Mold Damage After a Building Flood
• Read: Flood Response for Septic Systems what to do after a septic system has been exposed to flooding
2. Keep unwanted outside water out of the building. This means attention to the roof drainage system (gutters and leaders), surface drainage, and at some sites, unusual levels of ground water. In buildings where we find recurrent basement water entry, most of the time the underlying cause is inadequate maintenance of gutters and downspouts, with roof spillage against the foundation.
   
   Preventing indoor mold by keeping outside water out also means proper construction of all exterior components, roofing, siding, windows, doors, trim, steps, patios, exterior light fixtures, even downspout straps, to keep water out of building walls and cavities.
   
   See vapor barriers for a discussion of vapor barriers behind vinyl siding. The importance of flashing and house wrap on conventional construction pales next to the importance of property detailing when problem-prone building exteriors such as EIFS Synthetic Stucco are used since if workmanship is not exactly correct with those materials leaks into the building cavities trap water and often lead to costly damage, rot, or mold. See Exteriors, Landscaping, & Siding - Inspection, Repairs, Product Failures for a discussion of exterior building materials and defects.
3. Humidity: Maintain Proper Indoor Humidity Levels to avoid mold. See What indoor humidity should we maintain in order to avoid a mold problem?
4. Leak Prevention: proper roofing and flashing details are critical to avoid longer term building leaks at the roof and at other building penetrations such as windows, doors, plumbing vents. Indoors, replace corroded plumbing traps, use burst-resistant washing machine hoses and fixture supply risers. When possible, turn off water when leaving a building vacant for some time.
5. Mold-resistant Building Materials: should be used where possible, especially in high-risk areas such as basements and bathrooms. See Mold proof drywall ? You Must Be Kidding!
6. Mold-Friendly Building Materials: should be avoided in high risk areas. Do not put mold-friendly construction materials (stuff on which mold grows readily) into damp areas. In our experience this means the items listed at Interior Construction Product Installation Details below
7. Ventilation: to avoid indoor mold, proper venting avoids build-up of high moisture in building cavities and avoids, in freezing climates, leaks into building cavities from ice dams. See our discussion of attic venting and under-roof ventilation at Detecting and Correcting Attic Condensation and Preventing Ice Dam Leaks in Buildings and see our discussion of bathroom ventilation fans, ducts, and indoor moisture control at Bathroom Ventilation. Crawl space ventilation and dryout steps are discussed at How to Dry Out a Crawl Space.

IMPORTING MOLD - How to Avoid Bringing Problem Mold Into a Building

"Pressure Treated lumber" which is sold as resistant to rot and resistant to wood destroying insects is very often not only wet when purchased, but is often mold contaminated with several species of Penicillium sp. or Aspergillus sp. We confirmed this condition by a survey of building materials at several lumber suppliers in New York, using tape samples of visible mold on the surfaces of these products.

When using pressure-treated lumber for interior framing, clean off any visible mold. Simple power-washing would suffice. This step is not necessary and would be inappropriate for the same lumber when used outdoors, such as for a deck or an entry stair.

But inside, such lumber may be used for sill plates or in some cases I've seen it used to re-frame a rotted floor over a wet crawl space. Importing a large Aspergillus sp. colony on the floor framing surface over a crawl space provided an immediately-detectable high level of airborne Aspergillus sp. in the room above this area, as these spores move easily in convection air currents moving from the crawl area up through the building.

COSMETIC MOLDS - How to Avoid Unnecessary Mold Cleanup Expenses

As long as we are discussing not bringing mold into a building, it's important to warn against unnecessary expenses cleaning up "cosmetic-only" molds such as black mold commonly found on kiln-dried lumber which has been exposed to wetting in transit or storage.

Usually this is a cosmetic-only mold in the Ceratocystis/Ophistoma bluestain group. However in a few cases where this "black mold" on lumber was on wood framing later exposed to flood basements or crawl spaces, I've also found problematic mold growing mixed with this cosmetic inhabitant.

See Stuff that is Not Mold or Harmless Mold and review Harmless Cosmetic Molds in that document.

CONSTRUCTION DETAILS - Interior Construction Product Installation Details to Reduce Mold-Risk

• Avoiding Mold Growth on Drywall: when installing drywall in basements or in any location where a floor is at extra risk of becoming wet, keep the bottom edge of drywall at least 1/2" off of the floor. Avoiding floor contact means that a small spill which wets the floor, if cleaned up promptly, will not soak up into the drywall itself. We 'm not sure it's cost effective but you might want to consider using mold resistant drywall.
• Avoiding Mold Growth on Wood paneling: use the same floor clearance detail for wood paneling. We find severe mold not only on the exposed side of wood paneling but often, even if no mold is visible on the room side, the un-coated back surface of this material supports mold growth in buildings which have been wet or subject to high moisture.
• Avoiding Mold Growth on and Behind Floor trim: Even weeks after a "water damage company" had reported that they had "fully dried out a building" by "extracting the water" in it, we found very high moisture in the lower portion of the building walls.
  
  Pulling off floor trim showed (in several investigations) that the back of the wall/floor trim was not only still wet with visible water droplets, but it had already become moldy. The drywall behind the this trim, and the wall cavity itself as well as insulation in it were also quite wet and moldy. When installing floor trim in a basement below grade, or in a bathroom or kitchen where spills are likely, I back-prime and end-prime all trim boards.
  
  In bathrooms we caulk the trim to the floor, putting a small bead of caulk on the under-side of the trim boards as they are placed. The caulk won't be visible, but it'll reduce the chances of a small spill sending water into the wall cavity.
• Avoiding Mold Growth on Floor cabinets in Bathrooms and Kitchens: When we are installing them, we spray the under-side and backs of floor cabinets with clear lacquer before installing them. This takes only a few moments, as the lacquer dries almost immediately. (Proper venting needed.) By sealing these surfaces we reduce the rapidity of moisture up-take into them in damp or wet conditions. Reducing the moisture uptake in these materials reduces potential mold growth on these hidden surfaces and gives more time for building dry-out after a wetting event. In bathrooms we caulk the junction of the exposed cabinet bottom and the floor after the cabinet is in place.
• Avoiding Mold Reservoirs in Fiberglass insulation: Don't put fiberglass insulation under floors over damp or wet crawl spaces - we are conducting a study of this topic. Preliminary data shows that very often we find sever Aspergillus sp. or Penicillium sp. infection of fiberglass over damp crawl spaces and in damp building walls against below-grade foundations, even when there is no mold actually visible on the insulation or its kraft paper facing. On below-grade building walls that are to be insulated we prefer solid foam insulating board as it does not hold moisture and is less mold-friendly. Also see FIBERGLASS HAZARDS in buildings.
• Avoiding Mold Reservoirs in Indoor Carpeting: wall-to-wall indoor carpeting which has been wet should be discarded. We have never seen a successful clean-up of this material in place after flooding. Carpets form a particle reservoir, including allergens, mold, mites, etc. in buildings which can be a problem especially in basement areas which can be expected to be damp even when there is no water entry. We advise against using carpeting in these areas at all. If basement carpeting in a building has been wet, remove and discard it promptly. After building dry-out and cleanup have been completed, install a hard surface flooring or simply paint the exposed concrete slab with an epoxy paint. Nicer surfaces such as ceramic or vinyl tile work well. (Sheet vinyl may become moldy on its backside if the floor or slab below are damp.) If you are concerned about making a warmer or more quiet surface, use area carpets which can be sent out for regular cleaning.

• Bathroom renovations to avoid future mold growth are discussed in detail at Renovating to Avoid Mold Growth

MAINTENANCE - Interior Maintenance to Avoid Mold Suggestions from the U.S. EPA

This list based on a shorter EPA list, with additions and edits by the author.

• Fix leaky plumbing ... This means watching for corroded sink and tub traps and replacing them before they leak into the floor; find and fix loose toilets; use burst-proof washing machine hoses - this failure has led to some serious building floods. Insulate cold water lines to avoid condensation leaks into building cavities, basements, crawl spaces. ... and leaks in the building envelope as soon as possible - this means noticing roof leaks, roof flashing leaks, soffit leaks, leaks at windows, doors, plumbing vents, leaks where decks or patios abut the building wall.
• Watch for condensation and wet spots. Fix source(s) of moisture problem(s) as soon as possible. This means looking in your basement and crawl space for signs of water entry - which we discussed above.
• Prevent moisture due to condensation by increasing surface temperature or reducing the moisture level in air (humidity). - This means that you may find condensation on walls in cool closets or behind pictures hung on cool walls - places where air is not circulating. Heavy drapes in some buildings lead to mold on the walls behind them. To increase surface temperature, insulate or increase air circulation. To reduce the moisture level in air, repair leaks, increase ventilation (if outside air is cold and dry), or dehumidify (if outdoor air is warm and humid).
• Keep heating, ventilation, and air conditioning (HVAC) drip pans clean, flowing properly, and unobstructed. - we often find A/C condensate leaking into building walls and floors when drains are clogged. In climates where A/C systems remove a large amount of moisture from building air, watch for water blowing out of the condensate tray right into the blower assembly or even into ducts downstream in the air handler system - a common source of mold in air conditioning duct work.
• Vent moisture-generating appliances, such as dryers, to the outside where possible. Venting dryers into attics, basements, crawl spaces is a bad idea. If those areas are seasonally too dry, use a humidifier with a humidistat. When showering, use the vent fan - install one if there is none.
• Maintain low indoor humidity, below 60% relative humidity (RH), ideally 30-50%, if possible.
• Perform regular building/HVAC inspections and maintenance as scheduled. - Change air filters monthly when these systems are in use, and at that time, inspect for condensate leaks.
• Clean and dry wet or damp spots within 48 hours. - as we expound in more detail above.
• Don't let foundations stay wet. Provide drainage and slope the ground away from the foundation. -- But remember our advice from above. - most wet basements and crawl spaces are in that condition because of outside gutter and leader defects.

-- Initial Source: Mold Prevention Tips U.S. EPA.

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More Information on Building Diagnostic Inspections and Repairs

• Mold Action Guide: What to do about mold, mildew, and other indoor allergens. Review all of the sections of this document, particularly the section "Correct the Causes of and Prevent Mold or other indoor environment problems" also outlines the basic steps of mold prevention in buildings.
• How to Prevent Mold: how to avoid mold growth in buildings: priorities, repairs, products


• Condensation: Detecting and Correcting Attic Condensation and Preventing Ice Dam Leaks in Buildings
• Humidity: How Low Should You Keep Indoor Humidity to Avoid a Mold Problem
• IAQ or Anti Mold Products:Allergy/Asthma, Air Quality, Cleaning, Filtering, Ventilating, Dehumidification Products
• Ozone Warnings - Use of Ozone as a "mold" remedy is ineffective and may be dangerous.
• Products - Mold & Allergy Products: Allergy/Asthma, Air Quality, Cleaning, Filtering, Ventilating, Dehumidification


• Allergen Detection What various indoor allergens look like - identification photos to help identify pollen, dust mites, animal dander, toxic or allergenic mold - Common Mold and other Allergens, Irritants, Remedies & Advice
• FAQs on Mold - Frequently Asked Questions About Mold -- check this FAQ List & Site Map to see if you can find a quick answer to your mold concern
• Mold Detection - What Does Mold Look Like? Mold spores in the Home - a Photo ID Library for detection and identification of mold allergens
• Mold Detection - Stuff that is Not Mold but is often mistaken for it - things you may not want to test. Not all "black mold" is toxic or harmful.
• Contact Daniel Friedman, (author of this website) for expert field and laboratory investigative services

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