It’s Raining, It’s Pouring, Is Mold Growing While I’m Snoring?

/ / Healthy Building Inspections & Testing
It's raining it's pouring

It’s Raining, It’s Pouring,


Is Mold Growing While I’m Snoring?


Drought stricken California is getting some significant rain for the first time in over a year. As of today Oakland, California has received 3.22 inches of rain so far THIS MONTH, and 7.31 inches this season. If this continues, California’s drought woes should be significantly diminished, but mold growth is just getting started.

With heavy rainfall comes a tremendous amount of moisture; leaks and condensation increases, temperatures decrease and the amount of warm, drying daylight decreases.

These are optimal conditions for mold growth, both interior, and exterior. As exterior mold spores explode in number some of them are bound to settle in our indoor environments. Here’s an overview from the EPA on Mold growth in the home.

What can you do to reduce to likelihood mold will take hold?

I have some tips to minimize conditions conducive to mold growth and maximize you and your family’s health.

Mold needs 3 conditions for optimal growth:radiator_with Mold growth

  • The right temperature. Some mold species can grow at low temperatures (below 50 degrees F) and other species at high temperatures (above 90 degrees F), but most common indoor mold species grow ideally at 55-85 degrees F. Unfortunately this is the optimal temperature for human comfort, so it is unlikely you can keep your home at a temperature that is inhospitable for mold growth.
  • An organic food source. Different species of mold like to eat different things, but they all need something organic to munch on. Many mold species love cellulose, i.e. wood and paper. Molds are natural composters and when it rains these species start to eat the fallen branches and leaves in the forest, as well as our yards, emitting millions of spores that make their way into our homes. Inside our homes, molds like to eat wood. This is what “dry rot” is, fungi usually consisting of 2 species, Ascospores, and Basidiospores. Other species like to eat paper, such as cardboard boxes, books, and paper backed wallboard, such as sheet rock.   Pennicillium/Aspergillus and Stachybotrys (colloquially known as toxic black mold) are often found on wet or moist paper. Cladosporium, the species most often found growing on windowsills and in bathrooms, can eat a variety of biofilms (household dust consisting of epithelial cells (dead skin cells), insect parts, pet dander, natural fibers such as cotton and linen, etc.).   Some mold food sources cannot easily be removed from our homes such as framing lumber and wallboard, but others can, such as cardboard boxes.
  • Mold needs moisture. There is a common saying in our business: “Mold is the symptom, moisture is the problem”. Mold growth requires either liquid water or high humidity. Liquid water can come from condensation on windowsills and in bathrooms, or from leaks, either internal or external. Without liquid water mold will not become active unless humidity is high, usually 60-80% RH depending on the species. When humidity is high enough, mold can become active and grow by absorbing moisture directly from the air.

Here are tips to reduce both food sources and moisture in your home and reduce the likelihood and amount of mold that may grow inside your home:Control Moisture. Leaves clogged gutter

Let’s start outside. When it rains water can easily enter what we call the “Building Envelope”. It is important to make sure your site drainage system is clear of debris and working properly to move rainwater away from your home, foundation, and crawlspace.

  • Clean the roof of any leaves or other debris.
  • Clear gutters.
  • Make sure downspouts are in good repair, not clogged, and properly attached any extensions to the site drainage system.
  • Make sure all property drains are clear of debris and flowing freely.

Check the “Building Envelope” for possible sites of water intrusion, i.e. leaks.

  • Window and door frames are spots where water can intrude. Check all door and window frame caulk for cracks and gaps and repair where necessary.
  • Inspect the sealant around roof penetrations. Repair where necessary.
  • Check building siding for cracks, peeling paint, holes, etc., anywhere water may be able to get in.

After a heavy rain, walk around the entire house and look for standing water, and clogged drains. Look inside the crawlspace and make sure there is no hidden flooding. Carefully check the inside of the house, take a close look at ceilings, around windows and doors, and walls for small leaks. All big leaks start out as small leaks! Check under sinks and around tubs and toilets to make sure there are no plumbing leaks adding moisture to the interior of your home.

Assuming there are no leaks and your drainage system is working well, what other sources of moisture can you address?David Sasse Mold Lab Interpretation

Inside a home, occupants can produce a tremendous amount of moisture. On average each human occupant expires (breathes) and perspires (sweats) about 2 pounds of water into the air per day. Pets also add to this moisture source. During winter we often close our windows, and most residential heating systems have no way of bringing in fresh air or removing moist, stale, interior air. Thus interior humidity can often increase to levels above 60%, which is ideal for mold growth.

So what can we do about mold growth?

  • Monitor interior humidity. Small, portable humidity monitors are available for around $10-15 and can be placed around the home. If RH (relative humidity) is consistently above 65%, action should be taken. Ideally, interior RH should be between 45-55% RH. Below 40% RH mucous membranes start to dry out and can cause occupant discomfort.
  • Open windows when practical to help flush out moisture and other interior contaminants. Even 1 hour a day can make a big difference, although 3-4 hours is recommended.
  • Run ventilation fans in bathrooms and kitchens to help exhaust excess humidity from cooking and bathing. Run fans in bathrooms for at least 20 minutes after bathing. Timer switches can be installed on most bathroom exhaust fans and are highly recommended.
  • Wipe excess condensation from windowsills. Inspect windowsills often. Do not keep curtains closed as this can trap moist, cool air and promote excessive condensation.

What can do we do about reducing mold food sources?

  • Do not keep books, papers, or cardboard boxes in moist areas such as attics, garages, basements or crawlspaces. Attics and crawlspaces should not be used as storage areas, but if you must store items in a garage or basement, we recommend sealed plastic bins.
  • Keep areas mold likes to grow clean and dry. This means cleaning dust (biofilms) from windowsills, baseboards, and door frames. Vacuum carpet regularly with a HEPA vacuum. The recommendation is to vacuum and sweep one day per week PER OCCUPANT, including pets.
  • Check behind drapes and furniture for hidden condensation and biofilm. Allow airflow to reach these areas by opening drapes often and moving furniture a few inches from walls, especially exterior walls that can become colder and promote condensation.

Trust your nose, that musty smell is a sure indication of active mold growth. Musty smells are caused by microbial VOC’s, airborne chemicals that are a metabolic by-product of mold digestion.

If you think you have a hidden source of mold, call a professional Certified Microbial Investigator for a full mold inspection. Excessive interior mold can cause structural damage to your home and its contents, as well as allergic and respiratory reactions in some occupants. Take heed, be diligent, and you can survive a wet winter relatively mold-free.


Healthy Building Science is an environmental consulting firm providing mold testing and mold inspection services for commercial, residential, multi-family, buildings, offices, industrial and manufacturing workplaces, hospitals and medical facilities, and single-family homes in the greater San Francisco Bay Area and all of Northern California including the cities of San Francisco, San Jose, Oakland, and Sacramento.