Posted by: SherlockHomesSY | 14 Dec 2010

Avoiding Mold in Older Bay Area California Homes


Every year, during the rainy season between October and April, it’s common to find unhealthy levels of mold inside older houses in the San Francisco Bay Area of California.  Every house is unique, but the issues and solutions are pretty common.

Weather. The Mediterranean climate here keeps the temperature in the Bay Area relatively constant: between 40-70 degrees, with some days outside that range.  The Delta Region experiences hotter summer temperatures but similar winter ones.  Warm, dry summers don’t support mold growth so we’ll ignore them.

The winters are a different story!  Moist air is brought into the region from the southern Pacific, through various weather phenomenon including the Pineapple Express, El Nino, and La Nina.  In short, the Bay Area has humid winters—frequently over 70%.  Fog is almost a daily occurrence.

Typical House. Newer houses should not be adversely affected by the winter weather.  Older ones typically are.  Building practices improved throughout the 1970s and into the 1980s and continue to this day.

For the purposes of this discussion, these are the characteristics of the house:
– Vented Crawlspace
– Single-Pane Windows
– Wall Furnaces
– Under-Insulated Walls
– Air-Leaky House

Indoor Moisture. Mold grows on the surfaces of windows and exterior walls because they get cold enough to condense the moisture out of the air.  The dew point is reached.

Where does the moisture come from? First, it comes from outside.  It also results from living in the house: cooking, bathing, breathing, washing clothes, drying clothes, plants, pets—just to name a few.  When there is a vented crawlspace, especially without a vapor barrier over the dirt, a lot comes from this source too.

In the Bay Area, the temperature is higher than it is in most the rest of the country, and this air is carrying almost all the moisture it can.  It’s drawn into the house in a variety of ways.  The wind blows it in.  Warm air leaving the attic draws it in.

Behavior. Pacific Gas & Electric is the utility provider in the Bay Area.  The cost curve per unit of energy can easily get real steep for these houses.  It’s not unheard of to pay $0.40 per kWh of electricity because these houses are energy inefficient.  A typical response is to turn the thermostat down and wear warmer clothes.  Unfortunately, the indoor temperature doesn’t get high enough to lower the dew point.  The result is condensation on the windows and walls.

Goal. Since the condensation is on the walls and windows, the goal is to raise their surface temperature.  In addition, the relative humidity in the house needs to be reduced.  When that can’t be done, then a dehumidifier might be needed.

The ideal is to install ventilation, add storm windows or replace single-pane windows with dual-pane windows, fill walls with insulation, and decrease air leakage through the house.  Tenants aren’t going to make these types of improvements to the house, so temporary alternatives are provided here too.

Ventilation. Moisture levels get high in any house by bathing and cooking.  Exhaust fans to the outdoors should be installed in every bathroom and kitchen.  Nobody likes listening to these noisy things, so install a humidistat to turn the fan off when the moisture level drops sufficiently.  I keep the bathroom door closed while the fan is running.  I need the humidistat because I get busy, so I never get around to shutting the fan off—an energy costly move.

Windows. Single-pane windows get colder on the inside than dual-pane windows.  Dual-pane windows or storm windows cut the heat lost through the window by at least half.  Storm windows, or their plastic sheet equivalents, can be installed on the inside of the house.  Storm windows can also be installed outside.

A variety of window shades can be used to help keep warm, moist indoor air from reaching the window.  When it does get there under the right conditions, there needs to be enough air flow moving across the window to dry it.

Walls. Uninsulated or under-insulated walls allow the interior wall surface to get colder than it would otherwise.  A weatherization contractor knows how to insulate walls without tearing the wall apart.

Crawlspaces. The problem with crawlspaces is the holes in the floor put in them for plumbing, ductwork, and other utilities.  Older houses also use lumber, rather than sheathing (e.g. plywood), for the subfloor.   Air is easily drawn through the cracks.

Making matters worse is the moisture content in the soil evaporating into the crawlspace.  The wetter the soil, the worse it is.  I’ve found house where water drains into the crawlspace.  In the Bay Area, ground water is a typical problem too.  Ground water will migrate up through 16 feet of soil by capillary action.  This evaporated moisture is near 100% at around 50-60 degrees or so—nearly room temperature!

When the soil isn’t soaked or gets soaked, a sealed vapor barrier can be readily installed over the dirt and secured to foundation walls and pier columns.  Otherwise, the sources of water in the crawlspace need to be controlled first.  A home performance contractor ( is best trained to do this.   If you elect to ask a termite company or handyman do it, do your due diligence first!

Air Sealing. Moisture-laden air comes in from the outside through cracks in the floor, walls, and ceilings.  Perfect air sealing is impossible, so get the big holes first and continue sealing until you reach a predetermined target.  The practical limit is money.  We recommend sealing the house to around 0.35 air changes per hour.

However, any significant improvement is better than no improvement for this situation.  A weatherization or home performance contractor can help you with this.  If there are ducts in the attic or crawlspace, a heating and air contractor is needed to re-tune the system afterward.

Personal Items. Until the ideal improvements are made, keep furniture and other personal items away from exterior walls.  Leave the closet doors open when any of the walls touch an exterior wall.  Put away seasonal clothes so air can circulate around clothes hanging in the closet.

Dehumidifiers. Use dehumidifiers as a last resort, not the first choice—especially in PG&E territory.  Do what you can with ventilation, windows, walls, and air sealing before using a dehumidifier.  First, get a thermometer that shows both indoor temperature and relative humidity.  The goal is to keep relative humidity below 50%.  (At 60% mold starts growing.  50% will sustain it.)

Get an ENERGY STAR dehumidifier for best energy efficiency, and then run it at 50%.  You may have to bump the temperature up higher toward 68 degrees to help the machine out.   It will probably be more comfortable to put the thermostat at a comfortable level to allow the dehumidifier to run less. Experiment.


This special blog post was made to help explain to my clients in the Bay Area about what they can do with the typical older house in the region to avoid mold.  The principles should help most everyone Nationwide with similar circumstances.  I’ve noticed that this time of year, readers are visiting this blog because of concerns about window condensation and drafty rooms.

For more information, use the categories on this site to find what you’re looking for rapidly.  Of particular interest are the following posts:

Common Complaints

–  Drafty Rooms
–  Damp Basements
–  Damp Crawlspaces
–  Indoor Moisture Sources for Mold
–  Outdoor Moisture Sources for Mold
–  Window Condensation

Basic Solutions

Air Sealing
Duct Sealing
Rain and Groundwater Drainage
Vapor Management
Window Performance

California’s Utility Cost Structure

More posts concerning moisture control will appear on this site in the future.  We haven’t even begun to talk about moisture condensation inside walls.

In Conclusion

This article helps to avoid mold in the first place.  However, if you have it, you may need the assistance of Certified Mold Inspector or Industrial Hygienist to help you figure out what to do about it.  Most of the time, the services of a Certified Mold Remediator is recommended.  For more information, see our mold website.


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