Seattle Home Inspector's Blog


Moisture Control in Homes in the Northwest

I get lots of calls from people about moisture issues inside their homes.  Sometimes these moisture issues are related to the building itself, such as: roof leaks, plumbing leaks, foundation leaks, flooded crawl spaces, etc.  Obviously these sorts of water issues are typically easy to identify and generally not complicated to address—even if repairs could be quite expensive (like foundation problems).

Perhaps even more common, are issues related to the way the building is being used by the occupants. 

Sometimes both factors are involved and it becomes a process of elimination in fixing the issues.  I will refer to the first factor as “bulk water issues” while the latter factor is more of a “building science” issue.  Of course building science is involved with both, but moisture issues related to the occupants can require more head scratching as to cause and a solution will typically require “re-education” of the occupants--a seemingly daunting task.  It requires teaching them how the building functions and what they must know if they want to manage indoor moisture levels and its cousin indoor air quality. 

While much of this post is directed at those living in the coastal northwest, much of it will be applicable to other areas.  For the best information about proper ventilation for moisture issues in your area, you should consult a qualified party in your area.

Sometimes, maintaining good indoor air quality requires a means of mechanical ventilation that the occupants have no control over. 

This is especially true of rental properties, where regardless of the amount of information given, the occupants cannot be counted on to use manually operated systems.

It really does not matter too much whether we are dealing with too much humidity in the indoor environment, or too little humidity, we still must understand what we, as occupants, are doing to cause the moisture problems (obviously readers in Syracuse have to think  about too little moisture in homes--at least in the winter). 

Since we are not going to eliminate the occupants from the indoor environment we have to come to terms with managing the indoor environment.

It is my experience that the vast majority of home owners or tenants are clueless as to how to do this or that it is even necessary.

This becomes even more important as we seal homes up tighter and tighter to save energy. 

We can go ten steps forward to save energy, but then we must go back one step for health and indoor air quality.

In this discussion we are going to assume that there are NO “bulk water” issues in the home.  We are also going to assume that there is no air conditioning of the home—or that at least that air conditioning is very infrequent—as is common in the naturally air conditioned NW.  Perhaps I will revisit this information some other time for those of you that live in hot/humid areas that “require” air conditioning.

The key to controling moisture in occupied spaces in the Northwest is ventilation.  It is likely that if you have moisture issues in your home, you likely have inadequate ventilation for the amount of moisture being produced.  So how do we introduce moisture into our homes?

We do it by:


Gas ranges,


Wet towels/ bathmats,

Watering plants,

Number of occupants,

Activities of occupants,

Evaporation from toilets and pet watering bowls (maybe there is a good reason to close the lid afterall),


Clothes dryers,

Drying clothes indoors,

Clothes washers,


Sports equipment/clothing,

Firewood etc.


What are signs of excess moisture indoors?

Mold behind the toilet tank or on the bottom of the tank,

Condensation on windows—especially double pane windows,

Condensation on metal or vinyl window frames,

Mold growth behind bureaus, and other furniture placed against the walls,

Mold growth behind storage in closets,

Mold growth and condensation on bathroom walls and ceilings,

Mold growth inside tub and shower enclosures.

Some areas of high moisture can be very difficult to control with spot ventilation and a ventilation fan must be run for longer periods of time (showers with closed glass doors may never dry properly--so leave the door open if possible—to some extent this is a “design” issue).  Some newer exhaust fans can be run continuously at very low CFM and then increase as the need arises--even humidistat controlled.  For the average bathroom exhaust fan, if you are not running the fan for an hour after showering, the bathroom may still be under-ventilated.  Squeegeeing showers after use and raising the temperature in the bathroom will greatly reduce ventilation time and greatly reduce condensation in the room in general.

When we add moisture to the indoor environment by any or all of the above mentioned methods, we must ensure that adequate ventilation is happening or eventually the moisture will build up in the air to the point of saturation and then the moisture will leave the air and condense on the cooler surfaces behind furniture etc.  When this happens, surfaces become moist enough to support the growth of mold.  Keeping rooms throughout the home at relatively uniform temperatures is important, but becomes less important if we control humidity with adequate ventilation.

One of the most common locations where temperatures are not maintained adequately is behind tightly closed blinds/curtains.  The warm moist air is attracted to the cooler area of the glass and condensation happens.  Leaving blinds open a bit on the bottom will typically be adequate (unless moisture levels are really high) but even leaving them up a bit, will likely still be a problem with single pane windows (perhaps one of the best reason to upgrade windows). 

Obviously this practice decreases overall energy efficiency, but is a part of that step back that is necessary for indoor air quality.  It may not be possible to be 100% energy efficient and maintain good indoor air quality at the same time.

No amount of understanding about how to clean up mold growth will help prevent mold continually coming back if we do not address the ventilation problem or the moisture creation problem. 

Occupants of homes must learn how to minimize the ways that moisture is introduced into the indoor environment, as well as how to manage what is introduced.

So let’s get to the heart of the matter.

Are your exhaust fans functional?

If you have none, that alone is a big hint.

This does not simply mean that they make noise when you turn them on.  This does not mean just the exhaust fans in your bathrooms either.  How about your laundry exhaust fan?  What do you mean you never use it or do not have one?  How about your kitchen range hood?  But it only vents to the inside you say?  Kitchen exhaust fans must vent to the exterior and on my planet you would not be able to operate the range if the hood did not come on.  Cooking without a vent hood can add tremendous amounts of moisture to the indoor environment—depending on the types of cuisine being cooked—and especially if it is a gas stove. 

Do you know for a fact that the fans are pulling air from the building and exhausting it to a proper location at the exterior?  Are you sure that when you close the bathroom door that the door does not seal so tightly that function of the fan is reduced?  Are you sure that when all windows and doors (in the rest of the home--not just the bathroom) are tightly closed (all with nice energy saving weather-stripping) that the function of the fan is not reduced?  Is there “ghosting” around the edges of doors that show one of the pathways of how air is being drawn into the building? 

Newer and smaller houses have more problems with inadequate fan function than older bigger houses, because leaky houses provide make-up air for the exhaust fans easier.  The saving grace with newer homes is that they typically are going to have some means of guaranteeing (theoretically) air changes and a means of bringing in make-up air as well as changing the air in the home (at least in WA State since 1991).  It might be as simple as recommending that the occupant set the timer they didn’t even know they had.

As the damp in the indoor environment increases, so do other things related to a reduction in indoor air quality.  In this sense, the damp is the primary problem while the others can be more secondary issues.  Fixing the damp condition will alleviate some of the other concerns.  Obviously mold is one component of this damp that will be eliminated with proper ventilation and a reduction of moisture conditions, but so too will other larger components of the dirty air be reduced--such as dust mites, shedding pets, shedding humans and household dust. 

It is not the focus of this post to go into the long list of contributors to poor indoor air quality because they can happen irrespective of a moisture issue, but many can be exacerbated by high moisture levels and most can be greatly reduced by adequate ventilation.

Ventilation has to be able to keep up with the rate at which these offending elements are being added to the indoor environment.  Here is a partial list of such contributors to poor indoor air quality:

Interior building materials (including new carpet),


Household products (cleaners, body care products, soaps/detergents, fabric softener, disinfecting, cosmetic, degreasing, and hobby products.  Perhaps the movie “Hairspray” was actually a horror movie?),


Combustion by-products (Carbon Monoxide, Nitrogen Dioxide)


Pets (four dogs and three cats?),

Dry-cleaned clothing,


Pesticides (ant traps for example)

Moth repellants,

Wood burning stoves/appliances,

Air fresheners etc.

Many homes suffer from multiple items on this list and yet “mold” is deemed to be the culprit.  Mold is just a small component of poor indoor air quality and the aim should be to eliminate, or at least “manage,” all of the offending parties.

One of the common excuses I hear as to why people do not want to increase ventilation is that they are worried that bringing all that 35 degree, 100% humid, Seattle air into the home is going to make things worse.  Fortunately for homes in the NW, the building science does not allow it to work like that.  As that cold air comes in and warms up to the indoor temperature its humidity drops to below the humidity of the indoor air.  As it mixes with the indoor air, the overall humidity is actually reduced.  We can thus actually lower overly humid indoor spaces by bringing in that cold wet air.

The science may be counter-intuitive but I guess that is why it is called science—and why intuition may not be a good thing to rely on all the time.


Charles Buell, Real Estate Inspections in Seattle


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Comment balloon 13 commentsCharles Buell • July 02 2014 04:44PM
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