Posted by: SherlockHomesSY | 22 Oct 2010

Solution Basics: Vapor Management

Window CondensationVAPOR MANAGEMENT

Keep water out.
Let water out that gets in.
Water will get in, so let it out.

Water comes in three forms: ice, liquid, and vapor.   This one is about vapor and how to manage it.

We don’t think about vapor much—until we have mold, indoor air quality issues, or find building deterioration in weird places.

Where does vapor come from? There are indoor and outdoor sources.  Then there is condensation and building drying.

Indoor Sources of Water Vapor

Routine daily living fills our houses with moisture.  It comes from breathing, sweating, bathing, showering, cooking, washing dishes, washing and drying clothes, toilets, plants, pets, spills, etc.

Additional sources include crafts, drying paint, fish tanks, floor washing, carpet cleaning, unvented heaters and fireplaces, gas stoves and ovens, and other things that allows water to evaporate inside.

These sources of moisture are usually controlled by ventilation and dehumidification

Outdoor Sources of Water Vapor

The major outdoor vapor source is ever changing humidity resulting from weather changes.  Other sources are through the floor (slab or wood) from the ground, and water against foundation walls.

These sources of moisture are normally controlled by house wrap, crawlspace vapor barriers, foundation dampproofing, and proper drainage.  Crawlspace ventilation works in many places, but not the Southeast.


Condensation occurs when warm, moist air meets a cold surface.  Window condensation is familiar.  Summertime air flowing through vented crawlspaces condenses on wood, plumbing, ductwork because crawlspaces are cool.  This trapped moisture then evaporates—into the house.

Solutions to condensation sources are best solved with insulation, dual pane windows, and perhaps closed crawlspaces.

Building Drying

Building drying? Who thinks about that?!! My walls don’t get wet!  My siding is intact!!!  Sorry, rain always gets through siding and needs to dry.  But, any rain that gets in dries to the outdoors, doesn’t it?

Also, what do you mean my house dries inside out?

Controlling moisture due to building drying is hard for almost all of us to understand.  The reason is that the laws of moisture vapor can be counter-intuitive.  At least it was for me.

Laws of Water Vapor

1.   Water vapor flows from high vapor pressure to low vapor pressure

2.   Water vapor flows from the warm side of building assemblies to the cold side (thermally driven)

3.   When temperature differences are large, condensation can occur on cold surfaces

The first law is easy.  We understand heat flows from hot to cold.  We know wet air is dried by drier air.  High outdoor humidity is controlled by dehumidifiers, especially our air conditioners.

The second law seems insane on the face of it.  What difference does it make when a wall is wet after a summer rain storm and the sun is shining on it?  This pushes water vapor into the house?!!  Shouldn’t it be going the other way around?

Again, what difference does it make in my cold climate house?  Water vapor goes right through the wall trying to get to the cold exterior.

My head understands this law, but my heart doesn’t yet.

The third law is intuitive. Condensation occurs when warm, moist air comes in contact with a cold surface.  The solution is simple: don’t let the cold surface temperature drop below the dew point.

Water Vapor Movement

Add to the vapor laws the two methods it moves by: air transport and diffusion.  Water vapor will go through most things (diffusion).  However, humid air coming through holes in the shell allows much more moisture to pass than anything else.

For example, gypsum board (drywall) walls easily allow water vapor to go right through it.  When moisture levels are high enough to allow a 1/3 cup of liquid vapor through the wall, a 1 inch square hole in the same wall will allow 30 quarts of liquid vapor through it.  Vapor barriers and vapor retarders work best when they cover a tight air barrier.

Priority 1 for vapor management is air sealing.  The next step is the proper application of vapor retarders and vapor barriers.  Sometimes, the right answer is to not use either.


Understanding vapor management is hard, but it is manageable. A solid understanding of your environment is crucial. Home Performance contractors and consultants have to understand it because getting it wrong can mess up your house.

Mold is a result of water and vapor mismanagement.  My mold website is dedicated to diagnosing and correcting moisture issues.  My home performance website discusses them as part of indoor air quality.  Specific applications will be discussed in future blog posts.

An excellent resource is the Energy & Environment Builders Association’s (EEBA’s)  Joseph Lstiburek’s book, Water Management Guide.

In Conclusion

The home performance industry got started by trying to improve home energy and immediately ran into trouble with water vapor issues.  Don’t make the same mistake! It’s not intuitive to think about controlling moisture while trying to reduce energy bills.  However, they are tightly linked together.  Please pay attention to your consultant’s recommendation as it relates to vapor management.

Next time, we’ll consider the related subject: Drainage!

Improve your house!—but do heed our warning . . .

WARNING! Do not implement any of these solutions without considering the impact on IAQ, moisture control, & heat transfer. A combination of solutions are usually needed to improve health, building durability, comfort, and energy efficiency. Ignoring this warning may lead to disease, deterioration, high energy bills or worse!



  1. Gypsum board wall allow water vapor to go right through it, which makes gypsum board a moisture-resistant product. It is wonderful how many uses and applications gypsum boards can be put to use. It is fire-resistant and mold resistant. Using.gypsum board is a great idea and is a wonderful product for both commercial and residential purposes. I think that it is always important to begin by researching your options. McGraw Hill’s Sweets Directory of Construction Products and Manufacturers is a great resource. Though I now work for them, they are honestly the most complete directory I have found. I love the CAD details you can download from their site. I would definitely recommend them.

    • It true water vapor will go right through. In some applications, our houses depend on that very property to allow the walls interior of exterior walls to dry. For water though, drywall is sponge. It soaks up water very well. Sometimes, water damage restoration companies can dry it out before it warps or crumbles. Since the face of the gypsum board is paper, mold will be present in less than 48 hours. Water damage restoration companies and mold remediators routinely remove it after that.

      Gypsum board is an awesome product. It’s easy to work with, affordable, and has fantastic smoke and flame spread characteristics. In fact, in case of fire, the water bond in the material is released. Go show!

      If you’re looking for mold-resistant gypsum board, only the blue board line fits the category. We recommend it in bathrooms and kitchens. However, the green and white board lines do not possess this desired characteristic.

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