Traverse City Record-Eagle


Condensation: The silent enemy within

By Erin ParkerMoisture is the mortal enemy of all buildings, and air – or more precisely, condensation from the air – is its silent accomplice.  Moisture from a leaky roof, a foundation crack or a leak in the plumbing is usually pretty easy to spot, causing obvious wet spots, water stains on ceilings and walls, sagging drywall or bubbled paint.

Moisture carried in the air, on the other hand, is invisible and when condensation occurs in hidden spaces, it can go undetected for long periods of time, sometimes years.

When a condensation problem exists, the end result is oftentimes serious structural damage.  In fact, damage related to condensation problems can be more widespread and  serious than damage from an outright leak, simply because the leak is more often detected and corrected before it becomes a major problem.

All air contains moisture in the form of water vapor.  The warmer the air, the more water vapor it can hold.  Inside every occupied structure, moisture is continually being added to the air – there’s no escaping it. Every breath that’s exhaled, every time food is cooked, every time a load of laundry is done, every time someone washes the dishes or takes a shower, water vapor is added to the air.  Even pets and houseplants add moisture to the air.

Now don’t get me wrong – some moisture in the air is necessary and beneficial. If the air becomes too dry it becomes uncomfortable and causes all sorts of problems (e.g., dry chapped skin, scratchy throat, static electrical shocks). On the other hand, too much moisture in the air also causes serious problems (e.g., mold & mildew, fungal growth, musty smells and allergic reactions). However, even at optimum humidity levels, serious problems with condensation will occur if certain conditions exist.

So what causes condensation?

Air can only hold a certain amount of water vapor, usually expressed as a percentage, which varies with temperature and barometric pressure.  At a given pressure, warm air can hold considerably more water vapor than cooler air. So when warm moist air comes in contact with a cool surface, the air itself is cooled and some of the water vapor changes to liquid water, i.e., “condenses”, on the surface.

Think of a glass of your favorite beverage on a hot summer day. The cold glass is surrounded by warm humid air and, as the air touches the glass, it cools and can hold less moisture.  Consequently, the humidity in the air condenses into liquid water on the outside surface of the glass.

This exact process can occur on a much larger scale inside a home where the walls, floors and ceilings separate warm areas from cold areas.  When the right conditions exist, most often due to poor building practices, condensation will form on cool surfaces concealed inside the homes hidden spaces.  These hidden areas soon become damp, or even sopping wet, creating the perfect environment for mold, mildew, and fungus to grow.   If the situation is left uncorrected, structural damage is inevitable.

The good news is that there are simple, effective means in home construction and maintenance to effectively prevent or minimize condensation (and the damage it can do) from occurring. The three main weapons to used to control the problem of condensation are proper insulation, vapor barriers and ventilation.

Insulation – The First Line of Defense -
Insulation provides a thermal barrier between warm, humid air inside the home and places where it is cool. In the wintertime, warm and relatively humid air inside the building envelope must be insulated from cold outside air to provide a continuous thermal break. The effectiveness of insulation is measured by R-Value and the higher the R-Value the better.

The 2009 Michigan Uniform Energy Code (MUEC) outlines basic prescriptive requirements for insulation in new construction. While the code contains way too much detail to go into here, the MUEC is a good reference to check when determining the recommended minimum R-Values for insulation for homes in northern Michigan. For example, the MUEC specifies a minimum of R-49 for roofs/ceilings, R-20 for walls, and R-30 for floors over unconditioned spaces). Please note: these new code values far exceed the insulation found in most existing homes.

However, as indicated previously, insulation alone is not usually enough to control transmission of water vapor. This is where vapor barriers come into play.

Vapor Barrier – Stop Moisture in it’s Tracks -
A vapor barrier is usually a plastic, foil or treated paper membrane that prevents water vapor from traveling through the porous interior surfaces of the home and insulation.  While there are many types of vapor barrier now in use and used in the past, they’re all effective as long as they are installed properly and continuously (even the best vapor barrier is worthless if it full of holes).

The vapor barrier is important because typical interior surfaces as well as most of the commonly used types of insulation used in homes today (e.g., fiberglass, cellulose and open-cell foam) don’t impede the transmission of water vapor.  A vapor barrier solves this problem by stopping air and water vapor from traveling into and through the insulation.

Ventilation – Last Defense against Condensation -
The final element needed in order to control problems due to condensation is adequate ventilation.  The reason ventilation is important is because of the nature of typical modern construction.  Most all homes today are built with unconditioned (i.e., unheated/cooled) spaces such as attics, trusses, spaces inside exterior walls and crawl spaces.   Unlike in the past where construction was relatively drafty, modern materials and building practices have resulted in well-sealed exterior envelopes.

Since no insulation or vapor barrier is perfect, some warm, moisture-laden air will manage to travel through the insulation layer and enter the unconditioned spaces of even the most well-built homes. Without proper ventilation, water vapor then condenses out on the cool interior surfaces of the unconditioned space.

Steps to Prevent or Correct Condensation Problems –
Any structure, new or old, can suffer from serious problems with condensation.  To solve an existing condensation problem or to prevent a future problem from occurring, consider the following steps.

• Check to see that your home is adequately insulated. If not, consider adding additional insulation.   In addition to preventing problems with condensation, adding insulation increases energy efficiency and pays for itself in lower energy costs.

• Correct any air leaks into unconditioned spaces. Common culprits include the attic or crawl access hatches, cracks and seams, electrical boxes, and wiring, plumbing and duct penetrations.

• Check to make sure that all unconditioned spaces are adequately ventilated. In the attic, this usually means checking that soffit and ridge vents are not blocked with insulation or roofing materials. In crawl spaces, this, means checking to see that the vents are not blocked. Interesting note: many homeowners engage in the common mistaken practice of sealing off their crawl vents in the winter.

• If in doubt, consult an expert. It is far better and less costly to prevent a condensation problem from occurring than to fix serious damage after it occurs.

I hope you enjoyed this article and found it useful.  If you have any questions or comments about this or any other general engineering related topic, please feel free to contact me at


  • Anonymous

    That pretty well covers the subject. Everything possible has been said on the subject. . . then as Johnny Carson would say to Ed, ‘Wrong R-14 breath.’

    Actually, adding to the above, a practical problem – if a little is good, more is better NOT. I.e. packing insulation into tight spaces. Fiberglass bats need to be kept fluffy to insulate; the tiny air spaces in the insulation are what insulate. And, when dealing with tight triangular eave/soffits and knee-wall intersections between attics, leaving space on the outside of insulation, for cold air to circulate.   

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