This winter has been a little colder here than usual, but not by more than five degrees or so.  Why then have I been buried under calls from home builders looking for an answer to why they are hearing complaints about excessive condensation on windows.  I'm not talking about a few drops of water. These home owners are seeing big puddles on the window sills and even water running off and soaking carpets and wood floors.  All of this moisture day after day is also leading to the dreaded "M" word since the drywall around the windows is soaked and that leads to the growth of M***. 

The phone calls lead me into the field to do the necessary testing to determine what is happening.  In each case I found the same set of conditions:

  • The blower door test proved that every house was very tightly built
  • An inspection of the heating and cooling systems found that there was no provision made for regular or continuous ventilation
  • The owners either had a large number of people home all day or they worked from home and tended to be home most of the time
  • The houses were all less than two years old
  • The worst condensation was occurring on one particular side of the home

This is a set of conditions that did not surprise me in the least.  Each of these factors increases the likelihood that the home will experience high moisture levels in the indoor air and that is why the windows were condensing water. 

First, let's get a handle on why condensation occurs.  Condensation is a simple physical process.  It has two important variables.  The first is the moisture content of the indoor air or dew point temperature.   Dew point temperature is the absolute moisture content of the air and not a relative measurement comparing temperature to moisture content like relative humidity which is so hard for most people to understand that we are going to ignore it for this conversation.  The next is the temperature of the coldest surfaces in the home.  When double pane, Low-E aluminum frame windows are used, the window frame is almost sure to be the coldest surface inside of the home.  Research has informed us that the inside of an aluminum frame window will be about one or two degrees warmer than the outdoor air temperature!  That's right, on a 25 degree morning, the aluminum window frame in your home will be at about 26-27 degrees. 

The fact that these homes were very tightly constructed means that the moisture created inside of the home, stays inside of the home.  As the indoor air becomes more and more moist, the dew point temperature of this air rises.  As it rises, the likelihood that it will find a surface colder than it is, increases too.  It also means that it is not diluted by a lot of drafts that would in a leakier home allow the very dry, low dew point outside air to come in and reduce the inside moisture concentration.  Tight homes are more comfortable and they cost less to heat and cool, but they also change the old relationship between heat, air and moisture in your home.

When there are people in the home all day, they create moisture by breathing, bathing, cooking, and other human activities.  When they don't open the doors very often, they also keep that moisture they made trapped inside of the house where it builds up until condensation occurs. When the dew point of the inside air exceeds the temperature of the coldest surface, then condensation will happen. The windows most likely to see condensation are those on the North side of the home because they are the coldest.  There are only two possible solutions to condensation:

  • Warm the temperature of the cold surface
  • Reduce the moisture content (dry) the air which has the effect of lowering the dew point temperature of the air

We can't do much about how many hours a day we are in our homes.  We can't do anything about how many people there are in our homes either. But we can take low cost and reasonable steps that will both dry our indoor air and improve the quality of our home environment.  First, know that ventilation with fresh outdoor air is mandatory in the current building codes.  This is for a number of good reasons.  First, in winter it brings in fresh, dry air and that dilutes the moisture in the indoor air and lowers its dew point temperature.  Second, it helps to create healthy indoor air quality by flushing out particulates, and diluting the VOC's that all of that new carpet, paint, cabinet finishes and other stuff are putting into the air daily. 

Solution number one employs the concept of "reducing the indoor moisture content and thus lowering the dew point temperature:

  • Install a whole house ventilation system that regularly brings in fresh outside air
  • I like HRV's for far northern climates, ERV's for warm, humid climates
  • In any climate I will look favorably on a whole house combination unit that ventilates, dehumidifies if needed, and HEPA filters the air
  • I also like the systems that use the ECM blower of your high efficiency HVAC unit to ventilate the house
  • AprilAire, Ultra-Air, and the AirCycler are all good, reliable equipment to look at in these areas

The second solution is to warm the surface and it is usually more difficult to do.  Here are a few examples:

  • A couple called me to tell me that they were having mold grow every winter at the outside edge of the ceiling around their home.  The man of the house had developed a severe allergy to this mold and it made him sick most of the winter every year. 
  • This along with mold in corners is easily explained.  These are the coldest spots in most homes since they are places that are difficult to insulate properly.
  • The fact that the way we frame roofs means that the height of the attic at the outside edge of most homes is very low. 
  • That means that the depth of the attic insulation at the perimeter of the house is greatly reduced from the 10-12 inches it should be
  • This causes the ceiling to be cold in winter and that leads to a thin sheen of condensation which supports mold growth
  • At corners, there is no insulation, just a bunch of 2x4's bunched together and that lack of insulation makes the corners cold in winter and again, you get condensation and mold.

Solution number two employs the "warm the surface" process:

  • When you build, use raised heel trusses, which allow you to fully insulate your ceiling all the way out over the top plates or build an unventilated, attic a spray foam insulated roof deck
  • After that, you can remove the loose or batt insulation and spray the ceiling near the top plates with foam to establish a higher R value of insulation and stop any wind washing that was occurring before
  • The corners are more difficult to fix.
  • In new construction, use two stud (California corners) and insulate the corner fully
  • In retrofit, you can try to remove the dry wall (it is moldy anyway, right?) and fill the spaces in the corner framing with spray foam insulation. 

Now you can see why in retrofit, the best approach is to lower the dew point of the air.  It's a lot easier to ventilate than to tear a house apart and rebuild it right.  You also get extra benefits like a healthier indoor environment for your family. 

 One last caveat.  In climates where the summers are warm and humid, you need to understand that ventilation air brings in moisture when the dew point of the outdoor air goes up.  One way to know what your dew point temperature is to watch the local weather.  Another way is to understand that the morning low temperature is usually a few degrees above the outdoor dew point temperature.  So on a winter morning when the low was 20 degrees, the outdoor dew point temperature will be something like 15-18 degrees.  When your morning low in summer is 70 degrees, the outdoor dew point is probably in the range of 66-68 degrees.  The low air temperature can never drop below the dew point but it can and usually does get close to it.  Understanding this, you can see that the dew point temperature and the absolute moisture content of the air changes a great deal from season to season. 

In hot and humid climates it is necessary in many cases to add supplemental dehumidification equipment to handle the moisture load of the summer ventilation air.  Sometimes, if your contractor did not over size your equipment too much and you have an ECM blower, you can handle this extra moisture with the addition of a high tech control unit.  We all know how a thermostat works.  We set the temperature we want and the thermometer in the thermostat calls for the unit to come on when necessary to maintain that temperature.  But, what about indoor humidity?  The thermometer can't measure that so what to do? 

Today we have what are often called thermidistats.  They are both a thermometer and a humidistat in one unit.  You set both the temperature you want and the humidity you want.  These units call for the equipment to run in the correct mode to control both criteria to your wishes.  For temperature control the fan is operated at full speed.  For enhanced dehumidification, the fan is operated at a reduced speed which improves the removal of moisture. 

Now go forth and conquer your condensation and indoor moisture problems.  If you have a problem to be solved, let us know and we'll see what we can do for you. 

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