It's Not the Rain, it's the Humidity

The misunderstood part of water intrusion problems.

By William Decker, CRI, CMI

I was up in an attic, the other day (part of what we home inspectors do) and a funny, and unexpected, thing happened.  After I was in the attic for about a minute, the visibility went down to pretty much 6".  It was like, all of a sudden, I had been transported to Victorian London during the time of Jack the Ripper.  The whole attic was completely encased in fog.  What happened.


The day was hot and humid, 92 degrees with a 97% relative humidity outside.  Understandably, the homeowners had the air conditioning system set pretty low (70 F) and the house had been empty and holding that temperature for awhile.  At that temperature and humidity, the dew point is 91 degrees.  So, when I went into the ventilated attic, the humidity was suspended in the attic air and just waiting for a 1 degree temperature drop.


Did any of you make rock candy when you were young.  You heat up a pot of water and add a lot of sugar.  If the water was not hot, the sugar would not fully dissolve.  The water would become saturated with the sugar and the residual would settle on the bottom of the pan.  But, when the water is heated, the water molecules are moving faster and can support more sugar dissolving  in the water.  The same thing happens with water dissolved in air.  We don't really think of it in that way, but humidity is just water molecules dissolved in air molecules.  The hotter the air the more water it can hold.  But when the temperature of the air is lowered, the air can no longer hold the water and it condenses out of the air.  This is regularly seen when you have a cold bottle of beer on a hot summer day.


And this is what happened to me in the attic.  When I opened the hatch to the attic and went in, the cold air conditioned air from the house was drawn up into the attic (most probably, because of wind moving over the roof ridge, sucking the cold house air up into the attic) the air temperature dropped and the water dissolved in the air condensed out.  Water was leaking into the house, but not in the form of liquid.  It was in the air!


We only think in terms humidity and condensation in terms of hot summer days.  This is especially true for us Chicago area people because of Lake Michigan always providing water to dissolve in the air.  But condensation occurs in the cold as well.  I will always remember the many Halloweens I had experience when it was cool (68 F) and humid (98 % RH).  A drop of as little as 1 or 2 degrees would cause a great fog.  I saw this in action, just the other morning.  My new puppy woke up early (4:00 AM) and simply had to go out.  I took her out to the local Pooch Park and watched her roam around.  I looked down at my phone for a while, but when I looked up again, the large field of grass was completely covered with fog and I could not see my dog.  The reason?  The temperature dropped 3 degrees.


So, what does this have to do with home inspections and building science?  Plenty.


When people see water stains on their ceiling or around the recessed light fixture covers, they immediately think that their roof is leaking.  They call a roofer.  The roofer's jobs is NOT to solve their problem, it is to sell a new roof.  Can't blame them, it's what they do.


Then the problem reappears, even after they have replaced their perfectly good roof (and this has happened, many times, to my clients).


In many cases, these water stains and even active water leaks are not the result of a bad roof, they are the result of:



Improper or incomplete insulation on HVAC ducts installed in attics or ceiling / floor interstitial spaces.  If humid air comes in contact with exposed metal ductwork, water will condense and leak.


Lack of proper ventilation in attic spaces.  Moisture can concentrate in flat roof attic spaces, especially in masonry buildings, if the spaces are not either a) properly ventilated or b) properly sealed and flashed.


No air sealing.  I have seen water leaking from a recessed light fixture in a house, at the rate of 5 gallons a day, in the winter!  The cause was humid air from the house (humidity levels in houses during the winter should not exceed 35% RH) leaking through unsealed recessed ceiling lights into the cold attic space and condensing.


Is your house a "breather" or a "sealer"?  If older, your house will have multiple air leaks.  If you seal it up, say by covering the exterior walls with new vinyl siding with thin rigid insulation underneath, you stop all that air leakage, but you also lock in all the moisture.  If your house is newer and has been built "tighter", does it have an air recirculation system?  If not, where are you getting fresh air from and how is excessive humidity in the house being handled?


These concepts are usually not understood or considered by most house builders, and usually not even by the Architects who design them.  As construction techniques, materials and new energy standards change, unconsidered problems can arise.  Excessive condensation and water damage arising thereof is being seen, more and more.