Commonsense Moisture and Humidity

The Ides of July! Beware moisture! It’s not just March with Ides. During the hot summer months, thoughts of condensation are not always with us, but our life giving Aitch to Oh (H2O) can come into view in awkward places where warm moist air comes into contact with cold surfaces. Before we start just a quick commentary on the difference between water and water vapor. We sometimes think that what we see rising off a pond in the morning as water vapor, the white plume from the teapot, or clouds- if you can see it, it is water drops, water vapor is clear. Most important, water vapor does no harm- it is liquid water of which we are to beware.


  1. Avoid overall humidity analysis- it is at the exceptions the problems arise- when a channel of uncontrolled air hits the cold surface you have a problem, especially if that cold surface is sheet rock paper. Mold needs three things, spores (which are practically everywhere-note the Kurds in the desert making “sourdough” bread by leaving a pan of flour and water in the open for a few hours), liquid water, present through our poor planning or analysis, and food: cellulose is choice, although good old household dust can be quite nutritious.
  2. Remember that heat transfer is orders of magnitude faster during evaporation and condensation: a heat pipe is that because of these phenomona- moisture issues are huge heat highways.
  3. If you don’t ventilate you will suffocate and compost your structure. Cooking and bathing are prime culprits. Recirculating stove “vents” are the bane of the building scientist and fodder for forensics!


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