In this special episode of “If Walls Could Talk,” Editor John Wyatt speaks with Tom Harris.  They are less expensive than regular stick-frame construction, higher ceilings, open floor plans – a lot of good reasons to consider. The biggest concern is managing the interior moisture when you have a vapor impermeable shell.  It also depends on where in the country you build.

In the south, you mostly cool, so the temperature outside is typically higher than the temperature inside the building.  The moisture in the air (relative humidity) and the heat energy is trying to get into the cooler, less humid conditioned space on the inside of the building.  This is identified as the vapor drive.  Mother nature moves things from areas of high energy, high temperature, high RH to areas of lower energy, temperature and relative humidity.  If we have a metal skin on the outside of the building, and we insulate the inside of the metal, that metal should never get cool enough to form a condensation plane or surface.  It’s like the different between a glass of water and ice and an insulated cup of ice and water.  You’ll get condensation on the outside of the glass, but not on the outside of the insulated cup because the outer surface of the insulated cup isn’t cold enough to condense the moisture in the air.  Same thing with metal buildings in the south. The metal is the vapor barrier (glass) and protects the foam insulation from moisture in the air getting into the insulation and condensing… a good thing.

In the north, remember the vapor drive – from hot to cold and we’re now living in the inside of these metal skinned buildings so the relative humidity is constant and it could be high.

In the north, the outside temperature is predominantly colder than the inside of the building, so the vapor drive is from inside to outside, so we drive moisture into our insulation materials.  If these insulations are vapor permeable (they let moisture go through them) then the moisture will eventually hit the cold metal and condense and cause bigger problems within the wall.  

The SPFA is currently working to produce a “high humidity” applications guide to assist SPF installers and design professionals.