This image should look familiar, Most of us take a look at it daily, as our lives are affected by the weather.

Maybe its Spring and it's time for our fancy to turn to VAPOR, rain, flowers, the seasons change. If you are a home owner, builder, architect, specifier this concern for vapor is always present. One, because it is, and two) lots of litigation has sprung up with the damage vapor can cause. The first question is: "what is a discussion re: vapor doing on a Radiant Barrier Product site... "; I'll get to that!

First, a quick refresher, the daily/weekly forecast above is certainly wrong with one exception; the dew point and humidity. some time when the warm air migrates to the cold, it will pass through the dew point ( above, this day its 43F) if the warm air is in your home and is "escaping" to the cold outside, my guess, about 12:00AM +  to about 7:00AM tomorrow. During this time the moisture in the warm air is condensing when it reaches 43F and is dripping or saturating the material around it... usually Fiberglas. For all its virtues, Fiberglas is a great sponge  (The more water condensate the Fiberglas soaks up, the less effective it is as an insulator). The framing and other wall and attic materials also get wet and tend to stay that way for a while, sometimes forever, until rot weakens the structure and bad things happen. We live with moisture, period!

You home's walls, attic, and crawl/basement is the wrong place to store moisture or water. Most common building practices introduce a vapor barrier between the sheetrock wall and the Fiberglas, in the form of a film on the Fiberglas batt, a separate layer of visqueen, or poly something when blow in is used. The use of visqueen in the attic is usually debated, and the outcome is usually substantial venting to allow the attic space to breath. Breathing means vents, serious vents that let the outside in.

Back to the dew point: around midnight tonight, in this example, the dew point will be reached and moisture will be forming between the sheet rock and the vapor barrier, in this case it's visqueen. WHY, because the outside temp is now 40F and the Fiberglas has conducted most of it heat to the cold and the Fiberglas is 43F at the visqueen surface. So in this example the choice was to accumulate the condensate behind the wall's sheet rock. The Fiberglas has been protected from soaking and loosing its insulating properties, but the bad news is.... the wall is getting wet. This is a well protected place for mold, mildew, and rodents to take up housekeeping. In warm climates, where Air Conditioning is the way of life, the dew point can be in the 70's. while the outside is in the 90's and the airconditioning is set at 75F... you guessed it... now the Fiberglas gets wet, and the sponge begins to loose its insulating properties in a big hurry... (about 1-2 % moisture content reduces the Fiberglas about 30-40% from R-19 to R-12).

I apologize for the remedial tutorial, BUT it is lost on most of the architects and builders I encounter... or talk to!

As in most things, if you understand the problem, then you can figure a solution! So to restate: the problem is the temperature of the wall reaching the dew point in the interior next to the visqueen. If the wall did not reach the dewpoint, then obviously no condensate. So lets get about doing just that.

All of the materials in the wall we are dealing with, in this example, are conductors of heat, I.E. the materials conduct the heat form the interior through the wall to the exterior. This is heat conduction, hot to cold. That explains why the wall is going to reach that 43F at the visqueen about midnight. AND the vapor, moisture held in the interior air, is going to find its way to the visqueen... because the sheetrock wall assembly leaks. All Buildings leak.... Period! So, lets replace the visqueen with a radiant barrier, eliminate the heat loss or gain, and let the wall live forever without ever experiencing the dew point. It is that simple!

Ok, the answer is: the radiant barrier reflects 97.5% of the energy that hits it and sends it back to the interior. The space between the radiant barrier and the interior of the wall (the room side of the sheet rock where the point goes)  is the temperature of the interior room. The cold side of the radiant barrier is the temperature of the air space just before the Fiberglas, if any. in this example around midnight, the Fiberglas is still 40-43F, and the radiant barrier is about the same as it faces the Fiberglas. In severe cold climates a combination of Fiberglas and Super R is used or a sheet of TempShield is used. In coastal areas solid Super R is specified because the humidity is in the 90+% range, always. In less humid areas, I.E. 40-70% humidity, perforated Super R is specified because like Tyvek or Goretex, the small amount of permeability allows moisture to "dry"  out in low humidity times..( AKA: normalize to a moderate % humidity). The wall breaths!

The bottom line: stop the conduction of heat and you will stop the change in temperature that causes the dewpoint! It's that simple! Manage the vapor problem and get 30+%  energy savings free. OR is it manage the energy problem and get the vapor problem solved for free? Either way, its all good!