There are several types of insulation one can use to insulate a home. The most widely known is fiberglass (FG). The other kinds are rock wool, cellulose and foam. They come in different forms like batts, rigid board, loose fill and liquid. All insulation is rated for resistance to heat flow, which is represented in "R" and a number, such as R-11. This means that 1/11 of a BTU per one square inch is this insulation heat flow. This is based on the dominant heat transfer mechanism of diffusion and not radiant or convection. An R-19 would mean 1/19 of a BTU. What this illustrates to you is why the higher the R - value, the greater the insulation value you will have. In other words, there is more BTU's lost with R-11 than R-19.
The time and temperature difference was not included to illustrate the difference between 1/11 and 1/19 and which one is a smaller number. Vapor barriers were invented for insulation. That means it is only used with insulation. When homes were not insulated, there was no need for a vapor barrier. Once insulation was put in a home, it was found out that a moisture problem occurred within the insulation. Since all heat has moisture in it and the dominant heat transfer mechanism in heating is diffusion, it was apparent the moisture problem was a result of dew point being reached inside the insulation. There are basically two ways to prohibit heat from condensing inside insulation.
First is to reduce the volume of air inside the insulation, which was not feasible when insulation was first introduced because the thermal effect came from trapped air inside the insulation.
The second is to reduce the amount of humidity in the heat so that when dew point is reached inside the insulation it could not condense. Vapor barriers do not address diffusion, what it does is prohibit the heat inside the insulation from condensing by reducing the moisture content in the heat.
Vapor barriers are rated for performance just like insulation. It is recommended the rating for vapor barriers should be SP-15. This rating has nothing to do with rating the insulation. Though the vapor barrier may have some thermal effect, it will be rated in "R" values with the insulation, a separate and different performance rating.
Where and when should a vapor barrier be installed? This is where all the conflicting theories come from. In areas where you have cold winters, the vapor barrier should be between inside the home and insulation. In areas where the climate is dry, no vapor barrier. In areas where it is hot and humid, the vapor barrier should outside the insulation and the home. Where vapor barriers are applied the factor that you should be concerned about is heat is attracted to cold and heat condenses on cooler surfaces. In fact, surfaces inside the insulation qualify as a cooler surface. Where vapor barriers should not be applied, the factor you should be concerned with is equilibrium relative humidity. What this fancy term means is, if the air is very dry, it has the capacity to absorb the humidity in any heat present, thereby prohibiting condensation. It also means that if the heat is dry, it does not possess the ability to condense inside insulation. This is why in dry climates vapor barriers is not recommended.
In areas where you have cold winters, the heat inside he home is attracted to the cold outside. Since heat usually condenses against cooler surfaces and surfaces inside the insulation qualify as a cooler surface, the vapor barrier is put between the insulation and inside the home. This is done to prohibit condensation by reducing the moisture content in the heat.
In areas where it is hot and humid, the heat outside the home is attracted to the air conditioned space inside. So the cooler surfaces where the heat will condense is inward when cooling a home instead of outward when heating a home. Therefore the vapor barrier should be between the heat outside and the insulation. The problem with cooling is that the dominant heat transfer mechanism is radiant, unlike with heating, it's diffusion. To alleviate this problem, most contractors will use foil faced insulation, where the foil vapor barrier acts as a radiant barrier too.
I am aware that this raises more questions than answers. The sole purpose was to take away some of the confusion with insulation and understand the different applications with it. For those that require more information, you can e-mail me by clicking on ask an expert.