Putting It into Practice.
At the design phase, it is important to develop a vapor control strategy that is based on climatic conditions and material choices that meet the goals set for all of the control layers. This may involve a combination of vapor control strategies for different parts of the building enclosure.
Airtightness is of crucial importance for vapor control, as air leakage will move much larger quantities of moisture into the building enclosure than diffusion. Diffusion becomes more important once a building is relatively airtight, but the first concept in vapor control is air control.
Building codes in many jurisdictions use the language of vapor barriers, rather than vapor retarders, and the use of a strategy other than a vapor barrier may require the use of alternative compliance pathways.
At the construction phase, it is important that materials specified for the vapor control layer in the plans are actually used on site.
Competing materials that may seem to be equivalent from a construction perspective (for example, different brands of house wrap, sheathing boards, and paint, among others) can have very different permeability ratings, and the substitution of a product that does not meet permeability requirements will negatively affect the performance of the vapor control layer and may result in moisture damage in the building assembly.
Some materials used as a vapor control layer must be installed with the proper side of the material facing the interior in order to perform as intended.