The properties of vapor and moisture are complex. The following introduction is a cursory discussion only.
What is a vapor retarder?
A vapor retarder is a material that restricts or reduces the rate and volume of water vapor diffusion through the ceilings, walls, and floors of a building.
Building materials at a given thickness are tested and given a permeance rating. This rating measures the amount of water vapor that can pass through it. The thicker a building material is the greater its ability is to restrict vapor diffusion. Building materials with a permeance rating less than 1 are considered a vapor retarder.
What makes a vapor retarder different from an air barrier?
The vapor retarder should not be confused with an air barrier. A vapor retarder is designed to minimize the amount of water vapor passing through it. In comparison an air barrier is designed to stop air movement that could bring water vapor into a building assembly. Some air barriers are designed to allow water vapor to pass through to allow its evaporation and to allow the drying of the building assembly.
Why use a Vapor Retarders?
The main reason for retarding the transmission of water vapor through the building envelope is to prevent water vapor from condensing back into liquid form within the building structure cavities.
Where is a Vapor Retarder installed?
The local climate and the heating/cooling needs of building determine where the vapor retarder is installed. Where the vapor retarder gets installed primarily depends on the local climate and the heating and cooling needs of a building.
For buildings in a heating climate, the vapor retarder is placed on the inside or the warm side of the building envelope. The reason for this is that the cold air outside will hold less moisture then the warm air inside a building. It is the warm, moist air inside a building that can get into the building envelope and condense when it contacts the colder surface, usually on the backside of the exterior wall sheathing. This is called "the first condensing surface." With the vapor retarder on the inside and a vapor permeable air retarder on the outside, any water vapor that does condense inside will be able to evaporate and dry through the permeable air retarder to the outside.
In a cooling climate, the vapor retarder should be placed on the outside of the building envelope. In a cooling climate, the outside air is warmer, and has the potential of containing more water vapor than the inside air. Placing the vapor retarder on the outside will reduce water vapor movement from the outside from getting into the building envelope. Any vapor that does get into the walls or roof assembly can evaporate to the inside and therefore dry out before the moisture can lead to mold, mildew and rot problems.
Why is the very low permeance of closed-cell foam significant?
Vapor retarder and permeance research
The Spray Polyurethane Foam Alliance (SPFA) has published a short report, as an industry service, on the basics of water vapor transmission and how it affects the building envelope. The report is available for download in PDF format can be viewed using Adobe Reader.
Demilic, a major foam insulation manufacturer, asked the National Research Council of Canada (NRC) to conduct extensive testing on their Heatlok 0240 polyurethane foam product. The purpose of the tests were to evaluate the water vapor permeance of the foam insulation while applied to either gypsum or concrete block.
The first step in the testing process was to measure the permeance of each product separately, and then test the foam and gypsum or concrete block together. The permeance was tested using the ASTM E 96 (dry cup) method.
SPF on gypsum (sheetrock)
The results of the NRC test for Demilic:
SPF on concrete block
The results of the NRC test for Demilic:
Bynum, Richard, 2001. Insulation Handbook, McGraw-Hill, New York, NY
Demilec Inc, 1999. Typical details for the design of the building envelope: HEATLOK 0240
Lstiburek, Joseph and John Carmody, 1993. Moisture Control Handbook, Van Nostrand Reinhold, New York, NY
Lstiburek, Joseph, 1998. Builders Guide: Cold Climates, Building Science Corporation, Westford, MA