VAPOR BARRIERS
There are a ton of technical reports on that site on various issues. It might not address your questions directly, but you can probably start to piece it together. To figure out if and where you need a vapor barrier, you have to first know what the permeance of your building materials are, what the climate is and what the interior environment is (i.e. how much humidity is being produced inside the building). Humidity (vapor) will travel from higher to lower concentrations, or from hotter toward colder spaces. So knowing what the relative humidity and temperatures are between indoors and outdoors at various times of year is also important. Keep in mind that weather barriers may control 3 different things: water, air, vapor. Of the three, vapor can get through the smallest of holes. Air needs larger holes than vapor to get through. Water needs larger holes than air to get through. The prevailing wisdom is that mechanically conditioned buildings should be both air and water tight. If your buildings are not mechanically conditioned, then it would be good to talk about how your buildings are ventilated as that may have implications for your building envelope and whether or not it is acting as an air barrier (which may or may not be desirable). Moving on, vapor is an issue all its own...
Here is a quick primer from another random site that was talking about vapor relative to basement walls. I was trying to find a quick classification of the permeance of concrete and this one made reference to ASHRAE (you might try looking through the ASHRAE website directly) in quoting that a concrete mix of 1:2:4 has a permeability of 3.2 perms per inch (which they divide by the thickness... which I am not quite following, but whatever). That means that a 4" slab would rate as .8 perms for a "semi-impermeable" classification which is bordering on becoming "semi-permeable". So the thickness of the concrete matters as would the nature of the mix... and for the latter I'm not sure if or where you will be able to find the permeance ratings of various mixes. Again, maybe check to see if the ASHRAE Handbook might list permeability for different concrete mixes. For poured in place concrete another consideration is that the concrete will continue to cure over it's lifetime such that it is always letting off vapor. That means that you should plan for the concrete to either dry to the inside or outside or both.
Permeance of building materials
Permeance characterizes how easily water vapor diffuses through a material - the lower, the more resistance to vapor migration. Permeance is measured in perms(grains/ hr-sq. ft.-inch Hg), i.e. water vapor transmission rate (grains/hr-sq.ft.) per unit of pressure differential (inch Hg). (1 perm = 1.47 ng/sec-sq.m-Pa)
General classes of materials by permeance and examples:
Vapor impermeable - .1 perms or less
Vapor semi-impermeable – .1 to 1.0 perms: * Elastomeric or bitumen membranes (0.05–0.5 perms)
* Polyethylene film (0.1 perms)
* Foil-faced insulation
* Ceramic tiles (but not grout), VCT tiles, linoleum
* Epoxy, heavy urethane or oil-based paints
* Vinyl wallpaper
Semi-permeable to vapor – 1 to 10 perms: * Unfaced expanded or extruded polystyrene (2 or 1.2 perms/inch)
* Heavy asphalt impregnated building papers
* Fiberglass batt insulation with paper or bitumen facing
* Plywood, OSB
* Gypsum board painted with latex paint
Permeable to vapor – over 10 perms: * Unpainted stucco or plaster
* Unfaced fiberglass insulation, cellulose insulation
* Cementitious waterproofing coatings (20 perms)
* Interior waterproofing paints
* Lightweight asphalt impregnated building papers
* House wraps
* Unpainted gypsum wallboards (50 perms)
Avoid trapping water vapor, condensation, and molds behind walls in finished basements and under the floor covering on a concrete slab.
For more information see www.buildingscience.com/index_html
Vapor permeability of concrete
How about the permeance of concrete? According to ASHRAE Handbook, the permeance of concrete (1:2:4 mix) is 3.2 perms per inch of thickness. Then, a 4-inch slab would have (3.2/4=) 0.8 perms. But this is only for “good quality” moist-cured concrete with low water/cement ratio. Most basement slabs are much more porous. Although they may be almost impermeable to water, basement slabs are semi-permeable to water vapor (>1 perm).Newly poured concrete walls are classified as impermeable to vapor (8–10 inches thick, 3.2/8 = 0.4 perms for good quality concrete). However, once the exterior waterproofing coating disintegrates, water starts enlarging the pores in concrete and the walls will become permeable and later, may even start seeping water. Walls made of pre-cast panels have the lowest permeance because they are made of high-strength concrete under controlled factory conditions.
Hollow concrete blocks are a different story. They have only a 1 1/4-inch wall outside the hollow cores (3.2/1.25 = 2.6 perms). The concrete is very porous and the hollow cores fill up with water vapor, which then moves to the most permeable area to get indoors. The porosity of blocks varies widely. Some tests of good concrete blocks show 2.4 perms when the cores are filled or 4.8 perms for hollow blocks. Hollow concrete blocks are semi-permeable (say 5 perms) but lightweight CMUs, splitface blocks, “popcorn” blocks, and cinder blocks are permeable (>10 perms).
If possible, use poured concrete instead of standard blocks (concrete masonry units – CMUs) for foundations. Or make at least sure that the hollow cores are properly filled during construction. Apply parging on the exterior and a good waterproofing coating. Concrete block walls are also less able to resist lateral pressure of water and earth – high clay soil or expandable clay can cause structural damage.
All that said, you can design your envelope to dry only to the inside, only to the outside, or some combination of both (i.e. mostly to the inside but some to the outside, or equally to the inside and outside, etc). If you have a situation in which vapor wants to go one direction during the summer and the other direction in winter, then the general recommendation is not to use a vapor barrier. However, there are exceptions to that rule like if you have a masonry cavity wall.
Thanks to Shelly P
Here is a quick primer from another random site that was talking about vapor relative to basement walls. I was trying to find a quick classification of the permeance of concrete and this one made reference to ASHRAE (you might try looking through the ASHRAE website directly) in quoting that a concrete mix of 1:2:4 has a permeability of 3.2 perms per inch (which they divide by the thickness... which I am not quite following, but whatever). That means that a 4" slab would rate as .8 perms for a "semi-impermeable" classification which is bordering on becoming "semi-permeable". So the thickness of the concrete matters as would the nature of the mix... and for the latter I'm not sure if or where you will be able to find the permeance ratings of various mixes. Again, maybe check to see if the ASHRAE Handbook might list permeability for different concrete mixes. For poured in place concrete another consideration is that the concrete will continue to cure over it's lifetime such that it is always letting off vapor. That means that you should plan for the concrete to either dry to the inside or outside or both.
Permeance of building materials
Permeance characterizes how easily water vapor diffuses through a material - the lower, the more resistance to vapor migration. Permeance is measured in perms(grains/ hr-sq. ft.-inch Hg), i.e. water vapor transmission rate (grains/hr-sq.ft.) per unit of pressure differential (inch Hg). (1 perm = 1.47 ng/sec-sq.m-Pa)
General classes of materials by permeance and examples:
Vapor impermeable - .1 perms or less
Vapor semi-impermeable – .1 to 1.0 perms: * Elastomeric or bitumen membranes (0.05–0.5 perms)
* Polyethylene film (0.1 perms)
* Foil-faced insulation
* Ceramic tiles (but not grout), VCT tiles, linoleum
* Epoxy, heavy urethane or oil-based paints
* Vinyl wallpaper
Semi-permeable to vapor – 1 to 10 perms: * Unfaced expanded or extruded polystyrene (2 or 1.2 perms/inch)
* Heavy asphalt impregnated building papers
* Fiberglass batt insulation with paper or bitumen facing
* Plywood, OSB
* Gypsum board painted with latex paint
Permeable to vapor – over 10 perms: * Unpainted stucco or plaster
* Unfaced fiberglass insulation, cellulose insulation
* Cementitious waterproofing coatings (20 perms)
* Interior waterproofing paints
* Lightweight asphalt impregnated building papers
* House wraps
* Unpainted gypsum wallboards (50 perms)
Avoid trapping water vapor, condensation, and molds behind walls in finished basements and under the floor covering on a concrete slab.
For more information see www.buildingscience.com/index_html
Vapor permeability of concrete
How about the permeance of concrete? According to ASHRAE Handbook, the permeance of concrete (1:2:4 mix) is 3.2 perms per inch of thickness. Then, a 4-inch slab would have (3.2/4=) 0.8 perms. But this is only for “good quality” moist-cured concrete with low water/cement ratio. Most basement slabs are much more porous. Although they may be almost impermeable to water, basement slabs are semi-permeable to water vapor (>1 perm).Newly poured concrete walls are classified as impermeable to vapor (8–10 inches thick, 3.2/8 = 0.4 perms for good quality concrete). However, once the exterior waterproofing coating disintegrates, water starts enlarging the pores in concrete and the walls will become permeable and later, may even start seeping water. Walls made of pre-cast panels have the lowest permeance because they are made of high-strength concrete under controlled factory conditions.
Hollow concrete blocks are a different story. They have only a 1 1/4-inch wall outside the hollow cores (3.2/1.25 = 2.6 perms). The concrete is very porous and the hollow cores fill up with water vapor, which then moves to the most permeable area to get indoors. The porosity of blocks varies widely. Some tests of good concrete blocks show 2.4 perms when the cores are filled or 4.8 perms for hollow blocks. Hollow concrete blocks are semi-permeable (say 5 perms) but lightweight CMUs, splitface blocks, “popcorn” blocks, and cinder blocks are permeable (>10 perms).
If possible, use poured concrete instead of standard blocks (concrete masonry units – CMUs) for foundations. Or make at least sure that the hollow cores are properly filled during construction. Apply parging on the exterior and a good waterproofing coating. Concrete block walls are also less able to resist lateral pressure of water and earth – high clay soil or expandable clay can cause structural damage.
All that said, you can design your envelope to dry only to the inside, only to the outside, or some combination of both (i.e. mostly to the inside but some to the outside, or equally to the inside and outside, etc). If you have a situation in which vapor wants to go one direction during the summer and the other direction in winter, then the general recommendation is not to use a vapor barrier. However, there are exceptions to that rule like if you have a masonry cavity wall.
Thanks to Shelly P