I want to talk to you a little bit about vapor retarders, the building code requirements for vapor retarders and classes of vapor retarders.
Vapor retarders is one of the questions I get most often related to insulation and spray foam insulation.
The building code section for vapor retarders is IRC 702.7, it says that we need a Class I or Class II vapor retarder on the interior side, or the warm-in-winter side, of frame walls in climate zones 5, 6, 7, 8 and marine 4.
This means that in these northern climate zones a less than 1 perm vapor retarder is required on the interior side of the wall assembly.
Let’s talk a little bit about these classes of vapor retarders.
Building materials can be tested in accordance with ASTM E 96 to determine their vapor permeance rating.
Once a building material has a tested vapor permeance rating, it can be classified as either Class I, Class II, Class III or vapor open if the vapor permeance rating is beyond the classification limit for a class 3 vapor retarder.
A Class I vapor retarder has a less than 0.1 perm rating, this includes materials like polyethylene, aluminum foil, sheet metal; these products are considered vapor impermeable.
A Class II vapor retarder has a less than 1 perm rating, this includes materials like a properly installed craft faced batt and closed cell foam at appropriate thicknesses; these products are considered vapor semi-impermeable.
A Class III vapor retarder has a less than 10 perm rating, this includes materials like latex and enamel-based paints on gypsum board and most open cell spray foams when installed at typical thicknesses; these products are considered vapor semi-permeable.
Why does this matter?
We go back to what the code says, the code requires a less than one perm rating on the interior side of walls in climate zones 4a, 5 and higher.
It is important to understand these concepts to meet code compliance and make sure that you are designing and installing systems that are compliant with the building code in your local jurisdiction.
The code says the correct location of the vapor retarder is the interior side of the wall, but I like to think of it as the interior side of the insulation, because what you really want to do is keep the vapor from going through the insulation and finding a cold surface on the cold side of the wall to condense against.
As long as the vapor retarder is in contact with the insulation, or is on the warm side of the insulation, the vapor drive will be reduced enough to minimize the risk of condensation in that assembly.
This is the real intent of the building code, to mitigate and minimize the risk of condensation and moisture in the assembly that is being designed and installed.
When we talk about northern climates, we’re going to want to make sure we have a less than one perm vapor retarder on the inside of the insulation.
For cavity insulation applications, the vapor retarder should be on the interior side of the wall, typically between the insulation and the gypsum board.
If we move the insulation, for example, if we have all continuous insulation on the exterior of the building that becomes a critical design change and now we can have the vapor retarder on the exterior as well, as long as it is on the warm side of the insulation or it is on the same plane as the insulation, such as when we use closed cell spray foam on the exterior of a building.
There is an exception to this rule, IRC 702.7.1 which allows Class III vapor retarders when very specific construction guidelines are met.
I’ll put this table up right here so you can see it.
In the marine 4 climate zone,
- vented cladding over wood structural panels,
- vented cladding over fiber board,
- vented cladding over gypsum
- continuous insulation with R-value greater than R-2.5 in a 2×4 wall,
- and continuous insulation with R-value greater than R-3.75 in a 2×6 wall
All these designs qualify for this exception, where a Class III vapor retarder would be acceptable instead of a Class I or Class II vapor retarder.
You can go ahead and read the table for climate zones 5 and higher, whatever applies to you.
These are some of the different designs where a Class III vapor retarder can be used to meet building code.
One question that may come up is, what does the code mean by vented cladding?
Vented cladding generally refers to a type of exterior system that has a designed vent space between the sheathing and the cladding itself.
The most common types of vented cladding are masonry cladding materials.
These are systems that have a drainage plane between the cladding and the next layer to the interior – this includes brick, stone, concrete panel, something like this that is going to have a designed drain plane making it a vented cladding system.
One last note, you have probably heard the term vapor barrier before, but the building code never talks about vapor barriers.
The code only refers to vapor retarders, and it is generally understood in the building industry that a “vapor barrier” refers to a material with a perm rating of less than 1 perm, or a Class II minimum vapor retarder.
This has been an overview of the vapor retarder section of the code, IRC 702.7, and the whole point is to mitigate the migration of moisture into the wall assembly, minimize the risk of condensation and potential freezing of that condensation inside the assembly.
This allows us to design better structures and take better care of our structures, so we get longer-lasting buildings by using good designs.
