Building Science-Perm

Builders in northern Minnesota have been using polyethylene sheeting on the warm in winter side of wall assemblies for years.  Beginning in the 1960’s, we were taught that the poly would stop vapor diffusion in building cavity, keeping walls dryer.  During the 1990’s and 2000’s, building scientists determined that only a small percentage of wall wetness comes from vapor diffusion.  So, how does moisture end up inside building assemblies in a northern climate?  Mostly by air leaks!

Around the year 2000, builders began using mastics and tapes to seal the poly to the building framing, installing electrical boxes with gaskets, and taping seams in the poly to reduce air movements through the building assemblies.  Using these techniques have substantially decreased the amount of air naturally leaking in and out of our homes.  Our structures are becoming more air tight, but…

There is a down side to using polyethylene as an air barrier, it has a very low perm rating.  What is a perm rating and why is it important?  First, we need a definition.

One perm is defined as 1 grain of water passing through one square foot of material in 1 hour with a vapor pressure difference of 1 inch of mercury between the warm and cold side of a material.

Building scientist have known about the diffusion of water through building components since the 1940’s, but it wasn’t until recently that we started testing different building products for their permeance.  One of the lowest perm ratings for a building material is polyethylene sheeting at .04-.06 perms for 4-6 mil poly.  In comparison, raw drywall has a perm rating of 40.  So, why is this important?  Because the permeance of a building products used in our assemblies will affect the way a building will dry.

Let’s look at what the 2012 IRC, the current building code in Minnesota, has to say about vapor retarders:

R702.7 Vapor retarders. Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5,6,7,8 and marine 4.

The southern half of Minnesota is in Zone 6, and northern half is in zone 7.  The code is saying we are required to have a class 1 or 2 vapor retarder.  There are 3 classes of vapor retarders.

Class I – .1 perm or less. Polyethylene sheeting or unperforated                             aluminum foil.

Class II – .1 perm to 1 perm. Kraft faced fiberglass batts, vapor                               retarder primer.

Class III – 1 perm to 10 perms. Latex paint, 5 inches of open cell                              spray foam.

A class III vapor retarder is allowed only in a specific wall assembly in Minnesota, which will be discussed in a future blog post.

There are a couple locations in the 2012 IRC and the Minnesota building codes that discuss testing methods for determining the perm rating of a product. Test ASTM E96 Method A is the dry cup test and ASTM E96 Method B is the wet cup test.  These are both laboratory tests conducted by scientists and manufacturers to determine a perm rating.  When you use a product in a location where the permeance of the product is important, this information should be available from the manufacturer.

There is no place in the building code that specifically states polyethylene sheeting must be used as a vapor retarder, but it is commonly used in Minnesota.  Most of the time, the poly doesn’t hurt, but there are times when it’s use can be risky.

All exterior building assemblies that use polyethylene sheeting as the air/vapor barrier have only one path to dry, to the outside.  The up

Polyethylene sheet on warm in winter side of wall assemble

side to the poly is there is little potential for moisture to enter the wall from the interior (if the poly is properly sealed), but what about from the exterior?  It is very hard to construct a perfect wall and the exterior has the greatest potential for moisture leaks.  Bulk water, such as rain or melting snow often finds a path behind the siding and can potentially enter the wall cavity.  Some exterior claddings, such as brick, stucco, or stone, can drive moisture through the building assemblies during the summer months.  Other products used on the exterior of a building may also have a higher perm rating, limiting the drying potential to the exterior.  If there is limited drying potential both inward and outward, there is going to be a problem!  A great article on this condition is by Joseph Lstiburek titled Inward Drive-Outward Drying at  Sometimes it might be a good idea to consider some other vapor/air barrier other than poly.

We have other choices!  Finding another vapor barrier isn’t hard, convincing a building official might be a little more difficult.  There are several products that will achieve the code required 1 perm or less.

2.5 inches of closed cell spray foam has a perm rating of around .8. No additional vapor retarder will be needed.

Kraft faced insulation is a choice. Most Kraft facings have a perm rating under 1. This type of insulation is easy to install, just remember the facing goes to the warm in winter side of the wall, roof or floor assembly.

A smart vapor retarder, such as Certainteed’s MemBrain, ProClima’s Intello, or a new product by the Swiss manufacturer, Siga, called Majrex also meet code. These vapor retarders can vary their perm rating depending on the amount of moisture present within the building assembly. The drawbacks are Intello is expensive, MemBrain is fragile and difficult to work with and Majrex is new and may be difficult to find.

Personally, I like vapor retarding primers. We are painting the walls anyways, you can kill two birds with one stone.

You’ve convinced the building official that the vapor retarding primer or a smart vapor retarder is ok to use, now you need to pass the blower door test. How do you do that without the poly as your main air barrier?  This will be the topic of our next blog.

Here is a table of common building materials and their perm rating.

Aluminum Foil 1 mil 0.00                                                                                                    Polyiso with foil face 1 inch 0.05
Polyethylene 6 mil 0.06
Kraft facing, asphalt coated 0.30
Vapor retarding primer 0.50
MemBrain smart vapor retarder 1.00
Expanded polystyrene 1 inch 1.10
Closed cell spray foam 1 inch 2.00
Polystyrene insulation 1 inch 2.00
Tyvek housewrap 77.00
CDX plywood ½ inch 0.50
OSB plywood 7/16 inch 2.00
Concrete 1 inch 3.20
Concrete block 8 inch 2.40
15 lb asphalt felt 0.56 – 6.00
Raw drywall 40.00
Fiber cement lap siding 1.50
Traditional stucco 3.80

Next week: passing a blower door test without polyethylene sheeting.

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