Construction Materials-Smart Vapor Retarders

I have now had the opportunity to work with the three smart vapor retarders available in the United States.  Each has there own qualities. They all work in cold climate construction.  But…my in opinion there is a good, better and best.  This blog will outline my impressions of each.

Before we get started, lets review what a vapor retarder, or a better term, vapor control layer does along with when it’s used and where it is located (in a heating climate).  A vapor control layer exists to slow or stop the movement of moisture through a building assembly by way of diffusion.  Nearly all building products can move moisture through the material, how much and fast that moisture moves is called a perm rating.  (The definition for a perm rating is one grain of water moving through one square foot of material in one hour with one inch of mercury pressure difference between the two sides of the material.)  Materials like glass and metal move vapor either very slowly or stop this movement all together.  These materials are called vapor closed.  Materials like fiberglass insulation do little to stop moisture movement, these are known as vapor open.  Most building products fall somewhere in between.

There are three classes of vapor control layers, class I is less than .1 perm.  Materials such as solid metal and polyethylene sheeting fall into the class I category.  Class II products are between .1 and 1 perm and include kraft-faced fiberglass batts or drywall with a vapor controlling paint or primer.  Class III are products more than 1 but less than 10 perms.  Regular painted drywall, plywood or OSB sheeting are examples.  As I mentioned earlier, anything over 10 perms is considered vapor open.  The lower the number, the more vapor closed, the higher the number, the more vapor open.

Building codes require a class I or II vapor control layer on the warm in winter side of a wall assembly in climate zones 5-8.  Most building scientists recommend not using class I vapor control products, which is mainly polyethylene sheeting.  Builders in mixed climates use low perm paint on drywall or craft faced fiberglass insulation, which if installed and treated correctly, are a class II vapor control layer.  Move to a hot and humid climate and the vapor control layer is moved to the exterior of the building.  DO NOT USE THIS BUILDING METHOD IN MIXED, COLD OR VERY COLD CLIMATES.  Builders in my very cold climate still typically use class I, polyethylene sheeting or closed cell spray foam.  I prefer using, on exterior walls and some ceiling assemblies, one of the “smart” vapor control products.  There are three I am aware of available in North America, Certainteed’s Membrain, Pro Clima’s Intello and Siga’s Marjex.  The last two come from Europe.

6 mil polyethylene vapor control layer typical in Minnesota.

Before we get back to the topic of this blog, working with the three, I should talk a little about why I prefer using one of these “smart” vapor barriers over standard 6 mil poly.  Dryability!, not sure that’s a word, but I like that over breathability.  These three products can vary their perm rating depending on the moisture level.  Lets say there is a small water leak somewhere in a wall assembly (no one is perfect.)  Poly that has been installed on the warm in winter side of the assembly will not allow any drying to the inside.  The only drying potential would be during the winter months when the vapor drive is outward.  Another time moisture can accumulate in a wall cavity is during the summer while air conditioning.  Believe it or not, most northern Minnesota homes have some sort of air conditioning.  When the inside of the home is cool and the outside hot and humid, the inside of the wall can reach the dew point under the right conditions with moisture accumulating on the interior side of the poly.  No drying until winter.  The “smart” vapor retarder would recognize this increase in humidity and open to dry into the home.

Pro Clima’s Intello under negative pressure during a blower door test at the concreteless slab on grade project.

Finally…working with the products.  Intello and Marjex are similar in that they are made of a material the resembles a house wrap.  The durability of the two makes them easy to work with.  Both come in rolls that are 4.9 feet, (they are European, 1.5 meters), so you will be installing either vertically or at least two rows horizontally.  They roll out flat and are easy to attach to framing, even over your head.  I did have an issue with the Intello at the concreteless slab on grade home pull on the staples during the blower door test.  There were lots of staples installed and I think may have had a small affect on the blower door test.  This did not happen during the blower door test at the Code Minimum House where Majrex was used.

Siga’s Majrex used at the code minimum house installed between the attached garage and house.

Membrain closer resembles polyethylene sheeting or 6 mil poly and comes in rolls of 8’4″, 9’4″ or 10’4″ x 50′.  The difference is standard 6 mil poly is actually a little more durable.  Membrain tears and can be punctured easily.  Staples can easily pull through the product.  So, the major difference in the three products is with their workability, two being durable with the third, not so much.

Certainteed’s Membrain. The main vapor control layer used on the code minimum house.

So, why even consider using Membrain if it’s hard to work with?  Price.  The Certainteed product is much less expensive.  That being said, if I have any opportunity to use one of the European products on a project, I will, with my preference being the Majrex.  I actually enjoyed installing Siga’s product at the Code Minimum Home.  (I only installed Majrex on the wall common the the attached garage, Membrain was used on the rest of the exterior walls.)  Being it was the vapor control and secondary air control layer, I chose to staple the Majrex to the wood framing and taped and sealed all the joints and penetrations.  Another benefit of Majrex for a very cold climate, it is one directional.  As long as it is installed correctly, the Siga product will, for the most part, move moisture only out of the wall cavity.  The other smart vapor products may, under the right conditions, move vapor from inside the home into the wall cavity.  My understanding is that’s rare, but a possibility.

Final words.  The 6 mil poly that is common in my climate seems to work, that may change if summertime temperatures continue to increase.  A better choice is Certainteed’s Membrain or better yet, Pro Clima’s Intello.  If you want the best cold climate vapor control layer, choose Siga’s Majrex.  As good as Majrex is, there is still one better option, eliminate the class I or II vapor control layer.  Next blog posting, Randy’s dream wall assembly for a very cold climate and the move to a class III vapor control layer.   

3 Replies to “Construction Materials-Smart Vapor Retarders”

  1. Hey Randy! Great content here. Discovered you through the youtube video from the Greenhome Institute. I have a question that I think I know the answer… Trying to achieve passive house standards, I’d like to get a 2×6 wall to meet with 43 % exterior insulation up to R40+. Any way around not having 6-8″ of exterior insulation? Concerned about structurally supporting cladding etc.

    1. Hi Steve,
      The only two ways I’m aware of to eliminate thermal bridging and produce a super-insulated wall is either by continuous exterior insulation or double wall construction. Personally, I prefer the CI over double wall, even when trying to work with thick insulation attached to the exterior. In climate zone 7, the recommendation is 45% (per Building of the insulation total needs to be exterior, R-21 means R-17 CI is exterior (or R-23 cavity if using a high-density batt will require R-19). The 2021 IRC is a requires little less CI, R-15 on a 2×6 wall. Depending on your flavor of insulation, you might have up to 5 inches of continuous. If you can meet your metrics by using a 2 x 4 wall, you would only need R-12 on the exterior with R-15 in the cavity, but that only gets you a R-27 wall. If you are in climate zone 6, the ratio becomes 35%. FYI, these are only center of cavity R-values, actual R-values will be slightly less after figuring in the framing. Something else to remember, once you get to this level of exterior insulation, you can now move to a class III vapor retarder, painted drywall.

      As far as the structural support of cladding, the only cladding that becomes a concern is the really heavy stuff, adhered natural stone would be one. Wood cladding, and even fiber cement is considered light weight claddings. The better rigid insulation manufacturers have cladding support information including, fastening schedules for rainscreen applications depending on cladding type, listed somewhere on their websites. I know Rockwool had done a lot of testing on this subject, it’s not as complicated as you might think. Don’t be afraid to reach out to the manufacturer of the rigid insulation you are thinking of using, most have factory reps who can help.

      Thanks, great question!

  2. Thanks Randy!

    Just subscribed – great stuff.

    Thanks on the cladding. My reading shows that 16″ O.C. vert and horiz should be ok up to 6 inches CI. You are confirming from a real world point of view. Thanks.

    Thanks regarding the wall; that’s what I get from my reading. I’d have liked to do a double interior wall to get to R45 but I presume you are confirming that it’s either CI or exterior double wall to have any hope to get to 45%. I’ve been studying Joe L.’s material to better understand the condensing surfaces and just can’t find a way around it.

    So extending this thinking, it’s another level on the roof. It looks like my only hope for 61% CI would be 8 inches of xps (fun fact – 2 inches of exterior xps on my structure is about 1.5 – 2.0 million coffee cups worth of styro) . (?!) 8 inches (or more of RW) and I really have to second guess the structural integrity with a significant snow load (shear forces). Ever looked at that?

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