Construction Materials-Vapor Retarders

I recently wrote a blog post titled Building Science-Building Shell Layers-Vapor Control Layer.  This blog is going to expand on that discussion with information on several choices of vapor retarders. The choice of a vapor control product will depend on the amount and location of the thermal control layer, or insulation of the structure and the climate zone of the building.

Minnesota has modified the codes regarding the use of the different classes of vapor barriers. 

Minnesota’s code R702.7 Vapor retarders reads:

A class I or II vapor retarder is required on the interior side of frame walls in climate zone 6 and 7. Class II vapor retarders are permitted only when specified on the construction documents.

My interpretation of this code is that the use of a class II vapor retarder is going to require the approval of the AHJ or inspector during the review of the project.  A few of the products I am going to discuss in this blog are newer and would fall under a class II vapor retarder classification.  An inspector may not be familiar with their use.

Class III vapor retarders are allowed only when specific insulation values are achieved and when the insulation is installed to the exterior of the wall.  In zone 6, the southern half of Minnesota, the requirement is R-7.5 or more over a 2 x 4 wall and R-11.25 or more over a 2 x 6 wall.  In zone 7, the northern half of Minnesota, the requirement is R-10 over a 2 x 4 wall and R-15 over a 2 x 6 wall.

Let’s get started with the first product, polyethylene sheeting.  We have been using this class I vapor barrier for decades on the warm in winter side of our wall and roof assemblies.  Polyethylene comes in various thicknesses, or mils. The 4 and 6 mil sizes are commonly used thicknesses.  The perm rating, or the rate at which moisture moves through the product is .04-.06 perms or vapor impermeable.  This product works great during our cold winter months at preventing moisture from entering wall and roof assemblies by diffusion.  The downside is there is no drying potential inward if an assembly should become wet.  Warm climates (climate zones 1 – 4) do not allow the use of polyethylene because of the inward vapor drive and the high use of air conditioning.  If only there was a product that would stop outward vapor drive during the winter months, but then allow for inward drying during the summer.  Wait…there is such a product.  As a matter of fact, there are four that I am aware of.

Three of the products tout themselves as “smart vapor retarders” and if applied correctly, can also work as a continuous air barrier.  They work by varying their perm rating depending on moisture within the wall or roof assembly.  The fourth product also varies it’s perm rating but is not a good continuous air barrier.

Certainteed manufacturers a product called Membrain.  This product, along with the other two “smart vapor retarders”, will provide an air barrier if installed similar to polyethylene sheeting.  Membrain can vary it’s perm rating from 1 to 35 perms depending on the moisture level of the building cavities. The product comes in roll widths of 8, 9, 10, and 12 feet and lengths of 100 feet.  Membrain is installed the same as polyethylene, typically with staples spaced 12 to 24 inches apart.  Certainteed recommends the use of silicone-based sealants that conform to ASTM C920 or C834 specifications, acoustical caulks are typical.  The lap seams are sealed with a bead of caulk, then lapped approximately 6 inches.  An approved tape, which is not specified by Certainteed is used as a secondary seal on the lap.

CertainTeed’s Membrain

The next smart vapor retarder is made by Pro Clima, a German company that has been manufacturing air sealing products since the early 1980’s.  The product is called Intello and is more similar to a house wrap than interior poly. The perm rating for this product varies between .17 and 13 perms, allowing an inward drying potential during the summer months and has a very low air permeance rate making it a good air barrier.  Intello comes in 59 inch or 118-inch roll widths and 65-foot 7 inch or 164-foot lengths.  This vapor retarding system would be installed by stapling Intello to the walls and ceiling, taping the seams with Pro Clima’s tape called Vana, and using Contega HF, their air tight adhesive caulking at connections between Intello and other building products, such as a wood subfloor.

Intello by Pro Clima

The last smart vapor retarder is similar to Intello and is made by the Swiss company Siga.  Majrex has a perm rating between .17 and 3.8 perm and an air permeance rate lower than Intello.  This vapor retarder comes in 4.9-foot width by 164-foot length rolls.  Siga recommends using their double-sided tape called Twinet which would be applied to each stud or roof framing. This tape eliminates the use of staples or other fasteners which create small holes in the barrier.  All seams taped with Siga’s Rissan 60.

Siga’s Majrex

Kraft-faced fiberglass batts, which are available by several manufacturers, have been around for a long time and are considered class II vapor retarders. Perm ratings for Kraft-faced batts vary between 1 and 10, depending on cavity moisture.  Kraft-faced batts are allowed in Minnesota, as long as they are approved by an inspector.  Be aware that you will need a good air sealing strategy separate from the Kraft facing to pass a blower door test!

So, we’ve discussed the different options of class I and class II vapor barriers and retarders. How about a class III?  As I said earlier, the only time a class III vapor retarder would be allowed is when the insulation control layer is located outboard of the building’s framing. As I am reading the 2018 energy code requirements, this may be required in Minnesota for all new home construction as early as 2021.  Class III vapor retarders have a perm rating of more than 1 and less than 10.  Raw drywall has a perm rating of around 40, the addition of latex primer and paint will usually reduce the drywall to 3 to 5 perms.

Leave me a comment if there is a topic you would like to discuss, need further explanation, or disagree with something I’ve said. I’d like to hear your thoughts.

Leave a Reply

Your email address will not be published. Required fields are marked *