Vapour Barriers | A Comparison

Vapour Barriers | A side by side comparison

Many of our customers have asked us for clarification on this, so we thought we'd make a clear comparison of our most popular membranes to help you decide what is the best product for your project.

We recently finished a project in Toronto, that used the vapour barrier Ampatex DB 90.

In the beginning, we had to address a fairly common question regarding vapour barriers with variable permeability vs. those with a constant permeability:

Variable or constant? Which one is right?

The project is a renovation with some new 2x6 walls, and some existing 2x4 walls, that were stocked up to 2x6 thickness. Exterior sheathing is 1/2” plywood (and some of the original solid lumber ship lap sheathing) with an air barrier, then 3 inches of Roxul outboard, plus a rear vented rain screen.

On the inside, we find a vapour barrier, an installation cavity of 1.5” thickness (in most places), and then 1/2” drywall.

On the inside, we find a vapour barrier, an installation cavity of 1.5” thickness (in most places), and then 1/2” drywall.

Here are the two vapour barriers from Ampack that we compared for this project:


DB 90

Double layer, tear-resistant vapour barrier.

Double layer, tear-resistant vapour barrier (retarder) and airtight layer made of thermoset continuous fibres with PP fill layer (100% polypropylene). The membrane is made of high quality PP (Polypropylene) and contains no plasticizers. The standard moisture barrier for all roof and wall construction. Also suitable for cellulose insulation. Perm rating is 9,4 SI Perm or 0,16 US Perm (Sd 20m).



Moisture-variable vapour check

Moisture-variable vapour check and airtight layer made from polyamide and polypropylene. Ideal for use in dry internal work. Also suitable for roof renovation from the outside. Permeability range is 469 - 31 SI Perm or 8 - 0,55 US Perm (Sd 0,4 - 6 m).

So which membrane is right?

We’ve observed a trend with high performance buildings to use humidity variable vapour barriers almost universally in every situation. When we take a closer look and model the walls with respect to moisture, we come to the realization that this isn’t always necessary and the best approach. 

    “Humidity-variable moisture barriers are not a panacea.”

We design walls so that they can dry out, meaning that any vapour that enters on the inside can freely escape on the outside. For the majority of the time in Canada, the vapour pressure is higher on the inside than the outside, driving moisture through the wall from the inside out. Even during the summer, we are seeing vapour pressure equilibrium for much of the time, and only encounter the reversal of the vapour drive for relatively short periods.

What happens if we use a fixed membrane?

One could think that once the vapour pressure drive is reversed (back drying during the summer) issues with condensation in the wall may arise. This is not necessarily the case, and is dependant on the specific vapour barrier used in the wall. In the case of this renovation project with Ampatex DB 90, we are still allowing vapour to escape to the inside, just not as fast as a variable membrane such as Ampatex Variano. However, the wall is much safer during the majority of the time of the year, when a much stronger vapour pressure drive from the inside goes toward the outside. Much less vapour/moisture can enter the wall is if we were to use Ampatex Variano, the variable membrane

So when do we recommend Ampatex Variano?

So when do we recommend Ampatex Variano?

So when do we recommend Ampatex Variano?

Obviously there are scenarios when we recommend the use of Ampatex Variano, or else the membrane wouldn’t exist. One of the most common scenarios is the renovation of a house or roof from the outside. This type of renovation is very uncommon in Canada (I have never actually heard or seen one), and is when you tear down the outside of a roof or wall without disturbing the inside, and build it back up from the inside out. The humidity variable membrane is used, because because it has to be wrapped around the purlins in order to create one continuous layer, but the membrane is thus also located in multiple depths of the roof (in the insulation cavity it is located closest to the inside, around the purlin it is located much further outside). Due to the variable perm rating it can now accommodate multiple vapour permeability values and thus avoid blocking the moisture and provide optimal moisture behaviour of the wall.

Another scenario could be with a project where dark siding is used, that heats up in the summer when the sun shines on it, creating a very strong moisture drive from the outside in. 

Patrick Bruggman, application engineer at Ampack Switzerland explains: “This is generally the scenario when we talk about back-drying towards the inside, i.e. when the exterior layers are vapour tight on the outside and when we are observing drastic temperature differences between summer and winter. In the winter, moisture is pushed inside the wall towards the vapour tight layers on the outside. Since the moisture can only escape the wall on the inside, I need the dark surface on the outside to increase the temperature, which creates a vapour pressure drive aimed towards the inside. This is how we the vapour pressure drive is turned around and we achieve back-drying.” [translated into english]


We generally recommend that you use Ampatex DB 90 in new construction, combined with exterior sheathing that is a minimum of 10x times more vapour open (i.e. Agepan DWD or Agepan THD) on the outside. It’s a very safe set up. If in doubt, you can do a quick check of your walls behaviour with, or check it’s behaviour with WUFI. If you are unsure about how to do this yourself, please get in contact with us. We can also ask Ampack to analyse the wall and make the best recommendation for your specific construction project.
We hope this helps you to make the best decisions for your project but we are always here to help.

You can find the technical date sheets here

Ampatex DB 90

Ampatex Variano


Project Highlight | Babcock Residence

Our Project Highlight this month

Project name: Babcock Residence

Builder: Van Osch & Co.

Materials used from Pinwheel:

Agepan DWD exterior Sheathing

Ampack: Tapes: Ampacoll INT, Ampacoll XT, Ampacoll RS

Zehnder ComfoSystem (Not yet installed)

Ian Van Osch recently completed the construction of a prefabricated building shell. They were able to achieve their goal of building a healthy and energy efficient shell (passive house) using products by Agepan and Ampack. The DWD sheathing by Agepan allows for the construction of panels that are very rigid, withstand the pressure of blown in cellulose, are vapour open and stand up to the exertions of transportation. The panels had been prefabricated off-site and out of the elements and the entire shell was assembled on site during the winter. Ampack tapes were used in order to create an air- and wind tight shell. Ampacoll INT has been used on OSB on the inside, and Ampacoll XT on the outside. Ampacoll RS butyl rope was used to seal the panels to the foundation:


Building the shell

Walls are prefabricated and completed in the shop ready for transport.


On route to the site

The walls once completed are loaded on the truck and head to the construction site.


The walls (shell) are installed on site. This makes the job move much quicker in any weather.


Construction tape for overlaps and/or edge connections on strips and panel-type construction materials, or for airtight masking of any type of feed-through (protrusion/penetration), always on flat surfaces indoors.


Agepan 5/8" DWD Protect High Perm exterior sheathing for double stud or deep cavity walls. Prevent mold and rot due to cold sheathing and condensation build-up in high R walls.


Structural Details

The builder makes each project unique

Walls are Installed

The project is now well on it's way to completion