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