A technical guide to understanding the role weather barriers and rainscreens play in a wall, choosing the right products and installation tips for optimum performance.
They're officially known as water-resistive barriers or WRBs, sometimes weather-resistive barriers, but in practical terms they are more commonly called housewraps and weather barriers. With so many options I plan to randomly switch between terms just to alleviate the monotony and keep us all entertained.
A few other linguistic options to mention - cladding is the same as siding, furring strips are the same as strapping. Those and your weather barrier all installed properly are what defines a ventilated rainscreen.
There are many different potential wall assemblies, and many products with different characteristics that can be used in different cases. For the sake of simplicity and not getting off on a bunch of tangents, in this article we will stick with the bread and butter wall assembly we all know - a stud wall with exterior sheathing, housewrap, strapping and siding. Other options for wall assemblies can be found in our building guide.
Just like a canvas or nylon tent, water will drain down a water-resistive barrier but standing water would drain through them, albeit slowly. Since moisture can evaporate through WRBs, any humidity that accumulates in a wall assembly has a chance to escape.
There in a nutshell is the purpose of a weather barrier - to provide a drainage plane that protects sensitive materials from direct exposure to precipitation at the same time that it allows walls to dry out.
It's worth a mention at this point that weather barriers are often intended to act as an air barrier as well, but the most common installation techniques (staples, nails and hastily installed tape) mean it is a rare case when they actually do a good job at air sealing. Manufacturers will rightly claim their products are a suitable choice for an air barrier, and they most certainly are. The failure of weather barriers to perform well in that capacity lies entirely with how they are installed.
Randomly peppering your housewrap or home wrap with staples creates a lot of little holes where air can enter and escape. Also, when exposed to the extreme temperature shifts between summer and winter and the endless movement of a loose weather barrier due to wind and air pressure differences, the solvent-based tapes we most often use may become brittle over time and no longer adhere.
You'll have better luck using weather barriers as air barriers if you place staples where they will be covered by strapping, or put tape over them. Don't over-staple it for no reason, staples only have to hold it up until the strapping is on. And if you can wait for a calm day with no wind you will really only need to staple it at the top (where it will be overlapped anyway) and let it hang. Better still would be forgoing staples altogether and using nails with plastic washers.
Be meticulous when taping joints and try to use brands of tape that are more suitable for extreme temperature changes. Siga,Delta Multi-Band and Air Stop are three examples of tapes with much better adhesion.
An alternative to the commonly used non-adhesive housewraps are the 'peel-and-stick' membranes, and if you'd like a fancier word for them, go with this - self-adhering air and water-resistive barriers.
They're still vapour permeable, but membranes that stick to the sheathing are much less susceptible to damage caused by wind pressure.
Since a conventional housewrap is usually held on by staples, the back and forth ballooning of those loose weather barriers can over time enlarge staple and nail holes and reduce their ability to prevent air leakage. With no stapling needed and being fully adhered to the sheathing, wind and air pressure isn't a concern for air barrier durability with peel-and-stick membranes.
Depending on the substrate you're sticking the membrane to, manufacturers may recommend they be installed by first applying a primer to the surface for better and longer lasting adhesion. Including a primer in the mix will also affect the vapour permeability of your wall exterior; more on that in a moment.
Self-adhering membranes are not all the same:
Like so many other building products, it is important that you understand the properties of different membranes before choosing one, don't just pick one randomly off the shelf based on price or brand recognition. Some are vapour permeable and will allow walls to dry, some will not. They both have suitable applications but shouldn't be randomly interchanged.
For the standard wall assembly we are talking about here, make sure that whatever membrane you choose as your weather barrier is vapour permeable, and the more permeable the better. Look for the vapour permeance rating (how much moisture can pass though it); it should be on the product itself but also on the technical data sheets of company websites.
1 Perm and 60 Ng* (U.S. and Canadian ratings respectively) are the benchmark permeability rates for what defines a type II residential vapour barrier. Products rated below those numbers are vapour barriers, those rated between 1 and 10 perms are considered vapour retarders. The higher the perm rating or Ng, the quicker moisture can evaporate.
*Ng stands for Nanograms, or billionths of a gram of water. That 60 Ng number refers to the amount of water that will pass through a square meter of a material in a second [60 ng/(Pa.s.m2)].
1 U.S. perm is actually 57 Ng, that 3 billionths of a gram difference between 57 and 60 is what you call 'close enough for jazz'. And forget the 60 Ng for now, we''ll go with perms only.
Categorizations of building materials based on their permeability rates:
|Vapour impermeable||0.1 perms or less|
|Vapour semi-impermeable||1.0 perm or less and greater than 0.1 perms|
|Vapour semi-permeable||10 perms or less and greater than 1.0 perms|
|Vapour permeable||Greater than 10 perms
Understanding vapour permeability:
All we are talking about so far are the stand alone housewraps everyone is used to seeing, which will range from about 6 to 60 perms. To go further down this rabbit hole, you will find that your sheathing will impact vapour permeability of the wall assembly as well. [Read more about material permeability here].
Conclusions about the vapour permeability of sheathing seem to differ heavily from different sources, We will run with the pack on this one for now, using the numbers most manufacturers cite.
7/16" OSB (oriented strand board) has a rating of about 2 perms; compare that to half-inch plywood which is about 10 perms. And those rates will change with thickness - 3/4 inch OSB is rated about 40 Ng (2/3rds of a perm) which actually qualifies it as a vapour barrier, and can be used as such on the interior. [Here's an example of how osb can be used as a vapor barrier]
So you need to look at the wall as a whole, and don't assume that the perm rating of the weather barrier you choose will define the permeability of the exterior of your wall as a whole.
Starting off with such a low-perm sheathing as OSB means even a high-perm weather barrier may have little effect, so the added cost of plywood is something that may be worth considering investing in for your overall wall durability, as it is 5 times more permeable to vapour than OSB.
Also important to remember is that if you are putting a rigid board insulation on the exterior, that as well will have a vapour permeability rating that should be considered. As an example, 2 inches of Roxul ComfortBoard is rated between 30 and 40 perms, compared to 2 inches of EPS foam which is about 1 perm, and 2 inches of XPS foam is only about .5 perms. Meaning, by putting 2 inches of foam on the outside of the house you have effectively just installed an exterior vapour barrier.
Permeability is tested in two manners - 'dry cup' and 'wet cup'. Each method simulates the vapour drive in a different direction, so the resulting numbers vary, and manufacturers may not always tell you which test method was used.
To use Vent SA from Cosella-Dörken as an example (which to our knowledge has the highest perm rating of peel-and-stick membranes), their 'wet cup' test resulted in a rating of 31 perms, dry cup testing was 50 perms. Not all manufacturers will list both numbers or even tell you which result they are posting, assume they are putting their best foot forward and giving you the more favourable looking number.
A rainscreen allows walls to dry:
'Ventilated rainscreen' is a term that gets somewhat misunderstood and defined differently at times. It isn't an individual product or material, but a design feature. The combination of a weather barrier, strapping and exterior cladding and ventilation openings at the top and bottom together create a 'rainscreen'. The intention of a rainscreen is to protect walls from a driving rain while allowing any water that does penetrate to drain out, and letting air move through it so it can dry.
Installing siding directly against your weather barrier will greatly reduce the ability of a wall to dry. This isn't very common anymore but it does still happen. Always install strapping to separate the siding from the weather barrier. And a common practice to avoid is installing strapping horizontally. That prevents both the drainage and air convection that is essential for walls to stay dry. Water drains down and warm air rises - don't stop that from happening.
If your siding choice requires horizontal strapping (like board and batten), include some kind of spacer in back of it so water can still drain and air can still rise. Without that space behind it, horizontal strapping can lead to a small amount of standing water that can saturate building materials.
That gap in a rain screen doesn't need to be significant, just enough for water to drain and air to rise. If the weather barrier is tightly attached, as in the case of a peel-and-stick membrane or is part of a rigid board product, building scientists say as little as 1/16th gap provides a capillary break, that will let liquid water drain out.
Building walls that can dry © Ecohome
If you have a loose or bunched up weather barrier that might block that space, you may be wise to install a thicker spacer to ensure that space is continuous. Care during installation is what will determine how well your rainscreen blocks and releases moisture.