WSS, WRB, AB, & VP - Oh My!

After my last post, many of you are probably left wondering: "Has he done ANYTHING on his design yet?"

The truth is I have; not all my time over the last 4 or so years I have been ‘working’ on this project has been wasted.  Although I do not have any hard blueprints or floor-plans yet, I have continued to flesh out ideas for different components of the build by continuing to invest considerable time into research, seminars, technical courses, and discussions with colleagues.

One of the components I have invested the most time into (I would estimate at over 200 hours to date), is what will be best practice for my water shedding surface (WSS), water resistant barrier (WRB), air barrier (AB), and vapour barrier (VB).  There are probably as many opinions in this field as there are blades of grass in my front yard.  And it has taken me over five years to come to some conclusions of my own in this area (was interested in this subject long before I started this latest design journey) but even now, I am running lab tests to prove or disprove whether my decisions are wise enough to proceed on the real thing.

One of the key factors in deciding on a WRB, is first deciding on an AB strategy.  The most common strategy this (in my region) is an attempt to seal the vapour-barrier-poly on the inside surface of the wall (beneath the drywall) to also serve as the air barrier.  As a home inspector who has been in hundreds of attics, crawlspaces, and basements, I can honestly say I have yet to see a poly-vapour-barrier adequately detailed to form an effective air barrier.  It is next to impossible to form a lasting air-tight seal using poly and tape around penetrations like wiring, floor joists, knee walls, etc.  In most homes I have looked at, there hasn’t been a serious attempt to install this so important barrier.  The reason?  It is hard, takes a long time to do right, and is therefore expensive.  So it tends to just get a half hearted effort so that the contractors can tick off the requirement as they move forward in the build.

Let’s back up a moment: "Isn’t making a house more air tight why we are having so many problems with moisture issues and mould in North America?"  I am quite confident a large number of people reading this right now agree with this statement.  "After all, we did not start to have these problems until we started to make our building stock more ‘energy efficient’ – Right?"

The truth is, it is the process of adding additional insulation, and not ‘attempting’ to make our assembly’s air tight (I state attempting because we have actually done a really poor job in this field to date), that has led to the increased risk for our wall and roof assemblies.  I talked about this last March in my ‘New Education’ posting. You see, in the old days our houses were very inefficient but somewhat durable.  Sure the huge volume of air that flowed through our assemblies helped to keep some components dry, but it was the heat escaping through our poorly or un-insulated walls and ceilings that was doing most of the work.  This heat acted to ‘cook’ out any moisture in the wall and roof sheathing.  Once we started to increase insulation levels, we started to cool these surfaces down and eventually we cooled them down enough to reach the dew-point potential of our standard interior air (which for the Pacific Northwest at 21°C and 50-55% RH is around 10°C).  As we can reach this temperature for most of the fall through early summer in my region, the risk is actually very high and I see the outcome often, especially in attic cavities, in my inspection practice where a great number of homes I have looked at over the last four years that have had ‘modern levels’ of insulation in the attics, have also had moisture problems and mould in the attics.

As we have added more insulation to our dwellings to make them more comfortable and energy efficient, the air barrier, which has been the most ignored barrier to date in our building codes in my view, needs to become the star of the show.  In truth, it should have always been the star of the show, because reducing the air exchange through the assemblies automatically makes the home more comfortable and energy efficient without adding a stitch of insulation (read about my personal experience back on my January ‘We own a House’ entry).

To emphasize the importance of the air barrier, let me provide this graphic from Building Science Corporation in their paper RR-0412: Insulations, Sheathings and Vapour Retarders.  This provides a dramatic visual of how much vapour movement can occur through the smallest of holes in the air barrier compared to a large hole in the vapour barrier.

Fig 1: Over one heating season in a cold climate, 30 quarts of water can flow through a 1” x 1” hole in an air barrier by means of air movement, but only 1/3 quart of water can flow through a 4’x8’ hole in a vapour barrier by means of vapour diffusion.

As you can see, the air barrier is very important in keeping our walls dry.  Even small holes (imagine trying to prevent these holes with tape and poly) can lead to large amount of moisture, in vapour form, entering our wall cavities where it can then condense on the surfaces we have cooled down by adding more insulation.  This is because air movement is smart.  It moves in a three dimensional way from one hole to the next.  It can enter the wall cavity in one location and find its way out of the cavity to the outdoors on a completely opposite side of the house.  As a result, an air barrier has to be continuous (details are important), durable for the life of the assembly, and most importantly – buildable.  A designer or architect should be able to take a pen and draw the complete air barrier on the design without ever taking the pen off the paper.  If they can’t – its time to find a new one, because if they cannot draw it, how do you expect the contractors to build it!

One convenient fact about the air barrier, unlike the vapour barrier that must go on the high pressure side, is that it can be placed anywhere in the assembly.  There is no rule that states that the AB and VB have to be combined, and there are much easier ways to detail the air barrier than trying to seal the poly.  What IS important to pay attention to is the vapour permeability of the air barrier, and ensuring it is appropriate for the location installed.  In a heating climate with the vapour barrier to the inside, you want to ensure all components installed outboard of that barrier are as vapour-permeable as possible, to allow the assembly to breath to the outside.  So if you are creating an exterior air barrier, it is very important that the barrier has high vapour-permeability. 

When choosing your products make, sure you get independent data to support the manufacture’s claims of permeability.  We recently tested several sheet style sheathing wraps for vapour permeance at a Building Envelope Lab course at BCIT.  There was no surprise that our control wrapped in Poly was still soaking wet after 10 weeks, but what was a surprise was how long it took some of the sheathing membranes - Typar and Vapro Shield specifically - to dry out.  Both of these developed considerable mould on the samples because they had not dried out fast enough.  Even 2 layers of 60 minute building paper outperformed them and exhibited only a few tiny spots of mould.

Some options for creating an Air Barrier are as follows:
    •    Air Tight Drywall Approach
    •    Sealed Poly Approach (Combining with Vapour Barrier)
    •    Sealed Exterior Sheathing Approach
    •    Sealed Exterior Insulation Approach
    •    Sealed Exterior Sheathing Wrap Approach (cannot use building paper for an air barrier)
    •    Sealed Interior Sheathing Approach (Used a lot in the PassivHaus circles)
    •    Combinations of all or some of the above

Many in the scientific community advocate an external air barrier approach.  The benefits of an exterior air barrier approach are as follows:
    •    Can be detailed and tested early in the construction process to ensure you are on the right track and address if you are not.
    •    Is easily reparable during the build, up to the time the cladding or exterior insulation is installed.
    •    Is usually easier to make the barrier much more durable if the barrier is sandwiched between components.
    •    My favourite - Opens up the possibility of using a liquid applied barrier.
    •    But probably the most important - There are far fewer penetrations on the exterior of a wall assembly and most of those penetrations have square sides and are therefore easier to detail.

Because of these benefits, and my research to date, I have elected to implement an exterior air barrier approach on my build, and I'm currently testing a liquid-applied barrier from Prosoco called R-Guard Cat5.  This would act as both the Air Barrier and Water-Resistant Barrier.  If all goes well in testing, and I proceed with this approach, I will be applying the product to the exterior sheathing prior to installing my outboard continuous insulation.  I will discuss this a lot more in future postings but for now I have started to video diary my product tests on a mock-up which I have uploaded to You Tube.


I would welcome any comments or words of warning you may have.  Next month we will talk about the Water Resistant Barrier.   Thanks for reading!