Double Vapour Retarders are NOT Fine by Me!

Far too often, a poster on LinkedIn makes comments that defy good building science.  As I am often busy, I try to bite my tongue and just move on, but often the posts push too many of my buttons and I find myself in the position where I MUST comment or commit hari kari!

A resent post titled "Double Vapor Retarders are Fine by Me!" was one such post that pushed me to reply:

I find it interesting that often when I see what (in my view) is bad building science, the individual in the conversation is almost always in the business of selling or pushing foam. The facts are that Physics has not changed – EVER- and the rules are not being ‘smashed’, only disregarded – often to the building owners peril.

Re OP - I believe this is propagating bad language and bad science and (wish) to review my view of the basics.

VB or vapour control layers are to address vapour movement by diffusion. The requirement for the ‘tightness’ of this control layer is based on the vapour gradient across the assembly and location is based on the direction of the pressure. The VB should always be on the high pressure side, so generally inside in heating climates, outside in most cooling climates, and carefully designed and managed in the rest of the climates. Diffusion is a very small vapour driver, and while it cannot be ignored, having a pretty good VB is more than adequate. You do not need to sweat the details on a VB and do not need to worry about holes and untapped seams. A 90% effective or even 80% effective VB is going to be just fine.

An AB on the other hand is critical, and even small holes in AB’s can move large amounts of moisture by means of convection. The AB can pretty much be anywhere in the assembly and many (myself included) like to detail this layer on the exterior of the sheathing, where the number of penetrations are reduced and much easier to detail. When on the exterior, the one issue to address from a thermal performance point of view, is to reduce convection currents within the stud bays by use of a denser insulation.

Can you build a wall assembly with two VB’s? Yes in theory IF you have a perfect AB always. But in practice this is foolish, in my view, and will come back to bite you a significant number of times, because AB’s are almost never perfect in the field and or do not stay so for long even if they start out perfect. So conventional wisdom, based on decades of experience by many, pretty much always recommend that the assembly is generally vapour open from the VB control layer towards the low pressure side of the assembly. Careful design using modeling may show that you get away with a barrier on one side high side and a retarder on the low pressure side, but (in) my opinion, this is only going to consistently work in dryer regions where the load is low anyway. 

The reason a freezer works is because the two metal shells on each side of the foam are perfectly sealed as air barriers and the manufactures go to great lengths to ensure this during assembly with both sealants and gaskets.

The OP mentions “any mistakes are easily corrected by the air leaks”. Really? Are you promoting an ineffective AB? Are you promoting air movement that can move HUGE volumes of moisture into and through a wall??

The only time a dew-point does not exist, is when the temp and humidity of the air on both sides of an assembly never reach each other’s dew-point. Obviously, this is extremely rare. You can however negate a dew-point in various fashions. A) You can remove all of the air in and through an assembly so that there is no moisture to condense. (easy to do in a manufactured fridge – hard in a site built structure) B) You can design your assembly so that your thermal control layer is mostly/all on the low pressure side of the VB and WRB control layers (so that the condensing surface never reaches the dew-point temperature).

The spray foam crowd claim that filling stud bays with foam meets strategy 1. But I have yet to see a spray foam stud cavity where the foam has not pulled away to some degree from the studs (ore probably has been compressed away from the studs as the studs expand and contract into a material with no elasticity). SO, in these circumstances, the ‘air tightness’ has been lost and you better have a Plan B.


In my Linked-IN Post I also commented:

• Both OSB and Plywood are a Vapour retader in dry conditions, but only plywood opens up to over 6 perms in a wet-cup environment, that for instance all sheathing sees in my region. 
• My vapour diffusion holes question was a bit of a trap. Only way they work is when you have air movement through them, which is obviously something you do not want happening. I did a lot or research before posting on this subject http://goo.gl/0SAiSJ

Of course, I should have also added a third method to my post for negating a dew-point, and that is to ensure that all materials down stream of the high pressure side and VB control layer are vapour open enough to allow drying to the low vapour pressure side. 

It really frustrates me, that in order to sell foam (spray or rigid) to the building industry, manufacturers, vendors, and installers all try to twist the science to meet their objectives.  While this will help them sell their product or service, it will often leave the building owners with assemblies containing much higher risk for condensation, rot, and mould. 

I feel it is up to the rest of us to cry foul when we come across these inaccuracies and try to provide some protection to potential victims of bad science.

SENWiEco continues their test run of the Prosoco R-Gaurd product line

SENWiEco continues their test drive of the liquid applied WRB/AB system called R-Guard from Prosoco. This is a 4 part liquid barrier applied to the exterior sheathing to form both the Water Resistant Barrier and the Air Barrier.

The first part of their system is called Joint & Seam and is used at the  interface between any components that can move (either from moisture or temperature changes).  You would use this product (that contains fibres to provide strength to the joint under movement) at the interface between the plywood sheathing and the stick structure around door and windows, to fillet around plumbing and electrical pipes, to fillet between the wall and floor or between two walls, and along all sheathing seams.

The next product in the R-Guard lineup is the Fast Flash product.  Fast Flash is used in place of a self adhered membrane on the vertical surfaces of the sheathing and rough openings that may see a higher volume of moisture.  Unlike SAM, it is vapour permeable so will not contribute to rotting around the rough openings that is so often seen with dwellings where a high volume of SAM has been applied.  Although the manufacture recommends its use on window sills, SENWiEco cannot at this time recommend this application and instead recommend a water AND vapour tight membrane like PS45 at the window sill.  We will discuss this issue more in a future post and provide our test results where we performed an extended inverted water test using the Fast Flash product.

Once the Fast Flash has been applied, you are then able to treat the field of the walls with the R-Guard Cat 5 roll on product.  The product has gone through extensive testing and can withstand the forces of a Category 5 Hurricane.

Finally, as the final part of the R-Guard system, when the windows are installed you provide a air seal and water shedding surface at the interior face of the window frame using R-Guard Air Dam.

SENWiEco subjected our test assembly to -4500 Pa (equivalent of 185 Mph or 300 Kph winds) and did not experience any leaks through the sheathing joints and HVAC/Electrical/Plumbing penetrations.  We did have two pinhole leaks around window clips and the window frame itself started to leak at that pressure.  We will provide a follow up posting with links to videos of our tests once they are available.

In the meantime, we invite you to watch a series of videos capturing our trial run of the R-Guard system.

1) Application of R-Guard Joint & Seam
2) Application of R-Guard Fast Flash
3) Discussing the application of R-Guard Cat5
4) Detailing an exterior Electrical Outlet
(In hindsight, I would detail this differently to make it more attractive by making the wood 'box' only the width of the electrical outlet plus 3/8" each side for a caulking joint)
5) Detailing an exterior HVAC Termination/Intake
6) Discussing the consideration needed when installing a box style window
7) Detailing Window Sill using a Back Stop
8) Discussing the Head Flashing placement and Head and Jamb Trim considerations
9) Detailing Jamb Trims
10) Installing Head Flashing
11) Seal Sill Back Stop, Window Clips, and install Backer Rod
12) Installing R-Guard Air Dam
13) Discussing Failure at clips and ways to re-detail in a water tight fashion
14) Building a Poly Enclosure to test the Air and Water control layers in a wall assembly 
15) Pressurized Smoke Test
16) Depressurized Water Test 
17) Discuss Failure at Window Clip
18) BCIT BLDC 3060 Water Test - Passed at -1200 Pa 
19) Site built differential pressure gauge
20) Mock up assembly is tested to -4500 Pa or 18" of Head Pressure!

This was phenomenal results for the R-Guard system and for the Cascadia windows.   The only leaks around the windows were are locations where old sealant remnants had not been removed.  We also suspect that the window frame joint was damaged when the window was cut out of the BCIT Mock-up.  Even if it wasn't, this is very little water for a -4500 Pa pressure which represents winds this dwelling will never see!
  

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!