Tuesday, 20 November 2012

Why do we continue to drill holes into our buildings?

I was recently driving by yet another building with vapour ‘diffusion’ ports (VDP’s) and it got me thinking:

Why do we continue to let our buildings be drilled full of holes?

I have always believed that the ports did not work based on previous information I had reviewed, and they are certainly not a recommendation made in the Best Practices Guide published by the Home Protection Office (HPO) in collaboration with some of the best engineering firms in the North America, firms like RDH, RJC, and Morrison Hershfield.

So why do a limited number of buildings still incorporate these ports in their designs?

First of all, we need to try and understand the intent of vapour diffusion ports. As I understand it, the intent is to help walls ‘dry out’ by allowing the wall assembly to ‘breathe’ more easily.  The ports are intended to increase the breathability of the assembly by means of diffusion, over a solid sheathing base line. By ‘breathing’, we are referring to the wall’s ability to ‘lose’ moisture in a vapour form, by allowing that moisture to go ‘through’ a wall’s materials and evaporate into the outside air (low vapour pressure side). We say walls can ‘breathe’ if all of the materials on the low vapour pressure side (which can change direction depending on conditions) are vapour-open, or that they have a high permeability.  In simple terms, this means that the pores of the material are large enough to allow a water molecule in vapour form to pass through the material.  The mechanism of passing through the assembly is usually termed ‘diffusion’, but a review of Chapter 8 of Building Science for Building Enclosures by Straube/Burnett details that the actual mechanisms involved with moisture movement through a porous material are extremely complex and can involve surface diffusion, capillary action, evaporation, convection, absorbed moisture transport, etc.  Even today, the flow of moisture through materials is not fully quantifiable by scientists or fully predictable. What is a provable fact is that in order for vapour diffusion to take place there needs to be a difference in vapour pressure across the two sides of the material (or assemblies) in question.

So are these ports working as intended?   How wide a range does each port have? Are there other mechanisms at work?  Is this an effective drying strategy?

In researching this topic I came across two related studies performed in the late 1990’s after the leaky condo crisis.  The first study titled The Envelope Drying Rates Analysis Study looked at the abilities of various wall assemblies to dry out (in lab conditions) while changing up components like sheathing materials, capillary break depth, and sheathing membrane materials.  What I found most surprising was that none of the panels were dry to safe levels (below 20%) even at the end of the 2000 hour study.

The second study titled Evaluation of Vapour Diffusion Ports on Drying of Wood-Frame Walls Under Controlled Conditions utilized the same panels, but drilled out vapour diffusion ports on some of the panels and then re-ran parts of the experiment to look at the effectiveness of the vapour diffusion ports specifically.  What was again remarkable was that, although the VDP’s did provide a minor increase in the initial drying rate of the OSB panels (no change in the rate of drying in the plywood panels which was already higher than OSB with or without the VDP’s ), all of the panels (OSB & Plywood) again contained areas with dangerous levels of wood moisture at the end of the study (which repeated the first studies results).

After reviewing these studies and Building Science for Building Enclosures,  I now have a better grasp on the true mechanisms at work with vapour ‘diffusion’ ports, and only a little of the process actually involves diffusion.  Where the ports have been effective to any measurable level, some form of convection must take place in conjunction with lots of capillary movement. Let’s look at these mechanisms in a bit more detail.

Convection (or air movement) can occur because of air leakage through an assembly from the high to low pressure side.  It can also occur by means of the convective currents that can develop within each stud bay if a low density batt and fill insulation (i.e. fibreglass) is used.  Neither of these mechanisms is desirable from an enclosure performance standpoint. Convection forces within the wall assembly can deliver moisture-rich air derived from the ‘wettest’ parts of the wall assembly (studs, plates, & insulation) and deposit that moisture onto the back side of the sheathing (which can allow a generalized increased drying rate from the sheathing by means of diffusion to the outside air). Where that moisture is close enough to a port, there can also be a localized marginal increase in permeability over the base-line sheathing.  This can help dry an assembly if no additional moisture is introduced through the convection mechanism, but air leakage through an assembly can often bring with it high levels of moisture into the assembly from the interior of the dwelling.  And once in the assembly, that moisture can then condense on any of the wall elements with surface temperatures that are below the dew point of the leaking air. (See my July/2012 and March/2012 blog entries for more info on dew points and moisture in wall assemblies).  Stud bay convective currents often create ‘air pumps’ that can lead to cold interior walls and thermal bridging, causing additional heat loss (you are bypassing the insulation by delivering heat to the outside sheathing and taking it from the inside wall board).

Now lets look at capillary and diffusion forces in connection with these ports - as areas close to the ports dry out via diffusion through the holes in the sheathing (which have a higher permeability because the moisture has to only diffuse through the sheathing membrane and not though the membrane AND the sheathing), or by air leakage through the holes (because the sheathing membrane has been damaged or is not sealed as an air control layer), that moisture is replaced by wetter neighbouring regions.  The process continues until, over time, the moisture levels within the assembly equalize.  The assembly as a whole is also trying to equalize with its lower vapour pressure neighbours (so in this geographical region, usually to the outside).  The entire process continues until all regions have the same vapour pressure (something that in practice rarely occurs as there is always changing interior and exterior humidity and temperatures which lead to changing vapour pressures).

The problem however, with relying on the diffusion and capillary forces alone is that, although capillary forces are faster than diffusion, both forces will not dry a wall assembly out fast enough to be an effective deterrent to decay and fungal growth.   This was confirmed in both studies, where portions of every test panel still had wood moisture levels in the danger zone for promoting (not just maintaining) rot, even after close to three months, regardless if VDP’s were installed or not!  These studies only had one wetting period - What happens when you have repeated introductions of moisture (interior air leakage, exterior leaks, or plumbing leaks)?

What does all of this mean? Are these results important?

Let’s look at where moisture can originate and how effective VDP’s are at addressing that moisture source.  There are five causes that come to mind:
  • Initial wood moisture elevated during construction due to the rain forest we are building in, 
  • Bulk water entry (after construction) from the exterior around poorly detailed penetrations and even through vapour diffusion ports, 
  • Bulk water entry from water pipe and drain leaks within the assembly,
  • Air leakage through the assembly causing condensation,
  • Vapour diffusion into the assembly (usually caused by excessive RH% levels in the dwelling’s interior and/or inadequate vapour control layer)

The easiest way to address the first cause is to leave the drywall, poly, and insulation off the interior of the structure until the assembly dries out.  The problem is that construction schedules are so tight; we typically do not allow adequate drying time.  If we are unable to allow sufficient time for a wall to naturally dry out before boarding, then we need accelerate the process by adding heat and air movement to the mix (and in some extreme cases, de-humidification). If we rely on the VDP’s alone to allow the wall to diffuse the moisture out and close up the assembly in a wet state, we could be waiting many months for safe moisture levels within the assembly to occur, and by that time a healthy crop of fungi would most likely have already taken root leaving us with a sick wall assembly. 
VDP’s Score: F

The second and third mechanism only has one solution – stop the bulk water entry!  There is no effective strategy to manage bulk water entry once it has occurred. Even BC’s Rain-Screen requirements will not solve repeated bulk water entry into a wall
assembly.  You must stop it at the source. 
VDP’s Score: F

The fourth mechanism of moisture entry is also somewhat simple to address. Stop the air flow through the assembly by means of an effective air control layer.  Unless you stop the air flow and the resulting condensation, the rate of moisture input will again often overwhelm the slow drying rate of a wall assembly by means of diffusion only, with or without VDP’s.
VDP’s Score: C- down to F (depending on the incoming moisture levels and type of sheathing installed). 

The final mechanism also has an easy solution in most dwellings.  Control the interior humidity by means of mechanical ventilation and/or install an effective vapour control layer in the assembly.
VDP’s Score: C down to F (depending on incoming moisture levels and type of sheathing installed).

Let’s summarize this information.  If a VDP is sealed against airflow, we are generally relying on diffusion and capillary forces and possibly stud bay convection currents to help dry out a wall assembly.   However these processes are slow, and the VDP’s only account for a limited improvement of drying potential over base lines.  As a result the assembly would typically not dry out in time to prevent decay regardless of the presence of the VDP’s.  If we allow air movement to occur through the ports, the air movement that would occur through the assembly can often bring with it levels of moisture that are easily able to overcome the limited additional drying capabilities represented by the VPD’s.

Are there downsides to including VDP’s “just in case”?

Well, it turns out these holes are usually conveniently placed just below windows and other enclosure penetrations where any incorrectly detailed flashing or membrane (like the holes drilled through all the water-resistant layers in the photo - below left) could direct water along the surface of the sheathing and into the wall assembly through these strategically placed inward water highways!  Is this really a wise practice?

We need to stop this insanity.  I put a challenge out to all of the engineering firms that know better.  Please stop allowing your buildings to be drilled full of holes. Start taking the architects, building officials, and contractors to task to stop this poor building practice. Start educating the teams you work with and let’s start building smarter!


Figure 1 Vapour ‘Diffusion’ ports (holes in sheathing that act as inward water highways) on buildings under construction in North Vancouver, BC


Anonymous said...

There is a case to be made for using VDP in OSB sheathing. According to research testing VDP significantly increase drying in OSB sheathing but not plywood. See CMHC research documents:


SENWiEco Designs said...

Thanks Anonymous for visiting and for taking the time to post a comment.

If you review my post carefully, you will see that I already refer to this CMHC report and also point out that the study identifies that even after close to 3 months, the panels are still not dry to safe levels. This means that during our wet months, when these should be making a difference, there is no hope that they will accelerate the drying fast enough to dry to below danger levels between rain events. This tied to the many negatives that the ports represent, and the fact that OSB is never part of a best practice wall assembly in the first place, and you have pretty much killed any reason to incorporate the ports into your design.