Canadian Contractor

By Paul Duffy, M.A.Sc., P.Eng.   

The science of moisture control: Building science rules for contractors

Canadian Contractor

A summary of the three major mechanisms for moisture infiltration into a building - and what to do about them

This article first appeared in the July 2018 Bulletin supplement to Canadian Contractor.

The enemy of buildings is moisture. In the presence of moisture: wood rots, concrete spalls, steel rusts and gypsum turns to mush. For building contractors, controlling moisture is job one.

Too often moisture wins. Either some important detail gets missed, or flaws get covered up, or worse, the whole design is fundamentally flawed. A few years back, the condo crisis in Vancouver proved that a lack of understanding of moisture wasn’t just isolated, it was widespread. Much of Vancouver is in a climate zone that could be classed as a “temperate rainforest.” Though certainly not at the extremes in terms of temperature, Vancouver gets more than 200 days per year of rain. Things get wet and never dry out. So it is not surprising that a moisture-related crisis first appeared there.

However, moisture problems are prevalent across the country because the same lack of understanding of moisture control is not confined to the west coast. To fully control moisture, you must first realize that it moves via three major mechanisms, each of which require different methods of moisture control.


1. Deflect away bulk moisture through proper layering

First, moisture can obviously appear in bulk. Wind-driven rain and snow can be driven inward from the exterior, so anticipating a moisture assault on the exterior of your building and deflecting it away is the key function of exterior building elements. The single most important way of controlling bulk moisture is by lapping materials shingle fashion so that any water that gets past one layer gets caught by the next and gets deflected back to the exterior. This is also done through wall flashings at major intersections, such as changes in materials and at penetrations like windows and doors, so that whatever moisture does penetrate can be carried back to the exterior where it belongs.

2. Prevent moisture wicking through an air gap

Second, moisture can be sucked up like a sponge — wicking. Porous materials like wood and concrete are particularly vulnerable so innovations like air gap membranes around foundation walls have been widely accepted as necessary measures to control basement leaks. The simple genius of an air gap stops moisture from being sucked up into the concrete via capillary action. The air gap helps ensure water moves vertically downward towards the perimeter drain versus inward to more moisture sensitive materials. 

The air gap is not just important in below-grade construction. Above grade, many building papers are micro-perforated and can wick moisture inward if not applied with an air gap over them. Tyvek, Typar and even the lowly felt paper will perform best when an air gap exists over top of them. An air gap is standard behind brick veneer, but is also built into premium Exterior Insulation Finish Systems (EIFS—Stucco). Many siding manufacturers also recommend an air gap, accomplished by applying them over strapping, for example.

3. Stop moisture arriving in the air by employing a continuous air barrier

The third mechanism for moisture movement is via air leakage. Moisture is often entrained in air. An air leak carries the potential for that moisture in the air to be deposited in the interior of the building. Many energy efficiency programs emphasize the importance of an Air Barrier in controlling energy losses. I like to think of it as a measure for controlling moisture as well. 

A Continuous Air Barrier limits the potential for moist interior air from migrating outward, thereby condensing on cold surfaces such as sheathing and fasteners. It also makes the air gap and the layered materials on the exterior perform more effectively. By stopping air coming in from the exterior, the inward wind forces driving rain and snow into a building are also stopped. The air gaps and layered design of exterior elements, combined with suitable weep holes and strategic use of caulking, can work their magic protecting the interior, moisture-sensitive materials from damage.

The astute builder will realize I have come this far and haven’t once mentioned the words “Vapour Barrier.” Surely that must be a mistake? No, it is not a mistake! That is because vapour diffusion control, typically by means of a vapour barrier, is the final — and perhaps least important — of the moisture control mechanisms. The language of the Building Code notwithstanding, a vapour barrier, that is far from continuous, can do a more than adequate job of protecting your building (provided you have done the other stuff correctly). And nowhere in the Code is polyethylene mandated as a requirement in all building envelope assemblies. Vapour barrier paints, spray foams, extruded polystyrene foams and other materials can meet Code requirements.

The important takeaway is that it is extremely important to get the above three concepts correct. Proper layering, drainage gaps, and a continuous air barrier are the most important aspects of good design. A continuous vapour barrier less so.

At the leading edge of construction, builders are starting to question the wisdom of a continuous layer of polyethylene on the interior of buildings. If, for example, you build a wall with a high level of continuous insulation on the exterior (i.e. more than RSI 3.0 on the exterior of the sheathing), and you have used spray foam or a sealed “peel and stick” membrane on the exterior of the wall sheathing, you start to be concerned about whether a vapour barrier on the interior will limit drying of the wall. Ideally such assemblies should be able to dry to the interior. Eliminate the interior vapour barrier and it is much more likely that incidental moisture, from a minor defect in the wall, can dry to the interior. 

Good luck getting this type of innovation through the approvals process on your own however. My best advice is to go into your local building department early in the design process armed with a review by a competent Building Science engineer. Most building departments are not going to approve such an innovation without professional review—some kind of analysis of the potential for condensation, and perhaps use of a software package such as WUFI—to look at the assembly in a thorough way. Manufacturers reps can be a big help putting you in touch with experts who can help pave the way.

In buildings with very high insulation levels, there is another issue to be concerned about. Highly insulated walls and roofs have less thermal energy escaping to the exterior so there is less drying potential in exterior spaces. It becomes even more important that exterior elements be well thought out. 

A lot of the focus is on window and door penetrations. Immediately after hurricane Andrew a few years back, much of the damage that occurred was traced to leakage failures in window systems. A number of manufacturers have since come forward with systems for flashing and sealing rough openings around windows and doors. If the window or door element ever leaks, there is a secondary flashing and sealing system around the window to capture the moisture and re-direct it to the exterior. For high end builders, this represents an important differentiator in their houses. As one builder put it, I am confident in what I can control—caulking and sealing as necessary—I am less confident that window and door frames will not have the odd defect I cannot see.

As a final note, I thought I should say a few words about caulking. In my opinion, caulking should never be the primary defence against moisture. You should never attempt to achieve a perfect seal via caulking on the exterior. If you do, those caulked joints have to withstand all the wind pressure driving wind-driven rain inward. The result can be disastrous: a minor leak allows moisture behind the caulking and now the caulking actually prevents drying. Caulking should be used to bridge gaps, to minimize moisture getting into drainage cavities and to improve aesthetics. Always leave openings at the base of walls and at through wall flashings for drainage.

As we pursue ever more energy-efficient, near Net Zero and even off-the-grid housing, it’s important not to lose focus on moisture control in all its aspects. Without keeping wind-driven rain and absorbed, condensed and diffused moisture under control, the benefits of more energy-efficient construction will be short lived.


Paul Duffy, M.A.Sc, P.Eng., is principal and lead consultant of Paul Duffy Associates. He provides strategic management and engineering consulting, specializing in building science and performance improvement. He served as vice-president engineering for Icynene Inc., a building envelope product manufacturer. He was also president of BLP Engineers from 1989 to 2008. Paul can be reached at




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