Why The Vapour Curve Matters

Why The Vapour Curve Matters

How vapour variable materials stack up when it comes to mileage and protection

Intelligent membranes, or smart vapour retarders, can help prevent condensation in enclosure assemblies (walls and roofs) in winter, while allowing inward diffusion in summer. This transformation is important to assure the safety of an insulated assembly, by increasing its drying reserves so it can cope with (unforeseen) moisture -- both into and out of the assembly. But how and when the material goes from being a Class II vapour retarder (at 0.17 perms far below 1 perm and almost a Class I vapour retarder) to a vapour permeable material deserves a more detailed look.

The ICC building codes require a Class I or II vapour retarder on the inside of insulated assemblies (IRC 1405.3 and IBC R702.7) in Climate Zones 5, 6, 7, 8 and Marine 4. This is to prevent warm and more humid interior air from moving through the insulation and condensing on the cold "condensing surface" during outward vapour drive in winter. Typically the condensing surface will be the exterior plywood or OSB sheathing. Since an interior vapour retarder will be warm, no condensation will form on it while it prevents the humidity from reaching a cold condensing surfaces. But there's a downside to having a material below 1 perm at the warm, interior side of the insulation, when the vapour drive reverses in summer. With the vapour drive inwards (more humid outside than inside), a low perm material will not let any humidity through - effectively damming it. You can see that in the image below, where a polyethylene vapour barrier is showing the humidity trying to push inward - but ends up condensing on the inside because the material is vapour closed.

inward diffusion condensing on PE - proclima

It would be best, of course, if the material in winter were a Class I or II material when the vapour drive is outward, but then becomes as open as possible when this drive is reversed in summer. Hence the need for smart or vapour variable retarders was recognized - and as a consequence, Pro Clima developed INTELLO. INTELLO is a smart vapour retarder with the highest level of vapour variability available on the market. Just as important, it becomes permeable at the right time - not too early, not too late. More about that below.

How is a material vapour variable?

To understand how (airtight) materials have different vapour permeance at different relative humidities, let's take the example of wood sheathing. A 5/8" thick piece of OSB is a Class II vapour retarder at 30% relative humidity. It becomes more vapour open if the surrounding relative humidity increases. This can be understood as the wood taking up this humidity and the damp wood becoming more vapour permeable - absorbing humidity on one side, transferring it to the other side and releasing it there. You can see that OSB gets a bit more permeable (2 to 4 perms depending on the testing lab) after it goes over 60% RH and 80%RH, but it will also start to rot or mold at that point. Since OSB is pretty retarding to start with, it can be suitable as vapour retarder on the interior of the assembly. But to make sure that the assembly can dry outwards from the OSB, it should only have materials that are more vapour open than OSB to the exterior. This goes back to the 1:5 "perm ratio rule of thumb" that we have discussed before. The 1:5 rule shows that in winter the exterior should be at least be five times more permeable than the interior vapour retarder - for the safest assemblies. This ratio is also referred to by the DOE, German DIN 4108-3 regulations, and Robert Riversong on GBA (see quote on 3rd paragraph in 3rd section).

OSB humidty-rain changed permeability and structure Proclima 475
OSB that has seen different moisture content (Source: Ecological Building Systems - ecologicalbuildingsystems.com)

There are some considerations with wood sheathing and vapour permeance and their airtightness, which will influence their suitability as vapour retarders and airbarriers:

  1. WUFI Pro notes in the material data: "Since wood and wood-based products tend to swell and shrink, their material properties may depend on the current as well as preceding moisture contents. The applicability of WUFI has to be decided on a case-by-case basis".
  2. It has been demonstrated in Europe and in the USA that OSB is not reliably airtight. We have gotten at least 2 reports of this happening in the USA. Again, this probably differs from brand to brand, plant to plant and glues/species used. If a material is not manufactured to meet airtightness below 0.004CFM/sf, then using it as an air barrier is questionable. See photo above right that shows OSB leaking during a blowerdoor test. We have not seen this happening to date with plywood.
  3. The vapour profile of wood sheathing varies with thickness, production plant (amount and type of glue used), wood species in boards, and the list goes on. Consider also that Dupont has tested ZIP system panels for wet and dry cup permeability - which showed it remained below 1 perm in both cases.

The graph below shows the US perms of different materials at different humidities. Hardwood is too open to be a Class II vapour retarder, and it would take a lot of tape to make an air barrier out of sawn lumber. It also shows that OSB is not very variable - going from a low Class I to a low Class II material. There are even some OSBs that have a fixed perm rate in WUFI, in which case the moisture distribution (sorption/absorption) alone will account for the moisture transport through the material. Plywood becomes a bit more permeable above 50%, but does not go above 9 perms in 5/8" thickness. The exact numbers are also dependent on sheathing thickness, glues, the production plant, the age, and moisture cycling history of the sheathing.


The curve matters. When should smart vapour retarders open up?

Buildings experience high and low humidity on the interior during construction and occupancy. Pro Clima recommends that heightened moisture levels during construction should be avoided as much as possible, but we realize that is not always possible. Furthermore there are spaces in the house that have higher interior humidity such as kitchens and bathrooms. To prevent humidity from rushing into the assembly during those times, Pro Clima has established the 70%/2.2 perm rule for the construction phase and the 60%/1.64 perm rule for completed and occupied spaces.

Occupied increased humidity, the 60/1.64 rule

During regular use of the spaces, rooms like bathrooms and kitchens experience higher interior humidity, and when heavily used can result in relative humidity at the vapour retarder of 60%. If materials are less then 1.64 perms at that condition, these higher humidities are sufficiently retarded during these daily periods of higher humidity. If an air barrier has a vapour permeance that exceeds this 1.64 level, too much moisture can enter the insulation. This is shown in the graph below where you can see that, for instance, polyamid/nylon of CertainTeed's MemBrain with >3 perms far exceeds the 1.64 perm rule.


Construction humidity: the 70/2.2 rule

A large amount of moisture is created during construction, especially when you are pouring concrete, tiling, plastering, compounding sheetrock, etc. This can cause very high levels of interior humidity, both in summer and winter time. Even while controlling the levels with dehumidification and venting, you can have periods of greatly increased relative humidity. As a consequence, the interior vapour retarder/air barrier may experience RH as high as 70%. To make sure this humidity does not end up inside the insulated assembly and cause mold and decay, the maximum perm rating at 70% RH should be 2.2 perms. It is then still tight enough to keep the majority of the humidity from this one-time event out of the assembly. Pro Clima's INTELLO easily complies with this requirement with a vapour permeance of 1.6 perms at 70% RH.

The best curve opens up after 70%

Moisture issues in walls - such as rot, mold and rust - occur at 80% RH and above. Therefore, when relative humidity exceeds 70% in summer months, it is important that vapour variable retarders open up as quickly and as much as possible to facilitate inward drying. If a vapour retarder has a fixed permeability - such as polyethelene (below 0.1 perm) or Siga Majpell at 0.68perm - then unforeseen moisture can't dry out rapidly in summer. Nor, if you air condition the building, can you be sure that you won't get issues with inward vapour drive condensing on such fixed vapour retarders/barriers in humid summer weather.

INTELLO has the best-in-class smart vapour retarding profile, with a permeance that varies by more than a factor of 100 - twice the permeance spread compared to the next in class material. Pro Clima's smart retarder is very vapour retarding in dryer winter conditions (0.13 perms compared to MemBrain's 0.75 perms), while becoming vapour open at over 13 perms in summer. These features allow you to construct both of the following:

  • Highly insulated assemblies in practically any climate, with exterior vapour retarders such as OSB sheathing, Zip system, flat roofs, unvented asphalt roofs, etc. We perform complimentary WUFI studies in certain cases to make sure that the drying reserves are sufficient and/or when building inspectors need to be convinced (since the code does not understand vapour variability)
  • Best practice vented roofs and walls in mixed and humid climates - that are foam free and protected from condensation concerns in summer and winter.