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

Intelligent membranes, or smart vapor 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 vapor retarder (at 0.17 perms far below 1 perm and almost a Class I vapor retarder) to a vapor permeable material deserves a more detailed look.

The ICC building codes require a Class I or II vapor 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 vapor drive in winter. Typically the condensing surface will be the exterior plywood or OSB sheathing. Since an interior vapor 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 vapor drive reverses in summer. With the vapor 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 vapor barrier is showing the humidity trying to push inward - but ends up condensing on the inside because the material is vapor closed.

It would be best, of course, if the material in winter were a Class I or II material when the vapor drive is outward, but then becomes as open as possible when this drive is reversed in summer. Hence the need for smart or vapor variable retarders was recognized - and as a consequence, Pro Clima developed INTELLO. INTELLO is a smart vapor retarder with the highest level of vapor 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 vapor variable?

To understand how (airtight) materials have different vapor permeance at different relative humidities, let's take the example of wood sheathing. A 5/8" thick piece of OSB is a Class II vapor retarder at 30% relative humidity. It becomes more vapor open if the surrounding relative humidity increases. This can be understood as the wood taking up this humidity and the damp wood becoming more vapor 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 vapor 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 vapor 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 vapor 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 that has seen different moisture content (Source: Ecological Building Systems - ecologicalbuildingsystems.com)

There are some considerations with wood sheathing and vapor permeance and their airtightness, which will influence their suitability as vapor 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 vapor 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 vapor 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 vapor Retarders Open Up?

Buildings experience fluctuations in interior humidity during both construction and occupancy. Pro Clima recommends minimizing excessive moisture levels during construction as much as possible; however, we recognize that this is not always feasible. Additionally, certain areas of a building, such as kitchens and bathrooms, naturally have higher interior humidity. To prevent excessive moisture from infiltrating the assembly during these periods, Pro Clima has established the HYDROSAFE ® value—ensuring that at 70% relative humidity (RH), vapor permeance remains below 2.2 perms.

HYDROSAFE ®: The 70/2.2 Rule

Significant moisture is generated during construction, particularly from processes such as concrete pouring, tiling, plastering, and drywall finishing. These activities can lead to extremely high interior humidity levels in both summer and winter. Even with dehumidification and ventilation, periods of elevated relative humidity can still occur. As a result, the interior vapor retarder and air barrier may be exposed to RH levels as high as 70%.

To prevent this excess humidity from penetrating the insulated assembly—where it could cause mould growth and structural decay—the maximum vapor permeance at 70% RH should not exceed 2.2 perms. This threshold ensures that most of the moisture from these temporary spikes remains outside the assembly. Pro Clima’s INTELLO meets this requirement with a vapor permeance of 1.6 perms at 70% RH.

Humidity Increases During Occupancy

During regular use, spaces like bathrooms and kitchens often experience elevated interior humidity. At peak use, relative humidity at the vapor retarder can reach 60%. With a permeance of less than 2 perms at this condition, these temporary humidity spikes are sufficiently slowed, preventing excess moisture from reaching the insulation.

If an air barrier has a vapor permeance exceeding this limit, too much moisture can infiltrate the assembly, increasing the risk of damage. This is illustrated in the graph below, which shows how materials like CertainTeed’s MemBrain (a polyamide/nylon membrane) with a permeance exceeding 3 perms surpass the 1.64 perm threshold—allowing excessive moisture transfer.

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 vapor variable retarders open up as quickly and as much as possible to facilitate inward drying. If a vapor 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 vapor drive condensing on such fixed vapor retarders/barriers in humid summer weather.

INTELLO has the best-in-class smart vapor 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 vapor retarding in dryer winter conditions (0.13 perms compared to MemBrain's 0.75 perms), while becoming vapor 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 vapor 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 vapor 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.