Smart Vapour Control, Magical Thinking and Goldilocks

Smart Vapour Control, Magical Thinking and Goldilocks

Be sure to protect your building for its entire lifetime

In an endurance test, it’s critical to have capabilities in reserve to handle the unexpected: And the life of a building enclosure is an endurance test. The greatest threat to a building’s longevity is moisture damage. Consequently, moisture risks must be mitigated by maintaining a safe storage capacity; a safety buffer. It’s building science. And the really cool thing about science is that it isn't opinion: Science is verifiable. Unfortunately, on this subject, our competitor, Siga, has offered misleading data to the marketplace about our product, INTELLO, and has proclaimed its own product a "one-way" retarder. So let's review the context of vapour control, building science, and how our respective products perform.



Image 1.1: To avoid moisture damages the drying capacity must be sufficient to prevent wetting from exceeding the safe storage capacity of the assembly. The dynamic of safety from moisture damage is illustrated by the Building Science Corporation (BSC) in image 1.1. Our manufacturing partner, Pro Clima, provides this clear formula:

Drying Capacity > Unexpected Wetting = Freedom From Damages

And the scientist, Anton TenWolde, formerly of the U.S. Forest Products Laboratory, has stated:

“You need to assume that the building will get wet, somehow, at some point in time. Stuff happens. So you need a moisture-tolerant design.”

To build moisture tolerance, BSC tells us that the three most important things we can do, in order of importance, are: Shed bulk water; Provide an airtight enclosure; Provide vapour control. Joe Lstiburek writes:

“A point to this important thing here, if you can’t keep the rain out don’t waste your time on the air. If you can’t keep the air out don’t waste your time on the vapour.”

Most people understand bulk water, and today airtightness is better understood by professionals than in the past. But vapour control continues to confuse. 475 works hard to discuss building science plainly and equip our customers with the best information and components to make high-performance enclosures. Yet we see confusion in the marketplace. So, let’s try to clarify.
Drying Potential is Primary Protection
Like a football moving up and down the field – water vapour drives outward, through the building enclosure, in winter conditions, and inward, in summer conditions: back-and-forth, back-and-forth, back-and-forth. The trick is to prevent wetting in winter, like a stingy defensive line, while allowing maximum drying in summer, like an explosive offensive squad. And while a team needs a strong defense, its offense simply MUST score to win. As the illustration and formula above implore, drying capacity is the primary protection. Yet thinking about “typical” buildings, let’s acknowledge that, while often poorly insulated and leaky, they can achieve protection from moisture damage by simply baking themselves dry. These buildings may last forever, but they are terribly inefficient and uncomfortable, and consequently provide very low performance. Today, we’re demanding that our buildings be well insulated, efficient and comfortable – so the enclosures are colder, and colder things dry with more difficulty. And so, often vapour diffusion is the chief drying mechanism. Put another way, the drying potential through vapour diffusion, of a well-insulated assembly, becomes very important. We recommend that if you want an assembly to dry out – you should have an interior vapour open control layer, wide open; and in the end zone. If you’re spending good money on a team, it should provide every opportunity to win the endurance test – and a complete INTELLO system, at 13+ perms when open - a permeable membrane per IBC/IRC - and gets the job done.
Support Drying: Prevent Wetting
Of course, we must also look to prevent wetting – as a strong defense lessens the needed drying. By preventing wetting in winter and maximizing drying in summer - like running up the score - we see reserves growing; reserves that will help the assembly deal with the unexpected and assure a successful long-term result. There are important concerns about moisture loading of highly insulated assemblies, such as flat unvented roofs, and from wet construction sites during the winter. In these challenging conditions, we should maintain a minimum safety threshold. At Pro Clima and 475, we call the safety threshold HYDROSAFE. Developed from the German building code standard DIN 68800-2 which requires that at 70% average RH, the vapour variable membrane’s permeability should not exceed 2.2 perms nor fall below 1.3 perms to guarantee the long-term protection and durability during occupancy of the building. In wet conditions, at 70% RH, INTELLO is 1.6 perms – the sweet spot. In dry conditions, INTELLO is a low Class II vapour retarder (0.2 perms per ASTM E96) and at 45% RH per 6880-2 is below 1 perm. Consequently, the best strategies and the greatest teams combine strong defense (wintertime retarding) with unparalleled offensive power (summertime drying) – giving them the resilience to overcome the unexpected.

INTELLO is balanced to provide defensive protection in winter and maximizes drying potential in summer.

INTELLO is balanced to provide defensive protection in winter and maximizes drying potential in summer. In the real world, this drying power has proven itself useful. It is demonstrated in the below graph (image 1.3) showing Wood Moisture Equivalent percentage (%WME) in a vapour closed, hot roof assembly over almost 4 years in Maine. The trend is downward, showing decreasing moisture in the assembly with growing safety reserves.

Image 1.3: Corson vapour closed “hot roof” in Maine demonstrates building drying reserves over four years.

Image 1.3: Corson vapour closed “hot roof” in Maine starts with construction moisture and safely dries, showing the assembly's drying reserves over four years.

To help complete the circle of knowledge, we’ve also provided additional guidance with blog posts: 10 Golden Rules for Foam-Free Flat Roofs, The 8 Golden Rules for Foam-Free Unvented Asphalt Shingled Roofs, vapour Venting An Unvented Roof: In Praise of Belts and Suspenders, and A high performance roof should be vented – how to do this properly.


Curiously, our competitor Siga claims that a low, mid-range, Class III vapour retarder, (Majrex) will provide sufficient drying of unexpected wetting over the life of a building (image 2.1).

Image 2.1: When summertime drying is most useful, Majrex never becomes vapour open.

Image 2.1: When summertime drying is most useful, Majrex never becomes vapour open.

Kohta Ueno of BSC recently commented in the Journal of Light Construction about it:

Four perms isn’t very impressive when you compare it to all the other products that are designed to open up,

It is true that Majrex is more retarding in wintertime construction conditions, but the difference is slight. Unfortunately, we take note that Siga has sent the graph at below left to a 475 customer, which misrepresents the performance of INTELLO, exaggerating the difference. A more accurate comparison is in the graph to the right – where we’ve replicated the spacing of values and position of the arrow comparing products.

side by side

Image 2.2: Inaccurate data provided by competitor on left: Correct data on the right

Even more curious is the claim by Siga, that Majrex is a one-way retarder. Do the graphs indicate it’s “one way”? If street traffic moves more heavily in one direction than the other, for several hours a day, is it a one-way street?


Earth sits in a Goldilocks planetary zone that makes our lives thrive (not too hot and not too cold); INTELLO also operates in such an optimal place. Another competitor, CertainTeed’s MemBrain, opens early and wide, providing poor defenses; while Majrex never releases full drying protection. But INTELLO is unique. INTELLO doesn’t open too early, and then it opens sufficiently.

big graph membrain

INTELLO is engineered to hit the “sweet spot”: Not too open in winter or too closed in summer: Just right.

It’s no accident. As NASA planned the Kepler Mission to search for Goldilocks Planets to secure the future of humanity; Pro Clima scientists engineered the ideal vapour control membrane to provide long-term protection for the future of your building.

As Joe Lstiburek notes, about second generation vapour retarders, in It’s All Relative:

Today we have extremely “smart” vapour control layers…The “inflection points[5]” for the sorption curves of these layers are “engineered” to provide control that otherwise would not be possible. In this case “inflection points” translates into “valve” opening and closing. “Smart vapour barriers” on the interior are truly smart... … [5] Where the curve bends upwards….


If you expect your building enclosure to provide high performance for the life of your building, INTELLO provides the best protection against moisture damages available on the planet today. Let’s review what makes INTELLO work:
  • 100x range of permeability - maximum drying potential (<0.13 to >13.2) - with the strongest curve.
  • HYDROSAFE for winter construction conditions - retarding sufficiently when needed.
  • Certified by the federal Germany agency DIBt, for use in unvented hot roof assemblies.
  • Passive House Institute Certified Component (tightest membrane system tested)
  • Reinforced for dense-pack insulation.
  • Available as INTELLO X: a commercial grade version that may also be a WRB for exterior retrofit applications.
  • Available in 5’ and 10’ wide rolls.
  • 10 Year System Warranty: The best warranty available.
  • Part of a complete system of airtightness with TESCON Vana tape that has been independently verified by the University of Kassel to perform reliably for 100+ years.
  • A track record of proven performance in difficult applications across North America for over 7 years and in Europe for over 10 years.
  • Ideally suited applications include: historic masonry retrofits; metal frame construction, CLT and double-stud and other wood frame assemblies; and insulated assemblies vapour closed outboard.
  • Helps manage risk.
  • Comprehensive technical support from 475, including easy to follow installation instructions, ebooks, dwg files and videos.