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Building Drying Reserves: 8 Situations Made for INTELLO PLUS Intelligent Vapor Retarder

August 16, 2016
drying - subway

The MTA provides a clear yellow safety buffer. Sure you can stand in the buffer, but it’s risky. Provide a safety buffer with increased drying reserves in your highly insulated assembly.

Drying reserves? Call it a safety buffer. When we’re waiting for the subway we don’t stand on the track’s edge. Getting hit by the train is like getting hit with mold damage. To be safe we need a buffer of drying reserves.

As Anton TenWolde, retired physicist from the U.S. Forest Products Laboratory, and cellist, says “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.” Lothar Moll, Executive Director of MOLL bauöko­lo­gis­che Produkte GmbH, parent of Pro Clima, says also “There is always moisture risk present, but if we can maintain drying reserves that are greater than the moisture risk, then safety can be assured.” Is it safe to stand on the edge of the subway platform? Safety really requires a buffer: in this case, moisture-tolerance. We want to stay well away from conditions conducive to mold growth.

Per ASHRAE 160P, Criteria for Mold Growth: Mold growth can occur when the relative humidity (RH) of the material is above 80% while the surface temperature is between 41 and 104 F on a 30 day running average. That definition is the cliff’s edge, not a safety buffer. We make a safety buffer, we establish drying reserves, by saying we’d prefer that the RH never goes above 80%, no matter the temperature. Then in analyzing assemblies with WUFI Pro and real-world data collected, it may not be reasonable to stay completely below 80% at the coldest points of the year – then it is a question of how much risk is acceptable risk? Is a spike above 80% okay? Probably. Are weeks or months above 90% okay risk? Likely not.


The vapor curve of INTELLO PLUS and INTESANA build stronger buffers.

Like increasing the safety buffer space between cars when driving at higher speeds, the drying buffer shouldn’t shrink as the insulation levels grow. With highly insulated assemblies that are colder and more likely to stay wet robust tactics and tools are required. This means greater airtightness* and intelligent vapor control. Both are possible with INTELLO PLUS.

The INTELLO PLUS smart vapor retarder can be an excellent tool for building safe drying reserves, and providing maximum protection from moisture damage, as it limits wetting and increases drying potential. INTELLO PLUS, installed inboard of the insulation layer does this by providing airtightness and unsurpassed smart vapor variability – significantly retarding vapor movement in wintertime conditions while opening up and allowing vapor to dry inward in summertime conditions. See our blog post: Why The Vapor Curve Matters

When planning drying reserves, here are 8 situations made for INTELLO PLUS:

  1. Assemblies with significant outboard vapor retarding or a vapor closed profile. In assemblies that have limited outward drying potential such as vapor closed roofs (flat or asphalt shingles), foam insulation boards or exterior sheathing (OSB, plywood). Of particular concern are double-stud walls with dense-pack insulation and OSB or plywood outboard. See our blog posts: Keeping Sheathing Dry in High-R Double-Stud Walls – a WUFI study, The 8 Golden Rules for Foam-Free Unvented Asphalt Shingled Roofs and The Ten Golden Rules for Foam-Free Flat Roofs

    Drying - BSC masonry

    Building Science Corporation Illustration of Historic Masonry Retrofit with Smart Vapor Retarder

  2. Metal framed, or other enclosures, with significant thermal bridging elements. For example, in wintertime cold metal framing can otherwise be a magnet for condensation of interior air, like a cold Coke can on a humid summer day. It may not produce mold, but long term rust and metal deterioration is problematic.
  3. Historic Masonry retrofit with interior insulation. We’ve devoted a whole book to this subject: High Performance Historic Masonry Retrofits and are gratified to see similar solutions provided by the Building Science Corporation in How Buildings Age.
  4. Assemblies with batt insulations. The combination of air barrier and vapor control of INTELLO PLUS can transform wall insulated with batts into a high-performance solution. See our post: Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass – A WUFI Post


    INTELLO PLUS airtightness and vapor control can transform the performance of a typical batt insulation approach.

  5. Non-Toxic (non-spray foam) solution is required. INTELLO PLUS in combination with natural insulation materials is a robust no VOC solution. See Foam Fails.
  6. Super insulated assemblies that need risk mitigation. While applicable to all of the above, in analyzing different levels of insulation you will see risks rise or fall and INTELLO PLUS can be a powerful level in the outcome.
  7. When interior airtightness is required. Interior airtightness has many advantages, see our blog post An Interior Air Barrier Does It Better. The INTELLO PLUS can serve both air and vapor control. See also 5 Reasons Why Airtight Drywall Is Obsolete.
  8. Meeting Code for Class II vapor retarders. INTELLO PLUS can handily meet the code demands of a Class II vapor retarder.

Let’s limit the wetting and increase the drying. We can help. See our INTELLO PLUS Resource Library for more information.

* Airtightness should always be confirmed with a blower door test. Pressurize and depressurize. Find leaks and make as tight as possible.


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