Create a Safety Buffer with INTELLO

Create a Safety Buffer with INTELLO

Drive Safe, Build Safe


As we get ready to drive over the Thanksgiving and Christmas breaks, our minds turn to safety and the risks we take for granted every day. What's your risk tolerance?

When you drive, do you tailgate the car in front of you or do you leave a safety buffer? Although you are very unlikely to crash, do you bother putting on a seat belt? Do you increase your safety buffer as you drive faster? Like safe driving, as insulation levels rise, we want to increase drying reserves. Like properly installed airbags, INTELLO Plus has a uniquely strong ability to increase the safety buffer.

Increase Safety with INTELLO Plus

INTELLO Plus Installation
INTELLO Plus Installation

You want to build a well insulated high-performance building, even possibly achieving Passive House levels. You might be located in Vermont or Maryland, Atlanta or Seattle. You're looking for the right tools to mitigate risks and do it safely. In many circumstances INTELLO Plus just might be the perfect tool. Why? Two reasons:

  • First, INTELLO Plus is a great air barrier and it gets installed in the best place, inboard of the insulation - keeping the conditioned air in the conditioned space - providing durable airtightness. (see An Interior Air Barrier Does It Better)
  • Second, INTELLO Plus is the best vapor retarder on the market. It is a vapor variable membrane - a smart vapor retarder. If you're in a location with a winter season, all the way down to northern Florida (Climate zone 2), INTELLO works to increase drying reserves and prevent moisture damage risk by limiting wetting in the winter and allowing drying inward in the summer. (See Why the Vapor Curve Matters)

Generally speaking, INTELLO Plus decreases risk, achieving safety. However, there are two exceptions:

  • When you have a constant high humidity interior climate, like indoor swimming pool, sauna or greenhouse a vapor retarder such as DA should be installed inboard AND the assembly should be able to dry outward through vented rainscreen or roofs - See also 475 Video: Sauna Uses Solitex Fronta Quattro and DA)
  • When the building is located in Climate Zone 1 an outboard class III vapor retarder makes for dryer/more forgiving assembly. Use ProClima DA

Airtightness is good in any climate - and ideally protected from the elements/thermal cycling, located inboard of the insulation. Make sure you blower door your building and confirm your airtightness.

And the benefits of smart vapor variability increase:

  • as insulation levels increase
  • as the winters get stronger
  • as the outboard construction gets more vapor closed
  • when the insulation is not hygroscopic

Example Assembly

There are an infinite number of enclosure assembly types and each one must be considered on its own merits - its risks, its costs, its durability. And depending on what climate your are in, the assembly make-up can be greatly affected. (See Foam-Free Wall Assembly Guidelines by Climate Zone) One must also consider use of the building, exposure, solar radiation and more. It's complicated. But as we will show, with INTELLO Plus it is hard to go wrong.


In this post, to demonstrate the basic truths about INTELLO Plus listed above, we'll look at the humble double-stud wall, a cost effective mainstay of high-performance and Passive House construction. (see: The Double-Stud Wall Simplified: Low Cost, High Performance)

The construction, from outside to inside is often:

  • back-vented rainscreen siding
  • vapor open WRB SOLITEX Mento 1000
  • 5/8" OSB sheathing
  • 10" densepack fiberglass insulation (2.5lbs/CF)
  • INTELLO Plus membrane with a service cavity
  • gypsum board

Our post, Keeping Sheathing Dry in High-R Double-Stud Walls - a WUFI study looked in detail at such conditions in Boston - but here we expand the view to multiple climate zones.



In this classic configuration, there is concern that the sheathing, the OSB, might get too wet -which can cause structural degradation/rot or mold formation.

How do we assess risk? Like assessing climate change risks or car crash risks, we run simulations. In this case we use WUFI Pro. Like other simulations WUFI Pro will not give predictions with 100% certainty - but it is very good as helping assess relative risk. Is this riskier than that? What are the variables? What happens when we adjust variables. Is there a safety buffer in the results? WUFI Pro is our crash test.

To avoid mold damages we generally like to see test results regarding two things: Moisture Content % and Relative Humidity Levels.

  • Moisture content risk - There is much debate about what the upper absolute safe limits are. Hunt and Garret, 1938 and Viitanen 1996 indicate that maintaining wood moisture content below 20% completely inhibits fungal development. However the APA’s engineered wood handbook (2002) has a limit of 16% moisture content (mc) for OSB. OSB has higher risk due to the lower perm rating of wood sheathing and the higher risk of swelling/damage by glue degradation. Pro Clima, following German standards (DIN 4108), recommends staying below 15M% (±3%) for OSB and plywood.
  • Humidity levels risk - Per ASHRAE Standard 160P, on a 30 day running average we want to keep relative humidity below 80% when surface temperature is between 41 and 104 degrees fahrenheit. (Note: this again is conservative and currently being updated in 160p-2016)

Naturally, each measurement of risk is related to the other. 80% RH on surface of OSB for some time during a cold spell corresponds to Moisture content of 20% of the same sheathing. Below we're looking at the moisture content as it accounts for some buffering of the moisture in the board and is a widely used proxy for avoiding issues in wood framed walls.

Example Locations

So given all this, let's take a look at this example assembly in various locations: Boston, Baltimore, Atlanta and Seattle. Let's verify the basic premise that INTELLO Plus helps build drying reserves, and where we have a winter and summer it poses no risk, and makes for safer assemblies, when compared to the same wall without this smart vapor retarder.


Seattle is in Climate Zone 4 Marine and particularly susceptible to moisture damages (humid exteriors in all seasons, mild summers)

Without INTELLO Plus protection. Reminds us of Thelma and Louise:


Seattle without vapor control shows very high (and disastrous) moisture content in the OSB.

With INTELLO Plus the change is dramatic: Driving Miss Daisy...


With INTELLO Plus added the moisture content is brought well below 20%, almost to the preferred safety factor level below 15%.


Boston is in Climate Zone 5 Cool & humid summers.

Without INTELLO Plus the moisture levels exceed the safety buffer, exceeding 20% every winter - even after 5 years. Yes, they do dry to our safety margin before wetting again. But to continue with the car analogy, it's as if at each curve the car drifts into the breakdown lane before returning to the safety of the driving lane. But what if some unexpected wetting occurs? What if there is a car parked in the breakdown lane? There is no buffer, so it could be considered driving without a seat belt or airbag - if there is an air leak - there are no reserves and damages/rot will occur (WUFI Pro doesn't include those).


In Boston climate the moisture level in the OSB is also elevated well above safety levels of 15% and 20%.

With INTELLO the safety buffer is fully established.


Again, with INTELLO Plus the moisture content is drastically reduced, here to within the best practice zone of 15%.


Baltimore is in Climate Zone 4 Mixed & humid.

Without INTELLO Plus. Without as severe a winter as Boston, the dangerous drift isn't as deep but the safety buffer is still compromised.

]asdfasdf Baltimore, like Boston reaches above 20% moisture levels without vapor control.

With INTELLO Plus the safety buffer is fully intact:


With INTELLO Plus, the assembly in Baltimore is brought within the preferred safety factor of 15%.


Atlanta is in Climate Zone 3 Warm & Humid.

Without INTELLO Plus: same 20% threshold is exceeded every winter.

asdfasdfasdf Even in Atlanta Georgia, the winters drive the moisture levels to uncomfortable heights.

With INTELLO Plus: safety.


nd again the INTELLO Plus increases drying reserves in the southern capital city.


As we increase insulation levels and performance expectations, INTELLO Plus not only offers great airtight control but also powerful protection from moisture damages. If you are working on a high-performance project we are very happy to look at your assembly specifics and help determine what can be the most cost effective and durable solution. Please ask us.

Drive safe. Build safe.