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What’s New with Water-Resistive Barriers

We’ve come a long way from the early Tyvek housewrap: our experience with Pro Clima Solitex, a WRB from Germany

Posted on Jun 27 2013 by Alex Wilson

I remember years ago — I hate to remember how many; it must have been around 1982 or 1983 — writing for New England Builder (now the Journal of Light Construction) about Tyvek housewrap. It was then a fairly new product — and really a new idea: a material that would wrap over the outside of a house to provide an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. and improve energy performance.

Tyvek wasn’t actually new in the early 1980s — it was invented by DuPont in 1955 and first commercialized in 1967 — but it was new enough in the building industry that two technical experts from DuPont trekked up to Vermont to give me a dog-and-pony show about it. New England Builder was gaining a reputation as a leading purveyor of practical, on-the-ground information for builders, and DuPont wanted to get the message out.

Tyvek was the first plastic housewrap

I used Tyvek houswrap on our 1788 house, which I was then in the process of renovating. I had removed the old shingle siding, repaired some rotted sills, replaced some sections of board sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , insulated on the interior with fiberglass between the studs (plus an inch of extruded polystyrene on the interior of the studs), and I wanted to provide a reasonable air barrier on the exterior.

Tyvek seemed like the way to go. It is a spun-bonded polyolefin (polyolefins are polymers, usually either polyethylene or polypropylene, made up of long chains of carbon and hydrogen) that comes in a roll wide enough to provide a continuous layer on the outside of the house. It seemed ideal.

After 30 years, Tyvek disintegrates

Thirty years later, doing some repairs to drainage around the house (repairs that I described in an Energy Solutions column last year), I had the opportunity to remove some of that Tyvek, and I was struck by how much it had deteriorated (see Image #2, below).

It turns out that Tyvek — at least the formula that was being used thirty years ago — was significantly damaged by surfactants in wood siding. (I didn’t know enough then to provide a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. detail using strapping, which would have separated the wood siding from the Tyvek and improved the housewrap’s durability.) The material almost disintegrated in our fingers as we examined it.

The evolution of water-resistive barriers

A lot has happened with housewraps in the 30 years since DuPont paid me that visit. I was just on the DuPont website, and the first thing I noticed was that there are now nearly a dozen types of Tyvek: the standard HomeWrap, StuccoWrap, a roof product, a handful of commercial products, Tyvek tapes to seal one layer to another, plus all the non-building-related products for mailing envelopes, protective haz-mat suits, etc.

Following DuPont’s success in creating a new type of product, there were lots of entrants into the housewrap industry: Typar (cleverly named to confuse the user into thinking it was Tyvek?), various perforated polyethylene films, and some textured products that try to achieve a sort-of rainscreen (air space behind the siding).

In fact, in the building-science community the good-old housewrap has evolved into the “water-resistive barrier” (WRB). It’s either an attempt to impress clients with a far-more-impressive-sounding product that justifies the cost, or perhaps an effort to mirror the dry terminology found in building codes. These barriers are supposed to keep out rain and wind (air flow), yet they need to be permeable enough that any moisture that finds its way into the building enclosure can evaporate and escape to the exterior.

Enter the Europeans

So what did I use on our current house project? We ended up going with a German water-resistive barrier called Solitex Mento 1000, made by Pro Clima and distributed in the U.S. by 475 High Performance Building Supply in Brooklyn, New York. (475 is a specialized building material supplier serving builders of low-energy and PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. buildings.)

Solitex is one of a number of European WRBs that go beyond typical American products in their performance. The other that I’m somewhat familiar with is SIGA Majvest Exterior Wall Membrane, a Swiss WRB distributed in the U.S. by Small Planet Workshop in Olympia, Washington.

Solitex Mento 1000 is a high-performance WRB that offers both very good water penetration resistance and very high water-vapor permeability. According to the company and 475, the product resists a 33-foot water column even as it provides a permeance of 38 perms — excellent numbers in both cases.

Meanwhile, Solitex Mento 1000 goes a long way toward restricting air flow through the wall or roof assembly, since its air permeance is only 0.00004 cubic feet per minute (cfm) per square foot, according to a standardized test method (ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E2178).

The manufacturer claims that it's not affected by cedar surfactants

Technically, Solitex Mento 1000 is a three-layer monolithic TEEE film (Thermoplastic Elastomer Ether Ester) with polypropylene protective layers. By being monolithic, it has no pores, so it is more water-resistant than standard housewraps, while actively transporting vapor outward during the heating season. This means that the TEEE functions at a lower pressure differential between inside and outside than the more common microproous/woven products.

“With traditional housewraps,” explains Ken Levenson of 475, “the vapor permeance is from the microscopic tears in the woven membrane, which the vapor can push through; while with a monolithic membrane with no tears or pores, it is the actual molecular structure that is transporting the vapor.” As such, because traditional wraps resist vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. at lower pressures, there is greater chance of moisture build-up filling the pores which can block vapor movement, while the molecular structure of the monolithic membrane moves the vapor at very low vapor pressure differentials, avoiding the danger of blockage.

Also, the membrane’s performance will not degrade from surfactants in cedar siding, according to Levenson.

You can also install it on your roof

The product is recommended for exterior walls and roofs. With roofs, it can even serve as a temporary roofing layer until roofing is installed. In our roof system, we used a slightly different version of the WRB, Solitex Plus (which has a reinforcing grid), as a layer to achieve the air space under the roof sheathing. In this application, the WRB provides both a waterproof layer to shed any water that may get into the air space down to the soffit vents and an air barrier in the insulated roof system.

Pro Clima Solitex comes in 1.5-meter (59 inch) rolls that are 50 meters (164 feet) long. Edges and overlaps should be sealed with Pro Clima Tescon tapes.

Compared with the very lightweight, 9-foot-wide rolls of Tyvek, the installation may be more time-consuming (and it's certainly more expensive -- with a list price of $229 per roll for the Solitex Mento 1000), but I am confident that we have a water-resistive barrier that will do a superb job of protecting our house, while helping achieve the airtight construction we are seeking and allowing any moisture in the wall cavities to escape.

I’m hoping this WRB will still be doing its job in 75 to 100 years, when it will be time to replace the siding on the home.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.


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  1. Alex Wilson

1.
Jun 27, 2013 7:43 AM ET

Edited Jun 27, 2013 7:44 AM ET.

Tyvek articles from the early 1980s
by Martin Holladay

Alex,
Thanks for your article. I especially appreciate seeing the photo of 30-year-old Tyvek.

My old copies of New England Builder are up in the attic, and are hard to get to. It would be fun to read your old article on Tyvek. Do any GBA readers know the date of the article?

A web search reveals that you wrote an article for the August 1987 issue of New England Builder titled "Wrap Wars." I'm guessing that "Wrap Wars" was based in part on information in an earlier article by Ned Nisson, namely "The Housewrap Wars." Nisson's article was published in the April 1987 issue of Energy Design Update.


2.
Jun 27, 2013 4:43 PM ET

Has Tyvek been improved since then?
by Lucy Foxworth

Alex,
I am wondering if they have improved Tyvek since that time.

I'm asking because I was planning to use Tyvek under the mineral wool exterior insulation then a rainscreen over the mineral wool then hardiboard and I was hoping it would hold up better than that. I am using Intello Plus as the interior membrane for cellulose.

I think Roxul ComfortBoard has a wax coating, do you know if that would affect the Tyvek?

Thank you for all your articles.


3.
Jun 27, 2013 6:34 PM ET

Better Tyvek
by Alex Wilson

Yes, I know that Tyvek has been improved a lot over the years, but I'm afraid I don't know many specifics. Perhaps Martin or someone else reading this will know more about that. I believe that one change was the addition of a UV inhibitor so that it doesn't degrade as quickly in the sunlight before siding covers it. As I recall, UV degradation played a significant role in the long-term damage from surfactants in wood siding, and particularly cedar.


4.
Jun 27, 2013 6:53 PM ET

Better
by Dan Kolbert

Yes, Tyvek has improved the product. But even if not, Alex's situation was the classic degraded Tyvek set-up - cedar siding directly over housewrap. You wouldn't have anything like the cedar tannins anywhere near the product.


5.
Jun 27, 2013 7:18 PM ET

Spruce clapboards
by Alex Wilson

Dan,
Actually, we installed spruce clapboards on the house, not cedar--for other reasons. I was with Joe Lstiburek once when he exposed Tyvek under cedar that was in far worse shape than mine. I wonder how spruce and cedar compare relative to tannins?


6.
Jun 27, 2013 8:27 PM ET

Can't find
by Dan Kolbert

a tannin chart - poked around. Did find out that the word "tanning" comes from tannin, though.

Certainly cedar tastes a lot of worse than spruce, if you've ever sanded a bunch of both, so since tannin is associated with bitterness, seems like a reasonable guess that spruce has less.

http://www.ansci.cornell.edu/plants/toxicagents/tannin.html


7.
Jun 27, 2013 8:45 PM ET

back primed
by Richard Kinkel

I’ve made the same mistake myself and today would do things differently. Curious was the spruce siding back primed where the Tyvek failed? Looks like siding was close to the ground and prone to splash back. Was siding removed higher up the building with the Tyvek failing the same way?


8.
Jun 27, 2013 10:32 PM ET

It was a little surprising to
by Peter L

It was a little surprising to see the Tyvek fell apart so badly. It probably failed at the 15-20 year mark and let in all the moisture for 10-15 years before it was torn apart. Those interior boards were pretty badly rotted.

With the strapping I see a problem occurring with termites and other insects getting into the cavity between the siding and the wall. How do you stop this? Weep Screeds at the bottom would not stop the insects.


9.
Jun 27, 2013 11:08 PM ET

Edited Jun 27, 2013 11:14 PM ET.

barriers, barriers, barriers...
by Thomas Slater

Alex,

Could you elaborate more on why you went with the Solitex over the Siga Majvest? Was it just because of the Solitex is monolithic while the Siga is microporous? They are priced very similarly (~28c/sf).

Also, are you using any sort of interior air barrier and are you worried at all about vapor in warm interior air in winter condensing in the insulation/rafters?

For that matter, what does everyone else think about the wall systems from each company (Majvest + Majpell or Solitex Mento + Intello)? I can't quite wrap my head around the benefits of one over the other. Seems to me only difference is really the seasonal variation in permeability of Intello vs. fixed permeability of Majpell.


10.
Jun 28, 2013 6:34 AM ET

Changes to Tyvek over the years
by Martin Holladay

Alex,
According to my understanding -- I think the topic came up in a conversation with Joe Lstiburek -- DuPont changed the way it manufactured Tyvek after all the evidence of problems with surfactants emerged in the late 1980s. However, I don't know any specifics concerning the changes to Tyvek, and in fact these changes may be a trade secret. Nor do I know whether the re-formulated Tyvek will last 30 years.


11.
Jun 28, 2013 7:06 AM ET

UV
by Dan Kolbert

I can say from experience that Tyvek holds up better to exposure that it used to - it got noticeably brittle and yellow-ish if left exposed. Don't know what that means for the longer term, but they have at least improved that part.

I've uncovered plenty of old Tyvek that seems to holding up fine. Haven't tested it for water resistance, though.


12.
Jun 28, 2013 7:22 AM ET

Selection of Solitex
by Alex Wilson

Thomas,
Good questions.

I was impressed with the literature about Pro Clima's Solitex line and also with the knowledge of Ken Levenson at 475 High Performance Building Supply. I suspect that Siga's Majvest is also a very good product, but I am not as familiar with it. The argument that Solitex is a monolithic material, instead of microporous, was compelling to me.

Regarding the interior: for the walls, we will not be installing an interior barrier, as we have achieved a very good air barrier mid-wall with taped Zip sheathing (with extra air sealing provided by Knauf's EcoSeal product). We will have about seven inches of John Manville Spider spray-fiberglass insulation on the interior side of that air barrier and six inches of expanded cork on the exterior, with drying potential to both the interior and exterior.

On the sloped, insulated roof, however, where there will be about 15 inches of Spider fiberglass, we are installing the Pro Clima DB Plus variable-permeability air barrier on the interior and then airtight drywall. Terry Brennan ran a WUFI analysis for us and concluded that there would not be a condensation risk without the interior air barrier as long as there is venting under the roof sheathing (as we are providing), but above the dormers roof venting will be limited at best, so we opted for the DB Plus barrier as a precaution. I am going with the DB Plus interior barrier over Intello Plus partly due to cost and partly because it's paper-based, which was attractive to me. The variable vapor-retarding properties are better with Intello Plus, but in our situation I think the DB Plus will be fine.


13.
Jun 28, 2013 9:26 AM ET

rafters
by Bob Irving

I'm curious about the use of rafter trusses, especially the way they are supported at the plate. Are they also supported by a structural ridge as TJI's would be?


14.
Jun 28, 2013 9:50 AM ET

Edited Jul 3, 2013 11:43 PM ET.

Answer for barriers, barriers, barriers....
by floris keverling buisman

Thomas, Thanks for your question. There are indeed some important differences between Pro Clima;s intelligent airtight system with INTELLO Plus and SOLITEX MENTO's performance when compared to Siga;s Majpell-Majvest combination.

INTELLO Plus is as you pointed out vapor variable, allowing the assembly to be protected in winter against moisture (perm 0.17, so almost class I) , while allowing large amounts of humidity to dry inward in summer when it's perm is exceeds 13. This is very beneficial to the drying capacity of well insulated assemblies, especially when they have vapor restricting sheathings (Plywood, OSB, ZIP - that are perm 2-4 even at wet cup conditions). (see the INTELLO primer blogpost". We (www.foursevenfive.com) also just started to supply INTESANA, which is INTELLO with weather resistance, great for renovation projects with outboard insulation.

This benefit is not present when using Majpell, that has a fixed perm rating of 0.68. A medium high class II vapor retarder both in winter (letting much more vapor into the assembly then INTELLO) and to restrictive to have any significant inward drying capabilities in summer).

The strength of SOLITEX MENTO membranes is the monolithic vapor open membrane. This actively transports vapor outwards, as described in the article above. Conventional WRBs, including Majvest depend on micro-pores or perforations to be vapor open (see image). This means the vapor leaks out of the little stretched (or perforated) holes in the membrane. This compromises airtightness and also reduces the waterproofness of the membrane. If the water tension is broken by wood tannins, oils or dirt, the water molecules (just like vapor molecules) fit through the holes and make the membrane leak. This is why SOLITEX MENTO is far more airtight and waterproof (can resist a water column of 33', which is 10x more then Majvest resists). Further information regarding the SOLITEX MENTO difference in this blogpost.

Pro-Clima-SOLITEX-MENTO-difference-475.jpg


15.
Jun 28, 2013 9:57 AM ET

rainscreen
by Bob Irving

"With the strapping I see a problem occurring with termites and other insects getting into the cavity between the siding and the wall. How do you stop this? Weep Screeds at the bottom would not stop the insects."
The air cavity would let the surrounding wood dry so that insects like carpenter ants would not be attracted. I'm not familiar with what attracts termites, but you'd be able to see the tunnels on the foundation, as with any house.


16.
Jun 28, 2013 11:30 AM ET

About the rafters
by Alex Wilson

Bob,
I discussed the open-web rafter system we used a few months ago: http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/using-op.... Yes, there is a structural beam supporting the rafters at the ridge.

As for excluding insects at the bottom of the walls, we are using a fairly simply system of rolled-up pieces of bronze screening that fills the air space, spanning between the strapping.


17.
Jun 29, 2013 3:28 PM ET

Edited Jun 29, 2013 7:35 PM ET.

Strapping
by Peter L

A permanent way of preventing insects from entering the gaps at the bottom of the walls between the strapping would be mandatory. A sloppy detail in that area would introduce other problems and keeping your walls dry would be the least of your worries. In the desert SW a gap between the sheathing and exterior siding would prove a disaster as you would be infested with hundreds of scorpions living within your walls. All they need is the thickness of a credit card and they can make it in. If the bronze screening is applied correctly and tightly, it might work out in the SouthWest.

One still has to deal with carpenter ants and other wood destroying insects. Not all termites leave termite tubes and carpenter ants leave no trace besides the damaged wood.

I understand the need to leave an air gap between the siding and wall sheathing as it needs to dry out but I believe another problem is being created by solving one problem. The rustproof bronze screens at the bottom seem like a possible solution but I question how many contractors are going to take the time to install them correctly?

This goes to show that a construction technique & detail that works up north in a rainy climate does not work in a dry desert climate.

Right now the wood SIP industry debates itself whether or not one needs to furr or strap the roof to allow for drying between the wood SIP and finished roof assembly. Each SIP installer you call will give you a different answer. Does SIP rot happen? Yes it does and has been studied by Journal of Light Construction and Building Science. Both groups say to furr the wood SIP roofs. Yet when you call the SIP companies, they say it's not necessary. I assume they state that because the cost to do strapping on the roof would double the install costs and would price out most people looking to do it with SIPs.


18.
Jun 30, 2013 9:14 AM ET

Edited Jun 30, 2013 9:39 AM ET.

Should one use "variable" or "fixed" perm??
by albert rooks

The thing i like about Siga Majpell is it's well suited for it's job: Stopping air movement at the interior of a wall or roof assembly. It's fixed permeance is based on simple cold climate diffusion open to the exterior - or- wall or roofs that are designed to "dry to the outside".

Highly permeable layers that allow "drying" also allow "wetting". It can become an awkward feature if the indoor humidity remains high long enough to wet the cavity.

While the variable permeance ability of Intello or Membrain are useful in the right application, they can also cause issues in the wrong application. Just as low fixed permeance can cause issues when the outboard layer is not able to dry and vent. Predicting how an assembly with variable permeance will fair is also problematic in WUFI since it can only calculate based on fixed cases.

None of these products are poor quality. They are all really good. Like always, it's all about choosing the right characteristic for your application. Modern assemblies need to stop air movement and have the ability to dry. If your in a climate where it's suitable to dry to the interior than variable on the interior and low on the exterior can work. If you have a cold/dry winter the differential can cause the cavity and sheathing to wet.

Both concepts are good in the proper application and both can be bad in the improper application.

Btw... Siga has set up a US subsidiary as they add personnel and dealers in the US. Alex had found the Swiss website. The new US website, complete with ASTM test data where it is completed, and even a page for career opportunities is here: http://www.sigacover.com


19.
Jul 1, 2013 1:06 PM ET

Historical JLC article on housewraps
by Alex Wilson

Martin,
Sorry for the delay on this. I dug out my old copies of the Journal of Light Construction, and found the article I had referred to. You were correct, it was from August, 1987 (not the early 1980s). It looks like I referenced EDU in one place, but had begun work on the article before that EDU article came out.

I was trying to compare the emerging housewrap materials and was clearly frustrated with the lack of consistent reporting and testing. From the article:

"What I discovered, however, was a quagmire of conflicting numbers, and a raft of technical comparisons that were confusing at best and dishonest at worst."

I also see what inspired DuPont's come up to Vermont to talk with me about Tyvek. In my conclusion "Which is Best" in which I said that from the information I had access to, I felt that Tyvek was the best housewrap out there. And I added:

"In a way I regret drawing this conclusion, especially after wading through the shoddy technical literature provided by DuPont..."

I scanned the two-page, large-format article, but wasn't able to provide higher than 200 dpi. I'm attaching that here, though I'm afraid you'll have to rotate the view on the two pages, and the table is hard to make out. Hopefully you can read enough to see how dated it is! We've learned a lot in 26 years!

AttachmentSize
JLC_WrapWars.pdf 1 MB


20.
Jul 1, 2013 1:50 PM ET

JLC Wrap Wars
by Martin Holladay

Alex
Thanks for digging up the article. I am attaching the two pages as jpg files.

.

JLC Wrap Wars 1 copy.jpg JLC Wrap Wars 2.jpg


21.
Jul 1, 2013 8:34 PM ET

Did you consider a fluid applied WRB?
by Robert Jordan

Tremco makes Enviro-dri and Prosoco exhibited at NESEA. Seems like a much easier way to go than peel and stick.


22.
Jul 1, 2013 9:51 PM ET

Fluid-applied WRB
by Alex Wilson

All the WRBs I'm familiar with are installed by stapling (like Tyvek). Some of the tapes we used are peel and stick, but not the WRBs themselves. I'm curious about the fluid-applied WRBs; I'll have to look into those.


23.
Jul 1, 2013 11:49 PM ET

Fluid Applied Wrb
by Jonah Griffith

We are just finishing up applying a fluid WRB and chose to use the Prosoco brand of products on our house here in the PNW. We used three of their products on all exterior and window openings: the Joint and Seam, Fast Flash, and Cat 5. Definitely worth looking into when comparing against the other options.
We ended up choosing this product for our house for a number of reasons: we built a 2x8 wall and need to have a waterproof but breathable WRB, especially here in Seattle. The Bullitt center building which was just completed used the product along with a number of other multifamily developments, our contractor has had good experiences with the product, and the product can stay exposed to UV for a few months. We are installing a rainscreen on top of the CAT 5 and then finishing with cedar shingles.


24.
Jul 3, 2013 11:39 AM ET

SIGA Majvest / Majpell
by steven konstantino

I agree with Albert that choosing the right product for the particular wall/roof assembly is critical to proper performance.
SIGA Majpell is designed to be a very low perm interior air barrier for assemblies that will dry to the outside. SIGA Majvest is a rugged WRB and has one of the highest perm ratings at 68 perms for superior drying performance to the exterior.

You can find all of the SIGA products, technical specs and ordering information at www.sigatapes.com
We stock and ship all products from Portland, ME and offer free shipping on most orders.


25.
Jul 3, 2013 11:30 PM ET

Edited Jul 3, 2013 11:34 PM ET.

WUFI calculations do include vapor variablilty
by floris keverling buisman

Albert,
Your remark regarding WUFI is not correct. This program and its calculations do include vapor variable properties of materials. I have attached the INTELLO material properties from WUFI Pro for you (perm btw 0.17 and >13), since it is modeled at 1mm thicknesses you get the values in the shown graph in perm/in). As you can see it's perm rating drastically increase right after 70% RH, the exact point when trouble starts in construction (mold & rot) and back drying is needed. This gives you the assurance that in climate zone 3,4 and even 5 - that when the vapor drive reverses in summer (especially while running AC), you don't have a too restrictive layer on the interior. We are after all trying to avoid moisture issues in the wall.

In the winter, the humidity that surrounds an airtight vapor retarding layer is relatively low. On the interior it might be 40% RH, on the outside 80% and 32F. However when the 80% RH air reaches the air/vapor barrier it is probably 68F and its RH has dropped to below 30%. INTELLO will be more vapor retarding (below 0.25) than Majpell (0.68) and thus keep the wall drier/keep more humidity in the conditioned space (see psychometric chart for RH/temp relationship).

And unless you are building a pool or other constant high humidity use, a vapor variable membrane performs better - both modeled and in real world conditions. Shower humidity spikes,etc, are to short to effect vapor variable membrane performance in the winter. Of course airtightness is key in high performance structure, but given the assembly additional drying capacity makes the structure more forgiving against unforeseen moisture (from inside or outside). I have modeled many walls in many USA climates in WUFI Pro and time and time again INTELLO shows better performance in climate zones 3 and higher, both with vapor open and restrictive layers on the exterior (OSB, asphalt shingles, flat or greenroofs). Allowing high performance, high R assemblies to be build safely (and foam free).

INTELLO perm per inch - WUFI.jpg


26.
Jul 18, 2013 10:31 AM ET

Drying to the Inside?
by steven konstantino

Seriously. If you have 70% RH in your wall cavity then you really messed up on your exterior to allow moisture into the cavity. That will not ever dry to the inside through any barrier covered with sheetrock and multiple coats of latex paint. The variable perm rating is useless.

Interior air barriers are there to stop air movement into a wall or roof assembly. All drying needs to occur to the exterior. You can choose to use an air barrier or not. It is really based on the assembly design.


27.
Jul 19, 2013 12:22 AM ET

Edited Jul 19, 2013 12:23 AM ET.

Redux yet new?
by albert rooks

Alex,

It looks like the WRB battle, after all these years, has moved indoors to the "warm side".

It certainly warms my heart to see such attention focused on air barriers. No matter who's products people like... And which way it dries... people are talking about airtight and the need to dry. This wasn't the case a few short years ago in 2009.

Chalk one up for Passivhausers! :)


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