Image Credit: Alex Wilson Tyvek after 30 years. We installed Solitex Mento 1000 over 6 inches of expanded-cork exterior insulation, taping it to the wrapped window surrounds. Pro Clima tapes were used to ensure a continuous weather barrier at the window surrounds. Strapping installed over the WRB. The fully strapped house awaits window trim and siding. We installed a slightly different Pro Clima Solitex WRB to form the vent space under the roof sheathing. The WRB is caulked and stapled to the top chords of the rafters to form the air barrier. Sections of WRB material were cut to insert between the rafters to create the air space above the insulation. The WRB will protect the insulation from any moisture that gets in, while allowing the insulation to dry out if it does get wet. Solitex WRB was used to form the air space and air barrier beneath the roof sheathing. The rafters are ready for insulation.
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 barrier 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 sheathing, 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 rainscreen 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 Passivhaus 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 (ASTM 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 diffusion 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.