ZIP vs SOLITEX: a WRB Comparison

ZIP vs SOLITEX: a WRB Comparison

Why SOLITEX is the best WRB solution

We often get asked how Proclima's SOLITEX Mento compares to Zip and Tyvek for weather protection. Here's a short answer:

SOLITEX Mento ensures the most durable and long lasting walls. That's because it provides a continuous monolithic protective layer that is completely waterproof and airtight, while allowing the most reliable outward drying potential to keep moisture levels safely low. Zip and Tyvek can be more prone to failure - especially on super insulated assemblies such as double stud walls with inherently colder exterior sheathing.

A Short History of Weather Resistant Barriers

In the beginning there was tar paper. This could be considered the first weather resistant barrier (WRB).

In 1967, DuPont's Tyvek hit the market. Tyvek is a non-woven membrane made from spun-bond olefin fiber, creating the first "micro-porous" membrane that blocks bulk water but allows vapor to dry out through the pores. It is applied shingle-style for positive drainage.

In 2006, Huber Zip System walls were introduced. Zip is OSB sheathing treated with a water-resistant coating that has a 12-16 perm rating. The sheathing is installed butt-jointed, relying on acrylic Zip tape for both airtightness and waterproofing at the board edges.

Leading a new generation of more advanced, high performance WRBs is ProClima's SOLITEX Mento. Today's Mento 1000 is a 3-layer monolithic membrane that's completely waterproof and airtight (with TESCON Vana acrylic tape). Instead of pores that can clog, the inner TEEE film reliably transports vapor outward at the molecular level, with a 38 perm rating. Yet it's so waterproof that it resists a 33-foot water column. In addition, the airtightness has been confirmed by ASTM lab test at an extreme 0.00004cfm/ft2 - i.e. a factor 100 better than Tyvek Residential or factor 25 better than Tyvek Commercial.

Zip's OSB Problem

OSB Sheathing

Building Science teaches us that as exterior sheathing gets colder, it is more prone to wetting. Sheathing is therefore at risk of moisture damage in cold climates. We also know that more permeable wall coverings (think GUTEX) dry faster than less permeable sheathings.

When OSB and plywood are tested in dry-cup conditions, i.e. relative humidity at 25%, both are Class III vapor retarders ("vapor semi-permeable") at about 1 perm. At 80% RH, plywood is around 5 perms and OSB stays below 2 perms. At 95% RH, plywood becomes even more permeable (10 perms), while OSB barely budges to 2 perms.* Note that when sheathing gets that wet, the risk of rot and mold is high, if the permeability is low.

But Zip OSB has a composition that makes this discrepancy even more pronounced. Perhaps as a result of its high resin content, the OSB permeability may not even go up at all. Third party laboratory testing has shown that Zip has "vapor permeability of less than 1 perm, under both wet and dry cup measurement conditions."** It appears that Zip can't rid itself of moisture when it most needs to.

It's a well-known fact that OSB is more vulnerable to moisture damage than plywood. That's because it's pressure molded from thousands of wood splinters, all of which are potential moisture conduits. For this reason, building scientists recommend keeping OSB below 15% moisture content (M%), compared to plywood or lumber which should stay below 18 M%.

Zip has an OSB problem. The water-resistant coating is married to a very low permeability OSB. Not surprisingly, OSB-sheathed walls have a long history of failure. The failure can start with water leaking from an improperly installed WRB or flashing, or exfiltrating interior moisture that condenses at the cold sheathing. And when the moisture is deposited on the OSB, it can't dry nearly as readily as plywood. UMass building scientist Paul Frisette writes, "OSB responds more slowly to changes in relative humidity and exposure to liquid water. It takes longer for water to soak OSB, and conversely, once water gets into OSB it is very slow to leave. The longer that water remains within OSB the more likely it is to rot." And Zip's vapor open coating doesn't do much to help.

Zip's Flashing Problem

With its elevated moisture risk, it would seem prudent to cover OSB in a bullet-proof waterproof protective layer.

Instead, Zip's flashing system does the exact opposite: tape is "reverse shingled" over butt-jointed seams, especially the most vulnerable horizontal connections. While Zip tape has passed muster in backyard tests, adhesive can never achieve the practical advantage of shingling overlay. In his blogpost Zippety Do-Don't, Michael Connor writes, "When the adhesive breaks down, water plants its flag in the opening, eventually claiming its victory, the spoils being rotted OSB and framing beyond".****

The history of water failure is largely that of failed flashing. In the case of Zip, the system depends on its weakest link. One improper tape installation, tape delamination or other failure, and the system fails.

If Zip tape were vapor open like Proclima's TESCON Vana (8 perms), then the system might have a higher rate of recovery. But Zip tape is backed with an impermeable carrier, blocking any exterior drying potential. If you do the math, roughly 10% of every Zip System wall is covered with vapor closed tape, and often the percentage is much higher. More impermeable tape = less drying at the Zip panel butt joints where failure is most likely to occur.

The Zip coating is another weak link in the system. Nails driven through the coating - an everyday occurence - create thousands of craters in the system defenses, each a conduit for moisture to seep into the interior flesh of the OSB.

Higher Insulation, Higher Risks

The moisture risks of OSB/Zip walls - and likewise the requirements for robust moisture control - increase as walls become more insulated, and the OSB gets colder and wetter. In conventional (non thermally broken) 2x6 walls, heat from the building interior can help dry the OSB. But with added thickness and thermal breaks - as with a double stud wall - the heat available for drying can be cut in half.

This so-called cold OSB problem has lately gotten a great deal of attention, as the popularity of super-insulated and airtight projects has grown. Here are some recommendations when planning projects going beyond 2x6 construction:

  • If you're building a double-stud wall, use INTELLO Plus as your interior smart vapor retarder.
  • For additional drying potential, move from OSB/Zip sheathing to more permeable plywood sheathing.
  • Because cold sheathing can accumulate moisture, be sure to include a ventilated rainscreen gap to maximize drying.
  • If locally available, use solid wood lumber sheathing for even faster drying than either OSB or plywood.
  • Use a high performance monolithic WRB such as SOLITEX MENTO or FRONTA QUATTRO, taped at seams & edges with TESCON Vana
  • Avoid impermeable insulation that can prevent inward drying, e.g. closed cell foam
  • If shear can be managed with diagonal bracing or other structure, SOLITEX MENTO Plus or GUTEX can be applied at the exterior, without sheathing.
  • TJI walls bring the structural sheathing layer to the warm, interior side of the wall - where airtight rated ZIP sheathing is safe to use

Zip Don'ts 101

Connor Homes

For Connor Mill-Built Homes, a VT modular builder, plywood is their sheathing of choice, and "truthfully the best and only sheathing that should be on a house, and is what we use as standard on a Connor Home!" Zippety Do-Don't is an entertaining critique of questionable claims from the manufacturer:

  • "Added Structural Durability" - Added to what? It's OSB, in most respects inferior to the same thickness plywood
  • "Superior Moisture Protection" - Superior to correctly shingled membranes? Not so much
  • "Superior Air Protection" - No more airtight than taped Mento 1000
  • "Saves labor" - Depends on which marketing video you watch
  • "Saves money" - Zip costs more than ½” CDX five-ply fir plywood sheathing, which is far more vapor open, contains less glue and more wood.

DuPont Corporation also busts some myths about ZIP. As you would expect, it also makes a case for traditional (micro-porous) building wraps:

  • Housewraps are in fact easier and quicker to install well
  • Housewraps are more durable (ZIP panels must be handled with care and stored appropriately)
  • Housewraps have superior water resistance
  • Housewraps have better drying potential
  • Housewraps better resist air infiltration

The SOLITEX Difference

SOLITEX Mento 1000 over plywood
SOLITEX Mento 1000 over plywood

Traditional housewraps depend on microscopic pores in the woven membrane for vapor permeance. But at low vapor pressures, these small openings resist vapor diffusion. Hence, there is a high likelihood of moisture build-up filling the pores and blocking vapor movement.

Which brings us to truly high performance weather protection: monolithic, non-porous SOLITEX Mento and FRONTA QUATTRO WRBs.

Instead of tears or pores, with a monolithic membrane it's the molecular structure of the active TEEE layer that transports the vapor. With this advanced technology, the vapor moves reliably even at very low vapor pressure differentials, avoiding the danger of blockage, as demonstrated in the video intro to SOLITEX MENTO 1000 below:

And unlike traditional housewraps, SOLITEX's water transport is not compromised by surfactants found in cedar siding, oils, soap and dust - all common at the job site with the potential to reduce the waterproof properties of conventional micro-porous membranes.