Windows and Doors
For window and door installations, we use the same basic flashing techniques with Zip sheathing that we use for walls wrapped with housewrap. The one big difference is that we're ultimately relying on the adhesive bond between the tape and the sheathing to keep water out of the wall. First, we cover the rough sill with 6-inch Zip tape or less-expensive 9-inch Vycor or Moistop, lapping the flashing out and over the sheathing. At the bottom corners of the opening, we use either a flexible flashing tape or Vycorner prefabricated self-adhering corners (866/333-3726, graceconstruction.com). Then we caulk the back of the unit's nailing fins with a flexible adhesive and set the window into the opening. Next, we flash the fins to the sheathing, making sure to firmly J-roll the flashing tape to the wall and nail fins. Finally, we install the head flashing over the trim at the top of the window, taping the Z-metal to the wall with Zip tape.
Huber's 6-inch-wide Zip tape costs a little more than the 9-inch-wide butyl- and asphalt-based peel-and-stick flashing tapes we typically use, but I think it's a far better product. We've left windows and doors flashed with Zip tape for over a month without a single corner or edge peeling off the sheathing (a problem we've had with many other peel-and-stick tapes). Unfortunately, Zip tape is approved as a flashing tape only with Zip sheathing and might be red-tagged by your inspector on regular OSB or plywood.
Normally, we frame roofs 24 inches on-center with 7„16-inch roof sheathing, so we have to use H-clips. But the Zip roof panels we use are true 1„2-inch OSB with a 3 2/16 span rating and they don't require edge support. This simplifies installation somewhat but presents a potential problem: Since the long edges are self-spacing, the spacer lips on the sheathing can sometimes overlap, lifting the edge of the uphill sheet slightly. To avoid this problem, we begin nailing along the bottom of the sheathing at each rafter to pull the sheathing tight to the framing before nailing off the rest of the sheathing.
In dry weather, Zip roof panels provide plenty of traction, thanks to the grit in the overlay. The panels are slicker in wet weather, but not noticeably more so than regular OSB or plywood sheathing. In most conditions, we can comfortably frame up to about an 8/12 roof without using toeboards.
Taping. On roof pitches less than 8/12, taping roof seams is a very quick process. For example, it takes about 15 minutes to tape and J-roll a 20-foot-by-30-foot section of 7/12 roof.
On roofs steep enough to require toeboards, however, I think that the benefits of Zip sheathing are outweighed by the extra labor required to tape the seams. On a recent 12/12-pitch roof, for instance, we needed more toeboards to get leverage on the tape gun and J-roller than we needed to actually sheathe the roof. And, of course, all of the nail holes from the toeboards have to be taped. In fact, it probably takes twice as much labor to sheathe and tape a steep Zip System roof as it does to sheathe the roof conventionally and roll out underlayment. So in the future we'll stick to OSB sheathing for steep roofs.
Underlayment. Manufacturers and local codes may require an additional underlayment for shallow-pitched roofs, depending on the roofing material. There is a two-layer underlayment requirement in most building codes for roof slopes between 2/12 and 4/12, and a taped Zip roof replaces the first layer only. Also, when it comes time to strip off the old shingles and reroof, new underlayment will be required over the Zip sheathing regardless of the roof pitch.
Eaves, valleys, hips, and ridges. The wider 6-inch Zip tape is ideal for valleys, though Huber says that overlapping the 3 3„4-inch-wide tape by at least 1 inch is okay, too. In some areas, code may require an additional self-adhering valley flashing. Hips and ridges are also taped, and we tape step flashings to sidewalls where necessary.
At the eaves, ice barrier membranes may be required in certain climates. They're not required in our coastal climate, but my roofer tells me that they would be if we were building farther up in the mountains. Huber allows any self-adhering ice barrier with an ICC-ES report (or equivalent) to be used over the Zip sheathing.
In my area, Zip System sheathing tends to cost about twice as much as comparable OSB sheathing. Currently, we're paying $7.67 per sheet for OSB sheathing and $14.63 per sheet for Zip wall panels. Zip roof panels are twice as expensive as both AdvanTech and Edge Gold roof sheathing. After several houses, we've learned that the installed cost for Zip wall sheathing - including the labor to install the tape but not the cost of the tape itself - is roughly equal to the installed cost of OSB plus Tyvek. The extra cost, then, is in the tape.
About one box of tape (a box contains 12 rolls, at 90 lineal feet per roll) is needed to seal a unit (70 to 80 pieces) of 4x8 Zip panels; for a typical 100-sheet house, that's 16 rolls of tape. In addition, we use two or three rolls of Zip tape for flashing windows and doors and taping step flashing and Z-metal to the wall. Tape costs around $28 per roll, so using Zip panels adds about $500 to the cost of a 3,000-square-foot house.
We're not convinced that Zip roof sheathing is cost-effective (particularly on steep roofs), since our roofing sub can roll out underlayment and cover the roof completely the same day he starts shingling. If we were trying to build tighter homes in an area that had a colder climate and a stricter energy code, though, the added cost might make more economic sense.
But with or without the energy savings, we're sold on Zip wall sheathing. We can tape the walls while we're framing and rigging them to be lifted into place, so we don't lose any time; and we avoid the ladder work that would be needed to hang housewrap. In fact, we eliminate about a day in the construction cycle that would typically be spent installing housewrap, detailing it correctly, and then - later - fixing rips in it caused by wind or by other tradesmen stubbing pipes or wires out through the walls.
Also, since we protect the seams with tape as we frame, the panels don't absorb moisture. This saves both time and money, since in our climate we might sometimes spend as much as $1,000 to run kerosene heaters and dehumidifiers to dry out an OSB-sheathed house before insulation.
Tim Uhler is a builder in Port Orchard, Wash.
About Zip System Sheathing
Zip System sheathing combines OSB structural panels with a layer of resin-impregnated kraft paper. The .01-inch-thick overlay is permanently bonded to the outside face of the panels in the press as the OSB is manufactured. The panels are manufactured for use as wall and roof sheathing; the roof sheathing panels have a gritty surface texture to improve traction. Roof panels come in 1„2-inch and 5„8-inch thicknesses in a standard 4x8 size; wall panels come in 7„16-inch and 1„2-inch thicknesses, in sheet sizes from 4x8 to 4x12, according to literature published at the manufacturer's websites (huber wood.com, zipsystem.com). Wall panels are available either square-edged or with a self-spacing machine-edged profile; roof panels can be tongue-and-groove, square-edged, or machine-edged.
Zip wall and roof panels consist of OSB structural sheathing with a water-resistant, resin-impregnated kraft facing.
Getting rid of housewrap? The ICC evaluation report for Zip System wall sheathing states that it is "used as combination wall sheathing, air barrier, and water-resistive barrier" - no doubt a big reason for its increasing popularity in some parts of the country. While housewrap can be installed quickly by anyone, it's rarer to see it installed well, and not uncommon to see it torn and flapping in the breeze - which would be impossible with a bonded WRB. Of course, the "system" requires the application of tape, which is time-consuming and expensive. But for builders looking for an airtight shell, the tape is just another part of the attraction. And though housewrap may be omitted, some builders using Zip wall panels include it anyway, because they are reluctant to rely on taped joints to permanently keep out water and because they want the additional water-resistant layer for weaving in cap flashings over windows and doors (see "High-Performance Homes on a Budget," 01/11).
Huber claims a vapor permeance of 12 to 16 perms for the overlay on the Zip wall panels. That means a wall sheathed with Zip panels will have a vapor profile similar to that of a wall sheathed with conventional OSB and covered with Typar, which has a permeance of around 12 (by comparison, Tyvek is 58 perms). - The Editors