This past year I had the opportunity to build a SIP and timber-frame house designed by architects Aamodt & Plumb and built in conjunction with Bensonwood Homes, the famous timber-frame company in New Hampshire. (Side note: I first heard of Bensonwood while watching This Old House in high school. It’s a dream come true to build a house with their timber/SIP package.)
Bensonwood has been using Zip System sheathing in their SIPs (Structural Insulated Panels) for a few years now, and has really liked them for several reasons.
Why use the Zip System?
1. Easy installation. Huber Zip is pretty darn easy to install for your carpentry crew. The tape they sell for sealing the seams is really top notch, too. I’m very impressed with this tape after my first project. It’s an easy system to learn because they aren’t as concerned about the shingle effect or where to layer things. Simply butt the panels and tape them.
2. Airtightness. I believe that airtightness is really the missing link in terms of efficiency for many builders across the country. This house we did with Zip System sheathing blew a very good blower door test. (Stay tuned to find the results.)
3. Labor savings. The Zip materials themselves come at a premium cost compared to some other “housewraps,” but the big savings with Zip happens with labor. A Zip System house requires one trip around the house compared to two with traditional systems.
4. Early waterproofing. This is a big one for me, especially with the roof panels. Usually after my framers lay the plywood or OSB roof sheathing, I’m waiting a few days for the roofer to paper the roof for a temporary waterproofing till they come back to install the roofing. With Zip, you have them deck the roof and tape it the same day! Drying the house early in the framing process is really nice, and frankly pretty important if you’re building a house in under 6 to 9 months.
After using Zip on my first house, I’ve become very comfortable with the system overall. But I think there are some small modifications that need to be made to ensure maximum durability of your house for the next few hundred years. Let’s look at this Bensonwood project to see how we modified the Zip system to suit my desire for ultimate durability.
Tip #1: Peel-and-stick roofing underlayment is key
Early dry-in with Zip tape is great, but I’m a big believer in installing a full Ice & Water Shield product on your roof as underlayment, especially if you have an unvented roof assembly.
This SIP-framed roof has 12-inch I-joists filled with cellulose insulation, and I need to ensure they stay dry for the life of this house. I typically use Carlisle’s WIP 300HT for my roof underlayment. This 40-mil product is rated for high temperatures and has some self-gasketing abilities.
The peel-and-stick underlayment overlaps the Zip wall panels by about 3 to 4 inches. To create overhangs on this house, the architects designed a very cool “over-roof” with 2-bys.
The roof has continuous ventilation under the metal from eave to ridge.
Tip #2: Install base wall flashing
In almost every house I remodel, I find rot at the base of the outside wall sheathing. I believe you must use a base wall flashing to protect the raw edge of whatever sheathing product you use.
I’ve used Carlisle’s CCW 705 for a few years now. (Here’s a link to a blog post for more information about how to use CCW 705.) You’ll see that we used Zip tape on top of the Carlisle 705 to ensure it would stick properly to the Zip wall. That gave us about 16 inches of extra protection at the bottom of this slab-on-grade house.
This blue base wall flashing will add tons of durability to your Zip house.
Tip #3: Create a rainscreen gap behind all your claddings
The rainscreen approach calls for an air gap behind your cladding (siding, stucco, brick, etc.). In my opinion this air gap makes a huge impact to the durability of your house.
Any water that gets behind your cladding will encounter the air gap layer that encourages drainage and drying. Simple — yet it’s rarely done in this country. If you build with Zip and add an air gap (a rainscreen), I expect your house to be in great shape 100+ years from now.
This house has a mixture of stucco and wood siding, so here’s how we got a rainscreen air gap.
The 3/8â€³ rainscreen gap behind the wood siding was created by Cor-A-Plast battens with a folded over insect screen at the bottom. This wall is ready for siding.
Face-screwed siding makes it really easy to pull the siding off later if we ever had a reason. The wood siding has a charred finish called shou sugi ban in Japan.
In the photo below, the furring strips on the wall on the right will be covered with wood siding. The drainage mat (Delta Dry Stucco & Stone) on the left wall will be covered with stucco.
For a more in-depth look at Delta Dry’s rainscreen system for stucco, follow this link to a blog post from earlier this year.
Follow this advice and I think you’ll find that Zip is a great system. By the way, this house blew a 0.92 ach50 in our blower door test! Just shy of the Passivhaus standard and five times tighter than code requirements for Austin, Texas.
I like an exterior taped system — especially when I can add my extra tips for durability!
Here’s what the traditional three-coat stucco with LaHabre integral color finish looks like. The rolling cedar shades look awesome!
P.S. I didn’t make a specific mention of this in my list of tips, but remember always to set your windows with a sill pan underneath. Zip has directions on their site for making window sill pans with their products, or you go with my standard method using DuPont Flexwrap. No matter what your choice of materials, never rely on face sealing alone for windows. Make a pan so any future window leaks will drain to the outside and not get into your structure.
Matt Risinger is a custom builder and whole-house remodeling contractor in Austin, Texas, specializing in architect-driven design and fine craftsmanship. His company, Risinger Homes, uses an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy, and durable homes. You can read more examples of Matt’s writing at Matt Risinger and the Green Building Blog.