If you’ve been following us for a while, you probably know we’re big fans of building products that serve multiple purposes. Today we look at versatile sheathing products that go beyond simple structural support and contribute to the “Hybrid Prefab” method of building that we continually strive for. We’ll take a look at two products: Huber Engineered Woods’ ZIP System and DOW’s Structurally Insulated Sheathing.
ZIP System Sheathing by Huber Engineered Woods
The first product, ZIP System Sheathing, acts as (1) a structural sheathing and (2) a Water-Resistive Air Barrier in one product. You can see in the image below that we are using this on our current Skinny Project. The product consists of two different structural panels (one for the roof and one for the walls) that is coated in a liquid applied WRB (Water Resistive Barrier) on one side. The wall panels are green and the roof panels are a copper color. The system is completed by a proprietary butyl tape for the seams that makes the entire assembly water and air tight.
We decided some time ago that we would start taping all exterior seams in our OSB sheathing to improve the air-tightness of our homes in the quest to reach the illusive 0.60 ACH @ 50Pa that accompanies official Passive Houses. The ZIP system was the next logical step that allows us to eliminate the step of installing an additional building wrap this is prone to leaking anyways. The ZIP system adds a few hundred dollars to our overall cost that we most likely would be spending anyways on a better butyl tape to stick to unprimed OSB as well as a better WRB than Tyvek. Using ZIP allows us to completely eliminate this step, saving us time, money and aggravation. The framing crew couldn’t be happier either as they hate installing building wraps. We’ve included some geek specs for you below.
ZIP Geek Specs:
Assembly Air Infiltration: 0.0072 cfm/ft2 @75Pa
Assembly Air Infiltration: 0.0023 cfm/ft2 @75Pa
Permeance: 2-3 Perms
Recognized Water Resistive Barrier
ZIP System Tech Docs
SIS (Structurally Insulated Sheathing) by DOW
The next product is similar to ZIP in that is acts as both a (1) structural sheathing and (2) an airtight water-resistive barrier, but it adds (3) exterior insulation to the mix as well. The DOW SIS product is made up of a thin structural element that looks a bit like particle board, a layer of rigid Polyisocyanurate insulation (think closed cell spray foam) and a blue top layer for improved water resistance. The product comes in a 1/2″ thickness that is rated R-3 and a 1″ thickness that achielves R-5.5. Once installed, using DOW’s Weathermate Tape will give your building envelope an airtight WRB that is ready to clad.
What’s great about the DOW’s SIS is that it can create a continuous layer of external insulation on your building which is pretty much the most effective way to completely eliminate thermal bridging in your exterior walls. This continuous layer of insulation is already required by building code in Canada and some northern US states (I believe). It may not be long until the entire US building code starts requiring this and what better way to achieve it than with a product that actually reduces overall labor?
SIS Geek Specs:
Panel Air Infiltration: 0.05 cfm/ft2 @75Pa
Panel Air Exfiltration: 0.1 cfm/ft2 @75Pa
Permeance: <0.3 Perms
Recognized Water Resistive Barrier
Water Absorption (% by weight): <9
DOW SIS Tech Docs
So which product is best and should you consider using either on your next building project? If you are not using one of the prefab wall systems like SIPs or ICFs, then we would highly recommend incorporating one of these hybrid sheathing products into your designs. Both are carry premiums in materials price, but it’s really not a bad as you would expect once you factor in the improved building performance and reduced labor from skipping the building wrap application. I know we were expecting higher prices than we got back from our material supplier on Skinny, Shelly’s Lumber.
If air sealing is a top priority in your build, then most building wraps are just not doing that great of a job unless they are applied to meticulous commercial standards. The wrap is often tearing or pealing off in sections. Almost no one tapes all seams as directed and the installers punch hundreds of tiny holes in them when installing with a slap stapler. Using either of these products to achieve an airtight home is also going to improve the performance of any insulation installed within the wall cavities. Air infiltration, especially in fiberglass insulation, can decrease the effective R value of the insulated wall by 15 – 40%. Most homes in the US could reduce their utility bills by 30% if they used one of these products from the beginning.
Of the two products, we’re looking pretty hard at the SIS for the future. It will add a few hundred dollars to each home compared to ZIP, but we will gain continuous insulation on the outside of our building lot footprint. In other words we are gaining free real estate for our wall insulation that does not decrease the interior square footage of the home with a thicker wall. The cost is also far lower than having an insulation contractor come in and spray an inch of close cell spray foam on all of your walls. The only downside is that it only comes in a 1″ max thickness. Most of the US requires 1″-2″ thicknesses of exterior insulation in order to eliminate condensation concerns in the walls. We are much closer to 2″ in our climate.
Whatever you choose. seal it well and insulate the heck out of it. We hope this post helps in your decisions making process.
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