This post first appeared on the Green Building Advisor Website.
Right now, there’s a lot of interest in the residential construction market around continuous insulation (CI) strategies. CI adds a layer of complexity to a build, and with it, questions. How much insulation do I need? How to fasten the insulation? How to fasten the cladding? How to integrate windows and doors? What about vapor control? And, this question, should the water resistive barrier (WRB) be at the sheathing layer or outboard the insulation? The answer, it depends, but you have a lot of choices.
There are many different products and methods we can use for the installation of continuous insulation, choosing one will depend on the experience of the installation crew, the requirements of the project (R-value, air tightness, and even cladding choices). Budget and environmental impact of the products used can also affect decisions. Let’s start with a few options that make some of these decisions for you, continuous insulations integrated with the water control layer.
One of the most well-known options that combine an integrated WRB and insulation is Huber’s Zip-R , which has been around for several years. This product makes the decision of where the WRB should go for you. The panels consist of an OSB sheathing with an integrated (factory installed) WRB, bonded to a layer of polyisocyanurate insulation. Nail the panel to the wall (to Huber’s specifications), seal the seams and treat the planned penetrations, and you are done. For the most part, the installation is simple.
There are a few limitations to the product, areas subject to earthquakes or high winds may require additional structural support and I’ve heard of a few structural engineers who are apprehensive about having a layer of insulation between the framing and sheathing, but in most cases, this is a code approved assembly.
A product that is similar to Zip-R is LP’s NovaCore. For now, NovaCore does not have the integrated WRB, it is simply a sheet of oriented strand board (OSB) bonded to insulation, in this case, the insulation is extruded polystyrene (XPS). It’s installed like Zip-R, but you will need to add your own water resistive barrier.
Both the Huber and LP products are considered nail base panels, but there is a difference between these two products and a traditional nail base. The original nail base panel is one sheet of OSB (usually) bonded to a sheet of insulation with the panel assembly being fastened to an already sheathed wall. Let me repeat, a nail base panel will be installed to a wall that has already has sheathing, the first layer of sheathing provides the structural requirements, the nail base is the insulation and a surface to attach most claddings. The insulation is usually expanded polystyrene (EPS), but other insulations are available, I’ve seen insulation thicknesses of up to 12”. This type of CI will require a separate WRB to protect the exposed outer OSB.
Another product that combines sheathing and insulation is called structural insulated sheathing or SIS. There are several manufacturers of this product, OX is the brand I am most familiar with. OX states that their OX-IS is four products in one, structural support, water barrier, air barrier and insulation. The insulation layer is a polyisocyanurate rigid insulation (from R-3 to R-9) bonded to a “fibrous sheathing” panel that ranges from 0.113 to 0.135 inches (around 1/8”). The product is installed with insulation to the exterior, fastening to the wall requires penetrating the foam layer with the fastener every 3” on the edge and either 3” or 6” in the field. There may need to be additional structural connections based on engineering requirements. All seams are taped using the OX brand flashing tapes. OX claims that field fasteners do not require sealing. With SIS products, manufacturers claim the exterior insulation, after being sealed using tapes, meets the requirements of a water resistive barrier, no additional WRB is required.
How about a product that is not structural, that combines insulation and a dedicated WRB into one product. I am aware of two products currently on the market. One has been around for a few years and is called ThermalTight (from the makers of ThermalBuck). ThermalTight has a graphite polystyrene insulation (GPS) bonded to a WRB that acts as both the water and air control layers. Panels come in 4×8, 4×9 or 4×10 options and vary in R-values, R-5, 7, and 10. The WRB has a flap on two sides where it overlaps onto the adjacent panel. A sealant is then used to make the flap connection weathertight. ThermalTight will be installed over sheathing or some other structural assembly. Graphite Polystyrene GPS Insulation Experts | ThermalTight
A similar product that has just been introduced is manufactured by Henry, called VPTech. This insulation and WRB in one also features GPS foam insulation. The insulation is bonded to Henry Blueskin, a self-adhering WRB (Basically a giant piece of vapor permeable tape). There are self-sealing flaps on two sides that connect to adjacent panels, the way these panels are sealed between each other is through the adhesive in the Blueskin. No additional sealants are needed between the panels. For now, the panels are only available in 4×8 sheets at a 1-inch (R-5) thickness. Blueskin® VPTech™ | Henry Company
Many of the products that have built-in WRB’s will have a reverse lap seal around windows, doors, and other through-wall openings. Their main benefits are to simplify installation and to reduce labor costs by minimizing trips around the house.
How about a more common continuous insulation strategy of installing an insulation product over traditional sheathing. An example would be XPS, EPS, GPS or Polyiso insulation over a plywood or OSB wall sheathing. Where should the WRB go? Or can the insulation alone act as a WRB? Here’s where you have choices. Many of the foam insulation manufacturers have details about how the insulation can be used as a water resistive barrier. Seams will need to be taped, typically using a tape specified or manufactured by the insulation manufacturer. Penetration such as windows, door, and other holes through the insulation will also need to be sealed per the insulation manufacturer’s instruction. This will result in a reverse lap seal; tapes and sealants are the only products keeping any water that ends up behind the cladding from entering the wall assembly. Some builders are uncomfortable with this assembly. Not a problem, change the installation to a traditional WRB over the insulation.
Builders will be familiar with installing a mechanically attached WRB over sheathing, something we’ve been doing for decades. Installing over exterior insulation only has one difference, the length of fastener used to hold the WRB against the wall. Cap nails or some sort of long cap staple would be used. You’ll want to follow the WRB manufacturer’s fastening instructions for installing over foam. The rest of the installation should be the same as installing over standard sheathing, shingle lapped and integrated into windows, doors and other envelope penetrations.
What about installing a WRB at the wall sheathing layer rather than over the insulation? That will work too. You may want to consider a WRB product that addresses any moisture that may accumulate between the foam and WRB. A drainable WRB (one that creates a small space between the insulation and WRB, such as Tyvek’s Drainwrap or Benjamin Obdyke’s Hydrogap) might be a good choice. You could also use a WRB integrated panel (such as Zip or Weatherlogic). Another choice would be a fully adhered WRB (like Henry Blueskin or Siga Majvest SA). Even a roll-on or fluid applied product would work. All these products will work under a layer of CI. Something to remember, building codes do reference the integration of the WRB into windows, doors and other projections in cladding systems. Any water that finds a path behind the cladding (and potentially the continuous insulation) will need to be kicked back out to either the face of the cladding or WRB. This code language is found in section R703.4.
What if you choose to use an air permeable or fibrous type insulation (such as Rockwool ComfortBoard) for continuous insulation? Again, you have choices. The most common method (and the one specified in many of the detail drawings on Rockwool’s installation instructions) is installing the WRB at the sheathing layer. I’ve used this detail in the past. Most water that ends up inside the cladding will drain down the face of the insulation (Mineral wool insulations are hydrophobic in nature). Any moisture that does end up being absorbed quickly moves through, draining out the bottom. Just make sure you have a plan on how to address any penetrations.
So, Where Should the WRB Go?
The purpose of the WRB is to protect the moisture sensitive building materials in our wall assemblies. As long as it’s protecting those materials, the WRB can go pretty much anywhere you’d like it, inside the continuous insulation or outside. Personally, I prefer to put it where it makes it easiest to integrate windows, doors and other planned opens, unless the WRB is also your primary air control layer? Then my preference moves it to the sheathing layer. A more difficult assembly, but also more robust and durable. Some products make these decisions for you, others you will need good planning. Just remember, you have choices.