Continuous exterior insulation (CEI) is an added layer of insulation to the exterior of a wall assembly, installed to the outside of the structural sheathing and behind the cladding. The material options for CEI range from rigid foam to mineral wool to wood-fiber boards, and all have their own specific characteristics and advantages. There are also many ways to install CEI and integrate windows and doors, flashings, rainscreens, siding, and trim. There may be 10 perfectly good ways to build a wall with CEI, but the specific one that you end up choosing will depend on what your team is familiar with, what materials are available, the project budget, the project’s climate zone, and your risk tolerance.
Incorporating all the details required for continuous exterior insulation (CEI) can be a bit of a head-scratcher if you’ve never built this way before. It’s more complex than the typical 2×4 or 2×6 code-built wall that many builders are familiar with, and it can affect the placement ofthe four control layers. In this article I hope to clarify how to choose and install CEI based on some simple building science principles and share two examples of real-world installations with window details.
CEI adoptees
At Birdsmouth Design-Build in Portland, Ore., we believe there are five non-negotiables for any new home or remodeling project: durability, health, resiliency, comfort, and energy efficiency. We often include CEI as part of our wall assemblies because it addresses all these goals in our climate zone, Marine 4. But CEI can work in all climate zones. It is an effective strategy for builders in cold climate zones looking to improve durability, comfort, and energy efficiency, and those in hot-humid climate zones looking to manage interior moisture and create efficient and comfortable homes. In some areas, it may be the easiest path to meeting the most current energy codes in the 2021 International Residential Code (IRC). And it can be an effective way to improve an existing home, too, particularly when re-siding.
| Choosing continuous exterior insulation The choices for exterior insulation can be daunting, especially when considering how it will affect a particular wall assembly. There are rigid plastic-foam options such as expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso), with manyvariations in thickness and facings. There are also mineral wool and wood-fiber board products. Beyond these common types, there’s cork, fiberglass board, hemp, and wool insulation, among others. The characteristics to look for in a CEI material option depend on your project goals, but generally include R-value, permeability, hydrophilic/hydrophobic profile, compressive strength or density, air permeability, and embodied carbon emissions, among other environmental considerations. It’s also important to know how long a material can be left exposed to the elements before degrading, whether it experiencesthermal drift (the ability for its R-value to degrade with time or temperature), its fire-resistance class (typically denoted as Class A, B, or C with A being the most fire-resistant), its cost, and its availability. Other things to consider include how a material is cut and what tools are needed to install it, whether the material is difficult to handle (such as mineral wool, which requires gloves and a mask to install), the type of fasteners are needed to install it, and if the material has a profile such as an interlocking tongue-and-groove edge. Any one of these considerations may tip the scales in favor of one material over another. If you’re building in a Wildland Urban Interface (WUI) zone, the fire rating may take precedent over all others since it will be mandated by the local jurisdiction (and good sense). In other circumstances, a product may be unavailable locally, or the cost may be prohibitive. In others still, a particular vapor permeance may be beneficial to manage the amount of water vapor entering or leaving an assembly, or the environmental impacts may trump all others from the client’s standpoint. The labor needed to install a more compressive material such as Type 1 EPS or mineral wool, along with the cost of fasteners, may drive a builder toward a less compressive material such as XPS. Avoiding tiny white balls of EPS foam microplastics all over a job site might be a priority; the decision might come down to the ability to cut a crisp 11° angle in cork. All that is to say, the choice is often based on a variety of factors. Rigid foam: From the polyiso shown (a class I vapor retarder due to its foil facing), to EPS products which can be as vapor open as 5 perms per in., plastic foams represent a range of often readily available and affordable options for CEI. Their R-values range too, from about R-3.5 to over R-6 per inch. Mineral wool: This fibrous board insulation is vapor-open, hydrophobic, and, because it is essentially rocks, a good choice in wildfire-prone areas where including it in a wall assembly may help you meet specific fire ratings. It has an R-value of around R-4 per in. Wood fiber: With similar characteristics to mineral wool, plus high compressive strength and a relatively low carbon impact, wood fiber is another option for CEI. Long available in Europe and gaining popularity with U.S. builders, these boards also have tongue-and-groove edges, which offer installation advantages. R-values are around R-3.5 per in. C****ork: Another option for low embodied carbon, cork is a renewable resource and a viable option for CEI. It is vapor-open and has a Class A fire rating—the same as stainless steel. Its R-value is comparable to wood fiber and EPS rigid foam. We have also found it easy to work with. Ensuring building enclosure durability Among thebenefits of continuous exterior insulation, one stands above the rest: increased durability. Moisture is a home’s biggest enemy, and CEI makes a wall assembly more durable by controlling moisture accumulation at what is typically the least permeable layer of the wall: the sheathing. This isn’t bulk water, which is handled with a water control layer; of concern here is water vapor. In cold climates during the winter, the air inside of our homes is more humid than the outdoor air. Because of the laws of thermodynamics, water vapor wants to equalize and travel from high concentrations to lower concentrations—a phenomenon called vapor drive. This means that vapor drive in buildings tends to move from inside to outside during winter in cold climates. In hot-humid climates, vapor drives from outside to inside. In both instances, vapor will travel through materials until it reaches a surface that’s the least vapor-permeable, where it may accumulate. If it’s not able to dry, it can lead to mold, rot, and eventually, structural failure. In a wall assembly, the vapor control layer is the one that is the least permeable (more on permeability to come). In many walls, this is the sheathing, which can either get frost on it or get damp if it is cold enough for long enough. With the right R-value for the climate zone and wall assembly, CEI keeps the sheathing warm enough that this phenomenon is greatly reduced or eliminated, and the potential for mold growth and degradation is reduced, too. This is why a wall with CEI is one of the most durable assemblies you can build. Resisting heat transfer Of course, durability isn’t the only benefit of continuous exterior insulation. Another important benefit is increased R-value and reduced energy losses via thermal bridging. Thermal bridging is the movement of heat energy via conduction from warm areas to cold areas. In common wood-frame wall construction, with studs every 16 in. or 24 in. on center and insulation only in the stud cavities, the studs (and plates, headers, etc.) have a R-value of only about R-1.25 per in. Even though the cavity insulation can be as much as R-5 per in. or higher, heat energy will travel along the more thermally conductive studs and be lost to the universe if there is no thermal break. This is where CEI comes into play. In a typical 16-in.-on-center wall, about 25% of the wall is solid wood. This represents a large potential for thermal losses. By placing CEI to the outside of the wall, this thermal bridge is broken and the energy losses are stemmed significantly. This not only leads to a more energy-efficient building with smaller HVAC loads, but also creates a more comfortable home for the occupants. It’s like having a warm sweater wrapped around your building. Breaking the thermal bridge Installing flanged windows on walls with CEI can be a head-scratcher. How do you attach the flanges if there is insulation in the way? How do you flash the window to the water control layer? What about connecting the air barrier? If the CEI is compressive, how does the flange not squish into the CEI or get damaged during installation? ThermalBuck is a product designed to address all these issues and simplify the installation of flanged windows in a CEI wall while breaking a common thermal bridge. It extends the mounting point to be flush with the continuous exterior insulation or rainscreen battens and allows for a successful integration of the four control layers. The installation of the flanged window with this product is straightforward and should be familiar to builders. One difference is that the window fasteners are longer than a typical installation and penetrate through the buck and directly into the framing of the window rough opening. ThermalBuck has been lab tested and shown to carry large, heavy windows with very little deflection, and has passed lab testing for air infiltration and water penetration, so there should be no worries about its structural performance or issues with compromising the control layers. Planning for permeability The materials that you may consider for CEI have a range of properties from R-value to fire resistance, but one characteristic will have a significant impact on how a wall assembly works, and that’s permeability. There are some products, such as mineral wool or wood-fiber board, that are vapor open—meaning, water vapor can readily pass through them. Others, such as foil-faced polyisocyanurate, are considered vapor-closed—meaning, water vapor isn’t effectively able to pass through. The vapor permeability of a product is expressed in perms. The lower the number, the less permeable the material is. It’s important to note that the perm rating of a product is often related to its thickness, and increasing the thickness will decrease the permeability. For example, extruded polystyrene rigid foam (XPS) may have a rating of 0.52 perms per in. If you install 2 in. of the product, the rating halves to 0.25 perms. If you add another inch, it halves yet again to 0.125. At this point, water vapor will effectively be stopped from passing through the material. This isn’t necessarily a bad thing, but it is important to know that a wall with 3 in. of this XPS will not be able to dry to the exterior, so you’ll need to be sure that it can dry to the interior. Having a low-perm material on the exterior in hot-humid climates can be an effective strategy because here, vapor wants to move inward for much of the year. This mitigates water vapor diffusion from the moister outdoor air and the potential for accumulation on the cool back side of the drywall. A good rule of thumb is to create a warm-side vapor-control layer, which essentially means that the lowest-perm material is always on the warm side of the assembly, no matter the climate. Another rule of thumb is to avoid a wall that can’t dry in either direction (sometimes referred to as a “double vapor barrier”). The classic example is a thick layer of foam insulation on the exterior and vinyl wallpaper on the interior. Unable to dry, any moisture that gets into this assembly can quickly turn the wall into mulch. Finally, you may also see vapor permeability expressed in classes, particularly as the IRC refers to “vapor retarders,” which are materials used to slow or limit vapor drive. There are three classes: A Class I vapor retarder has less than 0.1 perms. A Class II vapor retarder has between 0.1 and 1.0 perms. A Class II vapor retarder is between 1.0 and 10 perms. Anything over 10 perms is considered vapor open—it doesn’t appreciably slow the movement of water vapor. So, to know if your CEI qualifies as one of these classes of vapor retarders, you’ll need to know its perm rating. Knowing relationship with water Another aspect of CEI materials is whether they are hydrophilic or hydrophobic. This refers to the material’s ability to either wick water (phylic, or “love”) or shed water (phobic, or “aversion”). Continuous exterior insulation such as paper-faced polyiso is hydrophilic and will happily lap up water. Others, such as some of the wood-fiber boards, are also more toward the hydrophilic end of the spectrum. When using insulation materials such as these, it is smart to place the water-resistive barrier (WRB) to the outside of the CEI as this will keep bulk water from damaging the insulation. Of course, you can place the WRB outside of any CEI, and there may be good reasons for it. For instance, if you want to install a flanged window and detail the WRB and flashings in a manner that most closely resembles a typical install, you can put the WRB to the outside. Other CEI products such as mineral wool or XPS are hydrophobic and their performance isn’t significantly affected by water. With these products, you could install the WRB to the inside of the CEI, but each material will not act exactly the same. Mineral wool will readily drain water through itself, while XPS can trap bulk water between itself and the sheathing. If you are using a plastic-foam product like XPS, assume that at some point in the building’s life, water will get behind it. This is why it is recommended to use a bumpy or crinkled WRB behind the foam, to create enough space between the insulation and the sheathing to reduce hydrostatic pressure and allow water to drain via gravity. This has no appreciable effect on the thermal performance of the continuous exterior insulation. Of course, if you are using a drainable WRB, you will want to detail the flashings to the WRB, so windows or window bucks will be installed at the structural sheathing plane. | --- |