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 many variations 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 experiences thermal 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.
Cork: 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 the benefits 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.

Thickness is about R-value

There are several factors that influence the optimal thickness of CEI. It may be the local jurisdiction’s energy code, the cost implications of using thicker continuous exterior insulation and long fasteners, the climate zone, energy-efficiency targets, program goals such as Passive House or LEED, the relative thickness of the CEI in relation to the thickness of the interior insulation, embodied carbon, or aesthetic choices. From a durability perspective, you want to make sure the CEI is thick enough to keep the sheathing warm and prevent moisture accumulation. And this is why it’s more important to think about R-value than thickness.

One thing to be cognizant of is that the greater the R-value of the interior cavity insulation, the colder the sheathing is likely to be in the winter. It is important to be sure that the proportion of interior to exterior insulation isn’t decreasing the effectiveness of the CEI from keeping the sheathing warm. It is possible to calculate this ratio based on the R-value of a wall’s cavity insulation, or you can reference Table R702.7(3) in the IRC, which provides minimal R-values for CEI by climate zone for 2×4 and 2×6 walls.

From a practical installation perspective, any exterior insulation more than 1-in.-thick will require a nontypical-length fastener, which can get expensive, require special tools, or be time consuming to install. To support most claddings on furring strips or battens, the fastener must be set at least 11/2 in. into the framing. Assuming at least a ½-in. rainscreen batten, 1 in. of CEI, and ½-in. sheathing, this would require a 3½-in. fastener—a common 16d nail that most framing guns are designed for. Going more than 1 in. thick with the continuous exterior insulation means switching to either a specialty framing nailer that shoots larger nails (our Bostich BRT130 nailer can handle nails up to 51/8 in. long) or using a structural screw, which can slow production and add both labor and material costs.

Continuity of control layers is king

In a wall with the structural sheathing as the outermost layer, the installation of the four control layers along with the windows and doors is straightforward, with many tried-and-true options to choose from. However, as soon you add CEI to the outside of the sheathing, you are potentially changing the locations of the control layers and affecting the how windows and doors are installed, how siding is fastened, and how penetrations are flashed.

For an “outie” window, for example (one that is installed at the same plane as the continuous exterior insulation), the WRB may need to be installed to the outside of the insulation, as mentioned earlier. This way, a flanged window can be used and integrated with the WRB. You can still locate the air barrier at the sheathing by taping the seams. If this wall has XPS CEI, it will be the vapor control layer too because it is less permeable than the plywood sheathing. The thermal control layer is the entirety of the wall from the drywall, through the cellulose insulation, including the plywood and the XPS itself because all of these materials have an R-value and are in contact with one another. This may be the method most easy for a first-time builder to adopt, particularly with the ThermalBuck used for window installation.

If a flangeless window is installed in the middle of the wall—an “innie” window—and mineral-wool CEI is used, the control layers might be arranged differently. A self-adhered WRB could be applied to the sheathing to act as both bulk water control and the air barrier. Because the mineral wool and the WRB are vapor open, the vapor control layer in this assembly is the plywood sheathing, as it is the least-permeable component.

Thermally, the mineral wool is still doing the job of keeping the sheathing warm enough to prevent moisture accumulation. Again, the thermal control layer includes everything from the interior gypsum all the way to the exterior edge of the mineral wool. This assembly, with 3 in. of mineral wool CEI, demonstrates a bit higher-performing foam-free wall that could be used in a cold climate. However, integrating the flangeless window in a site-made plywood buck with a self-adhering WRB and rainscreen details may be a bit more challenging to the uninitiated.

Both of these examples use different materials, arranged differently, but the four control layers are all present and the sheathing is kept warm enough by the CEI material to minimize moisture risks. No matter what continuous exterior insulation material you choose, and where you decide to install windows, this should be your goal.