Editor’s note: This post originally was published as part of the ProHOME series at Fine Homebuilding magazine. Michael Maines is a Certified Passive House Consultant and residential designer based in Palermo, Maine.
When installed over an otherwise conventionally framed wall (or a wall built using advanced framing), continuous exterior insulation, also known as insulating sheathing, is a great way to improve whole-wall R-values and to keep the framing and sheathing dry. There are some tricks to choosing size and type of materials, though.
Depending on the situation, I use different wall systems. For low-energy new construction, the builders I work with often prefer either double-stud walls with cellulose insulation, or Larson-truss type outriggers with infill insulation over a standard stud wall. Both can work well, but they have their downsides, including cost, wall thickness, and water-vapor management. Many builders, including Mike Guertin, prefer continuous exterior insulation. Since builders seem to have strong opinions on which wall type they prefer, and I think they can all work fine if designed and detailed well, I worked with Mike to design a durable, efficient wall detail at the ProHOME.
The wall is framed with 2x6s 24 inches on center and insulated with fiberglass batts. The R-value of fiberglass batts is typically a bit more than that of cellulose and a bit less than mineral wool, but they’re all pretty similar. I have to say that fiberglass batts are rarely my first choice for insulation, but they are readily available and inexpensive, and in the right conditions — installed tightly in an airtight cavity with limited temperature differential across the batts — they can perform as intended. (The common assumption that they should not be compressed is wrong; if you compress them, their R-value per inch increases, up to a point.)
It’s important to have at least one really good air barrier somewhere in the wall assembly. It’s OK to have more than one air barrier, but it’s never OK to have more than one vapor barrier. I prefer to use vapor retarders rather than vapor barriers, as I’ll describe below. Air barriers and vapor barriers (or vapor retarders) can be one product that does double duty, or they can be dedicated layers. For the ProHOME, we are using one air barrier and two vapor retarders (the sheathing is acting as both a vapor retarder and the principal air barrier).
Detailing for Climate Zone 5
To meet code requirements for limiting vapor transmission by diffusion into the wall in Climate Zone 5, we needed a Class 1 or Class 2 vapor retarder on the interior, but with at least R-7.5 exterior insulation, the code allows a Class 3 vapor retarder. (Class 1 is 0.1 perm or less; Class 2 is from 0.1 to 1.0 perms; Class 3 is from 1.0 to 10.0 perms. Over 10 perms is considered vapor-open.)
CertainTeed is now selling kraft-faced batts with its vapor-variable MemBrain vapor retarder adhered to the kraft paper. The MemBrain normally blocks most water vapor diffusion as a Class 2 vapor retarder, but when the relative humidity reaches about 60% (when bad things can happen), it opens up to be vapor-open, allowing the wall to dry to the interior. With airtight sheathing, vapor movement by diffusion is not considered to be a major issue by prominent building scientists, but the MemBrain gives us insurance.
If we needed a functional air barrier at the interior, as some builders try to do with poly sheeting, then MemBrain in sheet form would be a better choice. Because we are using Huber’s Zip System oriented strand board (OSB) sheathing as the airtight layer for the walls, we don’t need to worry about the interior of the walls being perfectly airtight, although there would be no harm if it were a second airtight layer.
Zip System sheathing is denser and more airtight than regular OSB (I learned that the hard way on a Passive House project), and Zip’s tape system is now familiar to most contractors. Like other OSB products, though, it is more sensitive to moisture than materials such as plywood or solid lumber. That doesn’t mean it’s a bad product; just that moisture control needs to be considered when using any OSB.
The phenolic coating on Zip sheds liquid water effectively and is open to water vapor. Huber does not publish data for the actual vapor permeance of its sheathing, but standard OSB is usually between 1.0 and 2.0 perms, with some third-party testing finding that Zip is below 1.0 perm. The permeability of OSB and plywood sheathing varies with relative humidity, so it’s hard to pin down. In any case, the Zip panel can be assumed to be borderline between a Class 2 and Class 3 vapor retarder, meaning that it will allow water vapor to pass through, albeit at a slow rate. If the sheathing temperature is below the dew point of the air, the water vapor will condense out of the air and onto the surface of the OSB. Capillary action will then draw the moisture into the OSB.
Exterior insulation protects the sheathing
To reduce the chances of the OSB becoming saturated, our main defense is to keep air from flowing through the assembly, which we’ve already covered. The next defense is to keep the sheathing above the dew point by installing continuous exterior insulation. One of the key details of a wall with exterior insulation in Climate Zones 3 and higher is to make sure the proportion of exterior to interior R-value is high enough to prevent condensation most of the year. The exterior insulation keeps the sheathing warmer than the outdoor air, but insulation slows heat flow in both directions, so the cavity insulation keeps indoor heat from reaching the sheathing.
The 2012 IRC has a handy way to make sure the sheathing stays above the dew point most of the year: table R402.1.1, which shows the minimum R-value of cavity and exterior insulation. [Editor’s note: Strictly speaking, the previous sentence is inaccurate. For more information on this topic, see Comments #10 and #11, below, as well as a GBA article titled The 2012 Code Encourages Risky Wall Strategies.] For Rhode Island (Zone 5), it’s 13 + 5. In other words, a minimum of R-13 cavity insulation with a minimum of R-5 exterior insulation. What’s not clear from that table is that if you increase the levels of insulation, it’s very important to keep the proportional relationship the same, or to err on the side of additional exterior insulation. You can’t just keep code-minimum R-5 exterior insulation and bump the cavity insulation up to R-21; if you do, the sheathing will be too cold during a typical winter and condensation will occur.
For Zone 5, the relationship is 38% exterior to 62% cavity. We have R-21 cavity, which means that we need at least R-8 exterior insulation to control condensation. Table R702.7.1 supports this math, requiring at least R-7.5 for a 2×6 wall if you want to use a Class 3 interior vapor retarder.
We could have installed more exterior insulation; from an energy use point of view, more insulation is always better, but there are diminishing returns. Energy modeling showed that going from 2 inches to 4 inches of exterior insulation would save about 250kwh/year — less than $40 worth of electricity and less than the annual output of a single PV panel. I have designed Passive Houses with R-40 exterior insulation, so I’m not afraid of using too much, but considering the added expense and hassle of installing thicker rigid insulation, we opted to stick with 2 inches. At R-8 for the 2-inch thickness, we meet condensation-control requirements.
Choosing the best exterior insulation
With those decisions made, what should we use for exterior insulation? The most common exterior insulation is rigid foam, either XPS (extruded polystyrene) or polyisocyanurate (PIC). A third option is EPS (expanded polystyrene).
Although my feeling is that foam insulation is probably the best use for oil, in debating what materials to use, I argued strongly against XPS. XPS’s blowing agents are nasty — they stay in the atmosphere, and even though XPS insulation saves energy, in terms of global warming potential, XPS is usually a net loss to the planet, according to research by David White. Additionally, as XPS ages, its R-value decreases, and at useful thicknesses, it’s fairly effective at blocking the ability of the wall to dry to the exterior. In the plus category, it’s readily available, has decent compressive strength, and is easy to work with. Until the industry changes to the more benign blowing agents used for XPS in Europe, though, it’s always my last choice for insulation.
Polyiso has fairly benign, pentane-based blowing agents, and it is advertised as having a high R-value per inch, but with its usual foil facing, it is a total vapor barrier. Not only does its R-value decline over time, but it also loses performance as the temperature drops, just when you need it the most.
EPS is vapor semipermeable at thicknesses less than 3 inches, has relatively safe blowing agents, and stable R-value, plus it’s generally less expensive than the other options. It can be crumbly to work with, and it shares with XPS a brominated flame retardant that is not safe to ingest, but if I had to choose a foam, my first choice would be EPS. My second choice would be polyiso.
Fortunately, there is an alternative that is good for the house and good for the planet: rigid mineral wool, in the form of Roxul ComfortBoard IS (the IS is for “insulating sheathing”). It will keep the sheathing warm and dry, and it has low global warming potential and stable R-values. It’s a lot like a wearing a wool coat compared to a rain slicker. It’s a bit of a pain to install compared to the other options, but it’s completely vapor-open. Not only will it allow the occasional raindrop that finds its way past the siding to drain freely, but it will let the wall sheathing dry to the exterior if and when it needs to. Installed with 1x strapping, it makes an excellent ventilated rainscreen that is open to air movement at both the top and bottom of the wall.
Considering my obsession with managing water and water vapor and my strong desire to build resilient yet energy-efficient homes, I am happy with the ProHOME wall assembly.