Many builders add one or more layers of rigid foam insulation to the outside of a house to lower heat losses. Rigid insulation has an R-value of up to 6.5 per inch, but it also can be an effective vapor retarder.
Ed Welch touched off an extended discussion in the Green Building Advisor’s Q&A section when he asked whether the foam would trap moisture inside walls, creating mold as well as the potential for structural decay.
His concerns seem well placed. In a cold climate during the heating season, moisture vapor inside a building is driven outward into exterior walls. When it reaches a surface that’s below the dew point, the vapor condenses into a liquid. That surface is typically the back side of the exterior sheathing.
Rigid foam board, especially foil-faced polyisocyanurate, creates a vapor-impermeable barrier, so the wall would have limited drying potential. Even more permeable types of insulation, such as expanded polystyrene, are vapor barriers when the installation is thick enough.
No one argues this point of view more forcefully than Robert Riversong of Vermont, who has been building high-performance houses for many years. Riversong notes the exterior skin of a house should be at least five times as vapor-permeable as the interior. Yet 2 in. of extruded polystyrene insulation — the amount it would take to keep sheathing above the dew point in a cold climate — create an effective vapor retarder.
Further, Riversong argues that long-term studies have shown foam-clad walls have no way of drying out in the event of even minor rain penetration. He suggests there are better ways of increasing R-values and reducing thermal bridging, such as double-framed walls or using foam board on the interior.
He also favors cellulose insulation and other natural materials over petrochemical plastics because they are more forgiving of the inevitable moisture problems a house will encounter over its lifetime.
A common practice, but get the details right
The practice of using exterior rigid insulation is increasingly common as builders look for ways to increase overall R-values and reduce thermal bridging, the transfer of heat through the house framing. R-values of rigid foam range from 3.2 to 6.5 per inch, making it possible to boost energy performance substantially.
Building scientists such as Joseph Lstiburek of Building Science Corp. in Boston argue that if the foam is thick enough for the climate, the back side of the sheathing never gets below the dew point. Hence, no condensation and no moisture problems. Additionally, the insulation raises the overall R-value of the wall.
And Riversong’s reservations notwithstanding, adding rigid foam insulation to the outside of exterior walls is a building practice that is probably here to stay.
Plastic vapor barriers sabotage this system
There are caveats. As senior editor Martin Holladay points out, it’s important to get the details right. That means controlling the intrusion of water (rain and snow) from the outside and skipping an interior polyethylene vapor retarder so any moisture inside wall cavities can dry to the building’s interior. “If your details are done right, a foam-sheathed wall will stay dryer than a conventional wall,” he says.
Holladay provides an overview of the issues that builders need to focus on in a useful article, How to Install Rigid Foam Sheathing.
More insulation is (almost always) better
A key question is how much rigid insulation is enough, and the answer depends on what kind of foam is used and where the house is built. Holladay provides a table to help builders choose the right foam thickness in his article, Calculating the Minimum Thickness of Rigid Foam Sheathing.
There’s some additional guidance on the question in Robust Walls, an article in Coastal Contractor by Ted Cushman.
Cushman leans heavily on the research that Lstiburek’s company conducted, including years of computer simulations aimed at predicting moisture levels in exterior walls. In the end, Lstiburek’s team came up with a fairly simple formula in which the average temperatures of the three coldest months are averaged to create a winter design temperature. The thickness of foam is then calculated to keep the condensing surface above the dew point.
It’s not foolproof, Lstiburek admits, “but it’s a very good approximation — it gets us 98% accuracy with one easy calculation.”
In the examples that Lstiburek uses, for a house in Boston, Massachusetts, several combinations of wall cavity insulation and exterior foam would allow the house to meet current energy codes while preventing condensation inside the wall. Both 1 in. of polyisocyanurate and 1 1/2 in. of extruded polystyrene would work. In general, the colder the climate, the thicker the layer of rigid foam board must be.
Moving insulation outside the box
Ironically, the use of insulation in wall cavities along with rigid foam on the building exterior can actually increase the risk of condensation if the wall system is improperly designed. The reason is that cavity insulation slows the flow of heat outward and has the effect of keeping the back side of exterior sheathing cooler, thereby making condensation more likely.
This has led some builders to skip cavity insulation altogether and put all of the insulation on the outside of the walls, what’s been dubbed “outsulation.” Holladay discusses a technique called PERSIST (short for Pressure-Equalized Rain-Screen Insulated Structure Technique) that was developed in the 1960s by the National Research Council of Canada as an example of just how far it’s possible to take the exterior insulation route.
PERSIST walls are framed with 2x4s, even in very cold climates, sheathed with plywood or OSB and then completely covered with a peel-and-stick membrane. The walls and roof are covered with at least two layers of foam insulation, up to 8 in. thick, before vertical strapping and siding is added. The peel-and-stick membrane is an air barrier, vapor barrier and water-resistant membrane all in one, and the framing stays dry in all seasons and climates.
Although the technique is expensive, it’s highly effective. You can read Holladay’s summary of the PERSIST superinsulation system in one of his blogs, “Getting Insulation Out of Your Walls and Ceilings.”
Riversong has been building energy efficient houses for a long time, and he makes a strong case for a non-foam approach. He advocates “building envelopes that are tolerant of occasional moisture,” as well as natural building materials like wood, cellulose insulation, straw bales, earthen plasters and natural paints.
Still, foam insulation added to a building exterior can do just what it’s supposed to — improve thermal performance without introducing moisture problems. The key is designing the house as a system, meaning the foam board is part of the overall design and not an idle afterthought.