We tend to put insulation into empty roof and wall cavities because, well, they’re empty. If a rehab gives access to the bare exterior or interior of framing assemblies, how do we decide what type of and how much rigid insulation to add, and what are the pros and cons of putting the rigid insulation on the exterior or the interior of the assemblies?

Design all assemblies to dry

Regardless of where the rigid insulation might go, the key to all enclosure rehabs is to detail the assemblies to resist bulk water (weather-resistive barrier and flashings) and then allow them to dry if, or when, they do get wet. We tend to focus on the vapor permeability of just one layer in the assembly—the vapor retarder—to keep water vapor from getting into assemblies. But much more important is the overall vapor profile of the assembly (based on the relative vapor permeabilities of ALL the assembly’s components), which determines how the assembly will dry. For a detailed explanation of this approach to designing and specifiying assemblies based on the vapor profile concept, take a look at this blog: Vapor Profiles Help Predict Whether a Wall Can Dry.

Exterior rigid insulation

The primary advantage of exterior rigid insulation is that it “warms” the cavity to its interior. In the Perfect Wall (according to leading building scientists Joe Lstiburek and John Straube of Building Science Corporation), ALL of the insulation goes on the exterior, leaving the cavity empty. Why? Because then the wall framing and sheathing “see” pretty much the same conditions we do (as occupants) and the framing assembly is less likely to get wet. And if it does, it dries readily. Perfect walls don’t need “warm-in-winter” vapor retarders because condensing surfaces are well above the dewpoint temperature of the interior air. Eliminating interior vapor retarders means greater drying potential, and ultimately, increased long-term durability of the assemblies.

We seldom get to build perfect walls, so we have to combine exterior rigid with cavity insulation. But this must be done with care; the ratio of the R-values of the exterior and cavity insulation, based on wintertime outdoor and indoor temperatures, along with indoor relative humidity, determines if moisture accumulation during the winter months will occur. Martin Holladay, GBA editor, gives excellent guidance on the issue of climate-based, exterior rigid insulation thickness in this blog: Calculating the Minimum Thickness of Rigid Foam Sheathing.

The main disadvantage of exterior rigid insulation rehabs is the associated window details—the flashing, jamb extensions, and decisions about just where the new drainage plane or weather-resistive barrier will be. Fortunately, GBA is chock full of details for window flashing details with exterior rigid insulation, including a set just for rehab.

Interior rigid insulation

When I did a lot of remodeling back in the ‘80s, it seemed like we got a lot more opportunities to rehab from the interior than the exterior of our projects. And we often added interior rigid insulation to the standard cavity fill because clients wanted to get as much insulation as they could into the walls while they were open. And dealing with interior jamb extensions seemed a heck of a lot easier than what happens with exterior rigid insulation retrofits.

The key to interior rigid insulation is selecting the rigid insulation such that you maintain or even improve the vapor profile (drying potential) of the assembly. Rigid insulations vary in their R-value per inch—easy information to find. But they also vary in their perm rating or vapor permeability—not typically as easy to find. You want to choose a rigid insulation that slows wintertime movement of water vapor into your walls and/or roofs, but also one that promotes drying to the interior, particularly if you have layers in the existing assemblies that don’t allow good drying to the exterior. Use the BSC Building Materials Property Table to evaluate the perm rating and the drying potential/vapor profile of each assembly you retrofit, regardless of where you decide or get to add rigid insulation, and even if your rehab does not permit the addition of any rigid insulation.

Environmental footprint of various rigid insulations

One last thing to consider: while the in-place performance of insulation is critical, we should not ignore the overall footprint of the material. Take a look at Alex Wilson’s recent GBA blog: Avoiding the Global Warming Impact of Insulation. You might end up taking a second (or first) look at certain rigid insulation materials such as expanded polystyrene (EPS), polyisocyanurate (PI), rigid mineral wool, or even FoamGlas (although currently this insulation is several times more expensive per functional unit than other rigid insulation products).

Rehabs should improve the thermal performance of the building as much as is possible, maintain or improve moisture management and long-term durability, and tread as lightly, globally, as possible.