How to Insulate a Wall from the Outside
The right combination of cavity insulation and rigid foam will ease worries about condensation
Gregg is renovating his 50-year-old house in Wisconsin and trying to devise the best way of insulating exterior walls from the outside. The house was built conventionally, with 2x4 walls, fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. , fiberboard sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , and hardboard siding.
He plans to tear off both siding and sheathing and remove the batt insulation, then apply 3 in. of spray polyurethane foam insulation into the stud bays. The existing kraft paper vapor barrier on the interior side of the wall will stay in place.
Several local contractors have recommended the addition of between 3/4 in. and 1 in. of rigid foam insulation over the studs before applying 1/2-in. OSB sheathing and lap siding. But the recommendation is giving Gregg pause for thought.
“While I like the idea of eliminating thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs,” Gregg writes in this Q&A post at GreenBuildingAdvisor, my concern is that there will be an air gap between the two layers of foam insulation (both of which have limited permeability). In this scenario, wouldn’t I be creating an ideal situation for condensation to form between the two layers? We obviously live in a very cold climate and winter temperatures regularly reach -20°F to -30°F.”
Making the wall structurally sound
One concern, voiced by GBA senior editor Martin Holladay, is that when the sheathing is applied over the foam, and not directly to wall framing, it can’t provide much structural rigidity. In that case, he’d have to install metal strapping for bracing before the foam is applied.
An alternative is to put the sheathing on first, followed by the rigid foam insulation.
“I wouldn't hesitate to install OSB (or, better yet, plywood) over the studs, and then rigid foam and vertical strapping to create a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , if that's what you want to do,” Holladay writes.
Chris Harris agrees, recommending either dense-pack cellulose or open-cell polyurethane. But he suggests that Gregg might need structural sheathing only in the corners and possibly a few other wall panels. That would allow him to run a combination of 2 in. foam where no structural sheathing was needed, and 1 1/2 in where it was.
Weighing the risks of condensation
Trapping organic material (in this case, wood framing) between two impermeable layers is asking for trouble because any moisture that does get into the assembly doesn’t have a way of drying out.
But here, the kraft paper facing left from the batt insulation will be less problematic than a polyethylene vapor barrier would be, Holladay says. And if Gregg builds an effective rain screen that is flashed correctly, he adds, then “the wall assembly can still dry to the interior through the studs if there is ever a small amount of moisture in your wall cavity.”
“My concern would be the wiring,” says Kricket Smith-Gary, “particularly the outlets and switch boxes in the wall. Remember that condensation is a function of the differentials — temperature, humidity, atmospheric pressure AND HEAT RESISTANCE of the assembly — the least insulated materials or areas (i.e. windows and electrical boxes, in this case) tend to provide an avenue for condensation and the moisture can come from everywhere, even what soaked into the studs while you were doing this project, or the humidity from the inside accumulating in the outlet itself…”
Smith-Gary suggests filling the wall cavities with closed-cell foam and move wiring inside behind baseboards to eliminate any through-the-wall penetrations that might contribute to the problem.
“Think small and aim at a perfect foam insulation with no penetrations,” Smith-Gary says.
How thick should the rigid foam be?
The thread also touches on a question that’s popped up before: how thick to make rigid foam insulation on the outside of the wall? The risk is that when the exterior foam is too thin, the sheathing isn’t kept warm enough in winter to prevent the accumulation of moisture in the wall cavity. Moreover, the moisture may become trapped inside the wall, unable to dry effectively either in or out.
Holladay refers to a blog he wrote earlier on this topic, in which he recommends adequate foam thickness to prevent moisture accumulation, and vapor permeable materials toward the interior: “Because foam sheathing reduces the ability of a wall to dry to the exterior,” he wrote, “all foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance — especially polyethylene — on the interior of a foam-sheathed wall.”
That blog also includes recommendations for minimum foam thickness.
But in Gregg’s situation, Holladay adds, the minimum thickness guidelines for foam insulation don’t apply because the insulation he plans to use in the wall is not air permeable. If he switched to cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection., the guidelines would apply.
Another good source of information, Harris adds, is at the web site of Building Science Corp., which includes a searchable database.
Our expert's opinion
We asked GBA technical director Peter Yost for his thoughts. Here's what he said:
The key in Gregg’s situation is the choice of cavity insulation. As Martin points out, if the cavity insulation is not air-permeable, then the first condensing surface is not the structural sheathing, and that is really what you are trying to avoid.
If the installation of the cavity and rigid insulation is airtight, then that 1/2-in. air space is not a concern; the amount of moisture in that isolated space is just not enough to worry about. But this assembly will have very little drying potential and therefore won’t tolerate much wetting. Drying through the wall framing will be very, very slow as well. Keeping bulk water out (drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. connected to flashing) will be critical. I would definitely employ a rainscreen claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. in this case. Wouldn’t hurt to choose a slightly more permeable rigid insulation such as expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.), either.
What could make this project tricky is the sequence of the steps. The spray foam installer is going to want to do the whole project at one time, but eliminating ALL of the shear resistance by taking off all the existing structural sheathing or boards to open up the cavities is not a good idea. Maybe Gregg will need to leave the corners sheathed for temporary shear resistance, and “Froth Pak” insulate those wall cavities as a separate step? And I would definitely keep the structural sheathing up against the wall framing and favor plywood, as Martin suggested.
- John Hartman, Fine Homebuilding Magazine #208
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