Walls With Interior Rigid Foam
Walls With Interior Rigid Foam
When remodeling an older home, it sometimes makes sense to install rigid foam on the interior side of exterior walls
There are two main ways of reducing 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 studs: you can build a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation., or you can install a continuous layer of rigid insulation on one side of the wall.
Most builders who install a continuous layer of rigid insulation use rigid foam (polyisocyanurate, expanded polystyrene, or extruded polystyrene); a small minority of builders use semi-rigid panels of mineral wool.
Builders who install rigid foam on the walls of a new building usually install the foam on the exterior side of the wall. There are several reasons for this:
- The exterior side of the wall has fewer interruptions (for example, electrical boxes and partition intersections), so it’s much easier to install an uninterrupted layer of insulation on the exterior than the interior.
- Exterior rigid foam is a good way to insulate rim joists.
- Exterior rigid foam helps keep wall 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. warm and dry.
Unlike new home builders, remodelers can’t start with a blank slate. If a remodeler wants to reduce thermal bridging through studs, the exterior foam option is usually prohibitively expensive — especially if the siding is in good shape. So many remodelers ask, “Why can’t I put the rigid foam on the interior side of the wall?”
The answer is, you can.
Back to the 1980s…
In the 1980s, most cold-climate builders were taught that walls needed an interior vapor retarder, and needed to be able to dry to the exterior. When a few pioneers started installing exterior rigid foam, some people thought the practice was risky.
“Rigid foam is a vapor retarder, so it belongs on the interior,” some old-time builders opined. “Exterior rigid foam is risky, unless you live in Florida. It’s a wrong-side vapor barrier.”
Eventually, researchers proved that these builders were wrong. (To learn more about the history of vapor barrier requirements and the origin of unjustified concerns about wintertime vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. , see “Do I Need a Vapor Retarder?”)
Exterior rigid foam works very well indeed, as long as the foam is thick enough to keep the interior face of the foam (or the wall sheathing) above the dew point during the winter. (For more information on this topic, see “Calculating the Minimum Thickness of Rigid Foam Sheathing.”)
In fact, research has shown that the type of wood-framed wall with the driest sheathing is a wall with an adequately thick layer of continuous exterior insulation.
Once builders understood the building science behind exterior foam, they readily adopted the practice — especially since exterior foam does such a good job of insulating rim joists and partition intersections. Moreover, installing exterior rigid foam is much easier and faster than installing interior rigid foam, because there are far fewer electrical boxes to work around.
These days, exterior rigid foam is much more common than interior rigid foam.
Do walls need to be able to dry to the interior?
Let’s say that you are a remodeler who wants to install interior rigid foam. The first question that you need to resolve is a building science question: Will there be any problems if your walls can’t dry to the interior?
The answer is no — as long as your walls can dry to the exterior. So if you’re planning to install interior rigid foam, you shouldn’t install any exterior rigid foam. Most other types of sheathing — including plywood, OSB, fiberboard, DensGlas Gold, and boards — are all vapor-permeable enough to work well on this type of wall.
Most wet-wall problems are caused by moisture that enters the wall from the exterior, not the interior. (In other words, the big problem is rain, not wintertime condensation — especially if the wall was built with attention to airtightness.) So walls that dry to the exterior work well, as long as the builder has installed adequate flashing. A ventilated rainscreen gap between the sheathing or WRB and the back of the siding isn’t mandatory, but it’s an excellent detail to encourage walls to dry to the exterior.
What about inward solar vapor drive during the summer? Interior rigid foam prevents any problems from inward solar vapor drive, because the exterior face of the rigid foam never gets cold enough to permit condensation. Even if the homeowners like to crank up their air conditioner until the indoor temperature is down to 66°F, the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the foam keeps the exterior face of the foam warm, so that the foam panel doesn't present a condensing surface to any moisture lurking between the studs.
What type of rigid foam is best?
Polyisocyanurate is the most environmentally friendly foam; it is manufactured with a benign blowing agent and it doesn’t include worrisome flame retardants. Its only drawback is that it doesn’t perform well at low temperatures — but this problem is irrelevant when the foam in installed on the interior side of a wall.
If you don’t want to use polyiso, choose expanded polystyrene (EPS), which is also manufactured with a benign blowing agent. The main environmental drawback to EPS is the fact that it includes worrisome flame retardants.
Extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) is usually avoided by green builders, because XPS contains both worrisome flame retardants and blowing agents with a high global warming potential.
What do rigid foam manufacturers say?
Most rigid foam manufacturers are silent on the question of whether it’s a good idea to install their products on the interior side of walls. However, at least two manufacturers endorse the method.
The manufacturer of Iso R Plus asserts that their graphite-infused EPS panels can be used as “interior insulation for walls and ceilings.” This product is available in three thicknesses: 1/2 inch, 3/4 inch, and 1 inch.
One manufacturer of polyisocyanurate sheathing, Atlas Roofing, asserts that their polyiso can be “used to insulate the interior side of metal and wood frame walls on retrofits, upgrades and renovations. In addition, many new construction projects using poured concrete or CMUConcrete masonry unit. Precast concrete block used to build walls. CMUs have hollow cores that can be filled with concrete onsite for additional reinforcement. The use of stronger, more lightweight types of concrete such as autoclaved aerated concrete (AAC) is becoming increasingly popular in CMU manufacture. block walls use polyiso board fastened or glued directly to the masonry wall.”
Is horizontal strapping necessary?
Many builders have installed gypsum drywall directly over rigid foam that is 1 or 2 inches thick. The drywall is secured with long screws that extend through the foam to the studs.
This method works, although it may result in more screw pops (small defects in the drywall compound that need to be repaired) than drywall that is attached directly to studs or strapping. Moreover, many drywall contractors hate installing drywall over rigid foam. The foam can be a little bit squishy — not enough to cause major problems, but enough to make contractors grumpy — and it’s harder to hit the studs when using 3 1/2-inch screws than when using 1 5/8-inch screws.
If you want to keep your drywall contractor happy, you may want to install horizontal 1x3 or 1x4 strapping (usually installed 16 inches on center) on the interior side of your rigid foam. This makes your wall thicker, of course, so you’ll lose some interior area. However, if you specified foil-faced foam, the 3/4-inch-deep air space will give you a small R-value boost (about R-2) — and that fact softens the blow of the lost interior area.
Horizontal strapping has another advantage: it can be used to help secure electrical boxes or wall cabinets.
If you are adding rigid foam as part of a remodeling project, you’ll need to make sure that the electrical boxes are installed at the correct depth. You have two choices: either install new boxes (ideally, special airtight boxes) at the required depth, or install electrical box extenders (sometimes called “goof rings” or “add-a-depth rings”) to the existing boxes (see Image #3, below).
How thick should the foam be?
Although some builders install interior rigid foam that is only 1/2 inch or 3/4 inch thick, these thin foam products have such a low R-value that they are hardly worth installing. (Most types of rigid foam have an R-value ranging from R-3.5 per inch to R-6 per inch, so 1/2 inch of foam has an R-value somewhere between R-1.75 and R-3.)
Most builders who install interior rigid foam choose foam that is 1", 1½", or 2" thick.
Pay attention to airtightness
Rigid foam is usually attached to studs with cap nails.
Whatever type of insulation you choose to install, it’s important to pay attention to airtightness. Since this basic principle applies to all insulation materials, it obviously applies to the installation of interior rigid foam. When installing the foam, it makes sense to use caulk or foam-compatible adhesive at the perimeter of each sheet of foam, and to seal foam seams with a high-quality tape.
Most housewrap tapes do a good job of sealing the seams of foil-faced polyiso. To seal EPS or XPS seams, use Siga Sicrall or Siga Wigluv tape; these tapes are available from 475 High Performance Building Supply.
Remember to address airtightness at electrical boxes. If you haven't invested in airtight electrical boxes, do your best to caulk the holes at the back of the box where the cable enters the box, and seal the crack between the rigid foam and the exterior of the box.
Are there any other drawbacks to interior rigid foam?
Interior rigid foam has a few more drawbacks in addition to those already mentioned:
- Interior rigid foam complicates the installation of shelves, wall cabinets, and trim.
- Remodelers who use this approach need to think about air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. continuity, especially at the perimeter of the wall in each room.
- Remodelers who use this approach need to make sure that rim joists are properly air sealed and insulated.
- Unless details are thought through carefully, partition intersections can represent thermal bridges.
Martin Holladay’s previous blog: “Solar Hot Air Collectors.”
- Image #1: Iso R Plus
- Image #2: William Lucrisia
- Image #3: Fine Homebuilding
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