Exterior walls framed with light-gauge steel studs solve several problems at once. They stay straight for decades, they do not warp or shrink, and they give the trades a uniform substrate to build against. The catch is that steel conducts heat far faster than wood, so the rest of the assembly has to make up the difference. A proven approach pairs metal studs with glass-mat sheathing on the outside, a layer of rigid foam over that, and a brick veneer as the finished cladding. The placement of rigid foam sheathing decides how much thermal bridging the steel still passes, which is why the layer order matters as much as the materials themselves.
The Wall Assembly from Inside to Out
The full wall section in the source build runs, from the interior face to the exterior face: half-inch drywall, R-11 fiberglass insulation inside 3-5/8 inch 20-gauge metal studs, 5/8 inch glass-mat sheathing, 2 inches of rigid foam board, a 2-inch air space, and 4-inch brick veneer. Each layer has a distinct job, and the order is not arbitrary. The decision about insulating inside or outside the framing sets the thermal performance of the wall for the life of the building, and it cannot be reversed later without tearing the assembly apart.
| Layer | Thickness | Role in the wall |
|---|---|---|
| Interior drywall | 1/2 in | Finished surface and fire protection |
| Fiberglass batt | R-11 | Cavity insulation between studs |
| Metal studs | 3-5/8 in, 20 ga | Framing and brick tie anchorage |
| Glass-mat sheathing | 5/8 in | Weather-resistant substrate and racking resistance |
| Rigid foam board | 2 in | Continuous insulation and thermal break |
| Air space | 2 in | Drainage and drying behind the brick |
| Brick veneer | 4 in | Cladding and weather screen |
Why Steel Studs Change the Insulation Math
Steel conducts heat roughly 300 times better than wood, so every metal stud acts as a thermal short circuit through the wall. A cavity-only R-11 batt in a steel stud wall performs far below its label value once the studs are factored into the calculation. Continuous exterior insulation is what restores real-world performance, because it covers the studs themselves rather than just the space between them. The 2-inch foam layer in this assembly adds roughly R-10 of uninterrupted insulation across the entire wall plane, which is why the exterior layer does the heavy lifting in a steel-framed wall.
Reading a Whole-Wall R-Value
Catalog R-values describe the insulation by itself. Whole-wall values include the studs, sheathing, air films, and fasteners, and they are always lower. When comparing assemblies, use the whole-wall number rather than the batt label, because that is the figure that shows up in heating bills.
Installing Rigid Foam over Glass-Mat Sheathing
Installing the foam is straightforward when the boards are cut to the stud spacing and the fasteners are sized for the total thickness. The build used 2-inch tongue-and-groove foam panels and 3-inch screws with large 2-inch diameter plastic washers. The washer spreads the clamping load so the screw head does not crush the foam, and the tongue-and-groove edges keep adjacent boards aligned on a flat plane. Getting the foam sheathing thickness and vapor barrier placement right at this stage determines whether the wall drains properly or traps moisture behind the cladding.
- Snap layout lines at the stud locations so every fastener lands in a stud.
- Start at a corner and set each panel with the tongue-and-groove edges fully engaged.
- Drive 3-inch screws with 2-inch plastic washers at the stud lines, spaced about 16 inches apart vertically.
- Leave the lower 2 feet of foam off the wall until the mason installs the base flashing.
- Tape every seam with weather-resistant tape, then seal around window and door frames with spray foam.
Fastener Pattern and Washer Size
The fastener pattern matters more than the screw count. Every screw must reach a stud, because foam alone will not hold the board in place during a wind event. A 2-inch washer on a 3-inch screw provides roughly ten times the bearing area of a bare screw head, so the foam stays flat and the insulation value stays intact. Countersunk or overdriven screws crush the board and leave a low spot that shows through the finished surface.
Sealing the Seams
Foam boards are only as airtight as their joints. Taping the seams with a weather-resistant tape, the same product used at sheathing joints, turns the foam layer into a continuous air barrier. Gaps at windows and doors get spray foam, which expands to fill irregular openings that tape cannot bridge. Run the tape the full length of each seam and press it down firmly so it bonds to both panels.
Choosing Foam Thickness and Board Type
Rigid foam comes in three common families: expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso). They differ in R-value per inch, moisture tolerance, and cost. The project used 2-inch XPS, which is why the panels carried a tongue-and-groove edge profile and a dense, moisture-resistant surface. A technical guide to EPS, XPS, and polyiso boards helps with the material decision before the order goes in.
| Board type | R-value per inch | Moisture resistance | Typical use |
|---|---|---|---|
| EPS | 3.6 to 4.2 | Moderate | Below-grade and exterior walls |
| XPS | 5.0 | High | Exterior walls and wet assemblies |
| Polyiso | 6.0 to 6.5 | Moderate with facers | Roofs and above-grade walls |
Thickness and Climate
Cold climates push designers toward thicker foam. A 2-inch layer delivers about R-10 of continuous insulation, which satisfies many commercial wall requirements in the northern United States when paired with a modest cavity batt. Warmer climates can use 1-inch boards, while very cold climates often step up to 3 or 4 inches to meet code and control condensation. The dew point moves outward as the exterior insulation gets thicker, which keeps the sheathing warmer and drier through the winter.
Storage and Handling
Keep foam boards flat, off the ground, and out of direct sun before installation. Prolonged UV exposure degrades the surface, and a warped board is difficult to fit tight against the sheathing. Store the stack under a tarp and cut with a sharp knife or a fine-tooth saw for clean edges that close up tightly at the joints.
Flashing, Brick Support, and the Air Space
Brick veneer does not carry its own weight on the wall. It rests on a support at the base, usually a concrete brick shelf cast into the foundation wall. In the source project the foundation was notched to create that shelf, and the brick sits on it with a 2-inch air space behind. Brick ties screwed into the metal studs hold the veneer back to the structure. Installing foam sheathing with the right thickness and vapor barrier placement keeps the cavity behind the brick dry so the flashing can do its job.
The Brick Shelf
The shelf transfers the full weight of the masonry to the foundation. For a standard 4-inch brick veneer, the shelf needs enough bearing width for the brick plus a small overhang, and it must sit below finished floor level so the first course is hidden from view. The top of the shelf slopes away from the wall so water runs out instead of pooling against the veneer.
Brick Ties and Air Space
Ties are corrosion-resistant strips screwed through the foam into the studs, then embedded in the brick mortar. Spacing follows the veneer manufacturer and code requirements, commonly one tie for every 2.67 square feet of wall area. The 2-inch air space behind the brick drains water that gets past the masonry and gives the wall a drying path. Keep mortar droppings out of that gap, because a blocked air space turns the cavity into a water tank.
Air Sealing and Final Assembly Checks
Once the foam is up and the seams are taped, the wall behaves as one continuous enclosure. The source project deliberately left the lower 2 feet of foam off until the mason installed the base flashing, then closed that gap so the assembly was airtight end to end. Taping every seam and foaming every penetration is the difference between a wall that performs and one that leaks. That level of attention to foam sheathing installation pays off in lower heating loads and fewer callbacks.
Sheathing choice also changes how much air a wall moves. Glass-mat products seal better than some panels, and taped seams improve any substrate. Builders who compare air leakage through oriented strand board sheathing against glass-mat options get a clear sense of how much the substrate alone contributes to the airtightness budget. A blower door test after the foam is in place catches the gaps that the eye misses.
- Every foam seam taped with weather-resistant tape
- Spray foam at windows, doors, and penetrations
- Fasteners driven into studs with washers seated flat
- Base flashing installed before the lower foam course
- Brick ties screwed through the foam into studs
- Air space kept clear of mortar and debris