When you look at a freshly installed siding job, whether it’s vinyl, fiber cement, or engineered wood, it’s easy to admire the clean lines and color options. But have you ever wondered what layers of protection and structure are hiding behind those panels?
Knowing what lives behind your siding can help you spot problems early. A small gap in flashing might not seem urgent, but give moisture a few seasons, and you’re looking at rotten sheathing, mold growth, or warped studs. By appreciating each layer, you can ask informed questions, schedule targeted inspections, and protect your home’s value. So, let’s peel back the layers and see what really holds your home together.
Siding Panels
The siding panels are the visible layer of your home’s exterior. Whether you’ve chosen vinyl, fiber cement, or engineered wood, these panels define your home’s aesthetic and first layer of defense. Before breaking down the lower layers, its important to remember that the siding serves three big jobs:
- Weather barrier: Shields against wind, rain, hail, and UV rays.
- Aesthetic finish: Gives curb appeal and lets you pick a style that fits your home.
- Initial moisture control: Keeps water from getting into your house.
But none of these panels work alone, so let’s explore the deeper layers of your home’s siding.
Starter Strips, Flashing & Trim
Before any siding goes up, the job starts with starter strips, which are horizontal pieces that anchor the first row of panels in place. You’ll find a starter strip at the bottom edge of your siding, and it helps hold the first row of siding securely in place and ensures proper alignment.
Around windows, doors, and at the roofline, you’ll see flashing. Flashing is a thin metal or PVC strip that directs water away from vulnerable spots and seams. Properly installed flashing prevents leaks where two materials meet, like at a window jamb or roof intersection.
Wall Sheathing (Usually OSB or Plywood)
Sheathing is the rigid surface board to which siding attaches. It’s typically made from oriented strand board (OSB) or plywood and is fastened directly to the home’s framing. This layer provides structural strength and serves as a base for the weather barrier. Sheathing has a few important features:
- Gives your wall rigidity, preventing it from twisting or bending in high winds.
- Provides a nailing surface for siding and other materials.
- Acts as a secondary weather barrier, stopping drafts and slowing wind-driven moisture.
High-performance sheathing products can include built-in insulating foam or enhanced moisture resistance. If your home was built in the last decade, you may have insulated sheathing that adds energy savings and thermal control to your home as well.
House Wrap & Underlayment (Moisture Barrier)
Under all types of siding you will find a moisture barrier, typically either house wrap (like Tyvek) or a breathable synthetic fabric. If you’ve ever seen a white or black sheet peeling back from an unfinished wall, that’s the barrier doing its job. The house wrap lays over the sheathing, blocking moisture. This layer:
- Blocks bulk water that gets behind the siding (think driven rain or wind-blown spray).
- Allows vapor to escape, preventing condensation from being trapped.
House wrap is fastened with staples or cap nails and sealed at seams with special tape. Underlayment or specialized membranes go around roof-to-wall transitions or beneath trim, creating a continuous barrier.
Studs & Framing
Behind the sheathing, you’ll hit the framing studs (typically 2×4 or 2×6 lumber planks), which are usually spaced 16 or 24 inches apart. These form the structure of your walls, and everything else is built on them. They hold your insulation in place, anchor the sheathing, and define the shape and strength of your house.
Insulation
Between the studs, you’ll usually find insulation. If your siding is being replaced, it’s a smart time to inspect and upgrade your insulation. This layer helps regulate indoor temperatures and lowers your energy bills. Insulation slows heat flow, keeping your home cozy in winter and cool in summer. There are 3 common types of siding materials:
- Fiberglass batts: The classic pink rolls that are easy to install but can leave gaps if not cut to fit precisely.
- Blown-in cellulose or fiberglass: Loose-fill materials that settle around pipes and wiring for a snug fit.
- Spray foam: Offers top-tier air sealing and R-value per inch but comes at a premium cost.
Equally important is air sealing around windows, electrical boxes, and plumbing penetrations; even the best insulation can’t perform if air is freely flowing through cracks.
Existing Siding (if re-siding)
In some cases, new siding gets installed over old siding. While it can speed up installation, extra layers can hide moisture damage or create awkward gaps. If you spot uneven walls or odd ridges under your new siding, ask your installer if there’s an old layer tucked beneath. This is especially common with aluminum or wood siding under vinyl panels. However, for optimal performance and a clean slate, most professional installations begin by removing the old siding down to the sheathing.
Signs the Layers Under Your Siding May Need Attention
Even if your siding looks fine on the surface, here are signs that there could be trouble brewing underneath:
- Warped or bulging siding panels
- Soft spots or rot near the bottom of walls
- Drafts or cold walls inside during winter
- Mold or mildew on interior walls
- Insect infestations (especially carpenter ants or termites)
Schedule a Siding Evaluation Today
