Learn & Understand

The Water Barrier Behind the Siding, and How Materials Compare

The companion calculator sizes the siding a wall needs after deducting openings. What it treats as the wall's exterior is really only the outer, visible layer of a two-part defense, and the layer you cannot see, the water-resistive barrier behind the siding, is arguably the more important one. On top of that, the choice of siding material itself, vinyl, fiber cement, wood, and others, involves trade-offs the square-footage figure does not touch. Here is what sits behind the siding and how the materials stack up.

Siding Is a Rain Screen, Not a Seal

The intuitive belief is that siding waterproofs a house. It does not. Siding sheds the bulk of the rain and takes the sun and wind, but water inevitably gets behind it, around windows, through joints, driven by wind. The real waterproofing job belongs to the water-resistive barrier, the housewrap or building paper wrapped around the sheathing beneath the siding. This barrier stops water that gets past the siding from reaching the structural wall, and directs it back out. Siding and barrier work as a team: the siding takes the weather, the barrier is the last line of defense.

The Layers Behind the Surface

Flashing around windows and doors is where most siding-related leaks actually occur, the openings interrupt both the siding and the barrier, and if they are not flashed to shed water outward over the barrier, water finds its way in. Getting the barrier and flashing right matters more than the siding brand.

Comparing the Materials

The siding itself is a genuine choice, with real trade-offs in cost, maintenance, and durability.

• Vinyl: inexpensive and low-maintenance, but can crack in cold, fade, and looks less substantial; it must be hung loosely to allow expansion.
• Fiber cement: durable, fire-resistant, and holds paint well, but heavy, harder to cut, and pricier to install.
• Wood: attractive and traditional, but needs regular painting or staining and is vulnerable to rot and insects.
• Metal: long-lasting and fire-resistant, but can dent and may need finishing.

There is no universally best siding; the right choice depends on budget, climate, and how much maintenance the owner will tolerate. The calculator computes the same area regardless, but the material decides cost and upkeep for decades.

Expansion and Installation Detail

Every siding material moves with temperature, and each has an installation quirk tied to that movement. Vinyl especially expands and contracts significantly and must be fastened loosely so it can slide, nailing it tight causes buckling in summer heat and cracking in winter cold. Lap sidings must overlap correctly so water sheds down and out. These details, more than the panel count, separate a wall that stays weathertight from one that leaks or warps.

Ordering Siding as Part of a System

Use the calculator to size the siding, then treat it as the outer layer of a system. Make sure a proper water-resistive barrier and correct window and door flashing sit behind it, choose the siding material for your climate, budget, and maintenance appetite, and install it with the movement and overlap each material requires. The area gets the wall covered; the barrier, flashing, and material choice decide whether it protects the house.