What to Put Under Cedar Siding: Best Subsurface Materials and Methods

Choosing the right materials beneath cedar siding is essential for preventing moisture intrusion, promoting airflow, and extending the life of the wood. This guide covers effective underlayment options, drainage strategies, and installation best practices tailored for American homes. It emphasizes moisture control, proper ventilation, and code‑compliant approaches to protect cedar from rot, mold, and insect damage while keeping the exterior aesthetically pleasing.

Key Functions Of Underlayment For Cedar Siding

The layer under cedar siding serves several critical purposes. It creates a drainage plane that guides water away from the structural wall, provides a stable surface for fasteners, and allows for ventilation behind the siding to reduce moisture buildup. A well‑designed underlayment also acts as a barrier against wind-driven rain, helps minimize pin‑hole leaks, and supports long‑term siding performance. When selecting materials, homeowners should prioritize moisture management, durability, and ease of installation.

Common Materials For Under Cedar Siding

There are multiple approaches to underlayment, each with strengths and tradeoffs. The following options are commonly used on cedar siding installations across the United States:

• Breathable House Wraps With Drainage Planes — Materials such as engineered breathable membranes create a ventilation gap while shedding water to the exterior. They’re often paired with a drainage plane to channel water down and out at openings. Pros: good moisture management, relative ease of installation. Cons: must be correctly installed to maintain the drainage gap.
• Rain-Resistant Building Paper — Heavy‑duty asphalt-saturated paper or synthetic wrap can provide a moisture barrier and some air movement. Pros: cost‑effective, familiar installation. Cons: less airflow than modern membranes, may require additional drainage components.
• Rigid Foam Board Underlayment — Exterior insulation board (EPS or XPS) can provide insulation and a stable nailing surface, often installed with furring strips to create an air gap. Pros: improves insulation value, reduces thermal bridging. Cons: must be properly vented to prevent moisture entrapment, potential for moisture traps if not installed with drainage plane.
• Foam Sheathing With Ventilation Furring — Polystyrene or polyiso boards installed over sheathing with vertical furring to create a cavity for air flow. Pros: high insulation, keeps cedar drier. Cons: careful detailing around corners, edges, and penetrations required.
• Specialty Drainage Mats Or Rainscreen Systems — Rainscreen products incorporate a continuous drainage gap and support a true capillary break. Pros: superior moisture management and airflow. Cons: higher cost and more complex installation.

Drainage, Vapor, And Ventilation Considerations

Effective cedar siding performance hinges on controlling moisture movement. Key considerations include:

• Drainage Gap — Maintain an open gap (typically 1/4 to 1/2 inch minimum, larger in damp climates) between the siding and the underlayment to allow water to drain quickly.
• Ventilation Space — Create a continuous air channel behind the cedar to reduce humidity buildup. Vertical furring strips or a rainscreen provide this channel.
• Moisture Barrier Placement — Install a water-resistive barrier on the exterior side of the sheathing, with seams sealed and flashing at all penetrations to minimize water entry.
• Termite And Insect Protection — Use approved termite shields where required and ensure the underlayment does not bridged with soil contact that could attract pests. Avoid gaps that create pest pathways.
• Code Compliance — Local building codes may dictate specific underlayment types, venting requirements, and clearance from soil. Always verify with the local inspector before installation.

Installation Best Practices

Following best practices ensures the underlayment performs as intended and extends cedar siding life. Key steps include:

1. Prepare The Wall Surface — Clean, dry, and free of protrusions. Repair any damaged sheathing and ensure framing is plumb.
2. Install A Water-Resistive Barrier — Secure a WRB over the sheathing with proper overlap and flashing at corners, windows, and doors. Seal all seams and penetrations.
3. Lay Drainage Planes Or Ventilation Furring — Install the chosen drainage/ventilation system following manufacturer guidelines. Maintain the recommended gap width and secure components firmly to the wall.
4. Provide A Clean Nailing Surface — If using rigid foam or furring strips, ensure a continuous, flat surface for cedar siding nails. Use corrosion‑resistant fasteners appropriate for exterior wood.
5. Flashing And Detailing — Install flashing at all openings and transitions. Seal joints neatly to prevent water intrusion. Use galvanized or stainless steel for longevity.
6. Inspect For Level And Square — Before attaching cedar boards, verify the wall is plumb and the underlayment is properly aligned to avoid warping or gaps in siding.

Maintenance And Inspection Tips

Ongoing maintenance helps cedar siding perform well over decades. Consider these practices:

• Regular Inspections — Check for signs of moisture intrusion, mold growth, or loose fasteners behind cedar boards. Look for staining or cupping that indicates trapped moisture.
• Clean Or Reseal As Needed — Clean cedar surfaces to prevent mold and mildew. Reapply protective finishes as recommended by the finish manufacturer and weather exposure.
• Gutter And Downspout Maintenance — Ensure proper drainage away from the siding. Clear gutters and downspouts to prevent water from pooling near walls.
• Repair Damaged Components — Promptly repair damaged WRBs, flashing, or drainage components to maintain moisture control and airflow.
• Seasonal Considerations — In humid climates, prioritize a robust drainage plane and ventilation to manage higher moisture loads; in dry climates, focus on preventing wind-driven moisture intrusion.

Frequently Asked Questions

Why not simply install cedar directly over OSB or plywood?

Directly applying cedar to a solid wall surface without a drainage plane or ventilation can trap moisture, leading to rot, warping, and reduced siding life. A proper underlayment and ventilation gap reduce these risks.

Do cedar houses require a specific underlayment in coastal regions?

Coastal regions with higher moisture and salt exposure benefit from high‑quality breathable membranes and robust flashing details to resist salt ingress and humidity, along with a well‑ventilated drainage plane.

Is a rainscreen system necessary for all cedar siding installs?

Rainscreens are highly effective for moisture management and long‑term durability, especially in damp, humid, or high‑rain areas. They are not strictly required but are a widely recommended option for modern installations.