Exterior XPS Retrofits Over Plank Sheathing

Let’s get this house properly retrofitted for energy efficiency in your Massachusetts climate! You’re making a wise choice with continuous exterior XPS insulation, especially in a cold climate like yours. Using reclaimed XPS from Green Insulation is also a great sustainable approach, reducing both costs and environmental impact. Here’s a detailed breakdown of the layers, materials, and methods focusing on best practices for energy efficiency, moisture management, and code compliance.

Looking for high-quality, reclaimed XPS insulation? Green Insulation offers cost-effective, sustainable solutions that help you achieve top-tier energy efficiency while reducing environmental impact. Contact us today to get the right materials for your retrofit project!

Understanding the System and Goals

Before diving into layers, let’s clarify the goals and principles:

• Continuous Insulation (CI): This minimizes thermal bridging through studs, significantly improving overall wall R-value and reducing heat loss. XPS is an excellent choice for this purpose.
• Moisture Management: In a cold climate, we need to prevent interior moisture from condensing within the wall assembly while also protecting against exterior bulk water intrusion. This requires proper vapor diffusion and bulk water control.
• Air Sealing: Airtightness is crucial for energy efficiency and comfort. We want to minimize uncontrolled air leakage through the wall assembly.
• Durability: The system needs to be robust and long-lasting, resisting weather, pests, and physical damage.
• Code Compliance: Always verify local Massachusetts building codes, which are based on the International Energy Conservation Code (IECC). Specific R-value requirements will depend on your climate zone and local amendments.

Understanding Plank Sheathing and Its Challenges

Older homes often use plank sheathing instead of modern plywood or OSB. Plank sheathing consists of wide, rough-sawn boards installed diagonally, horizontally, or vertically over wall studs. While it provides structural stability, its rough surface and large gaps between boards create significant challenges for air sealing and moisture management. These gaps allow uncontrolled airflow, leading to heat loss, drafts, and increased risk of condensation inside the wall assembly. Additionally, its uneven surface makes it difficult to properly adhere air barriers and WRBs.

One solution is to install OSB or plywood over the plank sheathing before applying an air barrier, which creates a smooth, continuous surface for sealing and fastening. This approach significantly improves airtightness and simplifies installation but increases material costs and adds thickness to the wall assembly, potentially requiring adjustments to window and door trim. If cost or space constraints are a concern, a fluid-applied air barrier directly over plank sheathing can work but requires meticulous sealing of gaps.

When dealing with board sheathing (plank sheathing) in older homes, you need to address air sealing, structural stability, and moisture control before adding continuous exterior insulation. Below is a step-by-step guide to properly retrofit insulation while ensuring a durable and airtight assembly.

Layer-by-Layer Breakdown (From OSB or Plank Sheathing Outward)

Layer 1: Existing Oriented Strand Board (OSB) or Plank Sheathing

Description: This is your starting point. It provides structural support and serves as the primary base for air sealing and moisture management.

Action:

• Inspection: Check for any signs of rot, damage, or delamination. Replace damaged sections as needed.
• Fastener Check: Ensure OSB or planks are properly fastened to the studs with appropriate nails or screws.
• Clean Surface: Remove dust and debris to ensure good adhesion for tapes and future layers.

Layer 2: Air Sealing with Fluid-Applied Air Barrier

Description: A robust air barrier at the sheathing level is critical for energy efficiency. A fluid-applied air barrier ensures a seamless and long-lasting seal, particularly important for rough plank sheathing where gaps must be sealed effectively.

Materials:

• Fluid-applied air barrier products (e.g., Prosoco R-Guard, Henry Blueskin, or Tremco ExoAir).
• High-quality sheathing tape for additional reinforcement.
• Sealants for penetrations (polyurethane or advanced polymer caulk).

Installation:

• Seal Sheathing Gaps: Apply fluid-applied air barrier across the entire plank sheathing surface, ensuring it penetrates and seals gaps.
• Seal Penetrations: Apply a thick bead of sealant around any wall penetrations (plumbing, wiring, vent openings) and use the fluid-applied barrier over the area.
• Window and Door Jambs: Ensure the air barrier extends into the rough openings to integrate with window and door flashing later.
• Bottom Plate/Foundation Junction: Apply sealant where the bottom plate meets the foundation to prevent air leakage.

Layer 3: Weather-Resistive Barrier (WRB) – Moisture Protection

Description: The WRB provides bulk water protection and a drainage plane, ensuring water sheds outward rather than soaking into the sheathing.

Materials:

• Vapor-permeable WRB (e.g., Tyvek CommercialWrap, Typar, or Henry VP100).
• WRB Tape for sealing seams and penetrations.
• Flashing Tape (Butyl or Acrylic) for window and door flashing details.

Installation:

• Install WRB Over Air-Sealed Plank Sheathing: Roll out the house wrap horizontally, starting at the bottom and working up in a shingle-lap fashion.
• Integrate With Flashing: Windows and doors should be flashed before WRB installation, ensuring water sheds away from openings.
• Seal All Seams: Tape all horizontal and vertical seams to create a continuous moisture barrier.

Layer 4: Continuous XPS Foam Insulation

Description: This is the heart of your energy efficiency retrofit. XPS provides excellent thermal performance and moisture resistance.

Materials:

• Reclaimed XPS foam panels from Green Insulation (R-5 per inch recommended for Massachusetts).
• Insulation fasteners with large plastic washers (cap nails or screws).
• Low-expansion spray foam for sealing small gaps.

Installation:

• Stagger Joints: Align XPS panels in a brick pattern to minimize thermal bridging.
• Tight Fit: Fill small gaps with low-expansion spray foam to maintain airtightness.
• Mechanical Attachment: Fasten XPS to studs using long screws (at least 3.5″ for 1″ XPS and 4.5″ for 2″ XPS).
• Seal Fastener Penetrations: Apply dabs of fluid-applied air barrier or patches of flashing tape over fasteners to prevent air leaks and moisture entry.

Why Use Reclaimed XPS from Green Insulation?

Cost Savings – High-performance insulation at a fraction of the cost.
Sustainability – Reduce waste by using recycled materials.
Local Availability – Convenient supply for Massachusetts builders.

Start Your Retrofit Today!

For high-quality, reclaimed XPS foam panels, contact Green Insulation—your trusted supplier in New England. Improve your home’s efficiency with a sustainable and cost-effective retrofit solution!