The Hidden Dangers Of Installing Plywood Siding Over Existing Siding
When it is time to update the exterior of a home, many property owners consider installing new plywood siding directly over the existing exterior. While this might seem like a time-saving measure, it introduces significant structural risks related to trapped moisture. Understanding how wall assemblies breathe and shed water is critical to preventing long-term damage, such as wood rot and mold growth.
Understanding Water-Resistive Barriers
To grasp why layering siding can be problematic, it is important to understand the role of a moisture barrier, officially known as a water-resistive barrier in building codes. Products like asphalt-saturated building paper or modern synthetic wraps are designed to be water-resistant, not entirely waterproof. This is a crucial distinction in building science. A waterproof layer, such as a thick plastic sheet, would completely trap moisture inside the wall cavity.
A proper water-resistive barrier stops liquid water from penetrating the wood framing but remains vapor-permeable. This allows the building to breathe. As temperatures fluctuate, condensation naturally forms within wall assemblies. Permeable barriers ensure that this trapped water vapor can slowly escape, preventing the interior wall cavities from growing mold or experiencing structural decay.
The Problem With Painted Siding
The primary issue with installing new plywood siding over old plywood siding is the creation of unintended moisture barriers. When the original exterior siding was painted, that layer of paint sealed the wood and acted as a vapor retarder. If you place a new sheet of plywood siding directly over that painted surface, you are sandwiching multiple impermeable layers together.
If water infiltrates the new outer layer, it becomes trapped against the old painted siding. Because the layers are pressed tightly together without adequate airflow, the moisture cannot dry out. Over time, this trapped moisture will absorb into the wood fibers, leading to accelerated deterioration. While adding an additional layer of synthetic house wrap between the old and new siding might seem like a protective measure, it can actually complicate the drying process by adding yet another barrier to the assembly.
Proper Air Flow And Drainage
If you are using a grooved product like T1-11 plywood siding, the vertical grooves can provide a very minor air channel behind the new material, assuming it is installed directly over a flat surface. This can theoretically allow small amounts of incidental moisture to drain downward.
However, for this minimal drainage path to work, the bottom edges of the siding must remain entirely unobstructed. If the bottom gaps are filled with caulking or sealant, any water that gets behind the panels will pool at the base of the wood. This trapped water will quickly wick upward into the plywood, causing the plies to delaminate and rot.
Conclusion
While installing new siding over existing siding is physically possible, it fundamentally alters the way a wall assembly handles moisture. To ensure the longevity of the structure, it is always safer to remove the original exterior covering, inspect the underlying framing for hidden damage, and install a single, properly integrated water-resistive barrier and siding system.
Three Key Tips
Recognize the difference between water-resistant and waterproof materials. Exterior wall assemblies must use water-resistant, vapor-permeable barriers so the structural framing can dry out.
Be cautious of existing paint layers. Paint acts as a moisture barrier. Placing new wood siding tightly against old painted siding creates a trap where condensation and intrusive water cannot easily evaporate.
Never caulk the bottom edge of grooved siding. Vertical grooves can act as minor drainage channels for incidental moisture. Sealing the bottom edge blocks this escape route and forces water up into the wood.
Bonus Questions And Answers
Do Building Codes Allow Installing New Siding Over Existing Siding?
While local building jurisdictions vary, the International Residential Code does allow new siding to be installed over existing exterior coverings under specific conditions. However, the code mandates that the new exterior wall covering must be securely fastened to the underlying structural framing, not just to the old siding. Additionally, all window and door flashings must be properly integrated to prevent water intrusion. Because achieving proper flashing integration is incredibly difficult when adding a second layer of siding, many structural engineers and building inspectors strongly recommend tearing off the old siding first to ensure the home remains watertight.
What Is A Rainscreen System And Why Is It Recommended?
If a homeowner insists on installing new siding over an existing exterior, utilizing a rainscreen system is the universally accepted best practice. A rainscreen involves installing vertical wood or plastic furring strips over the existing wall before attaching the new siding. This creates a continuous physical air gap, usually between a quarter-inch and three-quarters of an inch deep, between the old and new layers. This capillary break ensures that any water penetrating the new siding can freely drain down and out of the wall assembly, while constant airflow allows the materials to dry rapidly, preventing rot.
How Does Climate Affect Moisture Trapped Inside Walls?
Vapor drive is a building science principle describing how moisture moves from warm areas to cold areas, and from high-pressure areas to low-pressure areas. In cold climates during the winter, warm, moist air inside the heated home tries to push outward through the walls. In hot, humid climates during the summer, the reverse happens as heavy exterior humidity pushes inward toward the air-conditioned interior. If multiple layers of painted plywood and synthetic house wraps are stacked together indiscriminately, this natural vapor drive is blocked. The condensation is forced to drop out of the air as liquid water inside the wall cavity, which inevitably leads to mold and framing decay.
