Should I Use Foam Insulation Under My Vinyl Siding?

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Installing foam insulation under vinyl siding is a wise choice for homeowners who are seeking to protect their homes against moisture infiltration as well as improve energy efficiency. It also serves as an air barrier. Generally referred to as rigid foam insulation, this type of foam insulation is also called foam board, foam plastic, insulated sheathing and rigid foam sheathing.

Care must be exercised during the installation process to ensure that the insulation works effectively to control the penetration of water and air into the wall assembly.

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Types of Foam Insulation

While most insulation products are designed to fit between the cavities of 2 x 4 or 2 x 6 framing, rigid foam board provides continuous insulation across the wall surface The result is a higher R-value than what can be achieved using most other insulating products.

There are three basic rigid foam insulation products you can choose from: expanding polystryrene, extruded polystryrene and polyisocyanurate. Expanding polystryrene, which is also called “beadboard,” is a closed-cell insulating board with an R-value of 3.8 to 4.4 per inch. Extruded polystryrene has a higher R-5 per inch, is stronger and highly resistance to moisture infiltration. Polyisocyanurate has an R-7 value per inch.

Installation Methods

Remove any railings, mail boxes and light fixtures that are attached to the work area of the house. The house sheathing must be level and structurally sound before you install the foam insulation. Examine the surface of the wood sheathing for any weak spots and repair any damage.

Before you apply foam insulation, install a house wrap on the sheathing. This will provide the house with a weather-resistant barrier. If moisture gets behind the protective wrap, the wrap allows the water to drain away and not soak into the wall component.

Rigid foam insulation is available in different thicknesses. If you purchase foam insulation that is too thick it may pose a problem by pushing the wall system too far out from the existing windows and trim. If you are doing a remodeling of the entire exterior or new construction, this may not be a concern. Specialized equipment is not required for the installation of foam board. You can cut the material with a knife.

Always follow the manufacturer's instructions for installing rigid foam insulation. To secure the foam, use two-inch cap nails. The nails have a plastic cap that keeps them from damaging the foam. Some people use roofing nails, but if a strong gust of wind gets under the siding it may not hold. Fit the rigid foam insulation snugly and seal all seams with the specialized tape. Apply sealant around windows, doors and trim. Also, caulk the joint at corner trims.

At the bottom of the wall, install a strip of half-inch plywood. Line up the bottom edge of the plywood with the bottom edge of the water table. The water table is the trim fitted around the bottom perimeter of the wall, which directs water away from the wall system.

Not only does the plywood strip give you a solid foundation for the water table, it also provides a solid bottom wall assembly, which helps keep insects from burrowing through the soft foam insulation.


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