Fullback® Thermal Support System

Weather Resistant Barrier Statement

Vinyl siding has always been designed as an exterior cladding, not a weather resistant barrier. Vinyl siding is designed to allow the material underneath it to breathe; therefore, it is not a watertight covering. Because of its design and application, it provides a supplemental rain screen that enhances the weather resistant barrier system by reducing the amount of water that reaches the underlying weather resistant barrier.

The Fullback® Thermal Support System is a contoured foam underlayment that is designed to level the wall, and improve energy efficiency, while supporting the surface profile of the siding, giving the vinyl siding a more durable wood-like feel and appearance. Fullback is the most breathable foam insulation on the market and is also highly resistant to water absorption. Fullback is not, however, a weather resistant barrier.

Third-party tests have proven that wind-driven rain can penetrate an exterior cladding system comprised of vinyl siding with the Fullback Thermal Support System as an underlayment. Although water can potentially penetrate the siding envelope, the design of the Fullback Thermal Support System allows the water to freely exit through the drainage plane and the wall system will completely dry within 24 hours. (See Wind-Driven Rain Resistance Test Paper)

What is a Weather Resistant Barrier System?

It is a system that includes water shedding materials and water diversion materials. Weather resistant barrier systems commonly consist of a combination of exterior cladding, flashed wall openings and penetrations, weather resistant barrier material, and sheathing. Effective weather resistant barrier systems will shed the water initially, control moisture flow by capillary and diffusion action, and minimize absorption into the wall structure. The level of weather resistance required is determined by the applicable building code and climate.
Best Practice

To achieve designed performance, Progressive Foam Technologies recommends the use of a weather resistant barrier any time that the base wall is constructed from unfinished wood or water permeable materials. Vinyl siding with Fullback inside must be installed over a weather resistant barrier system that includes 1) a continuous weather resistant material and 2) properly integrated flashing around all penetrations and where vinyl siding interfaces with other building products such as brick, stone or stucco.

Refer to the VSI’s “Vinyl Siding Installation: A How-To Guide” or the manufacturer’s installation manual for specific product applications and recommendations. Whichever product(s) you decide to use as part of a weather resistant barrier system, be certain the materials meet the applicable code by contacting the manufacturer of the weather resistant barrier material(s).

Always consult the applicable building code for minimum weather barrier requirements in your area. Keep in mind that additional measures may provide better protection against water intrusion than the minimum requirements of the building code.