Reduce Thermal Bridging With Polyiso Continuous Insulation

May 2nd, 2013

There is growing interest in creating energy efficient commercial structures for a growing number of reasons. One of the keys to efficiency is a high performance building envelope. When heat or cold cannot penetrate the structure from the exterior or interior, there is opportunity to minimize energy loss and maintain high R-values.

Traditionally batt insulation or blown insulation was placed between the studs in commercial designs as a cost effective solution for keeping heat or cold from passing through the wall. The problem with this is that while the insulation serves as a barrier, the studs and sometimes other exterior sheathing components do not. This dynamic, known as thermal bridging allows heat to pass through the studs.

Thermal Bridging

Thermal bridging happens when materials that are poor thermal insulators come into contact with each other. Heat flows through the path of least thermal resistance. Thermal bridging reduces R-value, which is a measure of how effective a material is in reducing the conduction of heat.

With the growing use of metal construction as a cost effective framing method, temperatures pass more easily through metal studs than wood. The wall sheathing assemblies of choice to avoid thermal bridging are designed with continuous insulation (CI). That‘s because batt wall insulation between the studs can reduce a wall’s R-value by up to 50%.

Continuous Insulation

By definition continuous insulation in frame construction runs “continuously over structural members and is free from significant thermal bridging.” (The Buildings Energy Code). The purpose then of Continuous insulation is to reduce or eliminate thermal bridging.

Continuous insulation uses rigid foam board, usually polyisocyanurate (polyiso) as a combination sheathing board and insulation component. Polyiso wall insulation is dense so, while air doesn’t permeate the surface, it’s also possible to seal against air and water. Facer selection based on application is key to air and water tightness.

Continuous insulation is a requirement prescriptively in North American climate zones two though eight according to the latest ASHRAE energy requirements. Polyiso wall insulation panels butted end to end create a recognized continuous insulation assembly. The high R-value of polyiso wall insulation makes it an ideal component for applications designed to maximize thermal performance and reduce thermal bridging.

When the application calls for a vapor barrier or water resistive barrier, this can be accomplished by taping joints with an approved tape or sealant. Radiant barriers can be created using foil facers for reflective insulation properties.

Sealing the Building Envelope

Any time continuous insulation is penetrated there is a risk of thermal bridging, so the way rigid insulation panels are fastened is critical. Fastener length, cladding type and thickness determine how much thermal bridging still exists when the continuous insulation is penetrated for any cladding attachment.

Atlas Roofing Corporation began marketing polyiso wall insulation products more than 15 years ago. The company remains at the forefront of wall insulation product development for thermal performance in the building envelope. Atlas Wall CI Board (http://www.atlaswallci.com/) is positioned to meet the needs of architects, specifiers, building owners, dealers and contractors who use polyiso wall insulation in their commercial and residential wall assemblies to increase energy efficiency, including the reduction or elimination of thermal bridging.