Performance Across The Board

Extruded Polystyrene Foam Sheathing – From Manufacture To Installation

The most common components of a building envelope perform important functions, but they do not always meet their true potential. To that end, various types of plastic materials can help building owners achieve the efficiency they require. For example, extruded polystyrene (XPS) foam insulating sheathing offers many benefits due to the way this product is manufactured. Energy-efficient, easy to install, lightweight, and recognizable in its blue, pink, or green colors, the specification of extruded polystyrene foam sheathing may allow the use of a single product to create a continuous layer of thermal and moisture protection on a building’s walls, thereby contributing to energy efficiency.

Creating extruded polystyrene

Extruded polystyrene foam begins as a solid granule of polystyrene resin. The plastic granules are fed into an extruder, where they are melted and mixed with critical additives to form a viscous fluid. Next, a blowing agent is injected to enable the plastic product’s expansion. Under carefully controlled heat and pressure conditions, the plastic mixture is forced through a die into the desired shape. The rigid foam plastic is then trimmed to the final product dimensions and is usually recognized as boards.

This continuous process produces a closed-cell structure that looks like a mass of uniform bubbles with common walls between them. A continuous smooth skin on the top and bottom also forms.

The closed-cell structure of extruded polystyrene (XPS) foam imparts excellent long-term strength and durability. (See “Message from the APC” on page 4 for more on foam plastics.) Products are available in a range of compressive strengths to suit varied application needs. Due to its inherent physical properties, this strength does not depend on the use of facers or laminates, which can sometimes be compromised during installation. However, extruded polystyrene (XPS) foam faced-products are available to add extra strength when specified for a particular application. Extruded polystyrene plastic also comes in a wide variety of sizes, and up to a thickness of 102 mm (4 in.) to suit many applications.

XPS Energy efficiency

Extruded polystyrene (XPS) foam sheathing can have positive energy and air emission benefits when used in residential buildings. A 2000 Franklin Associates’ study shows far more energy is saved over the 50-year life of a home properly using XPS foam insulation than is consumed by manufacturing the insulation.1 Another study, presented at the 2004 Earth Tech forum, shows that after less than three years, more greenhouse gas emissions are avoided due to heating/cooling energy consumption than are emitted during the manufacture of extruded polystyrene foam insulating sheathing.2

Properly installed extruded polystyrene (XPS) foam can also improve a building’s energy efficiency by providing a complete layer of insulation on the wall. This reduces air movement through the wall that can rob energy. Insulation between studs does not necessarily offer complete insulation value because wood studs and other framing members are not insulated. (See “Plastics Takes Improvement to the Wall,” page 5.) This phenomenon is called thermal bridging and can dramatically decrease thermal performance of the building.

Since residential wood framing typically makes up about 25 percent of the wall area (taking into account window and door framing), a quarter of the wall is uninsulated when only cavity insulation is used. As such, extruded polystyrene (XPS) foam sheathing can provide insulating value to the entire wall area. Aside from its inherent insulating performance, extruded polystyrene (XPS) foam sheathing, when properly installed and seams taped, can also greatly reduce air leakage through walls, which can improve energy efficiency and comfort.

An essential attribute for sustainable building products, especially insulation, is the ability to function properly over its useful life without physical property performance degradation. In fact, good long-term insulation performance is needed to correctly design a building’s heating and air-conditioning systems.

Extruded polystyrene foam can also have advantages due to its ability to assist with moisture management, resisting both water absorption and freeze/thaw cycles. When traditional insulation absorbs water, its thermal performance can be compromised over time.

Reduce, reuse, recycle

Three strong environmental principles are to reduce, reuse, and recycle.3 Since extruded polystyrene (XPS) foam insulating sheathing can reduce energy loss in buildings, it can lower (i.e. reduce) the amount of energy (gas and electric) required to maintain comfortable living environments.

The more widespread the use of extruded polystyrene (XPS) foam insulating sheathing, the more profound its effect can be on reducing consumption of natural resources, such as coal, oil, and gas.

Polystyrene resin is a thermoplastic material, which means it can be melted and re-inserted (i.e. reused) into the manufacture of new extruded polystyrene (XPS) foam insulation. Extruded polystyrene manufacturing plants create virtually no scrap or waste materials because nearly 100 percent is recovered, ground up and re-pelletized for the production system (i.e. recycled). Some companies even seek outside sources of scrap polystyrene plastic resin to reuse.

Additionally, extruded polystyrene (XPS) foam can be created from post-consumer material. However, there is currently no infrastructure in the United States that makes it economically sound to collect construction material contaminated with nails, glue, etc. Should the situation change, extruded polystyrene (XPS) foam could be readily recycled once contaminants are removed from the product.3

Another choice for reducing post-manufactured material involves incineration, which is not widely used in the United States. Nevertheless, extruded polystyrene (XPS) foam could be feedstock for this technology, should it gain support. In any case, because extruded polystyrene (XPS) foam is used in structures with 15-to 50-year life spans, its effect on landfills tends to be small compared to traditional materials that may require more frequent replacement.

Extruded polystyrene foam installation guidelines

As more sheathing insulating options are made available, builders are constantly seeking easier and better installation methods. While traditionally following ongoing trends in residential building systems installation, changes in local codes and new product introductions challenge builders to erect better homes that offer more resistance to
moisture and increased energy efficiency.

Properly installed extruded polystyrene (XPS) foam insulating sheathing can provide excellent moisture and insulating properties. As mentioned, extruded polystyrene plastic products are light, versatile, and easily installed on residential walls as illustrated in the following step-by-step guide:

  1. The first step is to mark the extruded polystyrene (XPS) foam insulation boards by dragging the hook of a measuring tape along the surface of the plastic insulation board, while holding the other end of the tape at the desired measurement. This creates a small mark to follow when cutting.
  2. The plastic insulation board should then be placed on a flat, sturdy surface for cutting ease and safety. A pair of sawhorses is usually sufficient for waist-high work. Following the mark left by the tape edge, a deep impression should be made with a knife held low against the board (without cutting through).Once the entire measured line has been scored, the piece is simply snapped over the edge of the work surface. It is best to use a utility knife and straight edge to trim the extruded polystyrene (XPS) foam insulation board to conform to irregular wall angles, projections, or wall surfaces less than board width or height.
  3. Code-acceptable corner bracing (e.g. diagonal metal strapping, let-in wood, or wood structural sheathing) is then installed. If wood sheathing is used as a structural component, the building can be covered with extruded polystyrene (XPS) foam insulation boards to provide for full wall insulation.
  4. Extruded polystyrene (XPS) foam insulation boards that are 1.2 m (4 ft) wide should be installed vertically with long joints butted tightly together and bearing directly on the framing members. Horizontal joints should be minimized between boards, unless joints are positioned directly over a horizontal framing member. Taping the extruded polystyrene (XPS) foam insulation board joints seals them against air infiltration for improved energy efficiency.
  5. It is preferable to secure extruded polystyrene (XPS) foam insulation boards with 25.4-mm (1-in.) head plastic cap nails long enough to penetrate framing measuring at least 19 mm (0.75 in.). Another option is to use 9.5-mm (0.375-in.) head-galvanized roofing nails long enough to penetrate framing a minimum of 19 mm. Additionally 25.4-mm crown, 16-gauge wire staples long enough to penetrate framing a minimum of 12.7 mm (0.5 in.) can also be used. Nail heads or staples should not be over-driven.6. The field of the extruded polystyrene (XPS) foam board is then secured 406 mm (16 in.) on-center (oc) and around the perimeter 305-mm (12-in.) oc,or as required by the building code authority having jurisdiction.

In stick-built construction, one should consider using 1.2 x 2.7-m (4 x 9-ft) extruded polystyrene (XPS) foam insulation boards to effectively cover the sill plates, floor/ceiling junctures, bandbox, and headers in one application with fewer horizontal joints. Brick, wood, hardboard, aluminum, or vinyl sidings are fastened to the wood frame construction through the insulation in accordance with the siding manufacturer’s instructions. Shakes or shingles can also be applied by installing furring strips or a plywood nailer base over the insulation and attaching the shakes or shingles. The manufacturer should be consulted on the most appropriate methods.

Extruded polystyrene (XPS) foam insulation materials are available in either rigid board stock or fanfold sheet versions. The properties of extruded polystyrene (XPS) foam board stock are outlined in ASTM International C 578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. They are available in ASTM types IV, V, VI, VII, and X with square-edge, ship-lap, or tongue-and-groove edge treatments to minimize air leakage through the joints. Other extruded polystyrene (XPS) foam board options include slotted edge and boards with plastic, reflective, or perforated facers.4

About the Author

Susan Herrenbruck is the executive director of the Extruded Polystyrene Foam ssociation (XPSA), a trade association representing manufacturers of XPS foam insulation products and its raw material suppliers. XPSA members collectively manufacture more than 95 percent of all extruded polystyrene sold in North America.

Notes

1 See Franklin Associates’ Plastics Energy and Greenhouse Gas Savings Using Rigid Foam Sheathing Applied to Exterior Walls of Single-family Residential Housing in the U.S. and Canada—A case study (September 2000).

2 See “Energy and Environmental Benefits of Extruded Polystyrene Foam and Fiberglass Insulation Products in U.S. Residential and Commercial Building” by Merle F. McBridge, PhD, PE. This paper was presented at the 2004 International Earth Tech forum.

3 Recycling is not available in all areas. Check to see of recycling is available in your community.

4 For more information on XPS insulating sheathing, visit www.xpsa.com.