Ceilings, Attics & Roofs

Ceilings

The roof/ceiling of a house is one of the largest sources of heating and cooling loads. The situation is compounded when the HVAC system is located in the attic. Even small improvements in the energy efficiency of residential roofs / ceilings can result in significant energy savings. Extruded polystyrene (XPS) rigid foam can be used to enhance the performance of the insulating system for any framed ceiling assembly. The use of XPS in the ceiling assembly increases the value of any home by:

• Reducing operating costs (lower energy bills),
• Reducing the opportunity for water condensation within building assemblies and,
• Increasing comfort by maintaining a uniform temperature throughout the house by overcoming thermal short-circuiting.

For all framed ceiling assemblies XPS is attached to the underside of either the ceiling joists or the bottom chord of the truss. It is particularly beneficial when used in vaulted or cathedral type ceilings (see illustration) with limited space (depth) for insulation in the cavities between framing members. This application of XPS not only adds to the R-value of the cavity insulation but also minimizes the impact of “thermal short-circuiting” (accelerated heat flow) through framing members. This is important to both traditional wood frame construction and – even more so – when using metal framing members.

Note: Building codes require that any foam plastic insulation adjacent to habitable space be covered with a 15-minute thermal barrier. This is easily accomplished by simply using the same gypsum drywall interior finish as is found in the vast majority of new home construction.

Attics

When an attic space is finished off to create additional living space, “knee walls” are built to provide the vertical enclosure. Knee walls are framed just like exterior walls or interior partitions, with a bottom plate on the attic floor, studs spaced 16” or 24” on center, and a top plate fastened to the underside of the roof rafters. The stud cavities are insulated like standard frame walls and XPS sheathing is attached to the backside of the knee wall – facing the attic space – providing the same thermal and moisture control benefits as it does for exterior walls and for ceilings.

For both knee walls and exterior walls, when XPS sheathing is extended to the roof line of gable ends , the sheathing is exposed to unconditioned attic space. In these applications, building codes require that foam insulation be covered with an ignition barrier, unless the manufacturer has obtained special recognition for leaving the foam exposed. Consult the XPS manufacturer’s building code evaluation report to obtain specific information.

Roofs

For timber frame or post-and-beam construction, an insulated roof is in order. Here it is desirable to leave the framing members exposed to the living space below, so the foam insulation is installed over the roof decking. Since heat loss can occur through the wood framing – leading to ice damming, water backups, and leakage inside the home - XPS is, again, ideal for the following benefits:

• Reducing operating costs (lower energy bills),
• Reducing the opportunity for water condensation within building assemblies and,
• Increasing comfort by maintaining a uniform temperature throughout the house by overcoming thermal short-circuiting.