The use of HPFS has become an indispensable ingredient in energy-efficient wall construction. HPFS facilitates the attainment of high R-value walls with relative ease and low cost. Without these types of advancements in thermal envelope technology, it would be more difficult for consumers to absorb today's higher energy costs.

There are two primary advantages in using HPFS on exterior walls. First, it can help achieve a high R-value wall without having to convert to 2' x 6' wall construction. Using a HPFS in conjunction with today's R-15 high density batt insulation, you can construct a wall that meets an exemplary standard necessary for energy efficient buildings in our climatic region.

The second, and less obvious, advantage to using HPFS is that it can REDUCE the potential for moisture problems in exterior wall cavities. The HPFS, through its higher R-value, keeps the wall cavity warmer, therefore, lowering the potential for condensation forming in the wall cavity. We do not recommend using plywood, OSB board or other low R-value materials as exterior sheathings or corner bracing. Because of their low R-values, potential moisture problems may increase in the wall. If these low R-value materials are used, HPFS should be applied directly on top of them to decrease the potential for moisture problems in the wall.

Primarily, three types of HPFSs are used in residential construction in our area. They are available in a wide variety of sizes and thickness. The HPFS mostly used as wall sheathings are typically a 1/2" thick.

General Recommendations
We recommend using the highest R-value HPFS available. The R-values of the foams used in most HPFS are as follows:

• Polyisocyanurate: R- 7.20 per inch thickness
• Extruded polystyrene: R- 5.00 per inch thickness
• Expanded molded beads polystyrene: R- 3.85 per inch thickness

Foam Sheathing Characteristics

Phenolic Foam
This foam is typically pink in color with a foil laminated face on both sides. It’s available in 4' x 8’ and 4' x 9’ sheets in 3/4" and 1" thickness. It should not be used below grade where it is subject to any bulk water exposure.

Polyisocyanurate
This HPFS is typically a tan or yellowish color with a bright metallic silver facing/skin. It is also available with a black painted skin on one side. It is readily available in 4' x 8' and 4' x 9' sheets with 1/2" thickness, as well as 2' x 8' sheets with 3/4" thickness.

We recommend using an HPFS with a bright metallic foil face when installed behind brick veneer, on attic walls, and on any knee walls. The reflective surface must face an air space to benefit from the reflective properties of the foil facing. This added benefit can be R-2 to R-3.

Extruded Polystyrene
This HPFS is typically green, pink, or blue. It is readily available in 4' x 8' and 4' x 9' rigid sheets with 1/2" thickness, as well as 2' x 8' sheets with 3/4" thickness. It is used not only as an exterior HPFS, but also as an insulation for concrete slabs.

Expanded Molded Bead Polystyrene
This foam board is usually white in color. It has the lowest R-value, is easily damaged, and is used very little for exterior wall sheathing. It's used mostly as insulation in slab construction. Because of its ability to absorb and hold moisture, it is not recommended for use underground or in slabs.