An Insulation for All Reasons
I’ve included in this article a short catalog of some of the problem areas addressed on the Thermal Bypass Inspection Checklist that I find are frequently missed.

What stands out about all of these problem points is that they can be difficult to get right with inexpensive fiberglass insulation unless a builder is working with an experienced and service-minded insulation crew. However, these problems are easily avoided when using closed-cell spray foam (ccSPF) insulation. This alone provides a strong argument for always using ccSPF, but it’s certainly not the only reason.

There are many reasons why ccSPF makes particularly good sense in a coastal home:

• It has a high R-value of 6.5 to 7 per inch.
• It absorbs a negligible amount of water. It can even be used as an effective secondary rain barrier and is the only FEMA-approved insulation for flood-resistant construction.
• It does a good job of controlling diffusion.
• It has good air barrier qualities to reduce airflow into and out of wall cavities.
• It expands to fill voids in hard-to-
insulate areas.
• It provides some structural integrity to the frame (see “The Structural Properties of Foam,” page 26).

Steve Easleyis principal of Steve Easley Associates, a company based in Danville, Calif., that provides building-science training and quality assurance for builders nationwide. All photos by the author.

Attic Knee Walls
These are areas where the insulation on the back side of unsheathed walls is exposed to outdoor temperatures and airflow. They are often adjacent to ventilated attic areas. The Energy Star Thermal Bypass Inspection Checklist requires that an air barrier be placed on all sides of the insulation. This means that the back sides of knee walls need to be sheathed and sealed. Thin-profile cardboard sheathing with ccSPF works well here. Since ccSPF is air impermeable, the insulation does not have to fill the entire cavity, and it meets the air barrier requirement. Some codes require R-19 insulation, which is difficult to do in a 3 1/2-inch space with batt insulation, but 2 inches of ccSPF provides about R-19.5.

Band-Joist Areas
The band-joist area is typically a major site for air infiltration. These areas are usually very poorly insulated, causing one of the most significant thermal bypass areas. If the home is under a positive pressure (air pushing out from inside) in a heating climate, the air is likely to be at a high humidity level. This can cause frost, and eventually mold, to build up on the back side of the band joist. In a cooling climate that is under negative pressure (air pulled inward — a condition that’s commonly caused by leaky HVAC ducts), this can pull hot, humid air from outside, where it is likely to condense and lead to mold problems. The sealing properties of ccSPF will reduce these air-infiltration and energy-loss problems in this troublesome area.

Sloping Roof Areas
The sloping areas in a cathedral ceiling can be the sites for significant thermal bypasses. These areas are not only difficult to insulate but are difficult to ventilate. Yet ccSPF solves both problems. Placing ccSPF directly on the underside of the roof deck also creates a secondary rain barrier, and because of ccSPF’s high R-value and low permeability, moisture is not likely to condense on its surface, eliminating the need for cavity ventilation.