As part of the insulation system for the house, 1″ Polyisocyanurate (ISO) foam was installed on the exterior of the framed structure. Polyisocyanurate is the best performing (7.2R per inch / R=insulation factor) foam board that’s readily available , the foam’s foil facing creates a radiant barrier to help reflect heat back into structure in the winter and away from structure in the summer. As the panels are installed the seams are taped and counter-flashed to to ensure that the insulation layer is airtight and provides effective drainage plane.

Why Exterior Foam?

• Continuous Insulation – Installing the foam on the exterior of the structure allows for a continuous layer of insulation around the structure.  While interior insulation continues to play an important role exterior insulation has the unique ability to avoid the problems associated with interior installations specifically the complex spaces in which insulation must fit within the home.
• Fewer transmission points – One of the issues with conventionally-framed homes is that each stud creates a transmission point between the inside of the home and the outside, allowing cold to come in and heat to go out. or vice-versa in the warmer months. With the exterior covered in foam, the studs don’t make contact with the outside and the transmission effect is reduced.
• Framing becomes part of system – With insulation on the exterior, the framing (studs) become part of the home’s overall thermal mass because they are inside the building envelope. This means far less a problem with studs creating hot and cold points in the structure. As they become warm, the studs actually become part of the radiant structure of the house.
• Less condensation – Because the framing is always warm, condensation within the framing is likely to occur

On the exterior of the foundation 2″ extruded polystyrene was installed on the foundation walls (more on the foundation insulation).

Rainscreen

A rainscreen was installed on top of the 1″ foam. The rainscreen, created by 3/8″ thick strips of plywood nailed to the house (over the insulation), serves a couple of purposes. First, it provides a drainage plain allowing water that gets underneath the siding to easily drain from the home. Second, and perhaps more importantly, it creates an air gap between the sheathing (or in this case the foam insulation), promoting through drying of the home’s siding and allowing for water vapor to easily escape.

The system in a nutshell

First goes the house wrap…

Then the foam…

And the rainscreen!

Interior Insulation

The current plan is to use a closed cell spray foam in the walls and the ceiling on the interior of the house. Because we are using an unvented roof assembly with no ridge vents, the attic will technically act as conditioned space. This will allow us to put the HRV and other mechanical equipment in this area, and have them located within the conditioned envelope. We do plan to performance test the house once the insulation (interior an exterior) is complete to check for air loss.

Resources for Exterior Insulation

Harley, Bruce. (2007, July 1). Foam Sheathing, Inside or Out? Fine Homebuilding, 188, 96-98.
July 1, 2007

Arnold, Rick. (2006, August/September). Save Energy With Rigid Foam Insulation. Fine Homebuilding, 88-91.