Problems with Attic Trusses – GreenBuildingAdvisor – GWC Mag

Attic trusses, sometimes called room trusses, are factory-built systems that simplify a roof build-out and are engineered to meet the structural requirements of local codes and conditions. They are used to create additional living space without adding a full second story to a home. I commonly see them used for bonus rooms over garages and integrated into slab-on-grade homes to add living space without increasing building height significantly.

There are a few ways to insulate and air-seal these roofs. The most common method in my market is to use closed-cell spray foam on the underside of the roof sheathing from the eave to the peak. Some contractors will use some sort of chute to try to ventilate, others will create an unvented roof assembly (hot roof).

Another option is to use a batt or blown insulation and insulate the flat portion at the peak of the truss, down the short, sloped wall to the knee wall. Anytime fibrous insulation follows the pitch of a roof, ventilation is required. The insulation then runs across the lower-level ceiling. Achieving a good level of airtightness is difficult when using the floor, walls, and ceiling of the attic truss for conditioned space. It is especially tricky at the transition where the knee wall meets the attic floor/lower-level ceiling.

I’ve had a rash of building investigations where homes with this type of roof design—in our cold snowy climate—are having issues. All the recent issues included closed-cell spray foam installed at the roof sheathing. All were ventilated roof designs with the vent starting at the eave and ending at the roof peak.

Closed-cell spray foam and attic trusses

The first problem is the short, sloped wall that connects the knee wall to the flat ceiling. Most truss designs use either a 2×6…