Construction Design-Attic Truss

An attic truss is a type of roof truss with an integrated living space built into its design.  This truss is commonly used over garages to create a bonus room, but sometimes the space becomes bedrooms, offices, or even second living rooms. If the design is not well thought out, this added living space often has problems with air leakage and low insulation values.

I recently conducted an energy audit and blower door test on a home that was designed with an attic truss over a garage.  The space featured a direct connection to the rest of the home, two bedrooms, a half bath and second living room area.  The space looked nice, but did not perform well.  Cold in the winter and hot in the summer.  I’ve encountered several of these poor performing attic truss spaces through the years.

Problems with the design.  Lets start with the top cord of the truss.  This cord is often a 2 x 6, which will only allow a maximum of R-21 insulation in the sloped ceiling if using fiberglass batts without an air space above the insulation.  R-35 is possible if closed cell spray foam is used.  The home I tested had R-19 fiberglass with no air space.  This low R-value, no attic ventilation and several areas of air leakage contributed to ice dams and water damage during this past winter and early spring.

Another common problem is air sealing the knee wall areas.  Often the builder will install the subfloor directly to the bottom cord and drywall to the knee wall truss cord without installing any blocking or using an air sealing strategy.  (See drawing below.)  This allows unconditioned air from the knee wall attic space to flow between the subfloor and drywall at the area of the baseboard and into the conditioned living space, often creating comfort issues and high heating costs.  I’ve seen this problem in a new, high end home.

Thermal imaging of a room directly attached to an attic truss at a recent energy audit.  There is a connection between the unconditioned knee wall space of the attic truss and this common wall between the attic truss living space and this room.

How about heating the space.  The home where I recently conducted the energy audit used the knee wall space for a ductwork chase from the forced air furnace.  The heat loss from the ductwork would warm the unconditioned knee wall areas enough during the heating season to further contribute to the ice damming issues.  Any ductwork is recommended to remain in the conditioned space of the home.  Ductwork installed in unconditioned spaces of new construction require duct tightness testing and duct insulation, adding to the cost of constructing the home.  The home I recently visited had central air conditioning, but required a second air conditioning system (mini-split air conditioner) to cool the space during the summer and hot water baseboard heaters were added for comfort during the winter.

Design the air sealing and insulation details around how the space is used.  If the space is used as cold storage or only used during warm weather, install an exterior door that seals the space from the rest of the home and be sure the common wall between the attic truss and the rest of the home is both well insulated and air sealed.  The space then becomes an unconditioned attic.

When the attic space is connected with the rest of the home, air sealing and insulating the room within the attic truss must be well thought out.  A good air/vapor control layer needs to be on continuous and on all sides, including the floor!  In Minnesota, the vapor control layer needs to be on the warm in winter side of the assembly.  I have yet to see this vapor control layer installed on the floor.

Is the space below the attic truss conditioned?  If it’s a garage, additional fire protection may be needed.  Will there be a carbon monoxide concern?  In Minnesota, heated garages that store vehicles that are driven during the winter will have an elevated humidity level.  (Snow and ice that accumulates on the vehicles melt, causing high humidity in garages.)  This will need to be addressed as well.

Attic trusses are a relatively inexpensive way to get additional living space in a home.  Additional thought must be given to these designs to assure there won’t be future problems such as ice dams, high heating costs, comfort complaints, or CO or moisture problems from the space below.

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