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.

3 Replies to “Construction Design-Attic Truss”

  1. I have a garage with attic truss design in WA. I need R49 or R38 if the insulation runs all the way to the top plate of the exterior wall. I’d like to insulate that roof from ridge to top plate (vs insulating the knee wall and floor/ceiling) so I can build storage and book cases into that knee wall. My rafters are 2 x 12s until they get to the knee wall where the rafters/webs are reduced to 2 x 4s. How do I get R38 in that area of the roof where the webs are just 2 x 4s?

    1. Hi Glen, you didn’t mention what type of insulation you are using. I’ll assume it’s some type of fiber, fiberglass, mineral wool or cellulose. The best you’ll get is R-15 insulating a 2 x 4 cavity with fiber insulation. You have a few options to improve that, one is to add lumber to extend the roof framing to 12 inches. Another option is to use an interior rigid insulation. I’ve used Thermax, which is a polyisocyanurate foil faced insulation in the past. It has a little higher R-value than XPS or EPS foam and does not need to be covered to achieve a fire rating. We taped the seams in the insulation with a foil tape, the same type of tape used to seal ductwork. Don’t use duct tape. Unfortunately, Thermax it is kind of expensive. 4 inches will get you R-24. That, plus 3.5 inches of Rockwool Comfortbatt, which is R-15, would get you to R-39. Biggest trick to this type of roof assembly is to get it air tight. Air leakage in this area will cause ice dams during the winter months. I’d suggest a blower door test to make sure the roof has no air leaks. A continuous air chute from eave to ridge to improve roof venting also helps with the ice dams. I recommend chatting with your local code official before moving ahead with the Thermax. Last option is to use closed cell spray foam. You’d need around 5.5 inches to achieve R-38. The closed cell spray foam may need to be covered for a fire rating. Again, you may need to check with your local building official. Hope this helps! Randy

    2. Spray foam will increase R value over batting, but can’t get that high; that neds to be considered in design phase.

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