Construction Design-Problems with Attic Trusses

This post first appeared on the Green Building Advisor Website

Attic truss, also known as a room-in-attic truss are a way to create a living space without the need to add a full second story to a home.  These factory-built roof systems simplify a roof buildout and are engineered to meet the structural requirements of your local codes and conditions.  I commonly see them used for bonus rooms over garages and integrated into slab-on-grade homes to add living space without needing to add to the height of the building.

There are a few ways to insulate and air seal these roofs.  Currently, the most common method in my market is to use closed cell spray foam to insulate 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 works across the ceiling at the lower-level ceiling of the home.  Achieving a good level of air tightness when using the conditioned space floor, walls and ceiling of the attic truss is difficult, especially the transition where the knee wall meets the attic floor/lower-level ceiling.

Recently I’ve had a rash of building investigations where homes with this type of roof design are having issues in my cold and snowy climate.  All the recent issues used 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.

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 or 2×8 for the top rafter cord.  After the ventilation space is created, there is only room for 4-6 inches of closed cell foam, achieving an R-value of between R-28 to R-42 if the space is completely filled.  Spray foam insulation contractors do not want to remove closed cell spray foam that has expanded beyond any framing member, so, often the space is not filled completely, resulting in lower R-values.  This lower R-value and thermal bridge is seen in the photos below.

I’ve found a couple reasons for this higher heat loss, the first is closed cell spray foam that has separated from the truss cord, creating a pathway for conditioned air to move along side the truss and into the ventilation cavity and/or roof sheathing, as shown in the thermal image.

The second reason is often the truss design includes girder trusses where two or more of the roof trusses have been mated together for a structural purpose.  The metal gang plates that are installed at the factory often keep the trusses slightly spaced, creating a pathway for warm, conditioned air to move to the roof sheathing.  These photos show this condition.

The upper photo shows the gap created by two attic trusses being fastened together for structural reasons.  This photo was taken from the outside after a portion of the roof material and sheathing had been removed.  You can see some moisture on the foam that was installed for the ventilation chute (upper left side of the photo).  The lower photo is that same truss on the interior.  You can see where the insulation contractor has tried to seal the gap between the trusses, this additional insulation and air sealing was done as a repair after the snow melt was discovered on the roof after the homes first winter.

Another issue with this roof design is the thermal bridge that occurs between the roof sheathing and top rafter cord when installing closed cell spray foam at the roof sheathing.  What’s common in my market is closed cell spray foam being applied to encase the top cord, but the level of insulation isn’t sufficient to eliminate the thermal bridge.  When you get to the sloped wall portion that connects the knee wall to the flat ceiling, there is no insulation covering the truss, which results in even more heat loss to through the roof truss.

The condition is even worse if trying to use a fibrous insulation and a membrane to create an air barrier (a method I see in older homes using an attic truss).  This insulation strategy results in the top cord having no insulation on the bottom side of the cord to reduce the thermal bridge for the length of the cord.

The Fix.

I’ve had conversations with several different contractors who are experiencing these conditions on how to remedy the issue.  Several have brought up sealing the girder truss using a sealant before the truss is assembled and/or set.  Another suggestion is to increase the size of the ventilation space.  This may help some, but, in my opinion, there is a better way, the addition of continuous insulation above the roof sheathing.

This will result in an unvented roof assembly, that for the most part, will be thermally broken from the attic truss.  There are a few requirements for this assembly to work correctly.  The structural layer of roof sheathing needs to be airtight.  This can be accomplished by using the products in the illustration (Zip sheathing and tape) or applying an airtight membrane before the addition eave framing and the rigid insulation.  (This design results in the exterior wall and roof air seal being continuous, often called monopoly framing.)  The second requirement is that the ratio of interior insulation to exterior rigid insulation be followed.  The ratio is to ensure the structural roof sheathing stays above the temperature where condensation may form.

If you want to torture yourself, read the code language in the International Residential Code, Chapter 8, Roof Ventilation, specifically R806.5.  Two better places to learn more about this subject is in an article by Dr. Joe Lstiburek: BSI-100: Hybrid Assemblies | buildingscience.com

Or an article by Allison Bailes III, PhD: The Ratio Rule for Hybrid Roof Insulation – GreenBuildingAdvisor

My suggestion for reducing the issues with attic trusses is to insulate as shown in the Fine Homebuilding illustration above.  For my market, climate zone 7, and when using the 2021 IRC energy code and Table 806.5, we will need a total of R-60 roof insulation with R-36 being above the structural roof sheathing (vapor impermeable rigid insulation with offset seams) and R-24 below.  Because of the difficulties in insulating these factory-built trusses on an angle, I would use an open cell spray foam (6”-7” for climate zone 7) below the roof sheathing.  The top cord of the attic truss should be specified as 2×8 to allow enough insulation when a short, sloped ceiling connects the knee wall to the flat section of attic.  If you do not want to use the open cell spray foam, another option would be to install all R-60 above the sheathing.

Section R806.5 has a few other rules that need to be followed.

1. The unvented attic space needs to be completely inside the thermal envelope.
2. There cannot be a class I vapor retarder (polyethylene sheeting) either on the ceiling side (attic floor) or on the ceiling side of the unvented attic assembly.
3. Climate zones 5,6,7, and 8 require any air impermeable insulation required to be or have a class II vapor retarder in direct contact with the underside of the insulation. It cannot simply be a friction fit fibrous insulation batt that does not achieve a Class II vapor retarder.

This section of code has several other requirements and, is where information pertaining to vapor diffusion ports is found.  Refer to section 806.5 for more information.

I’ve probably had more than a dozen of these building investigations over the past two years.  Often the condition presents as water dripping from the insulated roof during the winter months while the home is under construction.  Other times it is the snow melt and ice dams that form after the home is completed.  A few of these investigations have included water damage in a completed, new home.  Repairs at that time can include removal of a portion of the roof from the exterior.  The lucky contractors find and seal the issues from the interior, neither never fully cure the snow melt patterns that form on the roof.  Changing the insulation strategy is really the only way to solve the problem.