Where to locate the thermal boundary is an important design consideration when designing the roof-attic-ceiling assemblies in new construction. The thermal boundary consists of the thermal barrier (insulation) and the air barrier (for example, caulked sheathing or mudded and taped drywall). These two layers should both be continuous around the building envelope (walls, ceiling, floor), and they should be in continuous contact with each other.

In a roof-attic assembly, the builder has two choices: the thermal boundary can be at the ceiling plane, leaving the attic space above uninsulated and vented, or the thermal boundary can be located at the roof line, creating a sealed, insulated, conditioned or semi-conditioned attic space. When the thermal barrier is located at the roof line, it can be either above or below the roof decking. Either way, the attic would be unvented.

See Figures 1 through 3 for examples of thermal boundary locations for vented and unvented attics for three attic designs: flat ceiling/gable roof, low-sloped shed roof, and 1-and ½-story home. 

Vented Attics

When the thermal boundary is at the ceiling level, the ceiling drywall serves as the air barrier; any gaps around penetrations (for wiring, flues, ducts, etc.) are carefully air sealed. The thermal barrier consists of the insulation (typically blown fiberglass or cellulose, sometimes spray foam and batt), which is piled on top of the ceiling drywall. Vents are installed at the soffits and at or near the ridge, and baffles are installed at each soffit vent to provide a pathway for ventilation air from the soffit to the ridge and to keep insulation from blocking the soffit vents. The lower soffit vents are intended to "supply" air to the attic space, and the upper roof/attic vents are intended to "exhaust" air from the attic space. Ridge vents are more effective than off-ridge vents such as "mushroom" or "button" vents in this regard. In general, gable-end vents should not be used in conjunction with a ridge or off-ridge vents because the gable-end vents are likely to "short-circuit" attic air flow. 

In hurricane and high-wind locations and fire-prone locations, the IBHS Fortified Home program recommends not installing gable-end vents in new homes. Use certified, properly installed ridge or off-ridge vents rather than gable vents. For existing homes that have gable vents, either permanently block the gable vents or provide removable or operable exterior or interior coverings or shutters that can be installed when a hurricane or fire threatens and removed when the threat has passed. Permanent mounting anchors should be installed for these coverings. To provide more robust wildfire resiliency, mesh screening should be installed at soffit vent openings. The recommended mesh size is 1/8 inch (3 mm) or less. Ridge vents that are compliant with Miami-Dade wind-driven rain requirements and are effective at wind-driven rain control are also effective in controlling the entry of airborne embers or cinders.  

Unvented Attics

The key to creating an unvented roof assembly is to keep the roof deck – the principle condensing surface in roof assemblies – sufficiently warm throughout the year such that condensation will not occur or to prevent interior moisture-laden air from accessing the roof deck. This is done by using air-impermeable insulation such as rigid foam board or spray foam. Rigid foam insulating sheathing is installed on top of the roof sheathing. For more information on this technique, see the guide Above Deck Rigid Foam Insulation for Existing Roofs. Spray foam is installed on the underside of the roof sheathing. For more information on this technique, see the guide Spray Foam Under Roof Sheathing. Both methods require a very high degree of airtightness for the avoidance of condensation due to the warming of cold surfaces. For the above-deck rigid foam, air tightness is achieved by installing a continuous membrane over the roof sheathing and under the rigid foam. This membrane could be a peel-and-stick or paint-on membrane or a carefully installed underlayment. For below-deck insulation, the spray foam will provide condensation control. However, all wood-to-wood joints in the framing must still be sealed. In both assemblies, air-permeable insulation (such as batt or loose fill) can be used to increase the overall insulation value. For the above-deck rigid foam option, the batt or blown insulation would go below the decking. The minimum required thickness of the "air-impermeable insulation" to manage condensation potential is stated in Table R806.5, "Insulation for Condensation Control," of the 2012 IRC. For cold climates, the air-impermeable insulation should be maintained at 50% or more of the total R-value of the roof system for condensation control. For example, if an R-80 unvented cathedralized attic is to be constructed in a cold climate, a minimum of R-40 (50%) should be air-impermeable insulation installed and layered according to Section R806.5 of the 2012 IRC (Figure 4). See the IRC or the guide Unvented Attic Insulation for details.

Choosing between a Vented and Unvented Attic

Factors to consider when determining where to locate the thermal barrier in new construction include cost, climate, desire for additional living and storage space, building design and configuration, and location of HVAC. Some factors are listed below. See the Retrofit tab for additional factors that should be considered in existing homes.

Vented attics are generally less expensive than unvented attics. There are three reasons for this. First, the types of insulation appropriate for insulating an attic floor (blown-in cellulose or fiberglass) tend to be less expensive than the types of insulation appropriate for insulating a roof and gable walls (spray foam or rigid foam board). Second, less insulation is required because the area being insulated is smaller. Finally, installation at the attic floor is often faster and less complex.

Depending on house geometry, an unvented attic can have 10% to 60% more square feet of surface area needing insulation than a vented attic. This larger area is due to the fact that the total area of a sloped roof plus gable and dormer walls is greater than the area of the attic floor. This directly affects not only cost, but also the efficiency of the home. All else being equal, an envelope assembly with 10% to 60% more area will have 10% to 60% more heat losses and gains, resulting in more load on the HVAC system.

Vented attics can reduce heat gains to the conditioned space by ventilating and exhausting heat from the attic. The insulation on an attic floor is essentially "shaded" from the sun by the roof above it. Attics, though hot in summer, are not as hot as the roof above them. Vented attics also allow fairly simple installation of radiant barriers.

While a vented attic has the advantages of less surface area and the ability to exhaust heat, the performance of the insulation itself can be much poorer than that of an unvented attic. Because the insulation in a vented attic is typically loose-fill insulation, it is susceptible to settling. This can result in a significant loss of R-value. Additionally, the fact that the loose-fill insulation in vented attics is open at its upper surface invites heat loss and gain through convection, especially in winter. Any movement of air through insulation greatly reduces its effectiveness. Air-sealing details can also be more challenging in a vented attic than an unvented attic due in part to more penetrations such as lighting, electrical wiring, and ductwork. Finally, if HVAC equipment is located in the attic, an unvented assembly will be much more efficient because the ductwork and air handler will be inside the thermal envelope. Ultimately, which type of attic assembly is more efficient depends on several factors including the difference in surface area, the effectiveness of any attic ventilation, the presence of a radiant barrier, the presence of a cool roof, the degradation of insulating ability through settling and open convection, the effectiveness of air-sealing details, and whether or not HVAC equipment is located in the attic.

A vented attic insulated at the attic floor makes it easier to detect and solve roof leaks or other moisture-related issues. If a leak does occur, moisture damage is less likely because the exposed and ventilated rafters and decking can dry out more readily. The underside of the roof decking and the rafters or top chords of the trusses are relatively easy to access compared to an unvented assembly where the decking and rafters might be encased in spray foam or sandwiched between the roof and the ceiling material (in the case of a cathedralized ceiling).

However, unvented attics have several advantages over vented attics. In coastal locations, hurricane regions, and areas prone to wildfires, it may be advisable to construct an unvented conditioned attic because soffit and roof vents can be an entry point for wind-blown rain and burning embers as well as salt-laden air which can corrode fasteners and HVAC equipment. Eliminating vents can also reduce the likelihood of entry into the attic by insects, rodents, birds, and other pests.

Other reasons for constructing an unvented attic include the following. The builder (or homebuyer if the buyer has purchased prior to construction) may want the attic space to be conditioned for additional living or storage space. The home design may have a complex interior design that would require many transitions between flat ceilings, knee walls, trey ceilings, and/or vaulted ceilings; in cases where numerous transitions make it difficult to achieve a continuous thermal barrier at the ceiling, it may be simpler and thus less expensive to insulate at the roof line.

A design decision to have a central ducted heating and cooling system with ducts and/or air handler located in the attic is another good reason to insulate and air seal at the roof line, as this will provide a conditioned or semi-conditioned space for the HVAC equipment. This can improve HVAC performance and longevity because the equipment does not have to work against the much greater temperature differentials of a vented attic. If HVAC is located in the attic, the energy efficiency advantages of using an unvented attic assembly will likely outweigh the energy losses through increased insulation area.

For installation guidance for vented and unvented attics in new and existing homes, see these Building America Solution Center guides. For more information, search “attics” or “roofs” in the Solution Center:

• Unvented Attic Insulation
• Attic Eave Minimum Insulation
• Attic Kneewalls
• Insulation Below Attic Platforms
• Blown Insulation for Existing Vented Attic
• Batt Insulation for Existing Vented Attics
• Spray Foam Insulation Applied to Existing Attic Floor
• Below Deck Spray Foam Insulation for Existing Roofs
• Application of Spray Foam Insulation Under Plywood and OSB Roof Sheathing
• Above Deck Rigid Foam Insulation for Existing Roofs
• Disaster-Resistant Roof Venting