Because most heat loss in a home is through the attic, increasing attic insulation is one of the most cost-effective steps homeowners can take to improve their home’s energy efficiency. High-efficiency and ultra-efficient insulation meets the insulation levels required by the International Energy Conservation Code (IECC).
- vented attics that are insulated over the ceiling deck with increased amounts of blown fiberglass or blown cellulose;
- unvented attics that are insulated on the underside of the roof deck with blown spray foam; and
- vented or unvented attics that are insulated above the roof deck with rigid foam.
High-efficiency attic insulation helps provide added thermal protection, lessens wasted energy, and increases enhanced comfort and quiet.
Unvented Roofs with Applied Spray Foam
Unvented roofs with spray foam applied to the underside of the roof deck have been used since the mid-1990s. Open-cell spray polyurethane foam or closed-cell spray polyurethane foam (ccSPF) insulation can be sprayed along the underside of the roof sheathing to provide a conditioned and insulated attic space that can be durable and efficient in all climate zones (see Figure 1).
Figure 1. This illustration shows a sloped roof with cavity spray-foam insulation sprayed on the underside of the roof deck and covered with sprayed-on thermal or ignition-barrier coating.
Moving the air-control layer and thermal-control layer to the underside of the roof deck has significant advantages in cases where the HVAC system is located in the attic. By locating HVAC equipment and ducts within the thermal envelope of the home, conductive thermal losses are minimized, and any losses due to air leaks still contribute to space conditioning (see Figure 2).
Figure 2. An attic that is unvented and is insulated along the roof line provides a conditioned space for HVAC equipment that is located within the home’s thermal envelope.
Insulating and air sealing at the roof line may also be a more effective means of providing a continuous thermal barrier in house designs that have complex coffered ceilings and numerous holes through the ceiling plane for lights, wiring, etc., which would otherwise make it difficult to achieve the airtightness needed below the insulation layer. In addition, it might not be desirable (in hurricane or wildfire areas) or practical (in retrofits) to add roof vents at soffit locations.
Advantages of Spray-Foam Insulation
When the choice is made to insulate along the underside of the roof deck, spray foams have advantages over other insulation types because of the ability of spray foams to effectively air seal complex assemblies. The spray foam can also serve as the thermal- and vapor-control layers in both new and retrofit construction.
Despite the advantages of using spray foam under the roof deck, there are some potential risks. The primary risks are rainwater leaks, condensation from diffusion, air leakage, and built-in construction moisture. Hygrothermal modeling sponsored by the U.S. Department of Energy’s Building America Program and conducted by the Building Science Corporation confirmed that even when the roof was modeled with rainfall leaks of up to 1% through the roof sheathing, or with the wood framing and sheathing having an initial moisture content (MC) of up to 18%, roofs insulated with open- or closed-cell spray foam could dry out sufficiently on a seasonal basis. Damage could occur if the wood moisture content was above 18%, or if repeated or prolonged leaks above 1% were experienced and the wood was unable to dry out. However, proper construction techniques, including the following measures, will minimize or eliminate the potential for moisture-related roof damage.
How Spray Foam Should Be Installed on the Underside of the Roof Decking
Make sure that the installation complies with the International Residential Code. The requirements of the 2015 IRC Section R806.5, “Unvented attic and unvented enclosed rafter assemblies,” are summarized here: Unvented attic assemblies and unvented enclosed rafter assemblies are permitted if all the following conditions are met:
- The unvented attic space is completely within the building’s thermal envelope.
- No interior Class I vapor retarders (e.g., plastic sheeting) are installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed rafter assembly.
- Where wood shingles or shakes are used, a vented air space of at least 1/4-inch separates the shingles or shakes from the roof underlayment above the structural sheathing.
- In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation is a Class II vapor retarder or has a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.
- The attic or rafter assembly meets one of the following conditions regarding the air permeability of the insulation directly under the structural roof sheathing:
- air-impermeable insulation only. Insulation is applied in direct contact with the underside of the structural roof sheathing;
- air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation is installed directly above the structural roof sheathing;
- air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation; or
- where rigid foam insulation is used as the air-impermeable insulation layer, it is sealed at the perimeter of each individual sheet interior surface to form a continuous layer.
- A leak-free roof membrane that is fully adhered to the roof sheathing should be installed.
- When open-cell spray foam is used, a low-perm Class II vapor retarder is installed where required.
- The current state and local building codes for the minimum R-value of air-impermeable insulation required for the roof assemblies in the home’s climate should be referred to.
- Inspect the roof assembly to ensure it has proper drainage protection above the roof deck.
- The moisture content of the wood prior to applying spray-foam insulation should be measured to ensure it has dried to below 18% or to the levels recommended by the spray foam manufacturer.
- Ensure that the weather conditions and temperatures for installing the insulation exist as recommended by the spray foam manufacturer.
- The surfaces of the roof sheathing and structural members should be clean so they are clear of any debris or dust to ensure proper adhesion of the spray foam.
- Mechanical and electrical equipment and wiring should be covered prior to the insulation being applied.
- Proper ventilation in the work area should exist during application.
- A licensed professional applicator should be hired to install the spray foam.
- Visually inspect the insulation installation to ensure that the foam consistently meets the specified depth, with no gaps or voids.
- Refer to the current state and local building codes for definition and requirements for the ignition and thermal barrier, as well as vapor diffusion retarder requirements.
- Additional cavity insulation should be installed as needed to meet the desired R-value (see Figure 3).
Figure 3. Open- or closed-cell spray foam is applied to the underside of the roof sheathing, and additional fiberglass or cellulose insulation is blown in as a cost-saving method for meeting high insulation requirements and filling in the cavity space between the rafters to the ceiling deck.
How Ducts in an Unvented Attic Should Be Installed
When HVAC equipment is installed in an insulated conditioned attic, sound HVAC design principles still apply:
- A compact duct layout with short, straight duct runs should be installed. Ductwork should be sealed and tested for air leakage.
- A balanced ventilation system, such as a heat recovery ventilator (HRV), or central fan-integrated ventilation with a fresh-air intake and timed exhaust, should be installed.
- Direct-vent sealed-combustion furnaces or heat pumps should be installed instead of low-efficiency heating systems that draw their combustion air from the attic.
The classes for vapor retarders, as defined by the IRC, are:
- Class I: 0.1 perm or less;
- Class II: 0.1 perm to 1.0 perm; andd
- Class III: 1 perm to 10 perms.
A very high degree of airtightness can be provided with a continuous membrane adhered to the top surface of the structural roof deck, along with a layer of rigid insulation over it to provide condensation control. The spray foam itself can provide adequate airtightness when it is applied under the roof decking between framing elements; however, all wood-to-wood joints in the framing must still be sealed.
The roof must be kept dry before the spray foam is applied. A fully adhered roof membrane will protect the roof before the cladding is installed and provide a secondary layer of drainage protection.
The ceiling plane must be air sealed, and openings around penetrations through the ceiling plane for ducts, flues, wiring, soffits, etc., must be sealed to prevent warm, moist air from entering the attic space.
The following conditions should be met to prevent the risk of moisture damage to the roof when applying spray foam under the roof decking:
- the installation complies with the latest International Residential Code;
- a fully adhered, leak-free roof membrane is installed;
- the roof sheathing and framing are dry (below 18% moisture content) before the spray foam is applied; and
- when using open-cell spray foam, a low-perm Class II vapor retarder is installed where required.
Spray foam was applied on the cathedralized attic of a home located in Climate Zone 6. It has an estimated R-21. All sheathing locations that were measured were less than 18% MC. No indications of moisture damage were observed.