Unvented conditioned attics and roofs with spray foam insulation applied to the underside of the roof deck have been used since the mid-1990s. Open-cell spray polyurethane foam (ocSPF) or closed-cell spray polyurethane foam (ccSPF) insulation is sprayed along the underside of the roof sheathing to provide an unvented insulated and conditioned attic space or roof assembly that can be resilient, durable, and efficient in all climate zones. Unvented insulated attics can provide a tempered space for HVAC equipment, conditioned storage space, and living space (habitable space) within the conditioned boundary of the building enclosure (“thermal envelope”).

Unvented conditioned attics and roofs are significantly more “fire safe” in wildfire areas and where neighboring buildings are close. Ash and embers enter vented attics through attic vents. No vents means no ash or embers entering attics and much less risk of fire. 

In high wind regions – particularly in coastal areas – wind-driven rain is a problem with vented attics and vented roof assemblies. Additionally, during high-wind events, vented soffits have been known to collapse, leading to building pressurization and window blowout and roof loss due to increased uplift. Unvented conditioned attics and roofs are safer during hurricanes; they outperform vented attics and roofs, principally due to the robustness of their soffit construction

In coastal areas, salt spray and corrosion are a major concern with steel frames, metal roof trusses, and truss plate connectors in vented attics and vented roofs. This is not an issue with unvented conditioned attics and roofs.

In flood-prone areas, provision should be provided to allow access to roof tops if refuge, rescue, and evacuation becomes necessary. This can be provided by installing operable skylights or dormers or other alternatives that allow egress.

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 from the ducted mechanical system still contribute to space conditioning (see Figure 1).  Insulating and air sealing at the roof line is typically a more effective means of providing a continuous air control layer (“air barrier”) and thermal control layer (“thermal barrier”) in house designs that have complex coffered ceilings and numerous service penetrations and holes through the ceiling plane for lights, wiring, plumbing, ductwork, etc., which would otherwise make it difficult to achieve the airtightness needed below the insulation layer. In addition, it might not be practical to construct vented roof/attic assemblies in retrofit applications – adding roof vents at soffit locations may not be possible.

When the design 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 control layer, if open-cell spray foam is used,  and both the thermal control layer and vapor control layer if closed-cell spray foam is used.  Note that open-cell spray foam is “vapor open” and cannot act as a vapor control layer. All spray foams contain polyurethane – open-cell, closed-cell, bio-based spray foams, sugar-based spray foams, and water-blown spray foams.

Wherever unvented conditioned attics or roofs are constructed, a means of moisture removal from the attic is necessary. The “conditioned” part of unvented conditioned attics or roofs is important. 

One of the best ways to remove moisture from the attic space is to provide air change. With the air change approach, air is exhausted from the peak of the attic using an exhaust fan ducted to the exterior. This creates a slight negative pressure in the attic and air is drawn in from the house below. To create “balanced” ventilation in the house, supply air is provided from the outside to the return side of the air handler (Figure 2). The quantity of this air flow should be based on the International Residential Code (IRC) 2018. To prevent over-ventilation or under-ventilation, a motorized damper is installed at the outdoor air supply of the system. The operation of the motorized damper is coupled or linked to the operation of the attic exhaust fan – the attic exhaust fan operates only when the motorized damper is open and the HVAC system blower is operating.   

Another approach is to provide balanced ventilation with heat recovery (an HRV) or energy recovery (an ERV). When heat recovery or energy recovery is used, exhaust air is pulled from the peak of the attic (as shown in Figure 2) and supply air is provided from the occupied portion of the house.

A third approach is to provide dehumidification (a “dehumidifier”) to the attic space.

A fourth approach is to provide ducted supply and ducted return air to the attic space using the HVAC system in the house. With this approach the attic or roof space is directly coupled to the occupied portion of the house and the moisture control strategy for the house is used to provide moisture control for the attic space.  Note that this fourth approach can only be used with spray foams that are not combustible or are covered or protected from fire.

Because this approach results in a sealed attic, any combustion appliances installed in the attic should be direct-vent sealed-combustion appliances that vent outdoors.

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 and air leakage, and built-in construction moisture. Hygrothermal modeling sponsored by the DOE Building America program and conducted by Building Science Corporation confirmed that even when the roof was modeled with rainfall leaks of up to 1% through the roof sheathing or with initial moisture content of the wood framing and sheathing 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 wood moisture content was above 18%, or if repeated or prolonged leaks above 1% were experienced and if 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. (See Grin, Smegal, and Lstiburek 2013 for more details.) 

How to Install Spray Foam on the Underside of the Roof Decking

1. Make sure the installation complies with the 2018 International Residential Code (2018 IRC; see the Compliance tab for more details). The requirements of the 2018 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:
    
   1. The unvented attic space is completely within the building thermal envelope.
   2. Interior Class I vapor retarders (e.g., plastic sheeting) are not installed on the ceiling side (attic floor) of the unvented attic assembly (i.e., under the ceiling) or on the ceiling side of the unvented enclosed rafter assembly.
   3. Where wood shingles or shakes are used, a minimum ¼ inch (6.4 mm) vented airspace separates the shingles or shakes and the roof underlayment above the structural sheathing.
   4. In Climate Zones 5, 6, 7, and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.
   5. The attic or rafter assembly meets one of the following conditions regarding the air permeability of the insulation directly under the structural roof sheathing.
      1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing.
      2. Where air-permeable insulation is installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control.
      3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item (a) and shall be in accordance with the R-values in Table R806.5 for condensation control.  The air-permeable insulation shall be installed directly under the air-impermeable insulation.
      4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperatures of the underside of the roof sheathing above 45°F (7°C).  For calculation purposes, an interior temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside temperature of the three coldest months.
      5. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.
2. Ensure, when using open-cell spray foam, that a low-perm Class II vapor retarder is installed where required – typically Climate Zones 5, 6, 7 and 8.
3. Provide appropriate fire resistance and flammability requirements for the spray foam selected and installed (“thermal barrier” or “ignition barrier”).  Note that not all spray polyurethane foams require thermal or ignition barriers.
4. Refer to the current state and local building codes for the minimum R-value of air-impermeable insulation required for the roof assemblies in your climate.  Also refer to the current state and local building codes – particularly in Climate Zones 5, 6, 7 and 8 – regarding the requirements for Class II vapor retarder coatings or layers.
5. Inspect the roof assembly to ensure it has proper drainage protection above the roof deck.
6. Measure the moisture content of the wood prior to applying spray foam insulation to ensure it has dried to below 19% or to the levels recommended by the spray foam manufacturer.
7. Ensure the weather conditions and temperatures for installing the insulation are as recommended by the spray foam manufacturer.
8. Clean the surfaces of the roof sheathing and structural members so they are clear of any debris or dust to ensure proper adhesion of the spray foam.
9. Cover any mechanical and electrical equipment and wiring prior to applying the insulation.
10. Provide proper ventilation in the work area during application.
11. Install the spray foam. It is recommended to hire a licensed professional applicator for the spray foam installation.
12. Visually inspect the insulation installation to ensure that foam consistently meets the specified depth with no gaps or voids.
13. 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.
14. Install additional cavity insulation as needed to meet the desired R-value. See Figure 3.
15. Inspect and air seal all penetrations through the attic floor.
16. Provide controlled ventilation, balanced ventilation with heat recovery, install a dehumidifier, or supply conditioned house air to the attic as described above.
17.  Ensure the home has good ventilation and that any combustion appliances installed in the attic are direct-vent sealed combustion appliances that vent outside.

How to Install Ducts in an Unvented Attic

When installing HVAC equipment in an insulated conditioned attic, good HVAC design principles still apply:

• Design a compact, duct layout with short, straight ducts runs. Seal and test ductwork for air leakage.
• Install a balanced ventilation system such as a heat recovery ventilator or central fan-integrated ventilation with a fresh air intake and integrated exhaust. (For more information, see Whole House Ventilation Strategies for Existing Homes.)
• Do not install low-efficiency combustion heating systems that draw their combustion air from the attic. Instead install direct-vent sealed-combustion furnaces or heat pumps. (For more information, see Combustion Furnaces, Traditional Split Heat Pumps.)