Using spray foam in an attic is a simple way to improve the energy efficiency of your home. It is particularly useful in for energy efficiency upgrades to existing homes. Most existing homes have an array of unsealed areas such as drop ceilings and bulkheads, plumbing and electrical service penetrations, leaky ducts and duct penetrations. Adding conventional air permeable insulation alone won’t solve the problem. Insulating and air sealing with spray foam can.
Spray foam can be applied to floor of the attic to air seal or insulate or the underside of the roof deck to encapsulate the space. If there is no ductwork in the attic and there are no plans to turn the attic into occupied space, you may consider adding insulation to the floor of the attic. However, if ductwork and equipment in the attic make it difficult to access areas of the attic floor, or if there are plans to convert the attic into living space, it could make more sense to insulate at the roof deck. Applying spray foam to the underside of the roof deck can create an “unvented attic.”
Insulating the Attic Floor
Traditionally, homes have used vented attics, which allow for air to move through the attic to control moisture. To meet code, homeowners and builders historically insulated the floor of the attic. That may still be the right option for your home, but it is important that it be air sealed to prevent conditioned air from leaking out of the house. Spray foam is an ideal choice for insulating the floor of the attic because it is air impermeable and a powerful insulator. This will keep conditioned air in the living area of the home, and the warm or cool air in the attic from entering the home where it can cause condensation, mold and other problems.
Unvented Attics
In an unvented attic, insulation is applied to the interior side of the roof deck. Spray foam is an ideal choice for unvented attics because it acts as an insulation and air barrier and, most importantly, it sticks to the surface where it is applied. Spray foam expands on application, filling gaps and sealing air leaks in the attic walls and roof. Because it is applied on site as a liquid, it adheres to the roof deck and will not sag or compress with time. Ventilation is not required in attics with spray foam because spray foam can control moisture levels. This protects the roof sheathing and allows the attic to function as conditioned space.
Unvented attics keep duct work in conditioned space. This means that conditioned air that leaks from the ducts will not be wasted. (In a conventionally insulated attic, duct leakage would have been lost to the exterior because vents provided for moisture control also allow energy to escape.) Air leaks from ducts will remain in conditioned space and can passively condition the air in the attic and occupied space. Duct air leakage commonly exceeds 20 percent of conditioned air flow, which results in a significant energy loss when ducts are located in unconditioned space like vented attics.
Unvented attics also help limit loss of conditioned air through the attic (stack effect). Stack effect is the natural tendency of air to rise up through a building and out through the attic.
Like a hat on a cold winter day, unvented attics help reduce energy loss through a home’s attic. An unvented attic can keep a home warmer in winter and cooler in summer, reducing the HVAC load.[1] Unvented attics in most cold climates decrease the heating load by about 10 percent.[2]
Spray foam can also act like an adhesive and help hold the roof deck in place during high wind events. The fact that an unvented attic has no vent openings limits the potential for ingress of wind driven rain and snow.
A Great Choice for New and Existing Construction
All homeowners have the opportunity to benefit from energy savings of unvented attics. Unvented attics work great in both new and existing construction. In a new building, the unvented attics can be installed during the construction process with spray foam applied directly on the underside of the attic walls and roof, leading to continuous coverage and maximum air sealing. In an existing building, the spray application for spray foam allows the product to be applied around any existing obstacles and into the hard-to-reach spaces of an unvented attics and unvented crawlspaces.
Energy Efficiency of Unvented Attics
Unvented attics bring duct work into conditioned space. This means that conditioned air that leaks from the ducts will not be wasted. It will remain in conditioned space and can passively condition the air in the occupied space. Duct air leakage commonly exceeds 20 percent of conditioned air flow, which results in a significant energy loss when ducts are in unconditioned space such as vented attics.
Unvented attics also reduce the heat loss/gain through mechanical equipment and as a result of limiting the loss of conditioned air through the attic (stack effect). Stack effect is the natural movement of cold air into the bottom of a structure, which is pulled by warm air exiting the top of the structure. In summer the process reverses and warm air comes in through the attic and exits through the lower floors; this tends to make upstairs rooms less comfortable.
[1] http://energy.gov/sites/prod/files/2014/01/f6/1_1c_ba_innov_unventedconditionedattics_011713.pdf