Attics with insulated roof decks benefit from tightly sealed gable windows.
Spray foam insulation is an effective choice for insulated roof decks and traditional insulated ceilings. The main difference between these two types of insulation is how they control the flow of air in an attic. Insulated roof decks are designed to create a warm, conditioned space in the attic, and insulated ceilings are not. Although an insulated roof deck eliminates a number of issues, it is generally more expensive than installing spray foam insulation above a ceiling.
Sealed Ceilings
Most homes have insulation applied above the ceiling to create an envelope that traps conditioned air inside the living areas of the home. Spray foam insulation applied above the ceiling plugs air leaks between the attic and the living areas, and it completely fills irregularly shaped sections of the attic. Unlike loose-fill insulation, sprayed foam does not shift easily and will not block vents or leave sections of ceiling bare.
Insulated Roof Decks
Insulated roof decks are sealed against outside air and typically have fewer moisture problems than attics with an insulated ceiling. An insulated roof deck eliminates the need to install vents and their associated hardware in the roof. Insulated ceilings are often difficult to seal due to gaps around wiring, ducts, fixtures and pipes penetrating drywall. Using an insulated roof deck eliminates the need to locate and seal those gaps. If you decide to insulate your roof deck, however, then the attic gables also must be insulated.
Cost
Spray foam insulation requires professional installation to ensure an even distribution of the product throughout the attic. Spray foam installed on the underside of the roof deck generally costs more due to the larger surface area of the roof deck and gables compared to the ceiling. If your home has air ducts or an air handler in the attic, an insulated roof deck will reduce the amount of energy lost from those systems and increase your home's energy efficiency. The need to heat and cool the additional space in the attic, however, somewhat offsets those gains.
Considerations
Insulated ceilings require 1 square foot of ventilation for every 150 feet of floor space in an attic. If the insulation has a vapor barrier, then the attic requires only 1 square foot of vent per 300 feet of floor space. The ventilation is designed to control the amount of moisture in the attic and cool the home during summer. Insulated roof decks do not require ventilation but do require regular maintenance to ensure that damage from weather, animals and insects does not penetrate the air barrier in the attic.
Tags: roof deck, foam insulation, insulated roof deck, above ceiling, deck eliminates, Insulated roof decks, roof deck eliminates