Attics and roofs present particular challenges for builders. As a first line of defense against inclement weather and a year-round building cap that plays a major role in climate-control, they have to be done right, and insulation is a big part of the job.
The most important thing to know about attics and roofing is that they can be either vented or unvented, and the choice has less to do with your climatic conditions and regional codes than with the design of the building itself. In either case, the main goal of a roof design is to make sure it stays free from moisture problems and controls air infiltration.
Vented roofs and attics are coupled to the exterior and create an air barrier at the ceiling line to isolate the attic space from the rest of the building. When installing vented roofs, it’s a good idea to keep ductwork and HVAC ducts inside the air barrier. In certain climatic zones, vented roofs constructed without spray foam insulation require a vapor retarder, but when spray foam insulation is used, no vapor retarder is required, regardless of climate. With vented attics, any moisture that infiltrates the attic is vented back out through attic ventilation.
“Supply ductwork and air handler leakage can be more than 20 percent of the flow through the system.”
Unvented attics are generally easier to design and construct. The more complex a building’s design, the more challenging it is to include a vented attic, which requires an air barrier at the ceiling plane. It’s also becoming standard for architects and engineers to build a house’s ductwork and mechanical systems into the attic space, which can create air leaks. In hot and humid conditions, ductwork can also attract condensation.
In unvented attics, rigid insulation (such as closed-cell spray foam) is recommended so that the roof deck stays sufficiently warm enough to prevent the moisture in the interior air from getting to it.
Unvented roofs have the following advantages:
- Health and comfort of the occupants—The thermal, moisture, and air-control boundaries of the house are in the roof deck, so the attic space has the same air conditions as the rest of the house, and air movement between the two doesn’t cause temperature fluctuations throughout the structure.
- Energy efficiency—Eliminating the need to insulate and air-seal ductwork and HVAC equipment means less air leakage to and from the living space below.
- Durability—Unvented roofs are not susceptible to wind, rain, and snow entering the attic through cracks, because there are no gaps for ventilation.
- Value—Finished, unvented attics create more living or storage space, which raises the value of the property.
“Air-impermeable insulations are typically low density or high-density spray foams. Netted or blown cellulose, fiberglass or rockwool insulation are not considered air impermeable.”
In cold climates or during cold seasons, roof ventilation’s job is to control ice dams that result from melting snow runoff, and to vent moisture as it moves toward the attic. Snow often melts not just in the spring but throughout the winter because of heat loss from the attic, both due to air leakage and conducive heat—often having to do with ceiling ductwork that’s at a lower R-value than the surrounding insulated roof.
In hot climates or during warm seasons, roof ventilation expels solar-heated air from the attic to cool the building—but this amount of cooling is generally very minor.
In windy conditions or climates, driving rain can be an issue with vented roofs. Unvented roofs tend to perform better during hurricanes because their construction is more resilient. In coastal areas, vented roofs can present further issues because of their metal frames and trusses, which are susceptible to corrosion from salt spray.
“The amount of attic cavity ventilation is specified by numerous ratios of free vent area to insulated ceiling area ranging from 1:150 to 1:600 depending on which building code is consulted, the 1:300 ratio being the most common.”
Spray Foam Insulation Versus Traditional Materials
In 2005, the Oak Ridge National Laboratory (ORNL) conducted a study on the actual R-value performance of various types of insulation under real-world (not lab) conditions. They found that traditional fiberglass batting which was labeled in the lab as R-19 actually performed at R-17 once installed in a building, and only if it was installed perfectly. More common installation methods resulted in an even lower actual R-value of as low as 13.7.
Spray foam insulation systems tested at a much higher level, maintaining 74% and 83% of their labeled R-value even during extremely cold conditions, compared to the 46% rate of fiberglass. During hot conditions, SPF systems maintained 61% and 67% of their R-value, versus the 51% for fiberglass.
Closed-cell spray foam formulations can be used in unvented attics to prevent air leakage even when HVAC equipment, ducts, recessed lighting, and other types of “penetrators” would ordinarily create gaps and cracks. Spray foam insulation creates an airtight barrier and easily seals a non-uniform cavity as it cures. This can be very important in driving wind and rain conditions, preventing internal pressurization of the roof and creating a waterproof barrier.
Closed-cell spray foam, used in unvented roofs, works in any climate zone and perform the following ways:
- Control heat flow—Closed-cell spray foam has superior thermal performance and a very high R-value, and can often exceed code when it comes to thermal performance.
- Control vapor diffusion—Closed-cell spray foam resists and reduces diffusion of water vapor so that condensation issues in the roof are avoided.
- Control rain leakage—Closed-cell spray foam is largely impermeable to water. The amount of water that can penetrate it is negligible, so it’s an excellent rainwater barrier.
- Control fire hazard—In windy conditions, spray foam prevents windborne ashes and burning particulates from entering the attic and posing a fire hazard. (Note that in some zones additional ignition barriers might need to be installed in order to meet fire codes.)