In many homes, attic spaces are constructed with a roof ventilation system that includes soffit vents, baffles, and ridge vents. Under the appropriate conditions, ventilating an attic can keep it cool and dry, which helps prevent moisture problems that lead to mold growth and reduced R-value of attic insulation.
However, if the majority of the home’s air handling equipment and ductwork will be located in the attic, the best practice is to construct an unvented, insulated, and sealed attic. This situation occurs most often in homes with slab-on-grade foundations, which lack basements or crawlspaces to house air handling equipment.
In an unvented attic, the insulation and the air barrier are installed on the underside of the roof sheathing and attic gable end walls rather than within the top floor ceiling assembly. Also, no soffit vents or ridge vents are installed. Insulating and sealing the attic in this way extends the home’s thermal envelope to include the attic. It allows conditioned air from the top floor to circulate into the attic space, making it a more hospitable environment for the air handler to operate in, compared to a traditionally vented attic.
Locating an air handler in a vented attic is problematic because it exposes the equipment to extreme temperature and humidity conditions. Heat gain in a vented attic during the hottest summer days forces air handlers located there to work much harder, which reduces efficiency, places more wear-and-tear on the system, creates a less comfortable interior, and costs the homeowner more money because of higher energy bills.
To compensate for decreased HVAC system performance, some subcontractors may recommend installing cooling system equipment that is far larger and more expensive than is required by industry-standard load design for a given situation. In sharp contrast, cooling equipment installed in an unvented, insulated, sealed attic can actually be of a smaller capacity, yet work more efficiently and effectively.
Installation
The first installation step is to be sure no vents at the soffits, ridges, or other areas are installed in the attic. Vented attics above attached garages, porches, etc. need to be separated from the home’s attic. This may require a knee wall to be built between the top plate of the exterior wall and the underside of the roof sheathing.
Next, install insulation on the entire underside of the roof sheathing to meet building code minimums. In most climates either open or closed-cell spray-applied foam insulation, typically polyicynene or polyurethane based, could be used. Closed-cell spray foam insulation also acts as a moisture barrier, preventing moisture-laden air within the home from reaching the roof sheathing where, in certain conditions, it can condense. When open-cell spray foam insulation is used in a cold climate, it may need extra protection from household moisture, as described below. Only in very hot-dry climates can blown-in cellulose insulation be used, because the moisture considerations in this climate are so reduced. Finally, some foam insulations can remain exposed in the attic without a fire barrier; be sure to check with the manufacturer and local building codes.
The final installation step is to install the vapor retarder. Its application is driven by the climate and the type of finish roofing material that will be used (asphalt shingles, clay tile, etc.).
Hot-humid climate. For asphalt shingles, install a vapor retarder that’s less than 1 perm (e.g. roofing membrane) to prevent solar heating from driving moisture into the attic from outside. For clay tile and concrete tile, which have a vented airspace between them and the roof sheathing, only typical building paper is needed. The same goes for finish materials that are installed on spacers.
Hot-dry climate. Only typical building paper is needed for all finish roofing materials.
Cold climate. For all roofing finishes, a vapor retarder must protect the spray foam insulation that covers the underside of the sheathing. The simplest solution is to spray vapor retarder paint directly onto the surface of the foam insulation. Another possibility is to paint the top floor ceiling with vapor retarder paint and make all penetrations airtight. On the exterior, only typical building paper is needed.
Costs
For a project completed in a hot-humid climate by building science firm, IBACOS, the net upgrade cost for constructing an unvented, insulated, sealed attic was $4,500 for a 6,506 sq. ft. home. The cost/savings breakdown is as follows: $9,000 for installing polyicynene spray foam insulation on the underside of the roof sheathing (4,500 sq. ft. @ $2.00 per sq. ft.); minus roof venting ($750 savings), minus air sealing at the top floor ceiling ($750 savings), downsizing the cooling equipment by 3 tons total ($1,500 savings), and minus the cost of traditional attic insulation ($1,500). Although constructing the unvented attic space is costlier initially, over time, the homeowner will see a return on this investment in the form of more consistent, comfortable indoor temperatures, lower energy bills, and increased longevity of the air handling equipment.
More Best Practices® for insulating your homes can be found in BuildIQ University’s online training course, Insulation & Air Sealing.
