The three keys to a high performance attic are insulation, ventilation and air sealing.
Attics are always spaces that are overhead and beneath a roof, whether one story, two stories or more. In most instances, the insulation is installed on the floor of each attic that is above a conditioned (heated and/or cooled) living area. The attic is usually vented to the outside and contains multiple forms of attic bypasses, all requiring sealing from within the attic.
Before 1979, there were no codes or standards for building insulation, including attics. Houses before 1945 were frequently uninsulated; houses constructed in the 1950’s, 1960’s and even 1970’s usually have three to six inches of blown or batt insulation: fiberglass, mineral wool, wood fiber or vermiculite. This ranges in R-value (Resistance value) from R-6 to R-15. With the establishment of national energy standards in 1979, attic insulation was raised to R-38, about 11 inches of cellulose or 15 inches of fiberglass. Because the perimeter of attics is too low for this much insulation, attics were insulated to R-44 so that the AVERAGE R-value throughout the attic will equal a minimum of R-38.
Attic insulation R-values are now R-50 or more, for both existing and new construction. This equals about 14″ of cellulose or 18″ of fiberglass.
Attic Bypass Sealing
All attics must be air sealed before insulation is added. Air sealing means closing all gaps, openings, cracks, holes between the attic and the heated area below. This is done from the attic using caulk, spray foam, sheet metal, drywall, rigid foam board and any product that will create a firm, airtight, permanent seal over the opening. Common locations:
- Plumbing pipes
- Chimneys and flues
- Interior top plates
- Outside top plates
- Electric wiring penetrations
- Ceiling fixtures/junction boxes
- Exhaust fans/range hoods
- Dropped soffits/ceilings
Attic ventilation is the movement of air between the attic and the outside, usually through roof vents, gable vents, ridge vents or soffit vents. Different roof designs require different ventilation approaches:
- One and two or more story houses — Roof vents, ridge vents, and soffit vents in conjunction with attic air chutes (“baffles”)
- 1 1/2 and 1 3/4 story houses — Gable and/or roof vents
The objective of attic ventilation is to keep the attic temperature closer to the outside temperature. The outside air keeps attics cooler in the summer and protects the roof shingles from overheating and premature aging. Protecting shingles from high temperatures is the original purpose for building codes for attic ventilation. Newer shingles warranties no longer require that roofs be vented to protect the warranty, especially because of the increasing use of high density, closed cell two part spray foam insulation applied directly to the underside of the roof decks. This raises the thermal barrier and pressure barrier to the roof itself, and no longer requires the ventilation of the attic to the outside.
Air chutes (“baffles”) protect and preserve attic ventilation
Air chutes/baffles are used when we have vented exterior overhangs or soffits. Some overhangs are not vented, and some house designs — 1 1/2 story and 1 3/4 story– do not have exterior soffits. Air chutes are installed at the perimeter of the attic, stapled to the underside of the roof deck between the rafters, fitted down over the outside top plate and down into the overhang through the small opening between the top plate and the roof, usually the height of the rafter, about 3 1/2″.
How does the chute protect the ventilation from the soffit?
Houses built before 1980 usually have lower pitched roofs where 3-12, 4-12 or 6-12 pitches are common. The attics usually had no more than four to six inches installed, so the insulation was usually lower than the roof deck, even at the outside edge. Now we want to add more insulation but the new insulation will actually fill the entire space between the attic floor and the roof at the perimeter –right where we rely on the rafter opening over the top plate to provide ventilation. The air chute is four or six feet in length and, installed snugly against the roof deck, provides a “U”-shaped corridor or “chute” so air can continue to flow even as we raise the insulation all the way to the underside of the air chute.