Attic Anatomy: Rafters or Roof Trusses?
One of an attic’s most important features is its framing. Many older houses (and plenty of new ones, too) are framed with rafters –2x or engineered lumber that extends from the ridge board at the roof peak down to the top plate of the exterior wall, where each rafter typically overhangs to form the eaves of a house. Rafters are cut and installed one “stick” at a time, so this type of roof is sometimes referred to as a “stick-framed” roof. Stick framing calls for the ceiling joists (that also serve as the attic floor joists) to be installed before the rafters. An attic framed with rafters will provide a fairly open space that can be used for storage, or even converted to living space. In contrast, a house built using roof trusses will have an attic space full of angled web framing that connects the top chords (analogous to rafters) to the bottom chord, which frames the ceiling beneath the attic space. The attic space beneath a trussed roof shouldn’t be used for storage –not just because of the constricted space, but also because the bottom chords of the trusses are not engineered to support substantial loads.
Attics and the Building Envelope
Where is the insulation in your attic? Most attics have insulation installed in the attic floor. This puts the attic space itself outside the “building envelope,” which we define as the “conditioned” space that is heated and cooled for comfort. But if you have a “conditioned” attic, the insulation will be installed between attic rafters, and perhaps directly beneath the rafters as well, to keep the attic comfortable when it’s uncomfortably hot or cold outside. When an unconditioned attic space is remodeled into living space, the position of the insulation has to change, bringing the attic inside the building envelope.
Attic Ventilation: Passive & Active Options
Any unconditioned attic requires ventilation. Without some means of ventilation, moisture and (in summertime) heat can accumulate to undesirable levels in the attic space. Poor attic ventilation can also be a contributing factor in the formation of ice dams in winter. The standard way to ensure adequate attic ventilation is to install soffit vents along the eaves of house (mounted in the soffit area) and a continuous ridge vent along the ridge. This passive ventilation system works by convection. As the warmest air in the attic rises naturally and escapes through the ridge vent, this exhaust action draws an equal volume of cooler air into the attic through soffit vents. Ridge and soffit vents can be retrofitted on houses that don’t have them. Vents are sometimes installed in gable-end walls as part of the passive ventilation system, but they don’t eliminate the need for soffit and ridge vents. In warm weather, the surface of your roof can get as hot as 160°, heating up the attic space and making rooms directly below the attic uncomfortably warm. This puts a big load on your air conditioning system, causing spikes in your electric bill. One way to minimize this problem is to install an attic fan. This active ventilation system is inexpensive and easy to install. An attic fan will turn on automatically when the attic reaches a preset temperature. The fan is typically mounted behind a gable-end vent, but other types are available that exhaust hot air through a roof vent.
Important Attic Elements: Air Sealing and Insulation
Unless an older house has undergone an energy upgrade, it’s certain to have too much air leakage and too little insulation. Nowhere is this more common than in the attic. To understand the major role the attic plays in your home’s overall energy performance, let’s talk about the Stack Effect. In wintertime, the warmest air in your house will rise by convection to the highest areas of your living space. But it doesn’t stop there; it can escape into the attic through many leakage points: recessed light fixtures in the second floor ceiling, gaps around the drop-down stair to the attic, and numerous clearance holes drilled from the attic to run wiring or install grilles connected to attic ductwork. When the air you paid to heat escapes into the attic, it creates negative pressure (suction) inside the house that draws in cold air from leakage points in lower sections of the house.
The Stack Effect doesn’t have a big impact in mild weather. But if you live in a cold climate, the impact of the Stack Effect can be significant. The warmest air in your house is constantly be lost to the attic (“exfiltration”), while cold air infiltration is forcing your heating system to work extra hard to maintain comfortable temperatures. To make matters worse, warm interior air that leaks into the attic can melt snow on the roof that freezes at the eaves to form ice dams. And warm air will also condense on cold attic surfaces like rafters or roof sheathing, causing mold and other moisture problems.
The good news is that you can short-circuit the Stack Effect cycle by air-sealing the attic and increasing attic insulation. Air sealing is usually done in the attic, before insulation is installed or with existing insulation temporarily pulled out of the way. An experienced insulation contractor can go through the attic and seal leaks with spray foam and other materials, but DIYers can also take on this task, using guidelines available at www.energystar.gov.
When air sealing work is complete, it’s time for insulation. In most cases, existing insulation (typically fiberglass batts installed between attic floor joists) can be covered by new insulation. The levels of attic insulation recommended today by ENERGY STAR are substantially higher than those used earlier, so it’s worth checking out what’s recommended in your geographic area. In most cases, the best type of attic insulation will be blown fiberglass or blown cellulose insulation. To keep the ventilation space clear above soffit vents, special baffles need to be installed before the insulation goes in. Some homeowners also take the time to build walkways and/or storage platforms in the attic prior to installing insulation. Elevating these surfaces above the planned insulation level makes it safer and easier to move around, or to store things in the attic.
In addition to upgrading the insulation that separates the upper floor of your house from an unconditioned attic space, it makes sense to insulate any exposed ductwork in the attic. Without insulation, attic ducts will get hot in the summer (when they need to carry cool air) and cold in the winter (when they need to transport heated air). Insulating ductwork is a DIY-friendly job, and while you’re at it, you can eliminate duct leakage by covering joints in duct fittings with foil-faced tape designed for ducts.
There’s one more item of attic intelligence that deserves mention: A radiant barrier. Remember how hot the attic can get on a sunny summer day? Well, you can reflect some of this heat back toward the outdoors by installing a radiant barrier. This thin plastic sheet covered with shiny aluminum foil works just like the heat shield you can buy for your car windshield. In most cases, an attic radiant barrier can be stapled loosely to the bottom edges of roof rafters. Once installed, a radiant barrier can cut your cooling costs by as much as 15% —a worthwhile upgrade if you live in a hot climate.
