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Natural Attic Ventilation
www.energystar.gov
At first it may seem odd to add insulation for warmth and then purposely allow cold air to
enter the attic through vents, but this combination is the key to a durable and energy-
efficient home. Here's why: in the winter, allowing a natural flow of outdoor air to
ventilate the attic helps keep it cold, which reduces the potential for ice damming (snow
that melts off a roof from an attic that is too warm and then re-freezes at the gutters,
causing an ice dam that can damage the roof). Proper insulation and air sealing also
keeps attics cold in winter by blocking the entry of heat and moist air from below. In the
summer, natural air flow in a well-vented attic moves super-heated air out of the attic,
protecting roof shingles and removing moisture. The insulation will resist heat transfer
into the house.
The most common mistake homeowners make when installing insulation is to block the
flow of air at the eaves. NEVER COVER ATTIC SOFFIT VENTS WITH INSULATION —
use rafter vents and soffit vents to maintain airflow.
Attic Fan Ventilation
Attic fans are intended to cool hot attics by drawing
in cooler outside air from attic vents (soffit and
gable) and pushing hot air to the outside. However,
if your attic has blocked soffit vents and is not well-
sealed from the rest of the house, attic fans will suck
cool conditioned air up out of the house and into the
attic. This will use more energy and make your air
conditioner work harder, which will increase your
summer utility bill.
You don't want your unfinished attic cooled by your
air conditioner. To prevent this, follow the air sealing and insulation strategies in this
guide and make sure the attic is well-ventilated using passive vents and natural air flow.
Doing the Job
Laying fiberglass rolls is easiest for a DIY job. If you have any type of insulation
between the rafters, install the second layer over and perpendicular to the first (again,
the second layer of roll insulation should be unfaced — with no vapor retarder). This will
help cover the tops of the joists and reduce heat loss or gain through the frame. Also,
when laying down additional insulation, work from the perimeter toward the attic
opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all
insulation at least 3 inches away from "can" lights, unless they are rated IC (Insulated
Ceiling). If you are using loose fill insulation, use sheet metal to create barriers around
the openings. If using fiberglass, wire mesh can be used to create a barrier.