Attic ventilation and insulation: what they do for your roof
Updated · KeystoneProGuide editorial team
What does attic ventilation do for a roof?
In ENERGY STAR's account, two jobs, one for each season. In winter, a natural flow of outdoor air through the attic keeps it cold, which reduces the chance of ice dams, the ridges of ice that form when snow melts off a roof over a warm attic and refreezes at the gutters. In summer, airflow through a well-vented attic carries super-heated air out, which ENERGY STAR says protects the roof shingles and removes moisture.
It can seem odd to insulate an attic and then let cold air into it. ENERGY STAR's answer is that the pieces work together: the insulation resists heat moving into the house in summer, and insulation with air sealing keeps the attic cold in winter by blocking heat and moist air from the rooms below. It calls that combination the key to a durable and energy-efficient home.
Do the experts agree on how much ventilation is needed?
Not entirely, and the trade association says so. The National Roofing Contractors Association (NRCA), the trade's association, says there are differing opinions on the amount of attic ventilation necessary. On current research, it says, the strongest case for ventilating an asphalt shingle roof with outside air is in cold climates where snow stays on roofs for long periods, where ventilation has been shown to remove excess moisture, prevent condensation that can damage the roof sheathing, and prevent ice dams. It adds that research has not verified a significant effect of ventilation on the average temperature of the roof surface, and that ventilation affects it less than the direction the roof faces or the color of the roofing.
The University of Minnesota Extension takes a related view from the ice dam side. It says that if the heat escaping from the house is cut back effectively, only small amounts of roof ventilation are needed to keep the roof evenly cold, though natural attic ventilation is still needed to dry the attic and remove summer heat.
Where should the vents be?
NRCA suggests a balanced system, with nearly equal ventilation area at the eaves or soffits and at or near the ridge, and about 50% of the area at the top. If a roofer proposes adding a ridge vent, that is the reasoning to ask about.
Pennsylvania's building code has its own rules on the amount of vent area. The state adopts the 2021 International Residential Code but amends its attic ventilation sections: it keeps the minimum vent area section of the 2015 edition in place of the 2021 version and changes two related sections. The details are a matter for your municipality's code office, which applies them.
What about attic fans?
ENERGY STAR is cautious. It says attic fans are meant to cool hot attics by drawing in outside air through the vents, but that if the soffit vents are blocked and the attic is not sealed from the house, the fans will instead pull cooled air out of the living space, using more energy and making the air conditioner work harder. It recommends passive vents and natural airflow. The extension says mechanical attic ventilation is not a recommended fix for ice dams in Minnesota, because it can create other moisture problems.
Why does air sealing come before insulation?
Because in many homes, the extension says, air leaking from the house is the main way heat reaches the attic. ENERGY STAR recommends doing an attic air sealing project before adding insulation, and says that combined, the two can help prevent ice dams. Its list of signs that a house needs attic air sealing includes drafty rooms, uneven temperatures, high energy bills, ice dams in winter and dust in rooms below the attic. The extension puts it the same way for ice dams: make the ceiling airtight first, then consider more insulation.
ENERGY STAR publishes recommended insulation levels by climate zone. For the zones from 4 through 6 it suggests R60 for an uninsulated attic and R49 where 3 to 4 inches already exist, based on the 2021 International Energy Conservation Code. Its page does not say which zone any Pennsylvania county is in, so check that before choosing a level.
Can insulation hurt the roof?
If it is put in wrongly. ENERGY STAR says the most common mistake homeowners make when installing insulation is blocking the flow of air at the eaves, and that soffit vents should never be covered. It recommends rafter vents, also called insulation baffles, stapled to the roof deck between the rafters, to keep a channel open from the soffit into the attic.
It also lists attic conditions that call for a professional before any insulation work: wet or damp insulation, which it says indicates a leaky roof; moldy or rotted rafters; kitchen, bathroom or dryer vents that exhaust into the attic instead of outdoors; little or no attic ventilation; and old knob and tube wiring, which it calls a fire hazard in contact with insulation. Some attics have vermiculite insulation, which may contain asbestos and should not be disturbed until it has been tested.
Does ventilation affect the roof warranty?
It can. NRCA says asphalt shingle manufacturers' warranties may make coverage conditional on minimum ventilation requirements, and suggests checking the manufacturer's requirements when choosing a shingle. When a roof is replaced, ask the roofer what the shingle maker requires and how the proposed vents meet it.
For storm-prone roofs, the FORTIFIED standard run by the Insurance Institute for Business and Home Safety requires wind- and rain-resistant attic vents, because high winds can pull ordinary vents loose or drive rain through them.
Is there help with the cost?
ENERGY STAR says many local utilities offer rebates for a professional home energy assessment and for some air sealing and insulation projects, and suggests asking your utility before starting. Ice dams on your roof covers the winter problem in detail, roof warranties the ventilation condition, and roofers serving the area are on the roofers directory page.
Sources
- About Attic Ventilation, ENERGY STAR
- Attic Air Sealing Project, ENERGY STAR
- Recommended Home Insulation R-Values, ENERGY STAR
- Resources (frequently asked questions), National Roofing Contractors Association (NRCA)
- Dealing with and preventing ice dams, University of Minnesota Extension
- 34 Pa. Code § 403.21. Uniform Construction Code., Commonwealth of Pennsylvania (Pennsylvania Code)
- FORTIFIED Roof, FORTIFIED (a program of IBHS)