Should an attic have ventilation?

Do all attics need ventilation?

Yes, if it was built as a vented attic, the kind with soffit, ridge or roof vents. The International Residential Code (IRC) requires cross ventilation in enclosed attics, and allows unvented attics only under a separate set of rules. If your report flags poor attic ventilation, such as blocked soffits, too few vents or frost on the roof deck, ask the seller to correct it. Back the request with a roofer's written bid.

Key takeaways

• The base code rule is 1 square foot of net free vent area per 150 square feet of attic floor. ARMA's shortcut, which runs a little high, puts a 2,000 sq ft attic at about 1,000 square inches of intake and 1,000 of exhaust.
• A ridge vent plus separate roof vents on the same roof can short-circuit the airflow. More vents is not always better.
• Adding a ridge vent and two soffit vents to an attic with neither, the smallest version of the job, runs $820 to $1,550 at national prices, before baffles, air sealing or deck repair. Most attics need more intake than two vents give, so the roofer's bid will usually be higher; ask for the bid amount, not this figure.

What are the signs of poor attic ventilation?

Attic vents work in pairs. Air comes in low, through soffit or eave vents, and leaves high, through a ridge, roof or gable vent. The International Institute of Building Enclosure Consultants (IIBEC) says both are needed for the system to work. In winter, per ENERGY STAR, venting helps keep the attic cold, which reduces the potential for ice dams. In summer, the airflow moves super-heated air out and protects the shingles.

Look for which of these your report names:

• Blocked soffit vents. IIBEC says to check for three problems: insulation laid without baffles, debris, or vents installed over solid wood. For insulation, ENERGY STAR says to fit rafter vents, also called insulation baffles, to keep the soffits clear. See attic insulation on an inspection report if both lines appear.
• Too few vents. IIBEC says a gable roof with only two or three static vents is not unusual, and that most of the time it is not nearly enough.
• No intake at all. A ridge vent with no soffit vents is "basically ineffective," per IIBEC, and can pull warm, moist house air into the attic.
• Missing or damaged bird block. A bird block sits between the rafters above the wall, with screened holes that let air in and keep birds and bugs out. The code requires vent openings be protected against "birds, rodents, snakes, and other similar creatures."
• Frost or condensation on the roof deck. Covered below.

What does it cost to fix attic ventilation?

A roofing contractor adds or changes the vents. Fixr puts a single vent install at one to four hours.

For the rest of the report, Kept prices every line of an inspection report for $79, usually under 4 hours.

Say the report finds a gable roof with no ridge vent and no soffit intake. A ridge vent at Fixr's range is $240 to $600. Two soffit vents add $580 to $950 (2 × $290 and 2 × $475). That's $820 to $1,550 before anything else. Baffles, closing old roof vents, air sealing the ceiling and deck repair are extra.

Two soffit vents is the smallest version of this job. A bigger attic needs more intake vents, each priced per unit, so the roofer's count decides the total. Call a licensed roofing contractor now. Ask for a written bid that names the vents, their net free area, and any deck damage. How to get repair estimates during the inspection period covers other ways to get a figure while your window is open.

Is attic condensation serious?

It can be. The Building America Solution Center, funded by the Department of Energy, says repeated or long condensation "can cause serious structural damage." The damage it names is rot, swelling and delamination of plywood and OSB sheathing, and corroded, failing fasteners. It can also quickly lead to mold.

In cold weather, condensation often shows up as frost. Frost itself generally does no harm, per the same guide; the trouble starts when it melts and the wood has to dry. Outdoor air is generally very dry then. Indoor air is warm and "often carries a much higher moisture load." When it leaks into a cold attic that isn't well ventilated, condensation is likely.

So the fix has two parts: seal the air leaks from the house into the attic, and provide balanced intake and exhaust vents, with baffles to keep the soffits clear. The same guide says sealing the ceiling gives the best chance of limiting condensation in extreme conditions. If the report notes stained, soft or delaminated sheathing, have the roofer price the deck repair in the same bid.

How much ventilation does an attic need?

For a vented attic, the IRC minimum is one square foot of net free area for every 150 square feet of attic floor. Net free area is the open space air can actually pass through, not the vent's outside size. The code allows half that, 1/300, only when upper vents supply 40% to 50% of the total. In climate zones 6, 7 and 8, it also needs a vapor retarder on the warm side of the ceiling.

The Asphalt Roofing Manufacturers Association (ARMA) gives a shortcut for the code minimum. Divide attic floor square feet by 2. That's the square inches of exhaust you need, and the same again for intake. ARMA notes the shortcut runs a little high.

For a 2,000 sq ft attic floor, that's 1,000 square inches each way. A typical static roof vent provides 50 to 60 square inches, per IIBEC. Exhaust alone would take 17 to 20 of them (1,000 ÷ 60 and 1,000 ÷ 50). Use the attic floor area, not the home's total living space.

What should I ask the seller for?

Write it as one line, with scope, trade and the bid behind it:

• Attic ventilation: correct attic ventilation to the code minimum net free area per the roofer's scope (for example clear soffits, add baffles, add or close vents), licensed roofing contractor, amount per the contractor's written bid.

Ask for the work done by a licensed contractor with receipts, or a credit sized to the bid. Put the line in your buyer repair request letter. What to ask for after a home inspection shows how it sits with the rest of your list.

What if the seller says no?

You can accept it, ask for a credit or price cut instead, or walk away. A vent fix with a bid behind it is a line to negotiate. Frost with soft sheathing is a bigger unknown until the roofer has looked at the deck. When to walk away after a home inspection walks through that call. Your contract sets the deadline and your options if the seller refuses, so confirm them with your agent.

For the other lines in that part of your report, see exterior findings on an inspection report.

Does an old house with no attic vents need them added?

Not automatically. Since the 2009 code, attics can be vented or unvented, and IIBEC calls it "a design choice." An unvented attic has its own list of code requirements, so an old attic with no vents may meet neither set. Ask the roofer which one yours is closer to, and ask your local building department which code edition applies at your address.

Can an attic have too much ventilation?

The wrong mix can hurt. IIBEC says a ridge vent combined with static roof vents "should be avoided," because the static vents short-circuit the airflow. Intake should also equal or exceed exhaust, or the attic can pull air from the house through openings like can lights. If your report lists a ridge vent and several roof vents, ask the roofer whether to close the extras.

If the attic is one line of many, upload your inspection report to Kept and put the roofer's bid next to a price for everything else.

Sources

• IIBEC, Attic Ventilation 101
• ENERGY STAR, About Attic Ventilation
• Building America Solution Center, Condensation Control in Attics and Roofs in Cold Weather
• Asphalt Roofing Manufacturers Association, The Attic Needs Ventilation, but How Much Exactly?
• Fixr, Roof Vent Installation Cost
• Dunn Lumber, Vented Bird Block