Building science
Attic ventilation is a pair. One half of it is almost always missing
Ventilation is the least visible part of a roof and the one most likely to be wrong. It costs little to fix during a replacement, it is awkward and expensive to fix afterwards, and it quietly sets how long the roof above it lasts.
What it is for
An attic sits between conditioned space below and weather above, and it has two jobs: stay dry, and stay close to outdoor temperature. Moving outside air through it does both. What it is not for is making the house cooler — that is what the insulation on the attic floor does.
- In winter
- Warm, moist air from the house leaks into the attic through light fittings, hatches, plumbing penetrations and gaps around the top plates. When it reaches cold sheathing it condenses. Ventilation carries that moisture out before it soaks the deck, rusts the nails and wets the insulation.
- In summer
- An unventilated attic under a dark roof gets extremely hot. The shingles above it cook from underneath, losing their volatiles faster, and they get brittle years earlier than they should.
- In between
- A cold attic keeps the roof deck at outdoor temperature, so snow on the roof melts because of the sun rather than because of the house. That is the entire mechanism behind ice dams.
Intake and exhaust
Air has to come in somewhere and leave somewhere else, low to high. The common failure in this market is a house with a continuous ridge vent — the exhaust half, easy to install during a re-roof, visible from the street as a job well done — and no functioning intake at all.
| Component | Which half | What goes wrong |
|---|---|---|
| Vented soffit panels | Intake | Painted shut, replaced with solid panels, or blocked from above by insulation pushed into the eave |
| Continuous ridge vent | Exhaust | Installed without a slot cut in the deck; or installed with no intake below it |
| Box or can vents | Exhaust | Too few, or mixed with a ridge vent so they short-circuit each other |
| Gable vents | Both, badly | They ventilate across the attic rather than up through it, and mixed with a ridge vent they become the easiest intake, leaving the soffits doing nothing |
| Powered attic fan | Exhaust, forcefully | With inadequate intake it pulls conditioned air out of the house through every ceiling gap. It is an energy cost pretending to be a solution |
| Baffles at the eave | Neither — they protect the path | Missing entirely, which is why insulation ends up in the airway |
More exhaust is not better. An attic with a ridge vent, gable vents and a powered fan, and no soffit intake, pulls its make-up air out of the heated house. You pay to condition that air twice and the attic still does not ventilate properly.
How much is enough
The conventional rule is one square foot of net free ventilating area for every 150 square feet of attic floor, halved to 1 in 300 where a vapor retarder is present or where the vents are balanced between high and low. Two details matter more than the arithmetic. First, net free area is not the size of the hole — louvers and insect screens cut it substantially, and the figure is published for each product. Second, the balance: intake should at least equal exhaust, and where they are not equal it should be the intake that is greater.
How to tell whether yours works
Go up on a cold morning
Frost or beads of moisture on the nail points and the underside of the sheathing means moist air is sitting in the attic instead of leaving it. Rust-stained nail points are the record of that happening for years.
Look at the eaves from inside
You should see either daylight or an obvious clear path above the insulation at every rafter bay. Insulation packed into the overhang means no intake, whatever the soffit panels look like from the driveway.
Look at the soffit from outside
Continuous perforated panel, or vented panels at intervals. Solid vinyl soffit with no perforation is decoration.
Check the ridge vent has a slot
A ridge vent installed over an uncut deck is a strip of plastic doing nothing. From inside, you should see a continuous gap at the ridge.
Feel the temperature in July
A ventilated attic on a hot afternoon is unpleasant. An unventilated one is dangerous, and that difference is obvious in about ten seconds.
What to fix, and in what order
- Air sealing first. Every gap between the house and the attic — around light fittings, the attic hatch, plumbing stacks, bathroom fans, and the top plates of interior walls. This is the cheapest and highest-value work in the whole sequence, because ventilation should not be a remedy for moist air that never needed to get up there.
- Make sure bath and kitchen fans discharge outside, not into the attic. A fan venting into the attic delivers a household of moisture straight onto cold sheathing.
- Open the intake. Vented soffit panels, and baffles at every rafter bay so the insulation cannot slide back into the airway.
- Match the exhaust to it. A continuous ridge vent with a properly cut slot is the usual answer on a roof with a decent ridge length.
- Then insulate properly. Insulation goes on the attic floor, up to depth, and it never blocks the eave path.
A finished attic, a cathedral ceiling or a spray-foamed roof deck is a different assembly with different rules, and the conventional intake-and-exhaust logic does not apply to it. Those need designing rather than venting, and getting it wrong puts moisture into a structure that has no way of drying.
The eave half of this work overlaps with soffit and fascia, which is why it is worth doing while a roof or a gutter run is already off.
Want the attic looked at as part of a roof inspection?
Request a free inspectionWill more ventilation make my house colder in winter?
No. The ventilated space is above the insulation, not inside the heated envelope. A cold attic in winter is correct. If ventilating the attic makes the house colder, the actual problem is that heated air was leaking into the attic, which is the thing to fix.
Are powered attic fans worth installing?
Usually not on a house with a working passive system, and actively harmful on one with poor intake, because the easiest air for them to pull is conditioned air from the house. Fixing intake is the better spend nearly every time.
Can I just add more soffit vents myself?
Cutting in vented panels is achievable, but it accomplishes nothing if insulation is still blocking the bays from above. Baffles at the eave are the part that keeps the path open, and they are fitted from inside the attic.
Does ventilation actually extend shingle life?
It does, and it is one of the better-supported claims in roofing. Shingles over a hot, unventilated attic get brittle earlier and crack sooner. It is also a condition in most manufacturer warranties, which tells you what the manufacturers think of it.
All work is performed by licensed and insured local roofing professionals.