Building science

Attic ventilation is a pair. Here, the intake is the half that is 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 in this climate it quietly sets how long the roof above it lasts. On the post-1960 slab-on-grade stock that dominates this metro — big attic volumes, shallow eaves — it is wrong more often than it is right.

What it is for, in this climate

An attic sits between conditioned space below and weather above, and here it has two jobs: stay close to outdoor temperature, and stay dry. 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, and what sealing the air leaks into the attic does.

In summer, which here is most of the year

An unventilated attic under a dark roof in direct sun gets extremely hot — well above outdoor air temperature. The shingles above it cook from underneath as well as from above, losing their volatiles faster, and they crack years earlier than they should. Everything in the attic cooks with them, including the ductwork and the air handler that most houses here put up there, which is why an unventilated attic shows up on the electricity bill as well as in the shingles.

The humidity half

Gulf Coast air carries a great deal of moisture. Ventilation is not only about temperature: it is about not letting moist air sit still against cool surfaces. The classic cool surface in a Houston attic is not the deck, it is the supply ductwork and the air handler, and condensation there is a common and badly diagnosed source of "roof leaks" that appear in summer with no storm behind them.

In winter, briefly

The cold-climate mechanism still exists here in mild form: warm moist air leaking up from the house condenses on cool sheathing on the coldest nights of the year. It is a minor problem compared to summer heat, and the fix is the same — air sealing first, then ventilation.

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 or a row of turbines — the exhaust half, easy to install during a reroof, visible from the street as a job well done — and no functioning intake at all.

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.

The unvented alternative, which is common here now

A great deal of newer construction in this region does the opposite: no attic ventilation at all, spray foam applied to the underside of the roof deck, and the attic brought inside the thermal envelope. Done deliberately and detailed correctly that is a legitimate and often better assembly in a hot humid climate, particularly where the ductwork and air handler are up there. Done by halves it is a trap.

How to tell whether yours works

1. Look at the eaves from inside

   The single most useful check in this market. 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.

2. Look at the soffit from outside

   Continuous perforated panel, or vented panels at intervals. Solid vinyl soffit with no perforation is decoration.

3. 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.

4. Compare the attic temperature to outside

   On a summer afternoon a reasonably ventilated attic runs above outdoor air temperature and an unventilated one runs far above it. A cheap thermometer left up there for a day tells you more than any inspection report. Do not spend long up there yourself in the afternoon.

5. Look for condensation on the ductwork

   Wet or stained insulation on the supply ducts, or a damp patch on the ceiling below a duct run, is humid attic air condensing on a cold surface. It is not a roof leak and replacing the roof will not touch it.

6. Check the nail points on a cold morning

   Frost or beads of moisture on the nail points happens here on the coldest mornings of the year, and rust-stained nail points are the record of it happening repeatedly.

What to fix, and in what order

1. 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, and in this climate it is also a cooling-cost measure rather than only a moisture one.
2. Make sure bath and kitchen fans discharge outside, not into the attic. A fan venting into the attic delivers a household of moisture straight into the one place you are trying to keep dry.
3. Open the intake. Vented soffit panels, and baffles at every rafter bay so the insulation cannot slide back into the airway. On a shallow eave this is fiddly and it is the whole ballgame.
4. 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. Do not mix vent types at different heights and expect them to cooperate.
5. Then insulate properly. Insulation goes on the attic floor, up to depth, and it never blocks the eave path. Consider a radiant barrier while you are up there.
6. Get the ductwork out of the equation if you can. Sealed, well-insulated ducts in a ventilated attic, or an unvented assembly designed for it. This is the biggest energy item in most houses here and it is the one nobody mentions during a roofing conversation.

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.