Attic temperature is the roofing subject homeowners hear least about and pay for most. A roof that cannot breathe bakes its own shingles from underneath, drives up the cooling bill and, in our climate, turns ordinary humidity into condensation on the underside of the deck. None of that is visible from the street.
Ventilation is a solvable problem with well established rules. It is also easy to get wrong, and the mistake is usually made on the half of the system nobody thinks about.
How a roof is meant to breathe
A vented attic works on a simple loop. Cool outside air enters low down, at the eaves, through the soffits. It picks up heat and moisture as it rises through the attic. It leaves high up, at or near the ridge. Warm air wants to rise anyway, and wind moving across the ridge helps pull it out, so the system runs without any moving parts.
That loop only works if both ends are open. Intake at the bottom and exhaust at the top are two halves of one system, and the smaller half sets the performance of the whole thing. Doubling the exhaust on a house with blocked soffits does not double the airflow. It just changes where the air comes from, which is usually somewhere you did not want it to come from.
Why Gulf Coast attics run so hot
Louisiana and Mississippi hand a roof two problems at once. Long hours of direct sun push attic temperatures far above outside air temperature, and the air itself carries a lot of moisture. Heat alone shortens the life of asphalt shingles, because the mat and the sealant are being cooked from both sides. Moisture alone causes condensation on cold surfaces and, over years, decking that softens and stains.
The intake side is where most systems fail
Blocked soffits
Soffit vents are the intake, and they are remarkably easy to close off by accident. Insulation blown in against the eaves is the usual culprit. Paint over the perforations is another. On older homes the soffit may simply be solid board with a few small vents that were never adequate. From inside the attic you should be able to see daylight at the eaves along the whole length of the house. Where you cannot, that section is not breathing.
Baffles fix the insulation problem. They are simple channels that hold the insulation back and keep an open air path from the soffit up over the top plate into the attic. They are cheap, and they are frequently left out.
Not enough open area
Vents are rated by net free area, which is the actual open area left after the grille or mesh is taken into account, so a large-looking vent with fine screening can offer very little. The conventional rule of thumb is one square foot of net free area for every 300 square feet of attic floor when there is a vapour retarder and the vents are balanced roughly half low and half high, and one to 150 otherwise. Your local code is what governs, and an inspection should confirm which figure applies to your house.
Exhaust options, and why they should not be mixed
Ridge vents
A continuous ridge vent is usually the best exhaust for a roof with a decent run of ridge. The deck is cut back a set distance either side of the peak, the vent is installed over the slot, and ridge cap shingles finish it so it reads as a normal ridge line. It exhausts along the whole ridge rather than at points, so the whole attic moves rather than just the bay under a vent.
Box vents and turbines
Static box vents are a reasonable answer on roofs with short or broken ridges, hips or complex shapes. They work in the bay they sit in, so they need to be placed and counted properly rather than scattered. Turbines add a spinning cowl, which helps in wind and adds a moving part that eventually needs attention.
Powered fans
Powered attic fans move a lot of air, which sounds like the obvious solution. The catch is that they will take air from wherever it is easiest. On a house with restricted soffits, that is often conditioned air pulled up through gaps around light fittings and the attic hatch, which means you are paying to cool the attic. If the intake is genuinely adequate they can work. If it is not, they make the underlying problem worse and hide it.
One firm rule: do not mix exhaust types on the same attic space. A ridge vent plus box vents on the same run turns the lower vents into intake, short circuits the loop and leaves the far corners of the attic stagnant. Pick one exhaust strategy per space and let it work.
Bath fans, insulation and air sealing
Ventilation cannot fix a moisture source. Bathroom and kitchen fans must be ducted to the outside, not pointed into the attic. Dryer vents too.
Air sealing the ceiling plane is the other half of the job. Gaps around recessed lights, plumbing stacks, the attic hatch and top plates let humid conditioned air into the attic all day. Seal those, keep the insulation at the depth your code calls for, hold it back from the eaves with baffles, and the ventilation has far less work to do.
Signs your attic is not venting
Rusty nail points on the underside of the deck, dark staining between rafters, insulation that feels damp, a musty smell, or shingles that have aged faster on one plane than another are all worth investigating. Upstairs rooms that never quite cool down point the same way.
What to ask for at replacement time
A roof replacement is the one moment when ventilation is cheap to fix, because the deck is exposed and the ridge is open anyway. Ask any contractor three things: what is the net free area of intake and exhaust on my house, how are you achieving each, and are you keeping to a single exhaust type. A clear answer takes a minute.
On our replacements the ventilation calculation is part of the written scope, and baffles and soffit work are listed as line items rather than assumed. If you want to know what your attic is doing before you commit to anything, our inspection covers it. Call the office on (225) 555-0142 or book a free roof inspection.
Related reading: how to read your own roof from the ground, including what to look for on an attic walk.