Why an Attic in San Jose Holds Moisture, and How It Dries

Where does the moisture in an attic come from?

The moisture comes from the house below. Cooking, bathing, laundry and breathing all add water vapour to the air, which rises into the attic through gaps around light fittings, pipes and the top of the ceiling.

A household adds a considerable quantity of water vapour to its air each day. An attic that is sealed loosely at the ceiling lets that vapour rise, and it collects in the roof space where the air is coolest.

A duct from a bathroom or kitchen exhaust that ends in the attic is a direct route for moisture. The technician checks where each exhaust vents, since a duct that ends in the roof space delivers warm, wet air to the coldest part of the house.

How does the valley's cooling make condensation worse?

The valley's nights cool the underside of the roof. Warm air meets the cold boards in the early morning and condenses, and the boards stay damp until the afternoon sun dries them.

The cycle repeats each night. The boards collect moisture before dawn and dry by midday, and the dark marks in the cold corners of the sheathing are the record of many such cycles.

The cycle is worse in the foothill towns, where the air cools further at night, and the technician takes readings there on a cool morning as well as a warm afternoon.

How is condensation told apart from a leak?

Condensation forms on the underside of the sheathing in the cold corners and does not follow a path from the roof surface. A leak comes from above, follows the framing and shows a stain that runs along a rafter.

A leak stain has an edge that runs toward a source above, and it is usually wettest near the entry point. Condensation is most marked in the cold corners and on the north faces of the attic, and it looks like a fine bloom or a dark patchy mark.

The technician reads the pattern and the timing. Marks that appear only on cool mornings and fade by afternoon point to condensation, and a stain that grows after rain points to a leak.

How is the attic dried?

The attic is dried by balancing the intake at the eave with the exhaust at the ridge and by redirecting any duct that vents into the roof space. The insulation is kept back from the soffit so the air can move.

The intake is cleared first. Insulation is pulled back from the eave, and baffles are fitted at each rafter bay so the channel stays open. The exhaust is then added or paired, so the air moves from eave to ridge.

Damp sheathing and insulation are dried and checked with a meter. The readings before and after the work are recorded, so the owner can see whether the change has worked.

What does a homeowner check each winter?

A homeowner checks the attic on a cold morning for frost on the nails, damp insulation and a musty smell. A check after the cold season shows the attic's condition at its worst point.

Open the hatch on a cold morning and look at the boards near the cold corners. Note any frost, damp or dark mark, and photograph it from the same spot each winter.

A check that finds the same damp each winter points to a cause that has not been corrected, and the technician is called to find it.

The owner who keeps the readings through the winter has a record the technician can use at once. A set of dated numbers, taken in the same places at the same hour, turns a general worry about damp into a pattern that can be checked and corrected.

How does a household's daily life add moisture to the attic?

A household's daily routine adds more moisture than most owners expect. Showers, the washing machine, the dishwasher and the cooking all release vapour into the air of the house, and the air carries it upward.

A single long shower adds a large quantity of vapour to the bathroom air in a few minutes. If the bathroom exhaust is switched off soon after, the damp air remains in the room and leaks through the ceiling into the roof space.

The laundry and the kitchen add their share. A dryer vented into the attic, or a dryer duct that has a gap, is one of the largest sources of moisture in a house, and the technician checks its route first.

Why do insulated attics need particular care?

An insulated attic holds the warm air of the house below its insulation, and the insulation can hold moisture against the boards. When the insulation is wet, it loses its value and keeps the boards cold, which makes condensation worse.

Insulation laid across the attic floor can settle and compress over time, leaving gaps where the air moves freely and cold spots where it does not. The technician checks the depth of the insulation and its position against the eave.

Wet insulation is replaced in the affected area, and the layer is kept back from the soffit so the intake stays clear. The record notes the insulation's depth and the readings taken before and after.

How do the readings show that the attic is drying?

A set of readings taken before the work and after it shows whether the attic has dried. The readings are made at the same time of day, in the same places, with the outside temperature noted.

A moisture meter on the boards near the cold corners gives a reading that can be compared from one visit to the next. A fall in the reading after the vents are cleared and the duct is redirected shows the change has worked.

The owner can keep a simple log of the same readings through the winter. A set of numbers across the season shows the pattern more clearly than a single visit does.

How do the foothill nights change the attic's moisture?

The foothill nights are cooler than the valley floor, and the attic's underside cools further. Condensation forms more heavily and lasts longer into the morning, so the foothill attic is read at its coldest hour.

On a clear foothill night the air can drop well below the daytime temperature, and the boards of the roof cool with it. Warm, moist air from the house meets the cold underside and condenses in quantity, and the wet remains until the sun reaches the roof.

The technician therefore takes readings in the early morning, when the moisture is at its peak, and again in the afternoon, when the attic has had time to dry. The difference between the two shows whether the attic is holding moisture overnight.

What is the role of the ridge vent in a damp attic?

A ridge vent lets the warm, moist air leave the top of the attic, provided the intake at the eave lets fresh air in beneath it. Without the intake, the ridge vent draws air from the house below and can make the damp worse.

The technician checks the intake before adding a ridge vent, because the ridge vent works only as the other half of a pair. A ridge vent fitted to an attic with a blocked intake is a vent in name only.

Where the attic is damp and the intake is sound, the ridge vent is the change that moves the most air. The readings before and after confirm whether the moisture has begun to clear.

What should a homeowner do when the moisture comes back?

A moisture problem that comes back after a repair is a sign that the cause has not been found. The owner calls the technician, notes when the damp appears and reports the readings from the log.

A repeat of the damp after the vents were cleared suggests a second source, such as a duct that was missed, a leak at a penetration or a vapour route through a gap in the ceiling. The technician tests each route in turn.

The owner's log of the damp, with its dates and the weather, is the fastest way to find the second source. The technician starts from the pattern and does not repeat the first checks.

How is an attic checked after a bathroom renovation?

A bathroom renovation often changes the exhaust duct, its length and its route, and the new run must be checked for where it vents. The attic is read after the work to confirm the duct leads outside and is sealed.

A renovated bathroom may have a new fan with a longer duct that runs through the attic, and a joint in the duct can leak warm, moist air into the roof space. The technician checks each joint and the route to the outside.

The attic is read on a cool morning after the renovation to see whether any new moisture has appeared. A clean reading is the sign that the new route works.