Attic Mold in Portland, OR: Roof Sheathing, Condensation, Bath Fan Ducts, and Ventilation

How an attic gets wet in winter

A vented attic is meant to stay close to the outdoor temperature. In winter, that makes the underside of the roof deck cold. Condensation forms when a surface falls below the dewpoint of the air touching it, which PNNL describes as surface temperatures dropping below the dewpoint of the air they are exposed to. A house in the heating season has more moisture in its air than the outdoors, from showers, cooking, laundry, and breathing, so any air that escapes upward carries water to a cold surface.

The path out of the house is rarely a visible hole. PNNL notes that a pressure difference is needed to force interior air through gaps in the air barrier, and that even the natural buoyancy of warm humid air in the building is enough to create it. In practice the gaps are the usual ones: around ceiling lights, the attic hatch, wiring and plumbing penetrations, and the top of interior walls. PNNL's recommendations for avoiding the problem are to ensure the integrity of the air barrier between the conditioned space and the attic, to identify sources of air leakage and seal them before insulating, and to keep indoor relative humidity between 30 and 50 percent in the heating season, which lowers the dewpoint and makes condensation less likely.

Ventilation is the second control. PNNL says the primary mechanisms for controlling condensation in a ventilated attic are limiting air leakage and providing proper intake and exhaust ventilation. Air moving under the roof deck helps dry out leaks or condensation that does occur. Ventilation does not substitute for stopping the moisture, but it helps the wood dry between wet spells.

Portland's climate sets up the winter risk. NOAA's 1991 to 2020 normals show the city's coldest monthly average is January at about 42°F, and 44 percent of the year's rain falls from November through January. Cool, damp weather with a heated house below is the combination PNNL describes.

Signs of a moisture problem in an attic

The first sign is often a smell or a stain on a ceiling below, but most of this is seen from the hatch.

• Dark staining or fuzzy growth on the underside of the roof sheathing.
• Frost on nails and sheathing in cold weather, which turns to drips when it warms.
• Insulation that is compressed, damp, or discolored, especially under the exhaust duct from a bath fan.
• A bath, kitchen, or dryer duct that ends inside the attic rather than at a vent to the outdoors.
• Water stains around a chimney, plumbing vent, skylight, or valley, which point to a roof leak instead of condensation.
• A musty smell in rooms below that gets stronger in winter or when the bath fan runs.
• Ceiling stains that appear in the winter on dry, calm days.

Condensation or a roof leak?

The two look alike. Both stain wood and insulation, and a house can have both. A few differences help narrow it down. A roof leak is local: it follows rainfall, shows near a penetration or a flashing, and leaves a stain with a clear edge. Condensation is broad: it appears across a slope, builds over weeks of cold weather, and does not require rain. Frost is a clue for condensation. PNNL says frost itself generally does not cause damage, but when warmer conditions arrive the moisture returns to a liquid state and has to be stored in the materials until it dries.

PNNL's assessment guidance for attics tells the inspector to look for water staining, damp wood, and other signs of roof leaks, and states that if the roof is leaking, it must be repaired before proceeding. The same guidance says that when mold is found in an area with water damage, a mold remediation professional should inspect and remediate it, and no work proceeds until the space has been certified clean. Water that has run in from a leak is handled the same way as other water losses; see water damage and moisture control for categories and drying.

Fans that vent into the attic

A duct that ends in the attic is the cause of a large share of attic mold complaints, and it is simple to check. PNNL's guide says to make sure your bathroom and kitchen exhaust fans are venting outside instead of into the attic or crawl space, and to look in the attic for the duct. It explains why: moist air coming out of the duct can condense on cold rafters, trusses, and sheathing. Ducts that end at a ridge, gable, or soffit vent, or that are lying on the attic floor, can cause the same problem.

The correct finish is a duct that runs to the outdoors. PNNL says the duct should terminate at a dampered roof cap vent in the roof, or at a vent in the soffit or side wall, be insulated, and attach to the vent with an air-sealed connection to properly vent moisture. A flexible duct that sags or is crushed collects condensation, and a duct that is not insulated invites condensation inside it. Fan size, run time, and the shower itself are on the bathroom page. This page owns the duct's path through the attic.

To test a fan, run it and watch the outside end. If the exterior flap does not open, check the duct in the attic for a disconnection or damage. PNNL's retrofit guidance goes further: bathroom fans that exhaust into the attic must be redirected outside before attic work begins, and mold near the duct outlets is checked for and remediated.

Attic ventilation: intake, exhaust, and how it is counted

A vented attic has openings low on the roof, usually soffit vents, for intake and openings high on the roof, such as ridge, gable, or roof vents, for exhaust. PNNL's attic assessment tells the inspector to count the soffit, gable, mushroom cap, and ridge vents, noting each vent's size, and use the figures to calculate the amount of upper and lower ventilation present in a vented attic.

How much ventilation the building code requires depends on the code edition in force and the roof design, so this site does not give a ratio. Ask the contractor which standard the proposal follows. The common failure is simpler than a ratio: soffit vents are covered by insulation or paint, and baffles are missing, so the intake air cannot enter. A flashlight and a look at the eaves from the attic shows whether air has a path.

Adding vents does not fix a moisture source. PNNL lists limiting air leakage first among the controls, so vents added without air sealing and proper fan ducts leave the moisture source in place.

How attic mold is assessed and cleaned

A typical sequence once the attic is accessible.

1. 1

   Safety first

   PNNL advises wearing gloves and a mask when entering an attic insulated with blown insulation, plus eye protection and disposable coveralls. Items under the hatch are moved and a tarp is laid down, because insulation may fall when the hatch is lifted.

2. 2

   Map the pattern

   The pattern points to the cause: a band along one cold slope suggests condensation, a stain beneath a flashing suggests a roof leak, growth above a fan duct points to the duct.

3. 3

   Check moisture

   A moisture meter on the sheathing and framing shows whether the wood is wet now. The USDA Forest Products Laboratory notes pin meters read less reliably above about 25 percent moisture content and read high on hot wood and low on cold wood, so readings in an attic are taken with the season in mind. Instruments are explained on the inspection page.

4. 4

   Fix the source

   Reroute fan ducts to the outside, repair roof leaks, and seal air leaks from the rooms below. OSHA's mold guide says the underlying cause of water accumulation must be rectified or the mold growth will reoccur.

5. 5

   Contain and clean

   Growth on sheathing and framing is removed using the HEPA-and-damp-wipe methods described on the remediation page, with the rest of the house protected from dust and from falling debris.

6. 6

   Replace what cannot be cleaned

   Insulation that was wet is replaced, as the EPA's cleanup table says to discard and replace cellulose and fiberglass insulation. Sheathing that is soft or decayed is replaced by a roofer or carpenter.

7. 7

   Verify

   Readings are retaken and the attic is inspected for fresh growth before insulation goes back. See post-remediation verification for written results.

What affects the price of attic work

The estimate follows the size of the attic, the extent of growth, and what must be replaced.

• Access A small hatch, low clearance, and a long carry limit tools and increase time.
• Area and extent Growth on a few feet of sheathing around a fan is different from a whole slope.
• Insulation Wet or contaminated insulation is removed and replaced, and attic area controls the quantity.
• Source repair Rerouting a duct is small. Re-roofing a leaking section is a separate trade and scope.
• Air sealing Sealing gaps under the insulation can mean moving insulation, which adds labor.
• Structural wood Decayed sheathing or rafters need replacement.
• Ventilation upgrades Adding baffles, vents, or fans is its own line item.
• Containment Protecting rooms below and passing equipment through hatches adds setup time.

Insurance and attic moisture

A roof leak after a storm is a different kind of event from condensation that built up over many winters, and an insurer may treat them differently. Photographs of the staining and of the leak location are the records adjusters typically ask for. The patterns and what to check in your policy are in Does Homeowners Insurance Cover Mold in Oregon?.

Questions to ask an attic mold contractor

These separate a source fix from a cosmetic cleaning.

1. Is this condensation, a roof leak, or both, and how did you decide?
2. Where do the bath, kitchen, and dryer ducts end?
3. Will you air seal the ceiling plane before insulating again?
4. What is the plan for the intake vents at the eaves?
5. Is the sheathing being cleaned or replaced, and what readings support that?
6. What happens to the insulation, and how is it disposed of?
7. Will you photograph the attic before and after?
8. Does the quote include the roofer or carpenter, or only the cleaning?

Frequently asked questions

Sources

• PNNL Building America Solution Center, Condensation Control in Attics and Roofs in Cold Weather
• PNNL Building America Solution Center, Look at Exhaust Fans for Venting to Outside
• PNNL Building America Solution Center, Pre-Retrofit Assessment of Attics, Ceilings, and Roofs
• USDA Forest Products Laboratory, R. M. Rowell, Moisture Properties
• U.S. EPA, Mold Remediation in Schools and Commercial Buildings (cleanup table)
• OSHA, A Brief Guide to Mold in the Workplace (SHIB 03-10-10)
• CDC, About Mold and Your Health
• NOAA NCEI, U.S. Climate Normals 1991 to 2020

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