Attic mold and roof ventilation
Almost every attic we get called about is dry when it rains and wet when it freezes, which rules out the roof before anyone climbs a ladder. The water is coming from inside the house.
The usual diagnosis
It is condensation, not a leak
A roof leak makes a stain that starts at a point and runs downhill: around a penetration, a valley, a chimney, a nail. Attic mold from condensation looks nothing like that. It appears as grey or black discoloration spread over the underside of the roof boards, usually heavier on the north slope, and it is often worse near the eaves or in a bay above a bathroom.
The mechanism is simple and it runs every night of a Michigan winter. Warm air from the house carries moisture upward, gets into the attic through gaps around light fittings, hatches and top plates, and meets a roof deck that is at outdoor temperature. The moisture condenses on the boards and the nail shanks, freezes when it is cold enough, then thaws on a mild day and soaks into the wood. Do that a few hundred times and the boards have been damp all season.
Which is why the seasonal question is diagnostic. If a homeowner has ever gone up in January and seen frost on the underside of the deck or ice on the nail points, that is the answer, and it has nothing to do with the shingles.
Where the moisture comes from
Four sources, in the order they turn up
- A bathroom fan discharging into the attic. The commonest single cause and the cheapest to fix. Guidance is consistent that bathroom fans, kitchen exhausts and clothes dryers should discharge outdoors rather than into the attic; a duct that stops at the insulation, or one that runs to a soffit and blows straight back into the vent above it, is putting a shower's worth of vapor into the roof space twice a day.
- Air leakage through the ceiling. Recessed lights, the attic hatch, plumbing stacks and the top plates of interior walls. Individually small, collectively a chimney. Sealing these is unglamorous work and it is usually the intervention with the longest life.
- Blocked or absent intake ventilation. Ridge vents only work if air can get in at the eaves, and insulation pushed into the soffits when a house was topped up is one of the most common ways an attic stops breathing. Baffles at the eaves cost very little.
- Indoor humidity that is simply too high, which brings the same 30% to 60% range that governs the basement. In humid months keeping the house below about 50% is the usual advice, and a whole-house humidifier left running through spring can single-handedly wet an attic.
The reason for that order is cost. The first two are inexpensive and fix most cases. Nobody should be quoting a new ventilation system before someone has followed the bathroom duct to see where it actually ends.
Doing the work
Cleaning it is the smaller half
Roof sheathing is wood, and wood that has surface growth but is still structurally sound is generally cleaned rather than replaced. That is genuine good news, because replacing a deck means taking the roof off. Cleaning is typically abrasive, with the affected surface taken back mechanically, followed by drying the space out properly.
The part worth being firm about is what happens afterwards. Attic mold is a symptom of an air and moisture fault, and cleaning without correcting the fault reliably produces the same attic in one or two winters. A quote that covers cleaning and an encapsulating coating but contains no line for the bathroom duct, the ceiling air sealing or the soffit baffles is quoting the visible half of the problem.
- Ask which fans were traced, and to where. Not whether fans exist.
- Ask what happens at the eaves. If insulation is against the roof deck at the soffit, air is not moving through the space, whatever is fitted at the ridge.
- Ask whether the coating is instead of the fix or after it. An encapsulant over a surface that will be wet again in January is paint on a symptom.
- Ask about the insulation. Material that has been repeatedly damp may need replacing, and in a house built before 1978 the question of what it is comes first.
Ice dams belong in the same conversation. They come from the same root cause, heat escaping into the attic and melting snow that refreezes at the cold eave, so a house with a history of ice dams and a house with attic mold are frequently the same house with the same fault, and one visit can address both.
Before you go up there
Reading the roof from the ground
A Michigan winter does most of the diagnostic work for you, provided somebody looks at the right week. These are all visible from the driveway, and any of them is worth a photograph before the thaw takes the evidence away.
- Snow melting in patches while the rest of the roof holds it. Bare stripes over the ridge or above particular rooms mean heat is getting into the attic at those points, which is the same fault that wets the deck.
- Icicles and a ridge of ice at the eaves. Snow melted by a warm deck runs down, hits the cold overhang past the wall line, and refreezes. An ice dam is a heat-loss symptom wearing a weather costume.
- Frost on the underside of the roof boards if you open the hatch on a very cold morning. That is the condensation itself, caught in the act, and it is as definite an answer as this subject offers.
- Staining that stops at a rafter bay. Damp confined to one bay usually sits above a bathroom or a laundry, which points straight at a duct.
Straight answers
Attic questions
My home inspector flagged attic mold on a house I am buying. How bad is it?
Usually less bad than it reads, and worth pricing properly instead of panicking about. The questions that decide the number are whether the sheathing is sound or delaminating, how much of the deck is affected, and what is causing it.
A house with staining on one bay above a bathroom and an unducted fan is a modest job. A house with the whole north slope affected and no intake ventilation at all is a larger one, and either way it is a fair thing to raise before closing.
Can I just add a powered attic fan?
It is a popular idea and it can make things worse. A powered fan pulling on an attic that leaks air from the house below will draw more warm moist air up through the ceiling, which is the thing you are trying to stop. Sealing the ceiling and opening up the intake at the eaves addresses the cause; adding suction to an unsealed attic mostly changes where the air comes from.
Is the mold in my attic getting into the house?
Air generally moves from the house into the attic during heating season, not the other way, which is the same pressure difference that caused the problem in the first place. That does not make it harmless. The direction can reverse, and any disturbance during work will raise spores, which is why the material gets bagged before it comes through the house and the access is controlled while the work is on.
A company offered to fog the whole attic. Does that work?
Fogging distributes a biocide as an aerosol, and it runs into the same objection every public health source on this subject raises about biocides: killing the growth does not remove it, dead mold still provokes allergic reactions, and none of it touches the moisture that produced the growth.
It also looks impressive and takes very little time, which is an unhelpful combination in a trade with no licensing. If it is offered, the question to ask is what else is in the scope—the duct, the air sealing, the baffles—and whether the fogging is in addition to those or instead of them.
Does it matter that my house is old?
For the diagnosis, not much, since condensation works the same in any age of building. For the work it can matter a great deal. Older Ann Arbor houses may have insulation and painted surfaces that bring lead and asbestos rules into a job that started out as a mold question, and those are separate regulatory regimes with their own requirements. Worth establishing early rather than mid-demolition.
Start with a survey, not a sales visit
Ask about an attic
Useful things to mention: whether the staining is on the north slope, whether you have ever seen frost on the nails or the boards in winter, and where the bathroom fans discharge if you happen to know. That last one solves a lot of these on the phone.
Related work we arrange
What attic work connects to
Mold inspection and moisture diagnosis
Finding the water before pricing the repair. Washtenaw County’s own position is that in most cases sampling is not necessary, which makes the visual survey the thing you are actually buying.
Basement and crawl space mold
Where most Ann Arbor mold lives. A below-grade wall can be wet from outside, wet from condensation, or wet from a leak, and the three want completely different repairs.
Water damage drying and material removal
The 48-hour window the EPA sets for drying wet building materials, and which porous materials have to come out rather than be dried in place.
Rental property mold and certificates of compliance
Ann Arbor inspects every rental unit every 30 months. Passing that inspection and fixing the moisture behind it are two different jobs, and only one of them holds until the next one.