When a Roof Leak Is Really Attic Condensation
Every winter, Des Moines roofers get called to a leak that isn't there. The homeowner has a wet spot on the ceiling, water dripping from the attic hatch, or a dark ring around a bathroom light. It hasn't rained in a week. There's a foot of snow on the roof and the shingles are fine. What they have is condensation, and a roofer who replaces shingles for it is taking their money. Here's how to tell the two apart, and what to do about the real problem.
How a house makes water in its own attic
A family of four puts several gallons of water vapor into the air every day — showers, cooking, breathing, laundry, houseplants. Warm air holds that vapor. When the warm air leaks up into an attic that's 10°F in January, it hits the underside of the roof deck and the nails poking through it, and the vapor turns straight to frost.
Then comes a sunny day. The deck warms, the frost melts, and a hundred nail tips drip at once. It looks exactly like a roof leak. It stops when the deck refreezes. That cycle can run for weeks, and by March the insulation is wet, the deck is stained, and there's mold on the north side of the attic.
Telling condensation from a leak
| Clue | Leak | Condensation |
|---|---|---|
| When it appears | During or after rain, or with an ice dam | After a cold snap, on the first sunny day, with no rain |
| Where | One spot, traceable uphill to a boot, valley, chimney or wall | Spread across the attic, worst on the north slope and near bath fans |
| What's on the deck | A single water trail along a rafter | Frost or a sheen across large areas; rusty nail tips everywhere |
| Insulation | Wet in one place | Damp on top over a wide area, or matted below a fan duct |
| Season | Any | December through March |
If it only happens in deep winter without rain, it's almost certainly condensation. If you want to confirm, go up on a cold morning and look for frost on the nails. Finding a leak from inside covers the leak side of that table.
Where the warm air comes from
On Des Moines houses the sources are predictable:
Bath fans that stop in the attic. One of the most common causes. The fan was installed with a duct that runs a few feet and ends in the insulation, or with no duct at all. Every shower puts a gallon of warm, wet air right onto the roof deck. Common on 1950s–70s ranches in Urbandale, Windsor Heights and the east side, and on a surprising number of 2000s houses where the builder never finished the run.
Recessed lights. Each one is a chimney from the living room to the attic.
The attic hatch and pull-down stairs. An unsealed hatch leaks more air than all the other gaps together.
Plumbing and wiring penetrations, the tops of interior walls, the chimney chase. Small gaps, lots of them.
Dryer vents run into the attic or disconnected at a joint.
Whole-house humidifiers set too high. If the windows are fogging, the attic is too.
Why ventilation alone doesn't fix it
The instinct is to add roof vents. More vents do help, but they're moving the water vapor out after it's already in the attic. The 1920s–40s houses in Beaverdale and Drake have almost no soffit overhang to put intake vents in, so exhaust vents just pull more warm air up from the house. The fix works in this order:
- Stop the air. Vent the bath fans and dryer outside through the roof or gable with insulated duct. Seal the hatch. Foam the penetrations and the top plates. Box the recessed lights. This is a day's work with a can of foam and a roll of weatherstripping, or $500–$1,500 to have done properly.
- Insulate to R-49, which is the target for climate zone 5. Insulation slows heat, but only after step one stops the air carrying moisture through it.
- Then balance the ventilation. Soffit intake with baffles, ridge exhaust. Roof ventilation in Des Moines attics works when there's intake low and exhaust high; a ridge vent with blocked soffits does very little.
The bonus is that the same three steps are exactly what stops ice dams, because the warm air that makes frost also melts the snow at the ridge.
A cold-morning check
Pick a morning after a night near zero and go up with a flashlight:
- Shine the light along the nails on the north slope. White frost on the tips means warm, moist air is reaching the deck.
- Look for frost or a wet sheen around the bath fan, the hatch and the chimney.
- Check the eaves from inside. If you can't see any daylight at the soffits, the intake is blocked.
- Feel the top of the insulation near the hatch. Damp on top means moisture is settling in it.
- Put a cheap humidity gauge in the living room. If the windows fog every morning, the indoor air is wetter than the house can handle in an Iowa winter.
What fixing it usually costs
Air sealing the attic floor and routing the bath fans outside runs about $500–$1,500 done properly, as above. A ridge vent cut into an existing roof is about $500–$900. Insulation is priced by the job and the depth; check MidAmerican Energy's current rebate list before you book. Compare that with new shingles, which don't touch the problem at all.
Mistakes that make it worse
- Adding roof vents without sealing the air leaks first.
- Running a humidifier on high all winter.
- Burying the soffit vents under new insulation.
- Ignoring a bath fan that's been "venting" into the attic for years.
- Replacing stained decking without fixing why it got wet.
When it's both
Sometimes a house has condensation and a real leak. The tell is a trail on the deck that doesn't match the frost pattern: one dark streak down a rafter from a pipe boot, in an attic that's also frosty everywhere. Fix the leak on the roof and the moisture in the house. A roofer who only looks at the shingles finds one; a roofer who only looks at the attic finds the other.
What a good contractor does
On any winter leak call, a good contractor starts in the attic. If it's frost, they say so, show you the fan duct that ends in the insulation, and give you a price for the air-sealing and venting instead of a price for shingles. If it's a leak, the roof repair page covers what happens next. Either way the answer isn't a new roof, and anyone who says it is should be asked to explain the frost.
Common questions
Is it dangerous to leave it?
Over time, yes. Wet insulation stops insulating, plywood can delaminate, nails rust, and mold grows on the deck. It's rarely an emergency, but it's worth fixing in the same season you find it.
Will a new roof stop the frost?
No. The moisture comes from the house, not the shingles. A new roof with better ventilation helps a little, but without air sealing and fans vented outside, the frost comes back the next cold snap.
Can I just open a gable vent or window up there?
More ventilation helps move moisture out, but it doesn't stop warm air coming up from the house. Seal first, then ventilate.
Should I run the bath fan longer?
Yes, if it vents outside. Running it 15–20 minutes after a shower clears most of the moisture. If it vents into the attic, running it longer only makes the attic wetter.
Does a dehumidifier help?
In a damp basement, yes, and lowering the moisture in the whole house helps the attic a little. But it doesn't replace sealing the air leaks. Warm air will still carry what moisture is left straight up to the cold deck.
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