Humidity, Dew Point and Why Attics Get Wet in a Great Lakes Climate
Relative humidity tells you almost nothing useful about an attic. Dew point tells you everything: condensation happens when a surface is colder than the dew point of the air touching it.
Attic moisture problems are almost always explained badly. Homeowners are told the attic is humid, or that the house needs to breathe, or that more vents will fix it. None of that is precise enough to be useful. Condensation obeys one rule and only one rule: water comes out of the air when the air touches a surface that is at or below its dew point. Everything else — ventilation, air sealing, fan ducting, vapour retarders, underlayment choice — is just a strategy for keeping surfaces above the dew point or keeping the dew point of the air low enough that it does not matter.
Relative Humidity Is the Wrong Number
Relative humidity describes how close air is to saturation at its current temperature. That makes it useful for comfort and useless for predicting condensation, because the temperature keeps changing. Cold outdoor air at ninety percent relative humidity holds very little water in absolute terms; bring it inside and warm it up and the same air is bone dry, which is exactly why houses feel dry in January and people run humidifiers. Two air masses at the same relative humidity can have completely different moisture content. The number that travels with the air regardless of temperature is the dew point.
What Dew Point Actually Means
Dew point is the temperature to which a parcel of air would have to be cooled, at constant pressure, for it to become saturated. It is a direct measure of how much water is in the air. If the dew point of the air in your attic is minus five and the roof sheathing is at minus fifteen, you get frost on the sheathing. If the dew point is minus fifteen and the sheathing is at minus five, you get nothing. The whole game is the relationship between one number that describes the air and one number that describes the surface.
In Winter, the House Is the Moisture Source
Outdoor air in a Southern Ontario winter is very dry in absolute terms. It cannot wet an attic. The moisture that frosts up the underside of a roof deck in January comes from inside the house, carried up through the ceiling plane by warm air that is leaking through it. Every one of the following adds water vapour to indoor air, and every bit of it wants to move upward.
- ✓Showers and baths, which are the single largest short-term source in most houses.
- ✓Cooking, dishwashing and kettle use, especially without a properly ducted range hood.
- ✓Laundry, and in particular any dryer that is not vented outdoors or whose duct is blocked with lint.
- ✓The occupants themselves. People exhale and perspire continuously, and a full house adds a surprising volume of water to the air every day.
- ✓Damp basements and crawlspaces, unsealed sump pits, and earth floors, all of which release moisture that migrates upward through the house by stack effect.
- ✓Firewood stored indoors, aquariums, large numbers of houseplants and humidifiers set too high.
- ✓New construction and recent renovations, where framing lumber, concrete and drywall compound are still drying out through the first winter.
Why Frost Forms on Sheathing and Nails
Warm, moist indoor air finds any hole in the ceiling — a pot light, the attic hatch, a plumbing stack, a wiring penetration, a dropped soffit over kitchen cabinets, the gap where a partition wall meets the ceiling — and rises through it. On a cold night the roof sheathing is close to outdoor temperature, and the exposed points of the roofing nails are colder still, because steel conducts heat out of the assembly and each nail point is a small cold fin hanging in the attic air. Air moving past those surfaces gets chilled below its dew point and gives up its water as frost. Come the first mild day, that frost melts, drips onto the insulation, stains the ceiling, and everyone assumes the roof is leaking. It usually is not.
Air leakage moves far more moisture into an attic than vapour diffusion through the drywall ever will. A vapour retarder that is full of holes for lights, wires and pipes is not doing the job people think it is doing. Sealing the ceiling plane is the first move, ventilation is the second, and adding vents without sealing usually makes things worse by increasing the suction on the ceiling.
The Summer Case, Which Runs Backwards
In July the Great Lakes basin delivers genuinely humid air with a high dew point, and the direction of the problem reverses. Now the moisture is outdoors and the cold surfaces are inside: ductwork carrying air conditioning through the attic, the top of a well-insulated ceiling in a house that is being cooled, or a basement wall. Humid air reaching those surfaces condenses on them. This is why sweating ducts in an attic are a genuine failure and not a nuisance, and why an air-conditioned house in a Great Lakes summer needs its ductwork insulated and its air barrier continuous just as much as it needs them in winter.
Night-Sky Cooling and Condensation Under Metal Panels
There is a third case that is specific to metal roofing and worth understanding. On a clear, calm night, any surface facing the sky radiates heat away to the cold upper atmosphere and can drop several degrees below the surrounding air temperature. That is why dew forms on a car roof and not under the car. A thin metal panel has very little thermal mass, so it does this readily. If the underside of that panel is exposed to humid air — because it was installed directly on purlins over an open, unlined structure, or because the assembly beneath it has no vent space and no capable underlayment — the panel's underside can drop below the dew point of the air touching it and water forms there. Homeowners describe it as the roof raining on the inside. The fixes are established: a continuous underlayment appropriate for metal, a ventilated air space above the deck where the assembly allows, a condensation-control or vented membrane on open-framed and agricultural buildings, and control of the moisture source below.
The Order of Operations That Actually Works
Moisture problems get solved in a sequence, and doing them out of order wastes money.
- ✓Cut the source. Duct every bathroom fan, range hood and dryer to the outdoors — through the roof or a wall, never into the attic and never into a soffit where the intake will pull it straight back in. Deal with a wet basement or crawlspace.
- ✓Seal the ceiling plane. Air seal every penetration between the heated space and the attic, including the hatch, before adding anything.
- ✓Restore and protect intake ventilation at the eaves with baffles, then match exhaust at the ridge so the attic is genuinely flushed rather than starved on one side.
- ✓Insulate to depth, continuously, right out over the top plate where the assembly allows it.
- ✓Choose the roof assembly to suit: underlayment appropriate to metal, a vent space where the design calls for it, and a properly designed unvented assembly if that is the route being taken.
- ✓Only then look at mechanical solutions such as balanced ventilation or dehumidification in the living space.
Frequently Asked Questions
Does a metal roof sweat more than shingles?
The panel itself does not produce water. What it does is respond quickly to radiant cooling because it is thin, so its underside can reach the dew point of the air beneath it sooner than a heavier assembly would. In a properly built residential assembly with a deck, underlayment and a controlled attic below, this is a non-issue. On open-framed barns, shops and agricultural buildings it is a real design consideration and it is handled with a condensation-control membrane or a vented assembly.
Can I vent my bathroom fan into the soffit?
No, and it is one of the most common defects we find. The soffit is where the attic draws its intake air, so a fan terminating there feeds warm, wet bathroom air directly back into the attic. It should terminate through the roof or through a wall, with an insulated duct, a working damper and as short and straight a run as possible.
Should I run a dehumidifier in my attic?
Almost never. A vented attic is connected to the outdoors, so a dehumidifier there is trying to dry all of Southern Ontario. If the attic is wet, the moisture is arriving from the house or from a source you have not found yet. Find it and stop it.
My attic frosts up but the roof does not leak. Is that a problem?
Yes. Repeated wetting of the sheathing degrades the wood over time, rusts fasteners, wets and mats down insulation so it loses its effective depth, and in bad cases supports mould growth on the sheathing. It is also a strong indicator of air leakage from the house, which is costing you heat and often driving ice damming as well.
Does spray foam on the roof deck change any of this?
It changes the design entirely. A foamed roof deck is an unvented, conditioned assembly with no ventilation and no cold sheathing, so the dew point question moves to whether the foam is thick enough and continuous enough to keep the first condensing surface warm. It works well when designed properly and fails badly when the foam is thin, discontinuous or applied over damp sheathing.
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