The water was already inside
People find water in a sealed shed and go looking for the leak. There usually is not one. Air holds water in proportion to its temperature, and the temperature at which a given parcel of air is saturated is its dew point. Open the shed on a warm humid afternoon, close it, and the building now contains air with a dew point somewhere in the sixties. That night the metal roof radiates heat to the sky, drops below the air temperature, drops below the dew point, and water condenses on it. In the morning it is on the roof, on the tools and on the concrete.
Two things follow. First, sealing the shed harder makes it worse, not better, because it traps the damp air and stops the building drying out during the hours when the outside air is drier than the inside. Second, the surface that gets wet first is whichever one is coldest and least insulated, which in a shed is almost always the underside of a bare metal roof, followed by cast iron machine tables. Neither is a construction defect. It is psychrometrics, and the layer-by-layer version of the same argument is worked out on the wall condensation calculator.
Two openings, at two heights
Passive ventilation works because warm air is less dense than cold air. Air inside the shed that is warmer than outside rises, leaves through a high opening, and pulls replacement air in through a low one. That is the whole mechanism, and it has three requirements: an opening low down, an opening high up, and a temperature difference between inside and outside.
Miss any of the three and it stops. A shed with two vents at the same height has no stack, so nothing moves except by wind. A shed with a big ridge vent and no low intake starves, because two openings in series are limited by the smaller one — which is why the calculator reports an effective area rather than a total, and why adding more high vents to a shed with small low vents does almost nothing. And on a still mild night when inside and outside are the same temperature, the flow is zero regardless of how much vent area you fitted. Those are the nights dew forms, which is the honest limit of the passive approach.
| Change | Effect on flow | Worth doing when |
|---|---|---|
| Double the smaller opening | Nearly doubles it | Almost always — the small one is the bottleneck |
| Double the larger opening | Very little | Rarely, on its own |
| Double the height between them | About 1.4 times | Gable or ridge vents against wall vents low down |
| Double the temperature difference | About 1.4 times | Not a lever you control |
The ratio is a target you choose
The 1 to 150 and 1 to 300 figures offered here are the familiar ratios of net free ventilating area to floor area used for attics, and they are offered because they are a sensible scale for a small building rather than because they apply to your shed by rule. Nothing on this page states a requirement. Pick the target you think the use justifies — the generous figure for anything storing tools that rust or wood that is drying, the reduced one for a garden store on a well-drained site — or enter your own square inches per square foot.
What matters much more than the ratio is the phrase net free area. A four inch round louvre vent is roughly 12 square inches of hole and considerably less than that of free area once the blades and the insect mesh are counted. Mesh alone commonly takes half. The figure to use is the one the manufacturer publishes for that specific product, and it is often surprisingly small, which is why sheds end up with far fewer vents than they need.
Removing the source beats diluting it
Ventilation is dilution, and dilution is the second-best answer. If there is a continuous moisture source inside, deal with it first.
Bare earth under a shed evaporates soil moisture into the building continuously, and it is usually a larger source than everything stored inside put together. A sheet of polyethylene laid over the ground under the shed, lapped and weighted, removes most of it for the price of the sheet. A slab without a barrier under it does the same thing more slowly. Bags of compost, damp firewood and a mower brought in wet are all sources; so is a person working in there with the door shut. Air movement handles the rest, and the dehumidifier sizing calculator is where you go if the contents genuinely cannot tolerate the still, mild nights when passive ventilation does nothing.
The rest of the shed
Vent area and free-area products are the same subject as attics, and the attic ventilation calculator goes further into the intake and exhaust balance and into what a specific ridge or soffit product delivers per foot. For air change rates expressed in the other direction, the air changes calculator converts between cfm and changes per hour for any volume. The volume itself comes from the footprint and the roof shape, which are set on the shed size calculator and the gambrel roof calculator, and the roof covering that decides how cold the coldest surface gets is chosen on the material takeoff.
One thing this page will not do is tell you where a vent may go. Openings interact with security, with driving rain, with wind-blown snow, and with what a building storing fuel or chemicals is required to have. That last one in particular is a local determination with real consequences, and the building department is the source.
Questions people ask
Why does my shed roof drip when it has not rained?
Because the underside of the roof fell below the dew point of the air inside. Metal is the usual culprit: it has almost no thermal mass and it radiates heat to a clear night sky, so it routinely sits several degrees below the outdoor air temperature. The moisture was already in the shed, brought in as humid air during the day. It is not a leak, sealant does not fix it, and the levers are ventilation to dilute the moist air and something on the underside that is either warmer or absorbent — sheathing with underlayment, a condensation-control membrane, or applied insulation.
How many vents does a shed need?
It depends on the free area of the vents you buy far more than on the count. This page turns your chosen ratio into square inches and then divides by the net free area of one vent, and the answer is often more vents than people expect, because a small louvre with insect mesh may deliver only a few square inches of free area from a hole several times that size. The more important rule is placement: half the area low and half high, on opposite ends where you can, because two vents at the same height do almost nothing.
Should shed vents be at the top or the bottom?
Both, in roughly equal measure. Air leaves high and enters low, and the flow is limited by whichever opening is smaller, so a large gable vent with no low intake performs like a small vent. The height between the two is the other lever: doubling the separation increases the flow by about forty percent, which is why gable or ridge vents paired with vents low on the walls beat two vents in the same gable. For fuel and chemical storage the low vent matters for a second reason, since several common vapours are heavier than air.
Will insulating the shed stop the condensation?
It can, and it can also make it worse, depending on where the insulation goes and what is behind it. Insulation applied in direct contact with the cold surface raises that surface temperature and can take it above the dew point, which is why spray-applied insulation on the underside of metal roofing works. Insulation with an air gap between it and the cold surface simply moves the condensing plane out of sight, where the moist air still reaches it and where you will not see the result for years. Which case you are building is exactly what the wall condensation calculator maps.
Is a powered fan better than passive vents in a shed?
It is more reliable, because it does not depend on a temperature difference. Passive stack ventilation delivers nothing on a still night when inside and outside are the same temperature, and those are precisely the nights when dew forms. A small fan on a humidistat or a timer sidesteps that. The cost is power, a moving part and an opening that has to be there whether you want it or not. For most garden sheds passive is enough; for tools that rust, in a humid climate, it often is not, and the honest comparison is between a fan and simply not storing those tools in an unheated outbuilding.