The room question shows up right after the first block. One bag sits on the kitchen counter and bothers nobody, five take over a windowsill, and ten have to go somewhere. That is when people start hunting for the perfect room, which does not exist.
Cornell Small Farms says it plainly: there is no single way to grow mushrooms, and usually no perfect place to do it. A successful grow depends on whether you can hold the conditions reliably, and on matching species, strain, and substrate to those conditions. The conditions themselves come from your climate, your seasonal temperature swing, and how well the building is insulated against those swings.
So “basement or garage” is really a different question: which four parameters do you have to hold, and how much work does holding them cost in this particular room. Here is the room by room version.
Two rooms, not one
The most common mistake in picking a space is looking for one room to cover the whole cycle. The cycle has two modes, and their requirements barely overlap.
Incubation. For the first 4 to 8 weeks the substrate sits at a steady 65 to 70°F (18.3 to 21.1°C). Two factors matter here, temperature and humidity. CO2 and lighting do not matter at all during this phase: Cornell notes they get adjusted for the comfort of the people working in the room.
Fruiting. Once a block is colonized, it usually moves to another space where temperature, humidity, light, and air flow all change. A fruiting room designed well holds 62 to 65°F (16.7 to 18.3°C).
The practical takeaway is a pleasant one. Incubation runs fine in a dark closet, a pantry, or any windowless corner with no ventilation at all, as long as the temperature stays even. Only the second room costs money and renovation. Most people start by looking for one space that does everything, and reject options that already solve half the problem.
Why the basement tops every list
Cornell gives the reason without decoration: in a basement and other in-ground spaces, temperatures stay in the 50 to 65°F (10 to 18.3°C) range with little or no supplemental heating and cooling. The upper half of that range is exactly the fruiting ideal, and the geometry of the building hands it to you for free.
The limits are spelled out too. Below 50 to 55°F (10 to 12.8°C) many species drop off sharply in productivity. Above 75°F (23.9°C) mushrooms turn thin, race past maturity, and lose quality. Between those edges the relationship is gradual: cooler air means slower fruiting and meatier texture.
The history of the crop backs this up better than any calculation. Penn State lists the buildings farms actually used: abandoned coal and limestone mines, old breweries, apartment house basements, natural and man made caves, rhubarb sheds. What they share is steady coolness rather than darkness. The same page states that mushrooms are usually grown without light, but darkness is not a requirement.
There is a flip side. Penn State also warns that despite articles claiming mushrooms grow in any dark hole, commercial growing requires purpose equipped houses with ventilation systems. A basement gives you temperature for free and gives you no air at all. That gets its own section below, because this is where most homemade fruiting rooms fall apart.
The garage: a one season room
A garage sits in Cornell’s list next to the basement and the retrofitted room, so the option is legitimate. The question is how many months a year it works without spending anything.
Cornell’s line about the integrity of the building applies to garages more than anywhere else. It is usually the worst insulated structure on the property, with a door that takes up half a wall. In summer it climbs past 75°F, in winter it drops below 50°F, and between those points mushrooms are either thin or stalled.
Two ways out. The first is insulation plus climate control, which turns a garage into a basement for money. The second is something Cornell describes as ordinary practice: shift the species with the season. Oyster growers run Pleurotus ostreatus in fall, winter, and spring, then switch to Pleurotus pulmonarius for summer, which tolerates a warmer grow room.
At home scale the second route is the honest one. A garage hands you all of spring and fall, then sits empty through the heat and the freeze. For early batches that is enough: you learn on real blocks without paying for climate control before you know whether you will keep going.
The greenhouse: the hardest option
Greenhouses are in Cornell’s list as well: high tunnels appear among the infrastructure growers retrofit for mushrooms. The source describes no fruiting regime for them, and the reason becomes obvious the moment you lay the crop’s requirements next to what a greenhouse is built to do.
A greenhouse is designed to collect sun and hold heat. Mushrooms want 62 to 65°F with a ceiling around 75°F. They need just enough light to read a book comfortably in the room, because a mushroom uses light to gauge how far to stretch its stem rather than to feed itself. Every strength of the building becomes something you have to cancel out: shade it, vent it, cool it.
Add glass and film, which hold almost no temperature, so the room tracks the outside air overnight and cold surfaces collect condensation. At the 85 percent humidity mushrooms need, a greenhouse turns into indoor rain.
Where a greenhouse does work: as a shell around a separate insulated chamber, and as incubation space in the shoulder seasons when it sits at 65 to 70°F. Putting blocks straight onto greenhouse shelving means working against the building.
Closet, container, warehouse: a question of scale
Cornell describes the range of spaces as a ladder: from something as small as a closet to a retrofitted room, garage, or basement, then a modified warehouse or a building designed specifically for mushrooms. Fruiting rooms themselves run from a single shelf to a tractor trailer. In a city the list adds shipping containers and warehouse space, both easy to retrofit.
What the ladder tells you is that any rung is a legitimate starting point, and moving up is a money question rather than a technology one. Active growers give Cornell an estimate of $1 to $3 per square foot in net income. The source names no time period for that figure, so it cannot be read as an earnings promise. The useful part is the shape of it: income scales with floor area, and a 40 square foot closet will never produce what a 400 square foot room produces, no matter how careful you are. What each rung of that ladder costs, and how many weeks it takes to come back, is worked out in our piece on mushroom farm startup budgets.
That is why early batches belong on the smallest rung. One shelf under a plastic tent already shows whether you can hold humidity and harvest on time. The minimum version is covered in the piece on growing oyster mushrooms on a windowsill.
Air: the number that breaks homemade rooms
Here is the arithmetic that eliminates half the “perfect” basements. Cornell works an example with a 10 by 10 by 7 foot room, which is 700 cubic feet, or about 19.8 cubic meters. Replacing all the air in it in one minute takes a fan rated at 720 CFM, roughly 1,223 cubic meters per hour. Growers put it on a timer for 60 seconds every seven minutes or so, usually as a pair: intake and exhaust running together.
Cornell’s general rule is a full air exchange every five to ten minutes, and blasting fresh air in over a minute or two beats running a weak fan continuously. The target is simple: CO2 below 1,000 ppm for most species and below 800 ppm for oysters. The mechanics of air exchange are covered in our ventilation wiki entry.
Now put those numbers into your own basement. A 700 cubic foot chamber needs a duct to the outside and a fan rated around 720 CFM. A sealed root cellar with no vent cannot do it, and opening the door for a while does not substitute. That cellar is excellent incubation space; fruiting there means cutting a hole in a wall.
The second half of that same number is the part people skip. Every seven minutes the neighboring space delivers its air into your chamber, along with its temperature and humidity. Cornell illustrates it directly: pulling 100°F (37.8°C) air into a chamber you want at 65 to 70°F means the cooling system has to work much harder. So it matters where the duct points, a shaded yard or a baking summer garage.
85 percent humidity and the walls you live in
A fruiting room is, by design, a space any building inspector would flag. For the first four days after pins appear, humidity stays above 85 percent, and the working target afterward sits around 80.
Compare that to the standard for housing. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. The reasoning is straightforward. Dry wet material within 24 to 48 hours of a leak and in most cases mold will not grow. Condensation on windows, walls, or pipes is already a sign of high humidity. The gap between 50 and 85 percent is the gap between a house and a problem.
Which gives you the governing rule for choosing a space: humidify the chamber, not the room. A tent, a shelf under film, a closed cabinet, or a standalone box inside the basement holds its 85 percent while the rest of the space stays dry. Vent the exhaust outdoors rather than into a hallway. Cold surfaces near the chamber, cold water pipes in particular, are worth insulating for exactly the reason the EPA gives: to keep condensation off them.
How indifferent mushrooms are to damp shows up in a story Penn State tells: one farm ran in an old dairy barn so wet that cows living in it had died of pneumonia. A building unfit for animals turned out to be fine for mushrooms. In that trade, your living space needs to stay on the side of the cows.
Contamination: the price of a roof
Walls and steady temperature come at a cost, and the cost is identical in a basement, a garage, or a greenhouse. Cornell states it directly. Step out of the woods into a contained space and the list of species you can reliably grow expands. In exchange you have to monitor the environment through every stage and, hardest of all, eliminate sources of contamination in your substrate. Otherwise nothing fruits. Outdoors that problem is almost nonexistent.
Three requirements follow for the room itself. Surfaces have to be washable, so cardboard, carpet, and old lumber work against you. Incubation belongs apart from fruiting, since a room full of mature mushrooms is full of moisture and spores. And during colonization, while the space holds 65 to 70°F, blocks need regular inspection for mold contaminants rather than a month alone in the dark.
Cornell counts sanitation as one of the four advantages of a controlled environment, alongside consistent temperature, automation, and predictable production per square foot. It becomes an advantage only in a room you can actually clean.
The last item on the list is about you rather than the crop. In the growing rooms of a shiitake farm, spore concentrations ran above one million per cubic meter, and all four workers there developed cough and measurable changes in lung function. A Japanese worker at a king oyster operation saw his symptoms clear in the hospital and return every time he went back to work. So the fruiting room goes as far as possible from bedrooms, and the exhaust goes outside. The topic is covered in detail in our guide to growing mushrooms at home.
Four questions to ask a room
Instead of hunting for the perfect space, take a thermometer and ask the room four questions. The answers are the decision.
- What does it read in July and in January? Two measurements, in the hottest and coldest week, tell you more than any advice. The working corridor is 50 to 75°F (10 to 23.9°C), with 62 to 65°F as the ideal.
- Where does the air go? A 700 cubic foot chamber needs a duct to the outside and a fan rated around 720 CFM running one minute out of every seven. No duct and no way to cut one means this is incubation space.
- What happens to the walls at 85 percent? If the answer is mold, humidify a box inside the room instead of the room itself.
- Can you wash the surfaces, and how far is the nearest bedroom? The first question is about yield, the second about your lungs.
No room answers all four perfectly, and that is fine: Cornell says outright that a perfect place usually does not exist. What exists is a room where you can hold conditions reliably and a species matched to those conditions. Button mushrooms, for one, demand their own regime and their own technology, covered separately in the piece on growing button mushrooms at home.
Start with the cheapest step. Put a thermometer and hygrometer in the basement, the garage, and the closet for a week, and write down the high and low in each. After that the choice takes five minutes, and no article is needed.
