How To Make A Still Air Box (and When You Need A Flowhood)

I've lost a fair few grows to contamination. Not from bad spores or culture. They died because I inoculated jars in a kitchen with the window open, and mould drifted in and reached the grain before the mycelium could. Annoying. Also completely avoidable.

The fix cost me about £20 and 20 minutes. A still air box. It's a clear plastic tub with two holes cut in the front, and it was the single biggest jump in my success rate that first year. Here's how to build one, how to use it so it works, and the honest version of how it stacks up against a flow hood, because I've spent years with both.

What a still air box does

Mould spores and bacteria are floating around you right now. They're heavier than air, so given a bit of time they settle and stay put. The problem is that they hitch a ride on moving air. A draught from under the door, the warm current rising off your body, you waving your arm about too fast. Any of it can carry a spore straight onto the grain you just sterilised.

A still air box does one job. It traps a pocket of air and holds it still. No fans, no filter, no draughts. You close it, let the air settle for a few minutes, then work slowly inside so you don't stir anything up. The spores sink to the bottom and your culture stays clean.

One thing to be clear about: a still air box is still, not sterile. It doesn't kill anything and it doesn't filter anything. It removes the air movement that contamination needs to reach your work. Your technique does the rest, which is exactly why two people with the same box get wildly different results.

What you'll need

None of this is precious. Most of it comes from B&Q and a chemist.

• A clear plastic storage tub with a lid, roughly 60 to 80 litres. Clear so you can see what you're doing. Deep so a grain jar stands upright with room for your hands above it.

• A roll of duct tape.
• A marker pen.
• Something to cut the holes. A heated tin can melts neat holes, a sharp Stanley knife works if you go slow, a hole saw on a drill is the cleanest if you own one.
• 70% isopropyl alcohol in a spray bottle. Not 99%, not vodka. 70% is the spot for killing things on contact.
• Blue roll or paper towel.

How to build it

Flip the tub upside down so the open mouth sits on the table and the solid base becomes the roof. You work underneath it, into the still volume.

1. Mark the holes. On one long side, mark two circles about shoulder width apart, roughly 4 to 6 inches across. Sit down in front of the tub and check the height before you cut, low enough that your hands rest comfortably on the table inside. You don't want to be hunched over it for an hour.

2. Cut them. Go slow. Cold plastic cracks if you force a blade, so score it a few times rather than stabbing through. If you're melting with a hot can, do it in a ventilated room and don't breathe the fumes. Tidy the edges whichever way you cut.

3. Tape the rims. Run duct tape around the cut edge of each hole, inside and out. It covers the sharp plastic so you don't shred your forearms, and it stiffens the hole so it keeps its shape.

4. Make the cuffs. Stretch a couple of strips of tape across each hole so it's mostly covered, then cut a cross through the tape with a knife. You get four triangular flaps. Push your arms through and the flaps fold inward and hug your forearms. Pull your arms out and they fall back to close most of the gap.

That last step is where people argue. Some growers leave the holes wide open and swear by it, and they have a point, an open hole can't pump air. What goes wrong is a loose flap sealing over an empty hole: every arm movement then works like a bellows and drags outside air in. So if you do the tape cross, keep it snug against your arms while you work, like a cuff. If it ends up floppy, cut it off and leave the holes open. Both are fine. A loose lid you punch through is the worst of the lot.

How to use it without wasting your time

The box is the easy part. The habits keep your jars clean.

• Wipe the inside down with 70% iso and let it dry. Wipe everything going in too: jars, syringe, lighter.
• Lay it all out inside before you start. The whole point is that you never reach out mid job. If you've forgotten something, take everything out, reset, start again.
• Close it and walk away for five minutes. You've just stirred the air carrying stuff in, so give it time to settle.
• Flame sterilise your needle or scalpel until it glows, then let it cool inside the box.
• Work low and slow, close to the table, with small movements. No big gestures, no fanning your hands. Don't lean in and breathe over open jars.

The discipline matters more than the box. I've watched someone inoculate ten jars in a battered old tub and get ten clean colonisations, and I've watched someone lose the lot in a lovely setup because they kept reaching out for the kettle.

Still air box vs flow hood: what I've learned using both

I started with a still air box. Years later I bought a flow hood, and I've run both side by side long enough to have a straight opinion. Here's the difference.

A still air box is passive. It costs around £20, runs on nothing, and lives in a cupboard until you need it. You work with your arms through the holes in a small, enclosed space. It's brilliant for inoculating jars, spawning grain to grain, and basic syringe work. You can do agar in one, it's just fiddly, because the space is tight and you're working slightly blind. Its entire job is to sit still, so everything rides on you not stirring the air.

A flow hood is active. A blower pushes HEPA filtered air across your workspace in a smooth, constant sheet, so clean air flows toward you and shoves contamination away. You work out in the open with a big clear area in front of you. It's faster, it's far nicer for agar and for any kind of volume, and you can leave plates open without sweating because the air at the filter face stays clean the whole time. The catch is the price and the footprint. A decent DIY one runs £150 to £300 once you've bought a proper HEPA filter and blower, a bought one costs a lot more, and it needs mains power and a permanent spot on a bench. The filter is the part you can't cheap out on, and it's the part that costs.

So which do you need? For your first year and your first hundred jars, a still air box is enough. The flow hood didn't make my technique cleaner. It made me faster. I switched when batch sizes got big enough that the box had become the bottleneck, not because the box was letting me down. And I still pull the box out for a single jar or a quick transfer when I can't be bothered firing up the hood. If anyone tells you that you need a flow hood to start growing mushrooms, check whether they happen to sell flow hoods.

Do you even need to build one?

Depends what you're after. If you want to learn cultivation properly, build the box. Sterile technique is the foundation everything else sits on, and a still air box teaches it to you in an afternoon for the price of a takeaway. You'll understand why grows fail, which makes you better at the whole thing.

If you mostly want mushrooms growing on your counter in a few weeks and the lab side sounds like faff, you can skip all of this. A grow kit has already done the sterile work in a lab, so the contamination-risky steps are behind you before it even lands on your doorstep. You inoculate it and wait. No box, no iso, no flame. Both routes are legit. Pick based on whether you want the hobby or just the harvest.

FAQ

Does a still air box really work?
Yes, when your technique is good. It reliably cuts contamination during inoculation and transfers. It won't save you if you keep reaching out of the box or working too fast.

Is a still air box sterile?
No. It's still, not sterile. It removes the moving air that contamination travels on. It doesn't kill or filter anything.

Should I leave the arm holes open or cover them?
Either works. The only bad option is a loose cover sealing over an empty hole, which pumps air every time you move. Open holes, or a snug cuff around your arms, both fine.

How big should a still air box be?
Around 60 to 80 litres, clear, and deep enough to stand a jar upright with room for your hands. Too small and you'll knock your work over. Too big and it's harder to keep the air settled.

Still air box or flow hood for a beginner?
Still air box, every time. Cheaper, simpler, and it teaches you the fundamentals. Move to a flow hood when volume makes the box slow, not before.

Can I do agar work in a still air box?
You can, it's just slower and more cramped than a flow hood. Plenty of people pour and transfer plates in a SAB. If agar becomes your main thing, that's when a hood earns its money.

How much does a still air box cost?
About £15 to £25 in the UK, depending on the tub. It's the cheapest serious upgrade you can make to your success rate.