Building a Martha tent changed how many blocks I could run at once. Before I built my first one, I was cycling through individual monotubs, each one needing manual misting two or three times a day. The Martha tent took that off my plate entirely. Set the humidity controller, check it once in the morning, and the system does the rest.
This guide gives you the exact build I use, including the sensor placement measurements that most guides skip. Getting those right is the difference between a tent that holds 88% RH without intervention and one that swings between 75% and 99% while your blocks dry out or rot.
What You Need (Complete Materials List)
Before anything else, here is the full parts list with realistic prices:
| Item | Notes | Approx. Price |
|—|—|—|
| 4-tier mini greenhouse kit (27″W x 19″D x 63″H) | Zippered PVC cover included | $28-$40 |
| Inkbird IHC-200 humidity controller | Standard (non-WiFi) version | $27-$32 |
| Levoit Classic 300 humidifier | 6L top-fill, basic knob version (not the 300S) | $45-$60 |
| Backup digital hygrometer | For sensor verification | $8-$12 |
| 4″ inline fan + speed controller (optional) | Budget brand such as VIVOSUN | $20-$30 |
| Short 4″ flex duct section + clamps | For fan port, or for external humidifier ducting | $5-$8 |
Totals: Without the fan: approximately $115-$122. With the fan: approximately $135-$152.
My recommendation: skip the inline fan for your first run and use passive FAE holes cut in the tent cover. You can add the fan later once humidity is dialed in. The fan makes CO2 control easier once you have 8+ blocks running. For a first build with 4-6 blocks, passive FAE is fine.
How the System Works
The Martha tent is a sealed microclimate. The Levoit humidifier produces ultrasonic cool mist. The Inkbird IHC-200 reads the humidity level via its external sensor and switches the humidifier on and off to hold your target.
The tent has three variables to manage:
Humidity (85-95% RH): Mushrooms need high moisture at the fruiting surface. Below 80% RH and you start seeing dry cracking on caps and slow pinset development. Above 95% for extended periods and you invite bacterial wet rot.
Fresh air exchange (FAE): Mushrooms exhale CO2. At high CO2 concentrations (above roughly 1,500-2,000 ppm) you get leggy stems and malformed caps. FAE holes or a slow-running fan pull in fresh air and push CO2 out.
Temperature (65-75F for most species): The tent traps heat. Do not put it in direct sunlight. Temperatures above 80F will stall most species and accelerate contamination.
The system I describe here addresses all three. Most budget builds address humidity and ignore the other two. You will notice the difference.
Step 1: Assemble the Greenhouse Rack
The standard 4-tier mini greenhouse kit goes together in about 20 minutes. The frame is a powder-coated metal tube structure with plastic shelf trays and a zippered PVC cover that wraps the entire frame.
Assembled dimensions: 27″W x 19″D x 63″H. Shelf-to-shelf clearance is approximately 14″. That is enough space for a standard mushroom fruiting block (typically 10″-12″ tall at full pin development) with a few inches to spare.
One thing I do before attaching the cover: wipe all shelf surfaces with 70% isopropyl alcohol. The racks ship with manufacturing residue on them. You do not want that in your fruiting environment.
After assembly, set the tent in its final location before loading anything. Moving a tent with blocks on it is awkward and risks toppling. Choose a location indoors, away from windows, with reasonable ambient temperature.
Step 2: Position the Humidifier
Place the Levoit Classic 300 on the floor of the tent, below the lowest shelf. Center it against the back wall.
The 360-degree rotating nozzle should point upward and slightly toward the center of the tent. Do not aim it directly at any shelf or at any fruiting block. Mist landing directly on the surface of a block or pin cluster causes bacterial wet rot. The goal is diffused mist filling the tent air, not direct spray.
Nozzle-to-nearest-shelf distance: Keep at least 12 inches of vertical clearance between the nozzle tip and the bottom of the lowest shelf.
Humidifier-to-back-wall distance: Push it as close to the back wall as the power cord allows, typically 2-4 inches. This keeps the front shelves accessible.
Use distilled water, not tap. Tap water through an ultrasonic transducer deposits calcium and magnesium on the vibrating disc, reducing mist output over time. It also aerosolizes mineral particles onto your substrate surfaces, which can alter pH and promote competing organisms. A gallon of distilled water costs under $1 at most grocery stores. The Levoit Classic 300 has a 6-liter tank (1.58 gallons); you will fill it every 2-3 days at 90%+ RH targets.
The external ducting method (optional but worth it): Some growers run a short section of 4″ flex duct from the humidifier nozzle through a hole cut in the bottom panel of the tent, keeping the humidifier body outside. This makes refilling dramatically easier and prevents condensation pooling under the unit. If you plan to run the tent long-term, this is worth the one-time setup effort. For a first build, just set it on the floor inside.
About the Levoit Classic 300 specifically: The reason I recommend this unit over cheaper alternatives is the 300 mL/hour mist output and the 6-liter tank. A smaller 2-3 liter humidifier needs filling every day at 90%+ RH targets, and it often cannot keep up with the tent volume during low-RH periods. The Levoit 300 is sized for 215-505 sq ft rooms, which means it is dramatically oversized for a tent. That is intentional. When the IHC-200 calls for humidity, the Levoit fires and hits the target fast, then shuts off. The humidifier cycles briefly rather than running continuously, which reduces mineral accumulation on the transducer and extends the unit lifespan. The top-fill design means you can refill without removing any trays or tipping the unit. On a stacked tent, that matters because the humidifier has limited clearance above it.
Step 3: Install the Inkbird IHC-200
The IHC-200 mounts outside the tent. The sensor probe runs through a small gap in the zipper or through a slit cut in the PVC cover.
Sensor placement: this is the section most guides skip.
Getting sensor placement wrong causes 80% of Martha tent humidity problems. Here is exactly where to put it:
- Height: Mount the sensor at the level of your top fruiting shelf, approximately 42″-48″ from the floor in a standard 63″ tent.
- Position: On the wall of the tent opposite the humidifier nozzle. If the humidifier is at the back, the sensor goes at the front. If it is centered, put the sensor on either side wall.
- Distance from humidifier nozzle: Minimum 18-24 inches of horizontal separation. Never let the sensor hang in the direct mist path.
Why the top shelf? Humidity stratifies. The air immediately around the humidifier will always read higher than the air near the top of the tent. If your sensor is near the humidifier, it reads 92% while your top shelf blocks are sitting at 78%. By putting the sensor at the top, far from the source, you get a conservative reading. If the IHC-200 says 88% at that point, you know every shelf below it is at least that humid.
The most common sensor mistake I see: Growers thread the sensor cable straight through the front zipper and let it hang a few inches from the nozzle. The controller reads 99% constantly because droplets condense on the capacitive sensor element. The humidifier never fires. Blocks dry to 70% RH in 6-8 hours. The grower assumes the controller is broken and buys a replacement. Moving the sensor 18 inches away fixes the issue.
A workaround for extended high-humidity runs: After a few months in 90%+ RH, even a well-placed sensor can drift or give erratic readings due to condensation accumulation inside the sensor housing. Wrap the sensor in a thin layer of breathable mesh or a light sock, which blocks direct droplet contact while allowing vapor through. I started doing this after my third build and have not had a false reading since.
The IHC-200 sensor cable is 6.5 feet (approximately 2 meters). You have plenty of reach to position it anywhere in the tent without splicing or extension.
Step 4: Wire the IHC-200
The IHC-200 has two controlled outlets labeled Work 1 and Work 2:
- Work 1 = Humidifier (Levoit Classic 300). This outlet fires when RH drops below your setpoint minus the low differential.
- Work 2 = Exhaust fan (if installed). This outlet fires when RH rises above your setpoint plus the high differential. When RH overshoots, the fan pulls in dry room air to bring it back down.
The sensor runs from the back of the unit and routes into the tent through the zipper or a small cut slit. The main power cable is 5 feet long. Mount the IHC-200 body outside the tent on a shelf or ledge next to it. Do not let it get misted directly.
Step 5: Program the IHC-200
Hold the SET button for 3 seconds to enter programming mode. The display will show your current humidity reading and a flashing setpoint.
Recommended settings for oyster, lion mane, and shiitake:
| Parameter | Setting | Meaning |
|—|—|—|
| Target humidity | 88% | Center of the ideal range |
| Low differential | 3% | Humidifier fires at 85% RH |
| High differential | 3% | Fan fires at 91% RH |
| Delay protection | 1 minute | Reduced from factory default of 3-5 min |
| Calibration (CA) | Adjust after verification | Set after checking against backup hygrometer |
At these settings the humidifier fires when RH drops to 85%, and if you have a fan on Work 2, it fires when RH hits 91%. The tent holds 85-91% RH continuously without intervention.
The delay protection setting matters more than most guides mention. The IHC-200 ships with a delay of 3-5 minutes, designed to protect air conditioning compressors from short-cycling. In a Martha tent, this delay means RH can swing 8-10% before the humidifier fires. Set the delay to 1 minute. Without this change, you will see wide swings on your backup hygrometer even though the IHC-200 appears to be working.
Calibration: The IHC-200 has a rated accuracy of plus or minus 3% and tends to drift in sustained high humidity. Always verify against a separate digital hygrometer inside the tent. If the backup reads 88% and the IHC-200 reads 92%, set the calibration (CA) parameter to -4. Run them side by side for the first 48 hours to confirm they track together.
Step 6: Cut FAE Holes
If you are not using an inline fan, cut passive FAE holes in the PVC cover. A box cutter works fine.
Standard passive FAE pattern for a 27″x19″ tent:
- Two horizontal slits in the top panel: each slit approximately 5″ long, centered in the panel, one on the left side and one on the right
- Two horizontal slits on each side panel at the level of shelf 2 (second from top): each slit approximately 4″ long
- Two horizontal slits on each side panel at the level of shelf 1 (top shelf): each slit approximately 4″ long
This gives you six FAE openings total. Natural convection pulls fresh air in at the lower slits and pushes CO2-laden air out at the top.
For a tent with more than 8 blocks, or for any species that is particularly CO2-sensitive, add the inline fan. Mount it at the top back corner of the tent exhausting outward. Set the speed controller to its lowest setting, targeting 30-50 CFM effective output. Running a 200 CFM fan at full speed in this tent will exchange the entire air volume in under 2 seconds and crater your RH.
The fan-plus-controller setup that actually works: Plug the fan into Work 2 so the IHC-200 controls both devices. The humidifier fires when RH drops too low. The fan fires when RH climbs too high. They cannot fight each other this way. Running a fan on a separate timer while the humidifier is on the IHC-200 creates a sawtooth humidity pattern where the timer fires the fan, dry room air dumps in, the IHC-200 runs the humidifier hard to compensate, and then the timer kills the fan and RH overshoots. Avoid that setup entirely.
Step 7: Load and Verify Before Adding Blocks
Before loading fruiting blocks, run the tent empty for 24 hours. Check your backup hygrometer every few hours. You want to confirm:
- RH holds between 85% and 91% consistently
- IHC-200 and backup hygrometer agree within plus or minus 3%
- The humidifier fires and cuts off on a predictable cycle (every 15-30 minutes at steady state is normal)
- No standing water pooling on the tent floor (if you see pooling, the nozzle is aimed too low or the humidifier is too close to the floor)
Once the 24-hour test looks stable, load your blocks. Keep at least 6 inches between blocks on each shelf. Block-to-block contact transfers contamination. If Trichoderma (green mold) hits one block and it is touching its neighbor, the neighbor typically follows within 3-4 days. Six inches of separation slows physical transfer considerably.
Common Problems and Fixes
RH stays at 70-75% no matter how often the humidifier runs
Most likely cause: sensor is in the mist path and reading falsely high, so the humidifier never fires at the right time. Fix: move sensor to opposite wall, 18″+ from nozzle. Verify with backup hygrometer.
RH spikes to 98-99% and holds there
Most likely cause: no FAE, tent is fully sealed. Fix: cut FAE slits, or confirm Work 2 fan is connected and programmed if installed.
Humidity is fine but pins develop long, leggy stems
Most likely cause: excess CO2 from insufficient FAE. Leggy stems are the mushroom reaching for lower-CO2 air. Fix: add or enlarge FAE holes, add inline fan on Work 2.
Pins develop but abort before reaching harvest size
Most likely cause: RH dropping during a vulnerable window, often caused by the delay protection being set too high. Fix: confirm delay is set to 1 minute; recalibrate sensor if drift has occurred.
Humidifier mist output declining over time
Cause: mineral deposits on the ultrasonic transducer disc. Fix: switch to distilled water; clean the transducer weekly with white vinegar on a cotton swab; descale the whole unit monthly.
Condensation puddles under humidifier
Fix: place the unit on a small plastic tray raised slightly off the tent floor, or switch to the external ducting method.
Maintenance Schedule
After every flush:
- Wipe all shelf surfaces with 70% isopropyl alcohol
- Empty and clean humidifier tank with white vinegar rinse
Weekly:
- Clean humidifier transducer disc
- Check sensor calibration against backup hygrometer
- Inspect FAE holes for any obstruction (mold growth around slit edges is common and should be wiped back)
Monthly:
- Full isopropyl wipe-down of tent interior, including PVC cover, frame, and floor tray
- Full descale of humidifier with a 30-minute vinegar soak, followed by thorough rinse
- Check sensor cable for physical damage near where it routes through the cover
Final Cost Summary
Standard first build without the inline fan:
| Item | Price |
|—|—|
| 4-tier mini greenhouse (27″x19″x63″) | $30 |
| Inkbird IHC-200 humidity controller | $30 |
| Levoit Classic 300 humidifier (6L) | $50 |
| Digital backup hygrometer | $10 |
| Total | $120 |
Add the inline fan kit ($25-$30) when you are ready to scale past 8 blocks or want tighter CO2 control.
The Inkbird IHC-200 and the Levoit Classic 300 are the two items worth spending on. The greenhouse kit itself is a commodity. Any 4-tier mini greenhouse with a clear PVC cover in those approximate dimensions will work. The cheapest one that holds together is fine.
Placement Reference Card
| Component | Placement |
|—|—|
| Humidifier | Floor of tent, below lowest shelf, nozzle pointing upward toward center |
| Nozzle-to-shelf clearance | 12″ minimum vertical |
| IHC-200 sensor | Top shelf level (42″-48″ from floor), on wall opposite humidifier |
| Sensor-to-humidifier distance | 18″-24″ horizontal minimum |
| FAE slits (passive) | 5″ slits in top panel; 4″ slits on each side at shelves 1 and 2 |
| Inline fan (if used) | Top back corner, exhausting outward, lowest speed setting |
| Block spacing | 6″ minimum between blocks on each shelf |
| IHC-200 setpoint | 88% target, 3% low diff, 3% high diff, 1-min delay |
Two years into running Martha tents, the IHC-200 sensor placement and the delay protection setting are still the things I mention first to anyone troubleshooting inconsistent humidity. Everything else in the setup is forgiving. Those two details are not.
Get your first run dialed in with passive FAE and no inline fan. Watch the humidity log on your backup hygrometer for 48 hours after loading blocks. If it holds 85-91% RH through several humidifier cycles, you have a working system. If it is swinging more than 8%, the sensor placement or the delay setting needs adjustment.
Once it is dialed in, the tent runs itself.
Scaling Up: Going From One Tent to Two
Once you have one tent running reliably, adding a second is straightforward. You need one IHC-200 per tent since each tent is an independent humidity environment. The Levoit Classic 300 is tent-specific. The wire rack, on the other hand, scales differently.
If you are running 12 or more blocks regularly, consider replacing the mini greenhouse kit with an NSF-rated wire rack (18″x36″ or 24″x48″) inside a dedicated grow tent. The NSF rack costs more ($60-$90) but the weight capacity per shelf jumps from 24-50 lbs to 300+ lbs, and the steel construction survives repeated isopropyl cleaning without warping or corroding. The mini greenhouse plastic frame starts showing wear after 6-12 months of regular use and high humidity.
For most home growers running 4-8 blocks at a time, the mini greenhouse kit at $30 is fine. When you outgrow it, you will know.