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Residential Marijuana Grow Rooms: 7 Minutes of BS

Extreme heat and humidity can drive moisture deep into framing cavities which can cause mold, rot, and structural damage
April 20, 2020

In this episode, we are joined by forensic carpenter Bart Laemmel of Crested Butte, Colorado to talk about the strange turns his investigations have taken (marijuana grow rooms) since marijuana legalization.

What a marijuana grow room is:

ma·ri·jua·na grow room​ | merə(h)wänə ɡrō ro͝om

​"A residential grow room—in my opinion—would be a specific area of the home that is set aside and dedicated for the growth of marijuana plants, and not for the standard occupancy of people and/or pets."

—Bart Laemmel, forensic carpenter

"In Colorado, we are allowed to have seven plants per person, so you can imagine in a residential home, say you have three people renting it, they can all grow seven plants, so no we have 21 plants, so you basically have indoor horticulture in a residential structure."


How marijuana grow rooms work

People have been growing houseplants in houses for a long time. They balance relative humidity, they clean the air, and they generally make the place homier.

But this is different. This is twenty-one large bushes filling a bedroom, demanding resources that typical houseplants do not demand.

This can produce a lot of unwanted situations from moisture and indoor air quality, water usage, electrical usage…

Basically, a grow room has powerful lights which produce a lot of heat and eat a lot of electricity. The plants also need a constant source of water, often automated, which can cause bulk water issues if there is a leak. Plants soak up water in their roots and transpire vapor through their leaves, which boosts the interior relative humidity—quite a bit in a hot room full of plants.

Most people build a specific area of the house to do this in. We’ve found them in crawl spaces, basements, attics, bedrooms± just about anywhere they can carve it out.

Why marijuana grow rooms matter

The problems begin with air sealing or the lack thereof.

These rooms are not set up to be completely air sealed chambers

So the hot moist air inside the grow room …

You might have a room that has 60% RH and 80-degree air temperatures

will quickly migrate to other parts of the house, including into the framing.

In a climate, such as where I live, Crested Butte, which is climate zone 7, so then you have big temperature differentials across that boundary.

So, you have cold dry air outside and all that warm moist air wants to do—it’s pretty dumb—it just wants to go in one direction, and that’s from inside to outside.

The thermodynamic laws of Physics tell us that—at least on earth—heat and moisture move from more to less. So the warm moist air wants to get out of the grow room.

So you’re injecting a lot of unwanted moisture and sometimes chemicals even, fertilizers and things like that, into the home, which drives to the exterior.

This matters a lot because we are creating massive drives across our building structure that put a lot of unwanted moisture into a lot of unwanted places.

Like wall cavities, roof cavities, or if the grow room is in a crawl space, the floor framing above.

So, we end up with condensation issues, we end up with moisture issues on windows, we end up with indoor air quality issues, mold problems, and respiratory issues.

From there, those compound into building failures as well.

A simple little grow room can sure snowball out of control if you don’t give it some serious thought.

All this to grow a few recreational marijuana plants.

Whoever said marijuana is harmless, right?


How to do marijuana grow rooms right

As a remodeler, if you live in one of the 31 states where it is legal for recreational or medical use, chances are, someone is going to ask you to build a marijuana grow room at some point. When they do, you should know what you’re up against.

The biggest two items on the ‘Doing it Right’ situation is that we need to concentrate on the air tightness of that space.

So, that needs to be, really a dedicated room for doing what you’re doing, and it needs to be fully air sealed from the rest of the structure.

Standard air-sealing techniques apply, though you can probably use weatherstripping to gasket the door rather than installing an exterior door into an interior room. But now you’ve got an airtight room that will be full of vapor that cannot escape into the framing, so you need to give it an exit path.

Once we’ve air-sealed that area, the next biggest thing is ventilation.

We need to provide proper ventilation out of that room to control the moisture, create a negative pressure compared to the rest of the surrounding building,

When a room is under positive pressure, it pushes air outward into other parts of the house. Negative pressure means that the grow room is basically pulling surrounding air into it because an exhaust fan is sucking the air out.

There is one hiccup on the ventilation side of this equation, though

Now, that creates ummm, smell issues outside the building so the next thing from there is you have to have filtration of the air going out in order to pick up those smells…

For a couple of reasons. One, the neighbors may not like the aromatic-enhancement to their neighborhood.

The other reason is that the pungency may attract enterprising young entrepreneurs, who are looking for a shortcut in the procurement pipeline, shall we say.

We haven’t had a huge amount on the residential side of people being broken into to steal their plants, but for security reasons on a grow operation? Yeah.

Charcoal filters can be added to the exhaust ducts to effectively neutralize the aromatic compounds in the exhaust air.

We don’t have a lot of regulation or code around this, so, there might be some code requirements, coming forward, that if you’re going to grow in your house you have to actually pull a permit and it has to be inspected and built to a certain standard.

To be fair, any major remodeling project requires a building permit, and a residential marijuana grow room may well involve plumbing, electrical, and waterproofing. Screwing it up can cause health, safety, and structural issues, so it is not outside the realm of possibility that building departments will want to get their arms around it.

We did have one in a condo complex with above and below units and the guy in the middle had an irrigation system, and it sprung a leak, and he had gone away for a week, and he came back and it had flooded the downstairs.

So, like any wet room, use stainless steel supply hoses, rather than plastic ones, and waterproof the floor, like you would in a shower or laundry room. Electrically, we are talking about multiple 1,000-watt bulbs which not only produce a lot of heat, they suck a lot of juice. So you may need to upgrade the service entrance.

Really your degradation to the building is coming from the heightened level of moisture, the heightened level of temperature, and those mixing and driving forces to the outside.

We hope you now have a heightened sense of what it takes to build a residential grow room for your customers without destroying the house that surrounds it. It’s important because you get paid for what you do and what you know. Even if it’s not something you’re going to do, you should know what is involved, in case they ask.

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Image: Cannabis Training University

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