Weatherization & Building Envelope for Indoor Grow Facilities

Why the envelope is a second-order term in a sealed grow

An office building's HVAC load is mostly weather: heat coming through the roof and walls in summer, leaving in winter, and the outdoor air brought in for ventilation. Insulation and windows are the main levers, which is why weatherization advice is written around them.

A sealed flower room is a different machine. The load is internal. Hundreds of kilowatts of lighting turn into heat inside the room, and the plants transpire most of the water you irrigate them with into the air. The chillers and dehumidifiers are sized to remove that heat and that water, and the numbers are large: the pages on HVAC sizing for grow rooms and dehumidification load walk through them. Against that, the heat conducted through a well-built wall is small.

That is the reason envelope upgrades rarely lead a grow's efficiency list. It is not a reason to ignore the envelope. Three things it does control are worth real money.

What the envelope does control

Winter heat balance. In a cold month, heat leaving through the envelope is heat the chillers do not have to remove, and a very well-insulated room holds heat the room is trying to shed 12 hours a day. Many grows in cold climates run economizers or fluid coolers that use outdoor air as a heat sink, and the envelope's job is to keep humid room air separated from cold outdoor air while that heat moves through equipment designed for it, rather than through cracks.

Where the humid air goes. This is the big one. A flower room at 75 to 80 degrees and 55 to 65 percent relative humidity has a dew point in the high 50s or low 60s. In winter, any surface inside the wall assembly colder than that dew point will collect condensation if room air reaches it. Building Science Corporation's vapor barrier digest makes the point that people confuse vapor barriers with air barriers because air often holds a great deal of moisture in vapor form, and when that air moves because of a pressure difference, the vapor moves with it [bsc-bsd-106]. DOE's building science material draws the same line: a vapor retarder reduces the rate water vapor moves through a material by diffusion, while moisture carried by air movement has to be handled by an air barrier system or air pressure control [doe-bsesc-air-vs-vapor]. Grow rooms are often run at positive pressure to keep pests and dust out, which pushes room air into every gap in the envelope. Without a continuous air barrier, that air ends up in the walls.

What comes in. Infiltrating outdoor air carries moisture in summer, dryness in winter, and dust, spores, and insects year round. Every pound of water vapor that leaks in is a pound the dehumidifiers remove at a cost of electricity.

Air sealing: the envelope measure with the best return for a grow

Air sealing makes the boundary between conditioned and unconditioned space continuous, so air only crosses it through the HVAC system. DOE's guidance is that air barriers should be deployed throughout the building, with vapor barriers added where diffusion is a concern [doe-bsesc-air-vs-vapor]. Where grows leak, in rough order of how often it shows up in audits:

• Penetrations. Refrigerant lines, condensate drains, electrical conduits, irrigation lines, CO2 lines, and data cables all pass through walls and roofs. Each hole is sealed by whoever installed the pipe, or not.
• Doors. Roll-up doors and man doors between rooms and to loading areas, with worn sweeps and no vestibule.
• Top of wall. The joint between the wall and the roof deck in a metal building, and the joint between demising walls and the deck in a subdivided warehouse. Room air rises, and this is where it exits. Retrofit partitions that stop short of the deck are the same problem.

A blower door test on a room, or on the whole building, tells you how leaky it is and, with a smoke pencil or thermal camera, where. That is a standard part of a good commercial energy audit. Fixing what the test finds is cheap work: sealant, gaskets, closed-cell foam at penetrations, door sweeps, and continuous tape or membrane at the top-of-wall joint.

Vapor control: get it right, or leave it out

A vapor retarder slows water vapor diffusing through a material. Building Science Corporation classifies them by permeance: Class I is 0.1 perm or less, Class II is more than 0.1 up to 1.0 perm, and Class III is more than 1.0 up to 10 perms; a vapor barrier is a Class I retarder [bsc-bsd-106]. Polyethylene sheet and foil facings are Class I. Kraft paper facing is around Class II. Latex paint on drywall is Class III.

Two rules from that source apply directly to grow rooms:

1. Do not build a double vapor barrier. The digest recommends avoiding vapor barriers on both sides of an assembly so it can dry in at least one direction [bsc-bsd-106]. A metal building with an impermeable exterior skin, insulated, and then lined on the inside with poly or a foil-faced panel is a double vapor barrier. Water that gets into that cavity, from a leak or from condensation, has no way out.
2. Be careful with impermeable interior finishes. The same digest warns against vinyl wall coverings on the inside of air-conditioned assemblies, a practice it links to moldy buildings [bsc-bsd-106]. Grow rooms are routinely finished with washable, impermeable wall panels for sanitation. That is a reasonable choice, but it makes the interior finish the vapor barrier, so the rest of the assembly has to be designed to dry outward and the air barrier has to be continuous behind it.

The direction of vapor drive in a grow is unusual. A flower room runs wetter than the outdoors for much of the year in most climates, so the drive is outward in winter and can reverse on a humid summer afternoon. The right assembly depends on climate, wall type, and room setpoints, and it is worth an hour of a building science consultant's time before you finish the walls. Getting it wrong shows up as wet insulation, rusted girts, and mold behind panels two winters later.

Insulation: what it does and does not do for you

Insulation resists conductive heat flow, and its R-value is the measure; the higher the R-value, the better the thermal performance [energystar-r-values]. ENERGY STAR's retrofit guidance for existing homes ranges from R30 for an uninsulated attic in the warmest climate zone up to R60 in zones 4 through 8 [energystar-r-values], which shows the scale of a code-level envelope. For a sealed grow, insulation's job is narrower:

• Roof. Roof insulation and a reflective membrane reduce summer gain and, more importantly, keep the underside of the deck warm enough in winter that room air reaching it does not condense.
• Walls. Enough insulation to keep interior surfaces above the room's dew point in winter. Thermal bridges through metal girts and studs are the cold spots; continuous exterior insulation over the framing fixes that.

What insulation will not do is shrink the chiller. If an HVAC sizing calculation shows envelope gain as a large share of the total, ask what lighting density and plant transpiration were used, because one of those inputs is probably low.

A worked comparison

Every number here is an input we chose to show the scale. Assume a 5,000 sq ft flower room, 12 feet high, with 200 feet of exterior wall (2,400 sq ft of wall) and the full 5,000 sq ft of roof exposed.

Under those assumptions the entire envelope is under 25 kW against 175 kW of lighting, and the largest envelope term is infiltration, most of it water vapor the dehumidifiers must remove. Halve the infiltration with air sealing and you have removed more load than doubling the roof insulation would. That is why the order is sealing first, vapor control second, insulation to the level needed for condensation control, and then stop.

Where this fits in your efficiency plan

Envelope work is cheap during build-out and expensive later. For a new or renovated facility: pick the lighting first because it sets everything else, size HVAC to the lights and the crop, specify a continuous air barrier and detail every penetration before the trades arrive, settle the vapor strategy with someone who models it, insulate to keep interior surfaces above the room dew point in the coldest month, and commission it with a blower door test and thermal scan before the rooms are loaded.

If the facility is already operating, an audit with a blower door and thermal camera is the place to start. The results usually point at a short list of penetrations and joints, not at re-insulating the building. Benchmarks in kWh per square foot for grow facilities will tell you whether the building is an outlier worth deeper investigation.