Condensing vs LGR vs Desiccant for Attic Drying Decision Tree

Why this matters

The attic is the hardest single drying chamber in residential restoration. Ambient grain runs high in summer and the cavity grain after a roof leak runs higher. Air temperature can swing 40F between morning and afternoon, and the only return air path is usually a pull-down ladder. Putting the wrong dehumidifier up there does not just stall the dry; it can actively drive condensation onto cold sheathing and cool framing as soon as the sun goes down. The picks among standard refrigerant, LGR, and desiccant are not interchangeable in this space, and the wrong call shows up as Day 5 sheathing reads no better than Day 2.

Symptom presentation

A typical attic loss after a wind-driven rain event or a pinhole supply line leak in an HVAC closet presents as wet insulation batts, wet sheathing along the leak path, and a wet top plate visible from below. Pin meter reads on sheathing run 18 to 24 percent MC at the wet path. Ambient attic temperature at midday in summer is 95F-plus and ambient grain is 95 grains per pound or higher. After dark the temperature drops to 70F and grain drops to 70 with it.

If a standard refrigerant unit was set the first night, the morning return finds the unit slugging water onto the floor or shut off on float, and sheathing reads have moved 2 points. If an LGR was set, the chamber held below 50 grains overnight but the daytime spike to 95 grains exceeds LGR rated effective range. If a desiccant was set, reads moved 4 to 6 points in 24 hours but the unit pulled 18 amps and tripped a 15-amp lighting circuit.

Quick checks

Measure cavity temperature at three points: ridge, mid-rafter, and at the top plate. Spreads larger than 15F across a single chamber tell you the chamber is not a chamber, it is a series of micro-environments, and one dehumidifier is not going to manage it without supplemental air movement.

Measure ambient grain at 8 AM, 1 PM, and 8 PM for 24 hours before settling on equipment. The grain envelope dictates the pick. If midday grain stays under 70 and night grain stays under 50, a standard refrigerant unit can work. If midday exceeds 90 and night stays above 65, you are in LGR territory at minimum. If grain exceeds 110 at any point or temperature drops below 60F overnight, refrigerant of any class loses efficiency and desiccant becomes a serious option.

Confirm power available at the attic. A desiccant that pulls 18 to 24 amps cannot share a circuit with anything; it needs a dedicated 20-amp or a 30-amp generator drop.

Isolation tree

Branch A: warm attic, moderate grain, accessible power. Standard LGR placed at the access opening with a 12-inch flex extension into the cavity. Two axial air movers, one delivering, one returning, set to create a horizontal sweep across the wet sheathing. Trickle vent the gable to manage solar-driven grain spike at midday.

Branch B: hot attic, high grain, accessible power. LGR plus supplemental dehumidifier. The first LGR cannot pull the chamber alone; pair with a second unit set at the opposite end of the wet path. Recalculate the dehu-to-affected-cubic-foot ratio every 24 hours.

Branch C: hot attic, very high grain, marginal power. Desiccant with ducted exhaust to outside. Desiccants are largely insensitive to ambient grain and can pull a chamber to 25 grains regardless of inlet. Run the wet exhaust outside through the gable; do not dump it into the conditioned space below.

Branch D: cool attic, leak from rain or snow event, low overnight temperature. Desiccant is the default. Refrigerant systems lose efficiency below 65F and lose it sharply below 50F. Desiccant performance is largely temperature-insensitive across the residential attic range. Branch D is the most common winter-leak pick.

Branch E: small cavity, surgical leak path, limited equipment access. Heat-and-vent or targeted drying with a small refrigerant unit plus heat. Do not size for the whole attic; isolate the wet path with poly sheeting and dry the chamber within the chamber.

Confirming the pick

Run a 12-hour pilot before committing the equipment for the full duration. Take grain pp readings at the dehumidifier intake every 2 hours. If grain drops at a rate of less than 2 grains per hour with current equipment, you have the wrong pick. Refrigerant slowing at high grain means upgrade to LGR. LGR slowing at high grain means upgrade to desiccant. Desiccant slowing means you have a continuing source or insufficient air movement.

Photograph the chamber psychrometric envelope at 0, 6, 12, and 24 hours. Adjusters increasingly ask for the chamber-grain curve as supporting documentation; a flat curve is the strongest argument for equipment upgrade.

Remediation criteria

Pick the smallest unit that can hold the chamber below 50 grains across the full 24-hour ambient swing. Oversizing a desiccant in a warm chamber will overheat the cavity and bake the insulation; undersizing an LGR in a high-grain chamber will stall.

Set the air movers to deliver, not just to circulate. The leak path is the drying target; the rest of the attic is overhead. Two delivered movers at 2,400 CFM each can sweep a 600-square-foot path; more movers on top of that just stir the chamber without lowering MC.

Vent the wet exhaust outside, never into the conditioned envelope below. The single most common Day 3 finding after a desiccant attic run is mold on the cool side of the bedroom ceiling because the wet exhaust came down through the access hatch.

References

1. ANSI/IICRC S500-2021 Standard for Professional Water Damage Restoration, Section 13.5.3 (Dehumidification Equipment Selection).
2. ASHRAE Handbook of Fundamentals, Chapter 1 (Psychrometrics), grain calculations and dew point relationships.
3. Phoenix Restoration Equipment, Dehumidifier Performance Curves at Low-Grain and Low-Temperature Conditions, OEM technical bulletin.
4. ASTM E96-22 Standard Test Methods for Water Vapor Transmission of Materials, sheathing vapor permeance reference.
5. EPA Document 402-K-01-001, Mold Remediation in Schools and Commercial Buildings, attic drying time triggers.