Ice-Dam Leak Inside a Brick-Veneer Cavity in a 1925 House
A January ice-dam leak at a 1925 two-story brick house entered at the second-floor rim joist and ran down inside the exterior wall cavity; plaster shows a watermark at the second-floor ceiling line. The exterior is brick veneer with a 1-inch cavity over sheathing, and the stud bays are filled with blown-in cellulose. Interior walls are plaster on metal lath. The first floor is 3/4-inch maple strip over 5/8-inch plywood on joists over a vented crawlspace. A bathroom has vinyl sheet over 1/4-inch underlayment on a slab-on-grade. Penetrating-meter readings: plaster 26 percent, maple 17 percent, plywood subfloor 24 percent, joists 21 percent. Unaffected maple in a dry room reads 8 percent. A sealed in-situ RH test in the bathroom slab reads 92 percent, while the vinyl surface shows no visible moisture. Indoor conditions are 68 F and 45 percent RH; the crawlspace vents are open.
1. Determine the class of loss and explain why a standard air-mover-and-dehumidifier layout will not dry this structure.
Read the worked answer
Class 4, specialty drying. Several indicators point the same direction:
- Water traveled inside a deep wall cavity filled with cellulose behind brick veneer, so the wet material is not exposed to room air at all.
- Plaster on metal lath is a low-permeance, high-mass assembly; it holds water and releases it slowly.
- Maple strip over plywood over joists reads 17 and 24 percent while dry maple in the same house reads 8 percent - the moisture is in the assembly, not on the surface.
- Vinyl over underlayment on a slab-on-grade with 92 percent sealed RH has no drying path from below and a vapor-retarding path from above.
A conventional layout of air movers plus an LGR unit works by evaporating moisture from exposed surfaces and removing it from the air. Here, every wet surface is either covered or inside a cavity. The predictable failure mode is surface drying while the core stays wet: the plaster face and the maple face drop to near-normal readings, the meter says "dry," and the cavity, subfloor, and slab remain saturated. That leads to callbacks, mold growth inside the bay, and finish failures weeks later.
Also, with crawlspace vents open in January, cold air under the first floor keeps the joist bays near or below the dew point of the interior air. Drying equipment upstairs can drive moisture into those cold bays where it condenses. The crawlspace must be closed and treated as part of the drying system, not left vented.
2. Describe the drying approach: what you would remove and what equipment and methods you would specify.
Read the worked answer
Removal and access first. Drying Class 4 assemblies is an access problem before it is a machine problem.
- Remove baseboards and any wet trim along the affected walls.
- Establish cavity access: small, controlled openings in the plaster for injection and inspection, sized to preserve the assembly where possible, and a flood cut where plaster is already compromised or contaminated.
- Remove the vinyl and underlayment in the bathroom to expose the slab; water under a sheet floor will not leave through it.
- Open the plywood or create inspection points for the subfloor and joists, and gain access from the basement/crawlspace side where the framing is exposed.
- Clean out saturated cellulose in the wet bays; wet cellulose is not a drying candidate.
Equipment and method.
- Cavity injection: manifolded dry air injected into each affected bay, with a return path so air actually moves through the cavity rather than pressurizing it. Verify with a thermo-hygrometer at the outlet.
- Mat drying on the maple: a mat system that pulls dry air across the floor face while the dehumidifier draws the moisture out is far more effective than air movers alone, and it avoids over-drying the face.
- Desiccant dehumidification or LGR with supplemental heat, depending on the target conditions and the day's weather; Class 4 assemblies benefit from warmer, drier air and from tight control of the dew point.
- Close the crawlspace vents and dehumidify the crawlspace.
- Keep the building closed; do not ventilate with cold outdoor air.
3. How do you verify the structure is dry when surface readings will be misleading?
Read the worked answer
Define the dry standard per material from unaffected material in the same building, then verify at depth - not at the surface.
- Maple flooring: use an insulated-pin meter driven to depth at a documented grid, and take readings at the back of the flooring where a board can be lifted at a transition or register. Compare against the 8 percent unaffected maple, and cross-check with the wood's equilibrium moisture content at the building's target temperature and RH. A face reading alone does not count.
- Plywood and joists: read from below where they are exposed in the basement or crawlspace; that is also the side that dries last when water came from above.
- Plaster and cavities: compare affected plaster to dry plaster in the same house, and place a thermo-hygrometer in the cavity to confirm the injected air is actually removing moisture rather than just circulating. A cavity reading that tracks the room's dew point means dry; one that stays at saturation means the cavity is still wet.
- Slab: use an in-situ RH probe to a documented depth, compare with an unaffected slab area and with the building's equilibrium conditions, and record the floor covering's manufacturer moisture limit before any reinstallation. A vapor-retarding sheet floor means the slab has to dry from the top, which sets the schedule.
- Whole-house: record temperature and RH inside and out daily so the readings can be interpreted, and take thermal images to find missed wet bays.
Do not demobilize on a single visit's surface readings; verify the trend across at least two consecutive days.