Case Study: Ice Dam — Recurring OSB Sheathing Mold from R-14 Attic Insulation, Sandy, UT

Event date: February 2024 | Property: 1983-era ranch, 10200 South corridor, Sandy, UT | Event type: Ice dam meltwater producing recurring Aspergillus colonization on attic OSB roof sheathing underside over at least three consecutive winter seasons | Insurance carrier: Allstate HO-3 | Approved amount: $6,780 | Deductible: $1,000

What Happened — Three Winters, One Root Cause

In February 2024, a Sandy homeowner on the 10200 South corridor contacted us after the second roofing contractor she had called told her that her roof was not leaking and had recommended she call a water damage contractor about the recurring ceiling stain in the master bedroom. The stain had appeared in January 2022 — she had it painted over. It had returned in January 2023 — she had it painted over again. It had returned in January 2024, darker and slightly larger, and this time she noticed a faint musty smell in the master bedroom that she had not noticed during the prior two winters.

The roofing contractors were correct: the roof was not leaking in the conventional sense of a defective membrane or missing shingles. The ice dam that forms at the eave of this 1983-era ranch during Wasatch Front freeze-thaw cycles in January and February is not a roofing defect. It is a thermal envelope defect. The R-14 attic insulation — the blown-in fiberglass insulation installed in 1983 to the then-current energy code standard — allows heat from the living space to pass through the ceiling plane at a rate sufficient to warm the roof deck above the master bedroom. The warmed deck melts the overlying snow from below. The meltwater runs toward the cold eave zone — the section of the roof that extends beyond the exterior wall plane, receives no heat from the building interior, and remains at the ambient outdoor temperature. The meltwater refreezes at the cold eave, building the ice dam. Subsequent meltwater pools behind the ice dam until it finds a pathway under the shingle laps or through a nail penetration and enters the attic assembly.

Each of the three winters had produced an ice dam at the same eave location — above the master bedroom exterior wall — and each ice dam had introduced meltwater into the attic assembly above the master bedroom. Each cycle had wetted the OSB roof sheathing in the same zone. The sheathing had never fully dried between cycles. Aspergillus had established in the OSB sheathing underside during the first cycle and had been expanding its colonized zone with each subsequent cycle’s additional moisture.

What We Found

Attic access above the master bedroom: Aspergillus colonization on the OSB roof sheathing underside in a fan pattern extending 29 square feet from the eave entry point — larger than the Sandy 2001-era craftsman case study in the ice dam series because this R-14 insulation had been producing a larger and more consistent roof deck warming than the R-22 insulation in the craftsman, and the ice dam had been forming more reliably across a wider eave zone over three seasons rather than the craftsman’s two seasons. Calibrated moisture meter readings at the sheathing: 19% to 27% at 11 monitoring points — above dry standard throughout. The mold pattern had expanded laterally with each season, consistent with three cycles of repeated wetting and partial drying.

FLIR thermal imaging of the master bedroom ceiling: cold zone extending 24 inches from the exterior wall — slightly smaller than the Aspergillus zone above it, consistent with the meltwater migration pathway from attic entry through the insulation to the ceiling surface losing moisture to the attic air as it migrates.

What We Did and What We Insisted On

Mold remediation of the attic OSB sheathing: HEPA negative air pressure containment above the master bedroom ceiling. Wire brush removal of colonized OSB sheathing surface material. Two EPA-registered antimicrobial passes at required concentration and dwell time. Film-forming encapsulant applied to the full remediated zone and 8-inch conservative extension beyond the thermal boundary. Master bedroom ceiling drywall replaced at the cold zone. Independent CIH clearance testing: normal background spore counts returned.

Before we would schedule reconstruction, we required that the attic insulation be upgraded to R-49 above the master bedroom ceiling — the IECC current recommendation for this climate zone. We do not typically make reconstruction conditional on homeowner decisions outside the restoration scope. This was an exception, and we explained it clearly: without the insulation upgrade, the same ice dam will form next January, the same meltwater will enter the attic through the same eave pathway, and the same Aspergillus colonization will re-establish on the same OSB sheathing — three to six months after we close the ceiling. We will not seal a ceiling over a mold recurrence waiting to happen in a 4-to-6-month timeframe. She had the insulation upgraded before reconstruction began. Allstate HO-3 covered the event. Total approved: $6,780. Deductible: $1,000. The insulation upgrade was not part of the HO-3 scope — she paid for it separately, approximately $1,400. She described it as the most useful $1,400 she had spent on the house.

Frequently Asked Questions

Why does the same ice dam form in the same location every year?

Ice dam formation is a function of heat loss rate through that specific ceiling section — not weather severity. If insulation allows consistent heat loss above the threshold that warms the roof deck above freezing during typical January freeze-thaw cycling, the ice dam forms in the same location every year. Upgrading insulation reduces heat loss through that section below the ice dam formation threshold.

What is the difference between Aspergillus and Cladosporium in attic mold events?

Cladosporium is typically the first colonizer — it germinates at lower moisture levels and shorter wet durations. In ice dam events with recurring wet-dry cycles, Aspergillus is frequently the dominant species in the established colonized zone because repeated wetting provides the sustained moisture conditions Aspergillus prefers. Both are addressed by the same ANSI/IICRC S520 protocol.

Does the insurance company require the insulation upgrade before paying the claim?

No — the carrier covered the restoration scope without requiring the insulation upgrade. The upgrade requirement was imposed by us as a condition of scheduling reconstruction: without it, Aspergillus re-colonization on the same OSB sheathing within 4 to 6 months after reconstruction was essentially certain given the three-year established pattern. Reconstructing over an unaddressed recurrence mechanism is not in the homeowner’s interest.

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True Day Water Damage Restoration | 11268 S 2865 W, South Jordan, UT 84095 | (385) 247-9359 | Utah Contractor License: #960332-3505 | IICRC Firm ID: #927354-5258