## Facts

- DuPont manufactured and sold an acetal resin (1983–1989) used in polybutylene plumbing systems.
- Widespread allegations arose that defects in the resin caused plumbing leaks, producing property damage and related losses; DuPont ceased selling the resin for that application.
- DuPont maintained a liability program with a $50 million self-insured retention (SIR) per occurrence and multiple layers of excess insurance (“towers”) across successive years.
- DuPont defended and settled a very large volume of claims arising from the plumbing failures and sought coverage from its insurers, recovering substantial sums from many carriers.
- Stonewall issued two excess policies within DuPont’s insurance program; each policy included limits and a non-cumulation of liability clause.
- The dispute centered on (a) how many “occurrences” the plumbing claims constituted under the excess policies (affecting the SIR and attachment), and (b) how the non-cumulation clause affected Stonewall’s liability given losses spanning policy periods and payments by other insurers.

## Issues

1. Whether the plumbing-system property-damage claims constituted one occurrence or multiple occurrences under Delaware’s occurrence-counting standard.
2. Whether the non-cumulation of liability clause reduced Stonewall’s limits only to prevent duplicate recovery for the same loss across policy periods, or instead eliminated Stonewall’s liability more broadly.
3. From what date prejudgment interest should accrue on amounts owed under the policies.

## Decision

- The court held the claims arose from a single occurrence under Delaware’s “cause” test, focusing on the manufacturing and distribution of the unsuitable resin as the operative cause.
- The court held the non-cumulation clause was unambiguous and operated to reduce Stonewall’s liability only to the extent necessary to prevent duplicate recovery for the same loss previously paid under prior coverage, not to extinguish coverage across all claims.
- The court reversed the prejudgment-interest start date, holding interest should run from a specific demand made in 2006 rather than from the 1999 complaint filing.
- The judgment was affirmed in part, reversed in part, and remanded for recalculation of prejudgment interest.

## Legal Principles

- Under Delaware’s “cause” test, the number of occurrences turns on whether injuries share one proximate, uninterrupted, and continuing cause; multiple injury events may be aggregated where they are effects of a single causal course of conduct.
- In product-related liability coverage disputes, widespread claims arising from the insured’s unified manufacturing and distribution of a defective product may constitute a single occurrence despite dispersion across many locations and claimants.
- A non-cumulation of liability clause, when unambiguous, is enforced to prevent double recovery for the same loss across successive policy periods and reduces limits only by amounts previously paid on that same loss.
- Prejudgment interest on contract damages accrues from the point payment is wrongfully withheld; in complex coverage disputes, wrongful withholding may begin when the insurer receives a clear, specific monetary demand and refuses to pay.

## Conclusion

The Delaware Supreme Court treated the nationwide plumbing claims as one occurrence under the cause test, enforced Stonewall’s unambiguous non-cumulation clause as a targeted anti-duplication mechanism rather than a blanket bar to coverage, and required prejudgment interest to be recalculated from DuPont’s 2006 specific demand date.


Stonewall Insurance Co. v. E. I. du Pont de Nemours & Co., 996 A.2d 1254 (Del. 2010)