Wisconsin Statutes Insurance (Ch. 600 to 655) § 631.81. Notice and proof of loss

(1) Timeliness of notice. Provided notice or proof of loss is furnished as soon as reasonably possible and within one year after the time it was required by the policy, failure to furnish such notice or proof within the time required by the policy does not invalidate or reduce a claim unless the insurer is prejudiced thereby and it was reasonably possible to meet the time limit.

(2) Method of giving notice. It is a sufficient service of notice or proof of loss if a 1st class postage prepaid envelope addressed to the insurer and containing the proper notice or proof is deposited in any U.S. post office within the time prescribed. The commissioner may expressly approve clauses requiring more expeditious methods of notice where that is reasonable.

(3) Meaning of insurer's acts. The acknowledgment by the insurer of the receipt of notice, the furnishing of forms for filing proofs of loss, the acceptance of such proofs, or the investigation of any claim are not alone sufficient to waive any of the rights of the insurer in defense of any claim arising under the insurance contract.