Fans of Felt To the Editor: Regarding the question “Is felt underlayment really necessary?” (“Asphalt Shingle Q&A,” 5/02): When a shingle is quickly heated by the sun on a cold morning, the resulting temperature difference above and below the shingle causes condensation under the shingle. Without the felt underlayment, this condensation ends up on the wood sheeting, causing delamination and eventual rot. The felt traps the condensation before it reaches the sheeting, until it eventually evaporates. As a builder and remodeling contractor in the North Hills of Pittsburgh, I have often seen what happens when the previous contractor failed to replace blown-off felt paper before installing the shingles: Those were the areas where the wood sheeting needed to be replaced. Mike Blumer Blumer Contracting Pittsburgh, Pa. To the