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For several decades now, every major building code has required roofs to be ventilated. While other areas of the codes have changed and evolved, roof venting rules have remained unchallenged. Recently, though, some building scientists have been questioning the need for roof ventilation. In researching this article, we pored over many pages of research data, including studies from the United States, Canada, and Sweden. We also considered the experiences of builders who have been trying out a variety of unvented, or "hot roof," designs. Our conclusion is that the strongest evidence, both from the lab and from experience in the field, continues to support the importance of ventilation. A Little History According to researcher Bill Rose