The natural alternative to spray foam. Oakum has been used for centuries in boat and shipbuilding but to seal gaps and joints in timber buildings, windows, and traditional construction. Made from natural hemp fibers treated with oil, it provides a flexible, breathable way to fill voids while allowing the building to move and manage moisture naturally. Unlike modern foams and synthetic fillers, oakum remains compatible with traditional materials, making it an invaluable tool for sensitive repairs and historic restoration work. When used as a filler between window frames and stone or brick, it can be finished with sand mastic.