Foil blocks heat transfer best if it stays clean, is adjacent to an airspace, and the delta is large enough. 

A woodworker finds out using a homemade test apparatus and peanut butter bait

Best Body of Work Winner

Gold Award Winner: Bathroom over $60K

Gold Award Winner: Kitchen $50K–$100K

Gold Award Winner: Whole-House over $750K

Gold Award Winner: Kitchen $50K–$100K

Gold Award Winner: Residential interior over $100K

Gold Award Winner: Bathroom $30k–$60k

Gold Award Winner: Whole-House $300K–$750K