Most UK buildings were designed to keep heat in, not keep it out. As summers get longer and hotter, that design assumption is being tested. There's a recognised tension between Part L of the Building Regulations, which pushes for airtight, thermally efficient buildings and Part O, which is meant to address overheating. A building can meet one requirement while struggling with the other. This roundtable brings together people from across glazing, ventilation, shading and cooling, alongside architects and specifiers where possible, to discuss what's actually happening on the ground and where the industry goes from here.