Advanced Thermal Concepts on Instagram: "Why Cellulose Performs Better Than Fiberglass in Your Attic When it comes to attic insulation, R-value alone doesn’t tell you how comfortable your home will actually be. In the summer, your roof deck can reach 130–140°F, and the heat entering your home isn’t just conductive it’s radiant heat. R-value testing focuses on conductive heat, but it doesn’t accurately measure how insulation handles radiant heat transfer. That’s where cellulose insulation shines.