ISO Boards

Hello Leo and thank you for all the great tips, recently I’ve been getting estimates for a 1700 square foot flat roof and asked for top roof insulation, three estimates later every one of these roofers came back with 1.5 and 2.0 inch ISO boards wether using a TPO or BUR system, after reading your website and others the recommended thickness of ISO boards to get a 20-25 R value is between 3.5 – 4.0 inches of thickness. My question is why are they pushing the thinner boards when they know the R value is not correct, is there more work to install the thicker boards or is there something else they’re not telling me. Please advise and thank you again.