How much insulation do I need on my roof? On flat or low slope roofs the insulation is usually installed above the roof deck and below the waterproofing layer. It is much easier to install and typically comes in 4′ x 8′ foam boards. Typically there is a state code that says if you remove all the roofing material to the roof deck, then based upon the amount of glass (windows/doors/skylights) that are present on the roof and walls, there is a minimum R value that needs to be installed. The number is typically around an R value of 20. The insulation of choice for the industry is isocyanurate, which has an R value of approximately 6 per inch. Insulating a building should be looked at in its entirety, not just for the roofing. If the roofing material has an R value of 100 and there are windows and doors that let all the hot/cold air escape – then all the money spent on insulating the roof is wasted. R values between 18 and 25 are typical for normal commercial buildings. We see R values between 40 and 50 installed on freezers and coolers. Be careful if you have a freezer or cooler because there are special roofing details that many normal roofers (and many engineers and architects) may not be familiar with.
The National Roofing Contractors Association has a tool for figuring how much insulation should be used – see energywise.nrca.net. If you really want to run the numbers for the whole building then look for an environmental engineer in your area. An engineer will utilize software designed to analyze and propose a solution for the entire building, including the roof.
A good way to be sure the building is insulated properly is to have an expert infra-red company perform an analysis with an infrared camera from the inside of the building.
