The basic questions are how much, how many layers, what kind and does it need to be tapered or sloped.
How much – if you are completely tearing off the old roof you have to install insulation whose R value meets or exceeds the state building code. For most buildings this is generally around R=20. The codes vary depending on your location and how much glazing (windows/skylights) are in the walls and roof. If you have special refrigeration requirements then you will need to go much higher. The National Roofing Contractors Association has a program that estimates energy savings as more insulation is added. There are other professionals that will also attempt to quantify savings of additional insulation. If your roof is completely insulated but your windows leak then the cost estimates may be inaccurate. The cost estimates can be used, with a level of skepticism, to generate payback for an increased amount of insulation. Obtain the cost for an additional inch or two of insulation and compare it to the anticipated savings – then decide. If you are going over an existing roof then no insulation may be required. A go over may be used as an opportunity to install more if your energy costs are high. Many times a thin layer of insulation is used to separate and/or provide a clean/known substrate from the existing roofing system to the new waterproofing layer
How many layers – two layers of insulation are better than one. The reason for this is the gaps between the insulation boards leak energy. If two layers are used the layers are staggered so the gaps are not over each other, resulting in better coverage. Two layers of insulation cost more than one layer. One 4″ thick insulation board costs less than 2 – 2″ thick boards. One 4″ thick board costs less in labor to install. It is difficult to determine the cost of any potential energy leaking from any gaps between the boards and the extra installation cost.
What type of insulation – the general answer to this is isocyanurate or polyisocyanurate. This insulation has the lowest cost and is one of the thinnest boards per R value, is closed cell so does not soak up moisture well and performs well in fires. It is the standard in the industry. There are a few other types of insulation that can be less expensive but come with drawbacks. The most common in re-roofing is fiberboard. Fiberboard is less expensive than isocyanurate but, and this is a big but, it soaks up water like a sponge. What typically happens is that a leak occurs in the waterproofing layer and the water dripping in spreads sideways through the fiberboard. This generates a number of bad consequences. First the leak isn’t noticed quickly and then a much larger area of roofing material gets saturated. Once the moisture is in the roofing system it constantly attacks downward to rot the bottom roofing system or deck and in hot weather it turns into a gas and attacks the weakest spots in the waterproofing layer. This will shorten the life of the roof and make it more costly to replace. Fiberboard is typically used when going over an existing roof and we do not recommend it. There are two common types of plastic based insulation that we see once in a while – extruded polystyrene (XEPS) and expanded polystyrene (EPS – also known as beadboard). The extruded polystyrene is more costly per R value than isocyanurate. The expanded polystyrene (same material as the cheap white coolers at the local market) is less expensive than isocyanurate but is only 2/3rds the R value per inch as isocyanurate and performs extremely poorly during a fire. Neither XEPS nor EPS can be placed next to a steel deck due to fire concerns.
Does the insulation need to be sloped – if the roof deck is flat the insulation needs to be sloped or tapered to force the water to drain off the roof. To determine if the roof deck is flat take test cuts at different points where the roof drains (a high and low point) and if the measurements result in a different thickness then the slope is in the insulation, not in the roof deck. If the slope is generated by the insulation and it is being removed then tapered insulation must go back on. Tapered insulation costs much more than flat insulation. A tapered insulation system is designed by the roofing manufacturer and then put together like a 3-dimensional puzzle by the roofing installer. Our experience is that 90% of buildings have the slope in the roof deck, making the roofing much easier (and cheaper). Even if the roof deck is sloped ponding will sometimes occur. Prior to any new roofing work careful attention should be paid to any existing ponding on the roof. Sometimes this ponding will be alleviated by new flat insulation. Most of the time in order to alleviate the ponding tapered insulation or crickets should be installed. If the ponding is in the middle of an area it may be more cost efficient to install an interior roof drain at the low point of the ponding. Ponding typically occurs between roof drains, at the high side of roof top units, skylights or other penetrations, and between scuppers/leader-heads and downspouts. All of these places should have crickets installed between them to facilitate drainage.