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Requirements for Minimizing Thermal Transfer
R-value is a measurement of a material’s thermal resistance or a measure of how effectively certain materials block or slow down the conductive flow of heat.
Up until the 1980s, attention to the thermal resistance of roofing projects had been mostly optional and consequently quite low. And because of the gradual rise in R-value requirements since roof insulation was made mandatory, combined with R-value degradation over the years that a roof has been in place, the older the roof system, the more prone these roofs are to higher rates of thermal transfer. I
n virtually all of the areas of the Southeast that Standard Roofing provides services, the R-value of above-roof-deck insulation of commercial buildings requires a minimum of R-25 continuous insulation, sometimes referred to as R-25ci. Two very important words in these requirements are “minimum” and “continuous”. Basically, if you are following the state guidelines for your area when reroofing a commercial building, if the roof deck is already providing the proper slope and the new roof insulation you plan to use provides an R-value of 3.6 per inch, then to achieve the minimum R-value of 25, you would need to apply a minimum of 7 inches (R-25 / 3.6-R per inch = 6.94 inches) of continuous roof insulation over the entire roof deck.
However, an important factor to consider is whether built-up roof insulation is being used on top of the roof deck to contribute to the total roof slope for proper drainage of your roof. If this is the case, since the minimum thickness of insulation at the roof drains/lowest point of the roof needs to be 7”, the new insulation will become thicker as you trace up the slope of the roof. This means that the further away from the lowest part of the insulation, the thicker the insulation becomes as it moves to the highest point. For example: if the roof drains (the lowest point of the roof) are at the R-25/7” thickness, and the farthest point of the inside of your parapet wall is 20’ from your roof drain, and your additional slope from built up roof insulation is calculated at 1/4” (.25”) per foot, the maximum thickness (the highest point) of the roof insulation will be 12” (20’ x .25” per foot = 5”) plus the 7” minimum of roof insulation at the lowest point of your roof will equal a total of 12” of roof insulation above the roof deck at the highest point of the surface of your roof. This highest point of roof insulation will be providing an R-value of R-43 (R-3.6 per inch x 12” of insulation), and the lowest point will be providing the required R-value of R-25 (R-3.6 per inch x 7” of insulation). This means that if your pre-bid inspection or core samples reveal existing insulation is tapered below 7” at the existing lowest point of current roof insulation, to maintain the same roof slope as your current roof, then your new roof will require roof insulation thickness that will exceed the highest point of the existing insulation.
Of course, properly-applied insulations at other locations in a building – walls, expansion joints, etc. – combine to enhance the R-value of the overall building envelope. Insulated windows, insulated doors, and insulation below sub-floors will also help prevent thermal transfer. Caulking exterior perimeters of windows and doors can also be surprisingly effective in the prevention of thermal transfer.
Keep in mind that, in addition to state-mandated insulation requirements, there can be a variety of local code requirements which must also be considered when planning for reroofing programs.