20 l ROOFINGBC l FALL 2018 FEATURE are two additional control layers in the envelope that comprise the work of roofers, which are critical components in the Step Code – the thermal control layer (insula- tion) and the air control layer. The roofing contractor needs to understand why these matter, and how their proper installation can affect the outcome of building model- ling and a whole-building air pressure test. Insulation, properly installed, regulates the interior environment of the building by keeping heat energy where it belongs. Staggered layers and offset joints avoid unnecessary energy-costly thermal breaks where heat can simply migrate through the roof system, unimpeded. The RoofStar Guarantee Standards already require stag- gered layers and offsets between adjacent panels. And those standards are always improving, as we continue to understand how different insulation materials respond to the cycles of heating and cooling. Thermal bridging is a related issue the roofing contractor should be versed in. While it is true that a small percentage of energy migrates through mechanical fasteners that bridge the space between the roof membrane and the supporting deck, an energy loss of five to seven per cent is perhaps still too much. Furthermore, thermal bridges may introduce unneces- sary temperature swings in the insulation itself, which can induce more expansion and contraction beyond what is acceptable or even desirable. Some insulation manu- facturers and roofing consultants actually recommend a hybrid approach to roof system design – mechanically fastening the lower layers, and adhering the layers on top – simply because this approach breaks the bridge. Of course, a Best Practices approach to the application of roof insulation plays no role in an air pressure test. That’s the role of the air control layer, commonly called the “air barrier.” And now, more than ever, it is the singular control layer that the roofing contractor must get right the first time, or face some very costly repairs. Here’s why: Heat energy migrates in and out of the building,alwaysfromwarmtocold,through conduction and convection. Conduction is a slow process. As materials warm up, they transfer some of that heat energy to adja- cent materials, simply through contact. Insulation slows or even arrests conduction, keeping the heat energy where it should be. Convection, on the other hand, is the movement of heat energy through a liquid or gas (air is a gas). Without air movement, convection is also a slow process. Turn on a stove element at home and hold your hand above it, and you’ll feel heat rising to warm the skin. That’s convection at work, but it will take a long time to warm up the entire kitchen. Put a fan behind that element and blow air across it, and the entire room will rapidly warm up. This is why the air barrier is the most critical control layer for energy efficiency. Air movement through any one part of the building envelope will inevitably carry with it heat energy, and regardless of the season, it will take considerable inputs (like elec- tricity or natural gas) to counter the loss or gain of heat energy caused by air leakage. And considerable inputs like natural gas produce greenhouse gases, which is why the Step Code places significance on the Photo features thermal bridging