How to Achieve the Most Efficient Thermal Roof Insulation

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We often get asked, ‘Can you make 5.2-inch thick polyiso?’ or ‘Can you make 8-inch ISO?’  

Even if we could, there’s a good reason to use multiple, thinner layers of insulation. Whenever you need a layer of insulation, you actually need TWO layers of insulation! If a single layer of rigid foam insulation boards gets used, there’s a high risk of heat gain or loss at all of the joints between the boards. These areas are called “Cold” insulation joints, and maybe you’ve even seen them: when the heat escapes up through a light dusting of snow on the rooftop, melting a neat 4’x8’ grid pattern.  

Use Multiple Layers of Polyiso with Joints Offset

The best practice is to always use multiple layers of insulation, with the board joints staggered and offset from the layer of boards below them. This staggered arrangement increases the efficiency of the thermal insulation “system.” 

The building owner is paying for a specific R-value, and using multiple layers of polyiso is the way to give them the most bang for their buck. The owner’s requirement for, say, R-30 insulation means 5.2” of polyiso, but that definitely does not mean a 5.2” thick board. At minimum, you want to provide two layers of 2.6” iso, with the top layer’s joints offset at least 6” from the underlying layer. 

Even better, it could be two layers of staggered polyiso boards followed by a layer of ½” GenFlex HD Polyiso coverboard on top, with its joints staggered and offset, too.  Many roofers have known this for a long time. The National Roofing Contractors Association made it the primary topic of their Bulletin #6 way back in 1980! When we’re reroofing existing buildings, though, it is not uncommon to find a single layer of insulation on the old roof deck.

Avoid Conductive Metal Fasteners

Another way to provide the most efficient continuous insulation system to the building owner is by considering how it’s attached. Sometimes the conditions will dictate the need for a particular adhesive or a particular mechanical fastener. Since metal is conductive, we have seen a trend away from using metal screws (and big, conductive metal insulation plates) that extend from the top layer of insulation all the way down to the deck. Metal screws and plates create a “thermal bridge” for heat to escape in the winter or invade in the summer.  

Instead of using long screws to commonly-fasten all the way through multiple layers of insulation, increasingly, we see design professionals and contractors only mechanically attaching the base layer of insulation. Then the subsequent layer(s) are adhered to that base layer. By setting those top layers in adhesive, it avoids having conductive metal fasteners running all the way up to the membrane.  

Deliver Value for the Building Owner

Achieving efficient thermal insulation has been a topical issue for years, and remains so. While you might spend a lot of time and attention to ensure that the roofing membrane is watertight, the roofing system is delivering more benefits to the building owner than that. Taking a moment to ensure that the roofing system includes the most efficiently laid-out and attached thermal insulation possible is a great way to show value to the building owner.