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Why is insulation not allowed to touch around a gas flue in the attic even if the insulation is not flammable?
Saturday, May 18, 2019
When a flue passes up through attic insulation there must be a shield/baffle that keeps it away from the flue surface at a distance specified by the flue manufacturer, which is usually one or two inches. Here’s how it is stated in the Residential Edition of the Florida Building Code (FBC), and the citation in the International Residential Code (IRC) is similar.
Although the cellulose insulation piled up around the gas flue in an attic in the photo above is an extreme example, we find missing shields around gas flues in attics occasionally. More often there’s a similar problem: insulation has been sprayed inside the shield, eliminating the required air gap. Air is an excellent insulator, although it would be more accurate to call it a “dissipator.” Convenction currents immediately form around a hot surface in contact with air and pull the heat away.
So, why isn’t an exception made for non-flammable insulation like fiberglass? Because, under the extreme heat of a fire in the flue, the insulation value of the fiberglass is eventually overwhelmed and it starts transmitting the heat to adjacent flammable surfaces and starts a fire in the attic. Shown below is an example of correct shields around two gas flues.
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To learn more about roofs and attics, see these other blog posts:
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