Insulation shield

Since fiberglass is non-combustible, is an insulation shield required or recommended in an attic when a water heater double wall flue pipe is going through fiberglass loose fill insulation?


Yes, it is.

The vent manufacturer requires clearances from “building materials,” not just combustible materials. The vents won’t draft as effectively when there’s a hot-spot created by insulation or other building materials.

The vent should work better as the insulation keeps the heat in the pipe allowing exhaust gases to stay hotter thus creating a lighter, more buoyant gas and better draft.

The fear with creating a hot spot is that if it is:

  1. hot enough,
  2. and can emit IR radiation to nearby wood products
  3. by pyrolysis over a longer period of time
  4. the ignition temperature of the dry wood can drop from 800-900 deg F to the 375-400 deg F range

From a paper on pyrolysis:
"However, it is possible for smoldering or glowing to exist prior to flaming ignition if the imposed heating causes the wood surface to reach 200°C (392F) or higher for the second regime of wood pyrolysis" *