Insulation shield

[Al D. Bolt]

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?

Thanks.

[Jeffrey R. Pope]

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.

[Brian A. MacNeish]

Originally Posted by jpope

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 (392*F) or higher for the second regime of wood pyrolysis"