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:

Why is my roof sheathing sagging between the trusses?

Why is granule loss a problem for an asphalt shingle roof? 

What are the mistakes to avoid when doing attic improvements?

What causes roof shingles to curl up at corners?  

What causes shingles to buckle along a line on the roof?

What causes leaks at a fake roof dormer? 

Why is it a mistake to replace a roof and not replace its flashings? 

Why is there no attic access hatch in the house?

What is the building code requirement for an attic access hatch, scuttle, or door? 

Does a roof with multiple layers of shingles last longer?

What can I do to prevent roof leaks?

Are roof trusses better than roof rafters (stick framing)? 

Why is a popped nail in a shingle roof a problem? How do I fix it?

What are the most common problems with wood roof trusses?

What causes a lump or dip in the roof? 

If my roof is not leaking, why does it need to be replaced?

How can I be sure my roofing contractor got a permit?

How many layers of roofing are allowed on a home? 

What are the dark lines running parallel to shingles on my roof?

Can metal roofing be used on a low slope/pitch roof? 

How can I make my roof last longer?  

What are the warning signs of a dangerous attic pull-down ladder?

How can I find out the age of a roof? 

Should I buy a house that needs a new roof?

Should I buy a house with an old roof? 

What are those metal boxes on the roof?

What does "lack of tab adhesion" in an asphalt shingle roof mean?

Why do roof edges start leaking?

Why do my dormer windows leak? 

Do home inspectors go on the roof? Do they get in the attic?

Should I put gutters on the house? 

How much of a roof truss can I cut out to make a storage platform in the attic? 

What's the difference between an "architectural" and a regular shingle roof? 

What does a home inspector look for when examining a roof? 

Do stains on the ceiling mean the roof is leaking?

How can I tell if the house needs a new roof?  

 Why does my homeowner's insurance want a roof inspection?

What are the hazards to avoid when going into an attic? 

    Visit our  ROOF AND ATTIC and INSULATION pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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