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Originally published in the September 2014 issue of Sweeping Magazine


Question: I am installing a 6” stainless steel liner in a clay tile lined flue for a new woodstove insert in a masonry fireplace. The flue is oversized, so I am downsizing by installing a full liner. The clay tiles are not cracked and it’s an interior chimney. Do I still need to insulate?


by Mike Segerstrom 


In almost every case, for solid fuel applications, the answer is yes. The only known exception is if the chimney structure itself meets all applicable clearance to combustibles requirements. These requirements are found in both the IRC and the NFPA 211. An interior chimney must maintain a minimum 2” air space clearance to combustibles. An exterior chimney must maintain a 1” airspace clearance. So if we can confirm that at all wall, floor, ceiling and roof penetrations, the required clearances are maintained, we might not be required to insulate. I say might, because certain brand and/or model liners may require a ¼” of wrap insulation, even if chimney clearances are met!

Over the years, there have been several myths, misconceptions, misunderstandings and even misinformation about insulation requirements when installing a stainless steel liner for solid fuel applications.

“I’m using heavy wall, so I don’t have to insulate.”

“The clay liners are not cracked, so I don’t have to insulate.”

 “It’s an interior chimney, so I don’t have to insulate.”

“The homeowner can’t afford it, so I don’t have to insulate.”

“Insulation won’t fit, so I don’t have to insulate.”

None of these determine if insulating is required. As stated above, it is required when chimney clearances are not maintained, and it may be required by the brand liner we use, regardless of chimney clearances.

In most cases, we would have to perform Level 3 Inspection by opening walls and/or ceilings to determine if chimney clearances are present. This level of inspection, repair of inspected areas, and then insulating the liner will cost the homeowner more than if we simply proposed insulation in the first place. I say insulating the liner after Level 3 Inspection because a well-known and common construction defect across the country is failure to maintain chimney clearances. Certain liner manufacturers will even state in their installation instructions “if chimney clearances cannot be confirmed, the liner must be insulated.”

Typical insulation methods include blanket wrap insulation, a poured type insulation and pre-insulated liner. Certain liner manufacturers also allow for a 1” air space between the stainless steel liner and the inside of the chimney as an “insulator.” Not all brands are UL Listed for all of these insulation methods. When using a poured mix, it must be a minimum 1” thick all around the liner. For liners approved with the 1” airspace, maintaining it may be nearly impossible, especially if the flue is not straight.

Whether required or not, insulating the liner for solid fuel applications will help stabilize flue gas temperatures, reduce condensation in the flue, reduce soot and creosote build-up, and/or improve draft. These improvements can translate in to better appliance performance and even reduced maintenance costs.

In summary, for solid fuel applications, ALL UL Listed liner manufacturers will require that the chimney meets clearance requirements, or insulating if the chimney exterior clearances to combustibles requirements are not met. And sometimes both. To accurately answer the original question, we must look at both the requirements of the liner we are installing and the chimney’s clearances to combustibles.

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