Chimney Liner Insulation

chimney liner insulation - CHIMNEYS.COM
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“Chimney Liner Insulation?”, is an important topic to think about with regard to your chimney. I’m going to try to answer a number of questions on this topic. 

If you have additional questions about Chimney Liner Insulation, please ask them in the comments area below this article.

The first question is ‘does it need insulation?’ The answer varies based on what fuel the chimney liner vents and the location of the chimney.

If the liner is for a wood or coal burning appliance then the answer is always YES. If there was a chimney fire or a chimney scan, made with a camera made for that purpose, shows cracks or open joints, then the flue tiles should always be removed. I have seen many chimneys that had creosote on the outside of the tiles.

Chimney liner insulation - CHIMNEYS.COMWood and coal insert liners need to be insulated down past the damper frame. I’ve seen houses burn because the insulation stopped in the smoke chamber and overheated the wall in that area.

For oil and gas the answer is maybe. There is normally no overriding requirement for insulation.

A high efficiency appliance will have comparatively low exhaust temperatures and so insulation will always help with the draft, reduce condensation and extend the life of the liner. An exterior chimney will benefit from liner insulation especially in cold climates.

With a very high efficiency appliance, insulation really should be considered necessary, even more so with exterior chimneys and cold climates, but is usually not required.

Liners going through a very large space should be insulated. You can get a convection loop in the space between the liner and the masonry shell that will cool the liner.

chimney liner insulation - CHIMNEYS.COM
Cracked chimney liner.

For any liner, if the flue tiles are badly damaged or deteriorated they should be removed to prevent them from collapsing in the future and crushing the liner. This may create the large space mentioned above.

Now for the confusing part.

The NFPA 31 oil code says to check the liner for condensation after the appliance has been run for a period of time. If there is condensation after the liner is up to temperature then it should have been insulated or it is incorrectly sized. Unfortunately after the liner is in place. it’s kind of hard to insulate it properly.

You really need to deal with someone who understands these variables as they apply to your climate.

In my climate, middle New Hampshire, I always recommend insulation for exterior chimney, high efficiency appliances. I also explain to the customer that if the liner fails due to corrosion, even if the liner manufacturer warranties it, which they don’t have to, the customer still pays for the labor to replace it.

As to the type of insulation. I always use either a wrap/blanket type or a pre-insulated liner. I prefer to use rigid liner for wood or coal when possible and that gets a wrap insulation. For wood/coal in a chimney with an offset I only use heavy wall flex liners with field wrapped blanket insulation.

For oil or gas I use the pre-insulated liners when needed.

I do a lot of chimney/hearth product fire investigation work and so end up taking apart chimneys that have burned down houses. Having taken down many chimneys and removed many liners with poured insulation in my 32 years in the trade, I will not use poured insulation around liners.

Chimneys with poured insulation need to have special attention paid to keeping water out of them in cold climates. The insulation will retain any water that gets in for much longer and increase the freeze/thaw damage to the masonry.

Remember, some day that liner will fail or need to be removed due to a change of use. Removing liners with poured insulation is usually complicated and expensive.

And as for the last question, ‘how do you know if it’s insulated the entire length?’ With poured liners mixed correctly no one knows, including the installer. I’ve taken down chimneys with poured insulation and found big spaces with no insulation as well as areas where the liner was contacting the masonry shell with no insulation between.

With wrapped or blanket insulation you have to either see the insulated liner before or as it’s installed or trust your installer.

There are very small, 1/4”diameter x up to 50’ long, sewer inspection cameras that can fit down the corners of the space around the liner that should be able to show the outside of the liner but they aren’t normally used in the chimney business.

If you have an exterior chimney a thermal imaging camera may show warmer areas that aren’t insulated but that’s harder to do with low exhaust temperature appliances.

Mark Jones

Chestnut Hill Chimney & Hearth

Farmington, New Hampshire
(603) 755-4835

Owner, Head Instructor
Chestnut Hill Chimney & Hearth Training Center
NFI Certified Master Hearth Professional
National Association of Fire Investigators

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