When it comes to the decision of whether or not to insulate a stainless steel chimney liner venting a solid fuel appliance venting wood or coal, there is really no option. It must be insulted with the proper type of insulation not only for the UL (Underwriters Laboratories) Listing, but it is usually mandated for the warranty coverage.
Masonry chimneys are built to convey flue gasses. Some standards and/or codes maintain that a chimney with a clay flue tile liner can be used to vent an appliance with a “continuous temperature of 1000 degrees F”. The problem with solid fuels is that the temperatures can reach much higher than this during a chimney fire – temperatures as high as 2000+ degrees F and even sometimes during high burn times. Combine with this with long periods of heating at higher temperatures during cold weather, and an uninsulated liner could spell a recipe for disaster.
Most masonry chimneys could not pass a UL Test because most were built right next to combustibles. A chimney on the interior of a house is supposed to have 2" of airspace clearance on all sides surrounding it and any combustible material. An exterior chimney is should have 1" clearance to combustibles on all sides. How often do you see this? In most of the country, it's almost never. This is why it's so important to insulate a solid fuel chimney liner.
Insulation holds in heat. This helps to prevent heat transfer to surrounding combustibles. It also aids in increased draft. The most important thing that liner insulation does is give you the UL Listing as long as you are using the correct insulation for the liner brand you use.
If this doesn't convince you just consider that a chimney brick thickness is 4”. A fireplace brick thickness is usually 12” minimum. A fireplace is built to contain fire. Again, a chimney is only built to convey flue gasses.
Another benefit of insulating a solid fuel liner is increased draft, better performance and a more efficient system. Holding the heat in helps to keep the flue gasses warmer, especially because with solid fuels the heat/cold cycle is always fluctuating depending on when the appliance of fireplace is loaded. A properly sized insulated liner will get your customer the best bang for their buck.
There are two common types of insulation for chimney liners. The Ceramic Blanket type insulation and the loose fill - TherMix Insulation. To insulate with the blanket, the liner is first sprayed with 3M Spray Adhesive or another approved adhesive. Then the liner is wrapped with the appropriate ceramic blanket and then covered with a stainless steel mesh. Then the ends of the insulation/mesh are clamped to hold them together securely. Some sweeps like to use stainless steel wire to "spiral" around the liner/insulation/mesh to provide more security.
The TherMix Insulation is a dry mix that must be mixed with water to be effective. It should resemble a damp potting soil when properly mixed. This insulation is them poured around the chimney liner once it has been secured in place.
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