How to Seal the Flue for a Fireplace Insert

Apply a generous amount of fire cement to the section of your fireplace insert that connects to your chimney flue. Some fire cement products come with an applicator, while others will need to be applied with a spatula. The outside of the hole in your fireplace insert slots into the bottom of your flue. Apply enough cement to ensure the two are completely sealed. If there is a slight gap between the opening in your insert and the bottom of your flue when your insert is fitted, more cement is necessary. If the two fit snugly, you'll be able to use less.

Apply fire cement to the inside of the bottom opening of your flue with your cement's applicator or a spatula.

Push your fireplace insert into position so its hole slides inside your chimney's flue. Smooth the fire cement with a spatula to create an airtight seal. Add additional cement if you can see any gaps in your seal.

Allow the cement to dry. Most fire cements dry in three to four hours. If the cement joint is visible through the opening of your fireplace insert, you can apply a coat of heat-resistant paint to improve the aesthetic. Since its use will be in the fireplace, use a paint that can withstand at least 900 F, depending on your needs. Generally, heat resistance goes up to 1,200 F.