To insulate a fireplace insert, place glass wool insulation within the gaps between the fireplace insert and the stones of the fireplace. Additionally, seal any gaps or cracks between the insert and stones with mortar.
Glass wool insulation is suitable for insulating a fireplace as such insulation is resistant to fire and can withstand heat up to a temperature of approximately 1200 degrees Fahrenheit. Use glass wool insulation with a layer of adhesive on one side for easy installation.
Before stuffing the gaps with the insulation, switch the unit off to cool it. Take the cover plates out and slide the insert out slightly to access the gaps easily. Insert the insulation with your fingers in the gaps with the adhesive side facing the insert. Replace the insert and re-affix the cover plates. Stuff any remaining gaps with more of the glass wool insulation.
To create a tight seal between the insert and the fireplace, mix mortar and water, and allow the mixture to thicken. Remove any old mortar and other debris in the space between the insert and the fireplace. Dampen the fireplace stones and mortar and slather the mortar on the cracks. If the crack is large, place wire mesh over it, and then slather the mortar on the mesh.
Scrape away any excess mortar. Allow the rest to dry for a minimum of eight hours before switching on the insert.