Fill the gap next to the fireplace insert

[kickflipfakie]

Hello!

We have installed a fireplace insert in our open fireplace, but the opening is a bit larger than the insert, leaving a gap (about 2cm) on both sides. What should I use to fill the gaps? I want it to look nicer, but I also believe it will improve airflow behind it and thus better heat distribution in the house.

I've considered fireproof mortar, but it may crack due to the high temperature differences.

 

[Roundup]

Heat rises mostly upward, so it should work to apply a couple of cm of plaster on the sides, like c-bruk or something. Personally, I applied about a cm of gypsum plaster on the sides, so the gap was 1-2 cm per side, as I understood there should be an air gap on the sides too, but this might differ from cassette to cassette. The gypsum plaster on the sides hasn't cracked, despite intensive burning at times. However, there are hairline cracks in the gypsum plaster above the cassette.

 

[myrstack]

Often, when installing a cassette, a hole is made higher up in the fireplace enclosure and a vent is installed. This way, cold air can be drawn in along the sides of the cassette and then, when the air is heated, rise up and out through the hole into the room. Because of this, vents are the most common.

If the circulation is arranged differently, it should be fine to seal the gap with fireproof mortar, but I would probably suggest leaving a few millimeters along the cassette's wall open to allow for expansion during heating.