What type of building material should NOT be used to seal chimney wall openings?

Using wood to seal chimney wall openings is not appropriate due to its combustible nature. Chimney systems are designed to handle high temperatures and must be sealed with materials that can withstand heat without igniting or degrading.

Brick, cement, and non-combustible materials are considered suitable options for sealing because they are durable and can resist the high temperatures produced by a functioning chimney. These materials are also effective in preventing the entry of drafts, moisture, and animals, which could compromise the chimney’s efficiency and safety. Therefore, wood, being flammable, poses a significant fire hazard and is not a suitable choice for sealing chimney openings.