Albert Barnes Commentary 1 Kings 6:15

"And he built the walls of the house within with boards of cedar: from the floor of the house unto the walls of the ceiling, he covered them on the inside with wood; and he covered the floor of the house with boards of fir." — 1 Kings 6:15 (ASV)

The description of this verse applies only to the main chamber of the temple, the holy place. The writer in 1 Kings 6:16 describes the holy of holies.

The marginal rendering of this verse is right, and not the rendering in the text.

Fir - Rather, “juniper.” See the note on 1 Kings 5:6.