Furring was the gap you built on purpose. Thin strips of wood nailed to the wall or ceiling to create a space between the old surface and the new surface. The furring strip was a shim with ambition. One inch deep. The whole point was the inch. The inch was the air gap and the air gap was the insulation and the drainage plane and the room for the wire and the room for the pipe. The inch was everything.

The carpenter furred a wall when the wall was not true. The old plaster bowed. The old masonry leaned. You could not nail new drywall to a crooked wall because the drywall would follow the crook and the crook would show. The furring strips went on first and the carpenter shimmed the strips until the strips were plumb and then the drywall went on the strips and the wall was straight. The furring was the lie that told the truth. The wall looked straight because the furring absorbed the crookedness.

Furring was the cheapest lumber in the yard. One-by-two. One-by-three. Finger-joint pine. Warped. Knotty. Split at the ends. Nobody cared because nobody saw the furring after the finish went up. The furring was behind the wall the way the stagehand is behind the curtain. The stagehand makes the show work and the audience does not know the stagehand name. The furring makes the wall work and the tenant does not know the furring exists.

The air gap mattered more than the strip. The strip was the means. The gap was the purpose. Moisture hit the exterior wall and the moisture wanted to come inside and the air gap said not today. The gap let the moisture weep down the backside of the wall and exit at the bottom. The gap was the wall drainage system. A wall without a gap was a wall holding water against its own skin. The gap was the building breathing. The strip was just the thing that held the breath open.

Nobody furs anymore. They glue foam board to the wall and call it insulation. The foam board is the gap and the insulation and the vapor barrier all in one product. The foam board does three jobs and does them all adequately and does none of them the way the furring strip did. The furring strip did one job. The furring strip held the gap open. The furring strip was so good at its one job that the gap never failed. Adequacy is not excellence. Three adequate jobs is not the same as one excellent job.

See also: Lath, Sheathing