How to insulate a stone fireplace

Q. The center third of our exterior living room wall is taken up by a floor-to-ceiling flagstone fireplace. A great deal of cold is inducted through the stones whether the fireplace is in use or not. As a test, for example, we hung a blanket over the top half of the masonry and found a 7-degree temperature difference behind and in front of the blanket.Can you suggest some kind of insulation, outside or inside, which is suitable in appearance and will reduce heat loss? A reader Nampa, Idaho

A. Interior insulation on the fireplace face is impractical. There is a certain amount of radiant heat which is emitted while the fireplace is in use.This heat would be negated if interior insulation were applied. And, as you note, the appearance of the interior stone fireplace would not be preserved by such an alteration.

One method would be to construct a 2-by-4-or 2-by-6-inch wood-frame wall against the exposed exterior chimney up to the plate or eave line. This would produce a support for any one of several kinds of insulation which could be applied between the studs.

Erected at 16-inch or 24-inch centers, the studs would accommodate stapled-in thermal insulation batts or rigid insulating board. Thereafter the insulated framing could be veneered with 4-inch, plus or minus, flagstone conventionally applied over the sheathed framing.

Naturally, watertightness of the framed area is essential for maximum thermal performance.

An alternative insulating method would be to erect the framework, including its exterior sheathing; then pour, blow, or pump any one of several kinds of insulation material between the studs. The same flagstone veneer would still be applied to the exterior as noted above.

The amount of heat passing through a fireplace masonry exterior wall is high. Fireplaces built on interior walls, by contrast, generate the dividend of warming the room on t'other side of the chimney.

Perceptive architects now are designing fireplaces on inside walls to save energy. Further, they may require that fireplaces be both functional as well as decorative by specifying circulation-type metal units made by such companies as Heatilator, Heat-Form, and others.