Q. For a brick fireplace and chimney on a gable end wall, how should the joint between the framing and the brick be sealed? Can a thermal break be included in the masonry to stop heat loss?

A. If the chimney and fireplace are solid masonry, the usual procedure to seal the joint is to bed a trim board in a good elastomeric caulk against the brick and the sheathing.

I do not know of any material that would provide a thermal break and that has the required fire resistance and bearing capacity to be included in a masonry fireplace. It is possible to insulate a brick fireplace by filling the cavity between the outer wythe of masonry and the flue with perlite or vermiculite (see illustration). A chimney-top damper can then be used to keep cold air from flowing down the flue.

The difficulty insulating a chimney chase and firebox is one reason prefabricated metal fireplaces, which can be installed in an insulated frame wall, are so popular.