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.