R1001.11 Fireplace clearance. All wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.12.
R1001.12 Fireplace fireblocking. Fireplace fireblocking shall comply with the provisions of Section R602.8.
R602.8 Fireblocking required. Fireblocking shall be provided in accordance with Section R302.11.
R302.11 Fireblocking. In combustible construction, fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.
Fireblocking shall be provided in wood-frame construction in the following locations: ....
5. For the fireblocking of chimneys and fireplaces, see Section R1003.19.
R302.11.1 Fireblocking materials. ....
5. One-half-inch (12.7 mm) gypsum board.
The requirements for the air space and fireblocking for fireplaces and chimneys is pretty confusing. R1001.11 referrences R1001.12 which referrences R602.8 which referrences R302.11 which comes back full circle to referrence R1003.19 which is a requirement for chimney fireblocking similar (but a little different) to R1001.11 for fireplaces where we started.
So what is required for firestopping?
Reading on to the fireblocking materials permitted - Section R302.11.1 - most are combustible - wood, particle board, etc. The only non combustible materials are drywall, cement board, mineral wool and fiberglass.
So we cannot fill the airspace with insulation unless we call it fire blocking.
In light of our testing that shows filling the 2" air space with insulation is much better at blocking heat transfer becuase it prevents the convection which is the primaary way heat is transftered through the air space, wouldn't it be better to simply say in Section R1001.11 ...
... The air space shall not be filled, except for non combustibl insulation and to provide fire blocking in accordance with Section R1001.12.
And rephrase Section R1001.12 to be the same as R1003.19 and/or Rreferrence Section R302.11 instead of R607.
Maybe R607 should be removed and maybe R302.11 could use some revision but this is a start.
Parallel changes should be made in the chimney section of the IRC, R1003.19 and in the IBC Sections 2111.11 and 2113.19.
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