Cleats vs Strapping Roxul Comfortboard 80 – tips wanted – GreenBuildingAdvisor

We want to put 2.5″ Comfortboard 80 continous insulation on the exterior walls of our home in the WUI (not adjacent to, but in an alpine forest valley adjacent to a creek).  This is an installation question.  We are committed to the strong fire resistance qualities of mineral wool.

Fire Context

We are in the highest fire risk zone, in California at 6600 ft, in an alpine forest valley, in an area that has one road, single-lane in each direction, no shoulder, with the nearest fire station 4 miles away.

As a community, we do serious defensible space, and land restoration (clearing out dead trees and fuel from the adjacent forest).  This area has not seen a forest fire in 60+ years, although some have come within 10 miles of the mountain ridge behind us in recent years.

Fire risk, safety and insurance are top of mind more than ever before.  We’ve been here for 30 years.

Construction

The exterior cladding will be reverse board and batten, using 3/4″ plywood as the “batten” and 1×8 red cedar as the “board” with 1″ gapping.  This cladding is obviously not the best choice for fire resistance (Class B), but it does look amazing and aesthetics are winning the day on this.

The cladding weighs about 4 lb/sqf (3/4″ plywood + 3/4″ cedar boards).  Not especially heavy, but the 2.5″ thick Comfortboard requires strapping instead of directly attaching the cladding with screws through the Comfortboard.

One contractor we’ve spoken to prefers cleating with a ripped 2×4 (i.e. 1.5″ wide x 2.5″ deep).  These cleats are attached to OSB sheathing (with an air barrier) over 2×6 framing with insulation.

The cleats would look like shallow stud bays with the Comfortboard between the cleats.  The cleats would be staggered from the 2×6 framing to reduce thermal bridging.  Because the Comfortboard is not continous, we would put 5/8″ Densglass over the Comfortboard for a 1-hour fire rated exterior wall.  Eaves will also have a layer of 5/8″ Densglass under the eave cladding (1×6 T&G doug fir planks).

Pros of cleating

strong

easy to true and plumb the cladding

quick construction

Cons of cleating

maybe 20% reduction in R-value of Comfortboard from thermal bridging (?)

2.5″ Comfortboard no longer 1-hour fire rated because of the cleats

extra cost of Densglass sheathing for 1-hour fire rating

Some contractors prefer strapping per the Roxul installation guide, but are not keen on strapping in general because it is hard to true and plumb.

Question

Cleating v Strapping.  Which way would you go and why?  Any installation tips to share for either cleating or strapping?