We’re starting the process of CLT build design and we’re trying to figure out the best way to design the window RO layers.

We’ll be installing about 3 inches of rigid wood fibreboard (Steico duo) on the exterior of the CLT and are planning our windows to be on the exterior edge of the CLT plane on a 7-inch thick wall (3 in of CLT, 3 in of Wood Fibre insulation, 1 in rainscreen batten). The CLT will be exact due to it’s CNC cutting, so I’m hoping to avoid needing plywood or timber bucks going into the CLT layer.

The plan would be to install the WRB to the CLT plane and attach a sloped sill to the top of the wood fibreboard on the bottom side of the rough opening bringing it out to the edge of the rainscreen layer.

My question is, would the wood fibreboard act as a suitable window buck if I attach the 1″ rainscreen battens around the RO perimiter, or do I need 4″ window bucks attached directly to the CLT?