Hey all. We are going to be installing our Selkirk chimney in a two story house. The first floor is has 11 7/8 I-joists. I was planning to frame a square opening for the finished ceiling support box with 2x8 between two I-joists. I may be overthinking this but does the framing need to match up with the height of the I-joist? Obviously 2x8 will not match the height of the I-joist and pretty much no dimensional lumber would be 11 7/8in.