wall thimble framing?

[cac4]

I was just perusing the install instructions on simpson duravent's website. they say you're supposed to "frame" the opening for a wall thimble. does anyone actually do that?? in a standard 15" stud cavity, you'd have to cut a 15" hole to do that properly.

 

[cncpro]

I cut some pieces just small enough to fit in the opening and screwed then in one at a time. I used 2 on the bottom, 2 on top, and one on each side then screwed from inside & out. I was imagining they wanted it framed up to keep insulation and/or foreign material from getting against the thimble so i figured my lame approach would be OK...

I'm not sure if it's correct or not... I'm just telling you what I did. ;-P