Sorry, probably out of the norm, but my husband Syncroid asked me to take a look at your dilemma. I work with contractors and builders on a daily basis (I'm a realtor, don't bust me too hard) and have worked with a lot of renovation homes, especially Eichler's in the South Bay. Ok, enough about the creds......
Since you have an already established composite material on the exterior of the roof, I have seen the best insulation between beams with a foam based material. I have a great local company that can talk you through the pros and cons.
With regard to the the T&G wood interior material on 6 foot span- it will sag. You need to install additional support.
Hope this helps.
Wife of Syncroid
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