Hi All – I just joined the community and have been tracking some similar questions here regarding insulation best practices for an unvented northeast climate zone 5/6 (outside of Boston,ma).  

Building science provides some suggestions on hybrid solutions  (figures 5 and 6, https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies)  … .

It doesn’t seem clear to me which would be the best preforming. We have no plans to finish the attic, so the foam or the net and blown will be exposed…

Insulation contractors have suggested a few options : 
– Just all Open Cell 12-14″ (most economical)
– Open Cell , topped with Closed Cell ( this I never heard of but was offered very confusing approach and against what building science suggests )

The two other options I am considering are: 
– 2-3″ Closed Cell topped with Open Cell  (most expensive)
– Lastly a flash and blow solution e.g.  2-3″ Closed Cell with a net blown cellulose.  (moisture barrier from the closed cell + the fire retardant and insect properties in this solution seem like clear benefits.)

We really want to avoid any humidity first and formost especially with an unvented attic but for me personally I like to limit the use of spray foam were possible… I appreciate any feedback.