I’m going to ask what may be a stupid question, but it wouldn’t be the first time…

I’ve read the discussion of vented and unvented attics, foam where less is more, and finally should ceiling insulation be removed when creating a unvented attic.

My attempt is to combine these questions into one large “big picture” grand unifying theory. My area is CZ 4 which is a mixed humid climate.

It is clear that less foam is more insulation (NOT) but there are several people who believe that a R20 under the roof deck is sufficient. Let’s not suspend the rules of physics, please.

There is a grand discussion of vented and unvented attics, the results of those designs on ducts, air sealing etc. Mainly, when the crux of the disagreement is when is an attic to big to economically constructed to be unvented. Since the my current is a truss built roof, under deck insulation other than foam would be difficult to install.

Lastly, there was a thread about renovating an attic from vented to unvented and the need or desire to remove existing blown in insulation. I think the general consensus was “leave it”.

Assume all these elements and the reality of cut up and steeply pitched roofs. Generally, because of a lack of basements, ductwork is in the attic in my area.

Would it be possible to combine a R20 foamed roof deck with R30 cellulose ceiling to produce the benefits of both? Do you get R50? Can it all be unvented? You receive the air sealing of the foam with the buffer of the attic space and easy installation of blown in insulation. You have a more economical combination than all foam.

What are the downsides?