Andy suggests:

The underside of the roof deck needs to be exposed to air in order to

allow moisture to dry out. There are two sources of moisture -- 1)
from
any small leaks that can soak the wood (and which you will never see
since it will quickly dry out) and 2) condensation on the inside of
the
attic which may take place as a result of temp differences on the
inside and outside of the roof...

You need to have air circulation. Wood rots really well when wet, and
lasts forever if kept dry....

One approach would be to buy the 4x8 sheets of foil backed styrofoam
for about
$6 a sheet, and nail them to the roof trusses, which are 2x4 or 2x6,
usually, in those areas where you are worried about banging your head.
That will give you about 4 inches or so of air which is free to move
between the roof deck and the bottom of the support beams, and
is strong enough to save your noggin.

Just a suggestion. There may be cheaper ways to accomplish the same
thing, but the air circulation under the roof deck is important.

Andy