Greetings. I am remodeling a 50 year old house in warm and humid central
Florida. The house needs additional insulation and I'm trying to figure out
the best thing to do. I feel that I have two practial choices. I could
just blow fiberglass on the floor of the attic like everyone else does or I
could insulate the top chord of the 2 X 8 rafters with fiberglass batts. I
don't see a spray foam option as very viable on the top chord since it is
difficult for them to get their gun down near the eaves in existing
construction, plus it is far more expensive than fiberglass for the same
R-value. If I choose the batts between the top chord rafters, I am
wondering how important it is that I leave air space above the batts for
ventilation from soffet to ridge. I will be installing a natural color
(mill finish color) galvalume metal roof on top which absorbs much less heat
than medium to dark asphalt shingles do.
My choice would be to put an R-25 batt in the top chord rafter and seal the
attic with no ventilation above the insulation. The reason for this is:
I have some old poorly insulated duct work that would be difficult to
replace in the attic and I have storage in the attic. I would like all of
that to be in the semi-conditioned temperatures. There are several ceiling
drops which means that I would have to cover over them to have a flat attic
floor surface for max efficiency of blown insulation. I suspect that there
is a fair amount of air leakage from wall cavity to attic. I have some attic
areas that have a floor down for storage that would be difficult to insulate
underneath. Finally, I am putting up new soffet material and would prefer
the look of various non-vented products.
So what is the scoop? Is it acceptable to seal the attic using fiberglass
on the top chord and to not vent it? Or, does one need to reduce to R-19
which will allow circulation above the insulation with vented soffet and
ridge. I also have the option of putting a foil barrier on after the
insulation to further reduce heat gain into the attic. With either of these
options, is it better to use insulation with a kraft paper vapor barrier or
does that depend on whether I put a foil barrier on underneath it? Or, is it
better to forget all of that and just blow in insulation on the floor?
Thanks so much for your thoughts.
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