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Heating & Air Conditioning

Duct Board For Ceiling Insulation?

11 years ago

Howdy. I closed in a porch. It has 2x4 rafters. I have some 2" pressed fiberglass (?)furnace duct board someone gave me. I'm thinking of cutting it into strips and using it between the rafters. It would still allow a 1-1/2" space above it for heat to travel from the soffit to the roof vent. The duct board has foil on one side but seems flimsy. Does my insulation plan make sense or am I heading for trouble? Thanks, Gene

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Comments (6)

candler
Original Author
11 years ago
Hello, again. I should clarify that the duct board is rigid and okay. Its outside foil is flimsy and peels off quickly. Gene
energy_rater_la
11 years ago
don't know what you hope to achieve..
ductboard isn't much insulation value
R-8 at most. adding batts would
give you more R-value and use the ductboard
somewhere else. air sealing these porches
to be able to heat and cool them is always
an issue.

I've gotten lots of calls to homes where they
closed in a porch to make it conditioned space.
and then couldn't condition it.
what type of cladding is your house,
is it on slab or on piers?
what type of windows did you use..vinyl double low e
single wood..double metal clear glass?
what orientation and shading does this room have?

best of luck.
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energy_rater_la
11 years ago
thought about it a bit more and realized
that you may be wanting to put layers of
ductboard for insulation.

ah so!

IF you remove the silver vapor barrier you can
use it for this purpose. you don't want
to have muptiple vapor barriers within the
same assembly as it will condnesate.

if you are hoping for a radiant barrier the
ductboard would have to be on the undersides of
the roofing assembly with the foil facing down.
or you could take removed fsk (reflective duct board
surface foil/scrim/kraft) and attach that to
undersides of rafter bays. foil has to have
a minimum of a 3/4" air space to work. if you
put the foil on the 'floor' of the ceiling dust
will stop reflectivity in a few years.

I pay about $30 per 10x8 sheet of ductboard
so I only use it for specific jobs.

best of luck.
candler
Original Author
11 years ago
Thanks for helping. Issues with trying to make conditioned space from a porch is very well said. Mine was a deck. The interior ceiling insulation matter and the interior wall covering is all that remains to resolve. The room is only around 170 square feet with eight insulated windows, an exterior door, and patio doors accessing the main part of the house. It's not on the primary heating system. Even before my first post I knew I couldn't insulate the 2'x4' ceiling as I wished. I have a carport beneath the room and I insulated the room's 2x10 floor joists pretty well. I've read what you said several times. I'll keep it in mind. Many thanks. Gene
busboy
11 years ago
Carport open or closed? Need vapor barrier under floor if closed for CO protection. (maybe if open too-check codes)
candler
Original Author
11 years ago
Thanks for the reminder, busboy. The carport beneath the closed in porch is open on three sides. I've given thought as to whether I might wish to ever enclose it so it would conform with the two car garage beside it. I did cover the carport ceiling with 4x8 sheets of luan. I'll keep the vapor barrier in mind. Regards, Gene

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