"HerHusband" wrote in message
...
Effi,
thought of shimming the rafters down (making them thicker) but didn't
like the idea of heavy hardie panel ceiling attached to 2x4's attached
to 2x4's, so thought about running 2x6's alongside the existing 2x4's,
The biggest problem I see with that arrangement is the overall thickness
of
the doubled up rafters. The wood framing is the weak link in the
insulation. In simple terms, the space taken up by the rafter is
uninsulated. If you double up the rafters, you'll have twice as much
uninsulated space (twice as much heat loss).
If you used construction adhesive and long screws every 12-16" or so to
attach the shim boards, I doubt you'll have any problems. I don't know
what
a Hardie-panel weighs, but even if it weighs 100 pounds, that's only a bit
over 3 pounds for each square foot (4x8 sheet). With 24" OC rafter
spacing,
you may end up with 7-8 pounds on each linear foot of the rafter. Not a
big
load.
Alternatively, or in combination, you could glue/nail plywood on both
sides
of the rafter and shim board. In effect, you would be building box
beams. This would actually allow you to increase the rafter depth as much
as you wanted. In fact, you could provide an air gap between the rafter
and
shim board, which would reduce thermal bridging. You could even insulate
that gap if you want to get really efficient.
wood blocks 1" x 1" square x 1.25" thick glued to underside of plywood
roof decking staggered every 12 or 18"
Sounds like a lot of work with little gain. Increase the insulation space
and use standard plastic insulation baffles if you need them.
radiant barrier (perforated brown paper with aluminum foil facing on
one side, stapled to thel 1".25 thick blocks of wood
If you use a radiant barrier, it should be on the heated side of the
insulation, not against the roof. You want to reflect the heat back into
the room, before it goes into the insulation.
(unless you live in a warm climate and are more concerned about keeping
the
heat OUT of the house).
may cut some leftover 24" wide 8 1/4" thick encapsulated fiberglass
in half using electric meat carving type knife
Sounds real itchy... Easier to just lay the mat over a board, compress it
with another board, and a quick swipe with a sharp knife. You'll end up
with a cleaner cut and less fiberglass dust floating in air. Be sure to
wear gloves and dust masks. Insulating is miserable work...
Anthony
plenty of food for thought here
also been looking at
http://www.owenscorning.com/around/i...sultoattic.asp
for ideas
am now considering following layers from underside of plywood roof decking
down:
layer 1.
Owens Corning Raft-R-MateŽ attic vents attached to underside of plywood roof
decking
http://www.owenscorning.com/around/v...mate_attic.asp
layer 2.
radiant barrier; yes in warmer clime so installed closer to roof to keep
heat out; am considering leaving radiant barrier out, pending research on
effect on asphalt composition roof shingles and ridge vent installed on that
section of roof to vent any extra heat from radiant barrier
layer 3.
pieces of 2x4 about 5" in length, one end attached to the underside of the
roof rafter 2x4, making it 5" + 3.25" = 8.25 inches from the underside of
the plywood roof decking, one 5" piece attached every 3' or so along the
roof rafter, by means of glue and press on metal braces which overlay and
pierce both boards being joined
layer 4.
a secondary 2x4 running the length of the roof rafters and attached to the
5" 2x4 pieces mentioned in layer 1.; the secondary 2x4 will be attached to
the 5" pieces with the 4 side of the 2x4 running horizontal, making the
length from the underside of the plywood roof decking now 8.25" + 1.5"
(thickness of secondary 2x4 attached horizontally) = 9.75";
may need or want to add some stability 2x4s between each new "extended
rafter" if they sway due to new length from roof
layer 5.
4" of fiberglass insulation between new "extended rafters"
layer 6.
hardie panel ceiling attached to new "extended rafters"
seems this arrangement might work, 9.75" thick air space created and used
for:
1.50" or so foam attic vents
8.25" insulation (this way can use encapsulated 8.25" thick
fiberglass without modification)
as to other poster's question, ceiling joists will stay in place, could be
removed later