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Note, I am not an engineer, insulation specialist, or anything...
I am in no way qualified to give any structural advice, and don't know
enough details from your situation to make an assesment if i were.
I'm a geek who has done some theatre work and lots of reading.
The following comments are just for consideration and possible picking
apart from those who might be qualified to make those judgements.
That said, I have a couple comments.
I believe the roof insulation technique you are suggesting (insulating
the entire cavity) is called a "hot roof". Google searches for that
term and insualtion, etc might give you some more information.
I've considered doing this in our attic, since we have no insulation
and the same sloped roofs and ceiling setup on our third floor
victorian, although, our peaks are trucated to ~7 feet, so we have a
flat ceiling where it's high enough. I couldn't find out if it was
appropriate for our climate, however. (Pittsburgh PA) I know it might
void the warrantee on some asphault roofing materials, but i don't
know how it would affect the metal roof. I think the way i'm going to
go is to put insulation baffels down first, then spray in foam.
The baffels will meet up in a plenum at the peak, and i'll deal with
the heat from there. (possibly with solar thermal collection :)
On to structural:
In theatre there is a type of platformed used called a stressed skin
platform. It's used to make a very strong but thin platform by
sandwiching two thin "skins" (plywood) over boards on their flats. (so
glue/screw the 3.5" sides of a 1x4 to the plywood). The resulting
platform is very thin, but has little deflection (sag) because the
deflection force is trying to compress wood on one side, and strech it
I believe there is a construction material made a similar way, using
two sheets of plywood, and a foam core. The curing of the foam adhears
the two sheets of plywood together and makes a fairly ridged board.
Perhaps removing the drywall from the ceiling, glue/screw-ing sheets
of plywood to the rafters, and filling the void with insulation will
cure several of your worries:
1: high insulation
2: new "stressed skin platform" roof has more strength than old roof.
If/when you get an engineer in to ask about this, perhaps you can give
him/her that option to consider. It might make them look at solutions
they didn't consider.
While we're on the subject... anyone use the DIY foam insulation
product from fomofoam.com. What kind of results did you get?
Any other DIY foam kits for sale?
Ich habe keine Ahnung was das bedeutet, oder vielleicht doch?
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