engineering cathedral ceiling with 3.25" thick airspace?

would like to create a cathedral type ceiling in a small area of 24" on center roof rafters 3.25" thick
within the 3.25 inches thick want: 1" airflow space; and 2.25" insulation
how can the 1" thick airflow space (between 2.25" insulation and underside of roof decking, plywood) be created?
seems just putting 2.25" thick of insulation without some means to keep the 1" airflow space open might be a mistake
have considered chickenwire with small 1" thick blocks of wood glued to underside of roof decking every foot or so to keep the space open
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On 12/20/2004 8:22 PM US(ET), effi took fingers to keys, and typed the following:

They make formed foam spacers that go between rafters that will keep the insulation away from the roof sheathing. Check at the big stores.
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considered those, they probably won't be structurally strong enough in this application
also considering 4" of fiberglass insulation with 1 1/2" or 2" pvc, perforated along the length by drilling lots of holes, run centrally inside the 4" fiberglass to provide air circulation instead of 1" open space above 2.25" of insulation
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On 12/21/2004 12:19 AM US(ET), effi took fingers to keys, and typed the following:

You've already ruled out solid foam panels then? They come in various thicknesses, and widths to fit between 16" o.c, or 24" o.c. framing. http://www.owenscorning.com/around/insulation/products/foamular.asp
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On 12/21/2004 12:19 AM US(ET), effi took fingers to keys, and typed the following:

I've already responded to this in another message, but I just had a thought. Instead of all these efforts to keep the insulation away from the sheathing with Rube Goldberg ideas, how about you nail 2-1/2" x 2-1/2" strips of wood to the current 2 x 4 joists, so that they are 2x5" joists, and then use common 3-1/2" insulation stapled to the 1-1/2" faces? That will give you a 1-1/2" gap between the insulation and the sheathing which is the recommended gap.
--
Bill

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the
Seems to me that 2.25" of insulation is a mistake! Greg
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ya, am leaning towards 4" of glass compressed into the 3.25" space, with perforated pvc pipe run centrally through it for air flow
hardie panel soffit board will be ceiling and provide some insulation factor as well
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Effi,

2.25" won't provide much in the way of insulation, especially in a ceiling where it's needed most. You might get R-5 to R10 if you're real lucky. That's a far cry from the R38 (in my area) that normally is required in cathedral ceilings.
To maintain the airspace, just use the foam air baffles you can get from Home Depot or most any lumber yard. The ones I used have little "bumps" in the middle to prevent them from getting pushed against the roof sheathing when you push the insulation in.
Alternatively, you could nail (or glue) some 1x boards (or strips of foam insulation) along the sides of the rafters and span the gap with a piece of plywood or hardboard.
If you MUST stick to that 2.25" insulation space, your best bet would be solid foam insulation. It will provide the best R-value per inch, and because of their rigidity will probably not need the air baffles above them.
Compressing 4" insulation into the space won't work very well either. It's the air that gets trapped in the fiberglass that provides the insulation ability. If you compress the fiberglass, you greatly reduce the air spaces and correspondingly the R-value.
If there's any way you can shim down the rafter space to gain more depth (i.e. nail 2x3's on edge along the bottom of the rafters), you could put in more insulation.
Alternatively, or in combination with the above method, you could go ahead and fill up the available rafter bays, then frame a second roof on top of the existing roof sheathing, providing the 1" air space there.
Finally, you may want to look into sprayed foam insulation. It'll provide the best insulating ability for the space, and will completely seal off the rafter bays. While this would also eliminate the 1" airspace, I hear it works well because no air currents can flow within the space. It's a lot like structural stress skin panels. Unfortunately, it's not a do-it- yourself application.
Good luck!
Anthony
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[...]
I've considered doing this. The resulting roof is then known as a "hot roof" and might not be code in the OP's area. It might also void shingle warranty.
We are looking at vaulting our third floor ceiling to open the place up. it is currently a combination of sloped and flat ceiling with an unaccessible "attic" space above the flat parts.
The way i've considered doing it is to use a small can of foam to "glue" the air channels in first... then spray with the foam to fill the remainder of the cavity. I havn't found an answer as to if this would be a "good idea" or not... but it seems it would seal well, supply vent space, and have the higest insulation value.

perhaps it is.... www.fomofoam.com is where i've been looking. I'd love to find some other sources for price comparison. Anyone know of any?
--
be safe.
flip
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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, resulting in 5.25" to work with (aka the 6 in "2x6"), but would have to tie 2x6's into existing framework at peak of roof and where rafters meet top of wall framing, an involved process so was gonna try to get by with 3.25", see now it won't work, may go with the 2x6's alongside, then:
layers from underside of plywood roof decking down as follows:
wood blocks 1" x 1" square x 1.25" thick glued to underside of plywood roof decking staggered every 12 or 18"
1.25" airspace
radiant barrier (perforated brown paper with aluminum foil facing on one side, stapled to thel 1".25 thick blocks of wood 1" x 1" in size glued to underside of plywood roof deck every 12 or 18", the blocks holding the radiant barrier and fiberglass back from the underside of the roof deck by 1.25") stapled and siliconed to the small wood blocks
4" fiberglass or foamboard (undecided on which) (may cut some leftover 24" wide 8 1/4" thick encapsulated fiberglass in half using electric meat carving type knife and use the +/- 4" thick halves between the 24" on center rafters)
hardie panel or soffit panels atached to 2x6 rafters as the ceiling
any ideas?
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Effi,

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.

Sounds like a lot of work with little gain. Increase the insulation space and use standard plastic insulation baffles if you need them.

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).

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
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plenty of food for thought here
also been looking at http://www.owenscorning.com/around/insulation/project/addinsultoattic.asp for ideas
am now considering following layers from underside of plywood roof decking down:
layer 1. Owens Corning Raft-R-Mate attic vents attached to underside of plywood roof decking http://www.owenscorning.com/around/ventilation/raftrmate_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
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Effi,

Yep, I visited there a lot when we were preparing to insulate our house too...

Depending on how much depth you finally end up with, you may only need them at each end of the rafter span where the insulation can get bunched up and block off the air flow. But, you can certainly use them the full length of the rafter bay if you wish. Just more cost (and slightly more restriction to the airflow).

Sounds fine. I have no experience with radiant barriers.

I personally would not trust the press on metal braces. I know they use them for roof trusses, but they have a big press in the factory to press them in firmly and evenly. I don't see how you could ever achieve the same results in the field.
If you want to use metal braces, I would get the types that have all the little holes in them, then nail them up with joist hanger nails (short lengths that won't go all the way through the 2x lumber).
I still think plywood supports would be cheaper and easier to install.

I was just looking at an old article from the Journal of Light Construction Online (couldn't find it online now) about "Energy efficient cathedral ceilings". One of their techniques was to use 4" wide plywood gussets nailed to the rafter with 8D nails and to the new bottom chord (2x3 in their case). By adjusting the length of the gussets, they could adjust the depth available for insulation. From what I could tell, they only nailed the gussets on one side of the rafter.
If it were me, I would probably use wider gussets or even full length strips of plywood, just for the added strength.

If each end is fastened securely, I wouldn't think there would be too much sway (don't know how long your span is), but I see no real problem with your solution.

Around here, I would try for R30 minimum in the ceiling if at all possible. But, you may not need that much in your climate. You would have to do some checking on that.
In any case, your current plan sounds much better than your original proposal.
Have fun!
Anthony
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The thing is, you don't really NEED all that much strength. You're only holding up the bottom chord, fiber insulation, and sheetrock, That's something like 7 PSF dead-load. The only problems occur in 6 years, when someone decides to hang a swing-chair from the rafters with lag-bolts...
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You don't eh? He's got a roof made of 2X4's on 24 inch centers. That's sufficient for a light roof in a place where it doesn't snow much and with a span of maybe 7 ft max. And that's assuming whoever built it used only quality construction grade 2X4's, not some random studs. Anyone taking that care likely would have used 2X6's to begin with.
The previous advice to build out the rafters and correctly reinforce them is the right advice. If done correctly, it will make it strong and fix the insulation issue at the same time.
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i see now, and will use them, might even be stronger

will use full length strips of plywood, good idea

for attic ceiling here http://www.owenscorning.com/around/insulation/rvalue.asp or here http://www.owenscorning.com/around/insulation/rvalue_results.asp the DOE reccomends r-49 in the attic, for texas!, and the attic only had 4" of glass (r-11 to r-15?) as originally built, a whole subdivision of houses the same way ! ! !
sounds like the fed govt should give us all a tax credit for getting our attic insulation up to what they specify since the houses were malequipped with insulation when built and the home builders can probably no longer be sued in a class action suit
then there's the cadmium problem with galvanized pipe, which the industry has known about for years...
this source
says "If you find cadmium in your hot or cold water, you will never be able to filter it out. Nor should you switch to bottled water. The amount of cadmium in your clothing from doing laundry with this water is already too much for your adrenals and kidneys. Change your galvanized pipes to PVC plastic. If you believe you already have plastic pipes or all copper (which leads to leukemia, schizophrenia and fertility problems) you will need to search every inch of plumbing for a very short piece of galvanized pipe left in the system! A piece as short as a 2 inch T or Y can be causing all the trouble."
modern life is so wonderful ; )

many thanks for the ideas
will post in the new year about results

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forgot to add this link last time about the galvanized pipe problem http://www.curezone.com/clark/high.asp

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If you're going to modify the rafters anyway, just convert them into the upper chord of a truss, using 2x3s as the lower chord, and regular metal or plywood gussets. That will give you all the joist-depth you can use. If you're converting a pre-existing flat-ceiling into a cathedral, that means you're taking out the existing ceiling joists and-or collar-ties, right? Have you worked out what that's going to do to the tops of your walls, in terms of resisting the outward thrust of the roof?
--Goedjn
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Something ain't right here. You have a roof that uses 2X4's on 24" centers for rafters? And you want to now turn it into a cathedral ceiling by insulating it and sheetrocking it? I think the first thing needed is some engineering calculation to figure out how the roof is going to support the load or maybe even how it's even still there before you do anything else to it.
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