roof jack "box" idea

[SilverLeaf]

Work has begun on my granary-to-sugarshack conversion (http://mapletrader.com/community/showthread.php?9316-Granary-to-Sugarhouse-conversion) (yippee!!!). As for where the stack goes out the roof, my dad had an idea that I've never heard of that I want to run by you all. The granary doesn't have trusses; just 2x6' studs creating the roof peak. We will be removing part of one of the 2x6's to make enough clearance for the stack; the stack will never be closer than 18" from the 2x6 on either side of it.

If I understand correctly, most people seem to have a double-walled pipe where the stack goes through the roof jack, and then the only "insulation" between the outer pipe wall and the nearest wood (stud/truss) is, well, air. In some cases maybe some additional insulation is affixed directly to the wood as an added precaution.

My dad's idea instead was to use tin to create a "box" of sorts; 6 inches thick to match the stud width, and then taking up the entire area around the stack all the way to the next 2x6 (so the box dimensions would be approx 6in x 4ft x 4ft). And in that "box" we'd fill it up completely with vermiculite insulation. It would seem at first guess that maybe this would insulate the wood from the stack heat well. But since I've never heard of it before, I'm uncertain - is this a good idea or bad idea?

Specifically:
1) Will vermiculite insulate better/worse than just plain old air? If it settles over time might that change its effectiveness?
2) Metal is an excellent conductor of heat. Might having this metal "box" end up drawing more heat toward the 2x6s than if we just skipped it? Essentially it would have 2 sheets of metal radiating out from the stack rather than just one (the roof)

[Scribner's Mountain Maple]

I'm not expert on roof jacks. I did install my own 2' stack through the peak of my sugarhouse last year.

I think you are over thinking and doing. Go simple and don't worry so much about the heat transfer. My stack is not double lined at the roof and I would guess most peoples are not.

A simple roof boot is all you need. Simple and water tight. If you are worried about clearance to the 2*6's, just put a piece of metal on insulators on the wood next to the stack. Simple, cheap and will work fine for a long time.

I didn't get to put in the metal on insulators because I ran out of time. Not that I think that is ok, but I watched the wood next to my stack when running and it never even browned a little. It is only inches from my stack. Overkill is good, but I don't think you need to go to crazy with it. Simple works too.

Ben

[maple flats]

You need to protect the wood. While Vermiculite is excellent insulation, I won't address that. While wood may not show signs of browning, hig heat near it slowly drives gasses out of the wood. Once these are driven out (time span depends on temperature and time) the wood gets to the point where it will burst into flame at 300 degrees. Don't risk it.
There are several ways to protect the wood, and your vermiculite may be good. I just wonder if the continuous tin holding the vermiculite may conduct enough heat around it, I don't know. Tried and proven methods are layers of tin, each spaced 1" on non combustible spacers, each layer cuts the required clearance in half. For a wood burning stove clearance should be 36", but since an evaporator runs full bore for many hours at a time, I think 42" might be safer. Then 1 layer tin spaced 1" cuts that to 21". another layer thus spaced cuts it to 10.5", 3 layers 5.25 and if needed another layer to 2.625". Ceramic insulation is also a good choice, ask for specs since there are different densities.
The last thing you want during a long boil is a fire in the rafters.

[Cabin]

I would not cover the rafters in any way. What type of roof decking are you using?

[SilverLeaf]

What type of roof decking are you using?

The roof is plywood with asphalt shingles, which was later covered over with tin. In that "box" area (between the stack and the closest 2x6's) we will be removing the plywood/asphalt so it's just the metal roofing with whatever roof jack contraption we end up coming up with.

Explaining this just made something else occur to me: I assume the melt temp of an asphalt shingle is less than the cumbustion temp of wood so I need to be extra careful to cut the temp down so I don't create a gooey mess just outside the "box" area where the shingles are still sandwiched between the plywood & tin.

[RC Maple]

I think you want to know that you did everything you could to make sure that you can enjoy you sugar house and boiling in it without having to worry about when the fire will start. There are many ways to accomplish that. The "box" idea is pretty much the one I went with. It can be bought at a place like Lowes fairly inexpensively. I ran black stove pipe up out of the evaporator then double wall up through the "cathedral ceiling box". The maker of that doesn't have you put anything in that box for insulation though. There are a lot of threads on here about roof jacks and running stacks out the roof and a lot of pictures to go with them. Getting the stack through the roof without starting fires or letting water in is pretty high on the list of things to get right. Just be comfortable with your solution.

[jrgagne99]

This is definately something you want to do right. Two seasons ago, I had about 6" of clearance to the roof decking and I caught the sugarshack on fire. Fortunately, it was only a 7-foot ceiling and I had a full 5-gallon bucket of steam-away condensate right there to throw at it to put out the flames quickly. But things could have got very bad in a hurry.