# Cathedral ceiling support box install question



## draughtdude (Jan 8, 2015)

Hi Guys,

Soon I will be installing a chimney support box in my cabin but I have a few questions that the manuals don't address at all.  

The support box dimensions for a 6" chimney are designed to fit in between rafters 16" OC, but any of them I have measured will leave a tiny gap between each rafter rather than fit tight, and the gap that's left wouldn't really be enough room to foam the space.  

How do typical installs deal with that? And what's the best way to deal with the roof vapor barrier near the support box? Tape it to the nearest rafter and foam between the rafter and box? Or just tape the vapor barrier to the box?

I had planned on using the Supervent 2100 kit made by Selkirk.  Readily available up here in Canada.

Thanks,

John


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## DAKSY (Jan 8, 2015)

If I had a gap & had to make sure the CSB was centered in the 14.5" between the rafters,
I cut some 2xs & scabbed 1/2" plywood to make up what I needed.
Your CSB should be 10"(+/-), so you'll need 2.5" on either side.
I always cut the top of the CSB to the roof pitch & wedged it tightly to the underside of the roof decking.
HTH


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## draughtdude (Jan 8, 2015)

DAKSY said:


> If I had a gap & had to make sure the CSB was centered in the 14.5" between the rafters,
> I cut some 2xs & scabbed 1/2" plywood to make up what I needed.
> Your CSB should be 10"(+/-), so you'll need 2.5" on either side.
> I always cut the top of the CSB to the roof pitch & wedged it tightly to the underside of the roof decking.
> HTH



Thanks for the reply! Ok, so basically make the wood framing tight against the box so it can be mounted on all sides.  What do you do with the vapor barrier?


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## cableman (Jan 9, 2015)

I framed mine out same size as the box. I leveled, screwed it in and cut the extra flush with the roof decking. Used a little high temp silicone against the box where i cut it flush, prolly could of been any silicone or not even needed. Also added high temp silicone around the support ring to make a gasket so i wouldnt have a draft coming through the box!


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## DougA (Jan 9, 2015)

Cut the barrier in an 'X', then fold into the box and tape with vapor barrier tape. Then, put thin plywood or whatever is needed on the sides of the rafters.  Trying to make this completely sealed is kinda silly when you will end up with a 6" hole in the middle of the chimney. 
On mine, my IR gun is reading very low temps at the box. Nowhere close to worrying about melting the plastic barrier.


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## draughtdude (Jan 10, 2015)

Thanks DougA, good to know about the vapor barrier against the box which is probably what I'll try doing.  Supervent sells a special insulation kit that goes around the Class A pipe inside the support box which I am also planning on using.  This should pull double duty and make the support box even cooler I would think.

Once I finally get around to it I'll post pics.


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## webby3650 (Jan 10, 2015)

draughtdude said:


> Thanks DougA, good to know about the vapor barrier against the box which is probably what I'll try doing.  Supervent sells a special insulation kit that goes around the Class A pipe inside the support box which I am also planning on using.  This should pull double duty and make the support box even cooler I would think.
> 
> Once I finally get around to it I'll post pics.


That insulation that Selkirk offers (JUSI) just looks like fiberglass. We use it in very tight, or spray foamed houses. It will prevent condensation from forming in the ceiling box. Also, your ceiling box is 12" square, not 10" that was mentioned above.


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## saskwoodburner (Jan 10, 2015)

This is the same kit I used, cathedral support kit from Canadian Tire, and it is in fact 14" and a hair wide. I went with generous clearances (overboard to keep missus happy), so my support box is sticking out 3" below the flat ceiling, and the Class A is sticking out 3" below that. The support box is cool even with a raging fire, like the same temp as my computer desk or random items in the home.

The Class A chimney sticking out of my support box is hot on the bottom 1.5"- 2", but I can hold my fingers on it all day where it enters the ceiling support box. I had considered the insulation you're talking about to prevent a cold down draft, but haven't noticed any so far.


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## webby3650 (Jan 10, 2015)

saskwoodburner said:


> This is the same kit I used, and it is in fact 14" and a hair wide. I went with generous clearances (overboard to keep missus happy), so my support box is sticking out 3" below the flat ceiling, and the Class A is sticking out 3" below that. The support box is cool even with a raging fire, like the same temp as my computer desk or random items in the home.
> 
> The Class A chimney sticking out of my support box is hot on the bottom 1.5"- 2", but I can hold my fingers on it all day where it enters the ceiling support box. I had considered the insulation you're talking about to prevent a cold down draft, but haven't noticed any so far.


The box was 14"? It must be an 8" system or triple wall?


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## saskwoodburner (Jan 10, 2015)

webby3650 said:


> The box was 14"? It must be an 8" system or triple wall?



it's a 6" system. Double wall to the support box, and the chrome (Class A or triple?) up through. I'll go look online and find the exact one.

http://www.canadiantire.ca/en/pdp/s...eiling-support-kit-0644200p.html#.VLFdptLF-So

This is the cathedral support kit I have.

http://www.canadiantire.ca/en/pdp/telescopic-pipe-6-in-diameter-0644202p.html#.VLFeBdLF-So

The double wall pipe from stove up.

http://www.canadiantire.ca/en/pdp/supervent-2100-chimney-length-12-in-0644246p.html#.VLFe5NLF-So

This is the same chimney type I have.


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## webby3650 (Jan 10, 2015)

saskwoodburner said:


> it's a 6" system. Double wall to the support box, and the chrome (Class A or triple?) up through. I'll go look online and find the exact one.
> 
> http://www.canadiantire.ca/en/pdp/s...eiling-support-kit-0644200p.html#.VLFdptLF-So
> 
> ...


Unless something is different in Canada, that box should measure 12"x12".


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## DougA (Jan 10, 2015)

The specs on their instructions do say 14x14"  I'd measure it myself before building the box. Not the first time something is wrong.
http://www.selkirkcorp.com/WorkArea/showcontent.aspx?id=1604


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## saskwoodburner (Jan 10, 2015)

Funny thing about the box size, I recall assuming it was 12", and when I looked at it, thought it was a generous 12" so I measured!


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## cableman (Jan 10, 2015)

Mine is 12x12. I have no condensation what so ever on how i installed it. I do have ice sheild under this whole part of my roof. Do you plan to cut the excess box off or fold it over?


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## draughtdude (Jan 12, 2015)

I think they make them the 14" and a bit so they fit in between rafters 16" OC without having to modify the rafter.


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## draughtdude (Jan 12, 2015)

cableman said:


> Mine is 12x12. I have no condensation what so ever on how i installed it. I do have ice sheild under this whole part of my roof. Do you plan to cut the excess box off or fold it over?



I have no idea yet.  It would seem to me that folding it over is better?? Any thoughts?


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## draughtdude (Jan 12, 2015)

DougA said:


> The specs on their instructions do say 14x14"  I'd measure it myself before building the box. Not the first time something is wrong.
> http://www.selkirkcorp.com/WorkArea/showcontent.aspx?id=1604



This is good advice.  I had planned on buying the support kit and having the box ready for when I start installing the rafters, then I will just make it tight against the box on the two sides, no fuss.  Waiting for it to warm up and stop snowing so I can actually get something done instead of plowing and shoveling snow.  This January for us is full-on winter.


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## draughtdude (Jan 12, 2015)

saskwoodburner said:


> This is the same kit I used, cathedral support kit from Canadian Tire, and it is in fact 14" and a hair wide. I went with generous clearances (overboard to keep missus happy), so my support box is sticking out 3" below the flat ceiling, and the Class A is sticking out 3" below that. The support box is cool even with a raging fire, like the same temp as my computer desk or random items in the home.
> 
> The Class A chimney sticking out of my support box is hot on the bottom 1.5"- 2", but I can hold my fingers on it all day where it enters the ceiling support box. I had considered the insulation you're talking about to prevent a cold down draft, but haven't noticed any so far.



Ok, cool.  That's two reports of a cool ceiling box so I'm definitely going to go ahead and tape vapour barrier to it for the best seal.  Really looking forward to burning wood in an Englander 17!!


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## webby3650 (Jan 12, 2015)

draughtdude said:


> I think they make them the 14" and a bit so they fit in between rafters 16" OC without having to modify the rafter.


We install several different brands, including Sekirk/supervent/Hart&Cooley. All of the 6" class A pipe we use has a 12" box, 8" pipe has a 14" box. That's the only reason I brought it up, a 14" box is uncommon for a 6" system.


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## saskwoodburner (Jan 12, 2015)

Keep us posted how you make out.


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## saskwoodburner (Jan 12, 2015)

draughtdude said:


> I have no idea yet.  It would seem to me that folding it over is better?? Any thoughts?



 What I did (which doesn't make it right, just saying how I went about it) was had the square hole cut through ceiling and roof, knew I wanted 3" of box hanging below ceiling. Take your support box, and using your white pencil crayon, tape measure, and level, make a line all the way around at 3". Mark each side with it's orientation, north south east and west, and an arrow facing up. Or remember big hole to the bottom!

 Have a helper hold the box inside the house to what depth you want it. Go up top, and mark with white pencil crayon, where it needs to be cut. Or folded I suppose.....seems pretty heavy duty stuff to fold over, and I didn't have room in my install, as my old trailer roof just isn't that thick. Remove, and cut with side grinder. Continue on from there. On a house roof, you may not even need to cut or fold very much.

Maybe there's a better way, this is just one of many I suppose.


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## Woody Stover (Jan 12, 2015)

draughtdude said:


> Ok, cool.  That's two reports of a cool ceiling box


They probably weren't having a chimney fire at the time, though...


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## bholler (Jan 12, 2015)

Woody Stover said:


> hey probably weren't having a chimney fire at the time, though.


Yeah but the box should never get hot enough on the outside to cause a problem even in a chimney fire


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## draughtdude (Jan 13, 2015)

Thanks for all the tips guys! I really appreciate it.


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