# Cathedral Ceiling Redo



## Nealtw (Jun 22, 2017)

What are the venting limitations?


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## arcticranger (Nov 3, 2017)

Nealtw said:


> What are the venting limitations?



The room with the vaulted ceiling has only 2 outside soffits. An 11 foot wide floor to ceiling fireplace occupies the wall between them.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> The room with the vaulted ceiling has only 2 outside soffits. An 11 foot wide floor to ceiling fireplace occupies the wall between them.


 Is there a cricket on the roof to divert the water around the chimney?


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## Bud9051 (Nov 11, 2015)

Local codes can vary but as far as I know you can't use fiberglass or Roxul insulation (air permeable) directly against the bottom of the roof deck. Air gets in and finds a cold surface and you have mold and rot.
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

If you can fill in your location of tell us your climate zone we can be more specific.

Bud


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## ryansdiydad (Aug 16, 2015)

arcticranger said:


> What is the actual difference in R performance between fluffy and solid and why is solid foam stacking so rare?


https://www.todayshomeowner.com/insulation-r-value/

It is significant which allows you to get more r value in less space with foam. I stacked foam board insulation in my cathedral ceiling to hit the required r value in a 2x10 rafter bay and still have room for venting.. 

The reason for not doing it is cost.


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> Local codes can vary but as far as I know you can't use fiberglass or Roxul insulation (air permeable) directly against the bottom of the roof deck. Air gets in and finds a cold surface and you have mold and rot.
> http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
> 
> If you can fill in your location of tell us your climate zone we can be more specific.
> ...



I'm in Sullivan County NY, it's zone 6. Both hot and cold.


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## arcticranger (Nov 3, 2017)

ryansdiydad said:


> https://www.todayshomeowner.com/insulation-r-value/
> 
> It is significant which allows you to get more r value in less space with foam. I stacked foam board insulation in my cathedral ceiling to hit the required r value in a 2x10 rafter bay and still have room for venting..
> 
> The reason for not doing it is cost.





Yes it will cost for sure. Can you describe your assembly from roof deck down and what solid foam product you used?


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## ryansdiydad (Aug 16, 2015)

arcticranger said:


> Yes it will cost for sure. Can you describe your assembly from roof deck down and what solid foam product you used?


So it goes 

- Decking
- Baffles to ensure I didn't push the foam boards up against the decking and cut off air flow.. 
- 4 layers of foam board with spray foam applied with each layer to fill in any gaps
- Drywall

I am pretty sure I used these boards and I did reach out to the manufacturer to confirm it was okay to stack these..

https://www.menards.com/main/buildi...949-c-5779.htm?tid=3383740433499869921&ipos=7

Given the R-value required the only other way to hit the mark was spray foam and the cost for that was way higher than my just doing the boards myself. 

Passed inspection the architect just needed to note the use of foam boards in an "as built" letter for the city.


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## arcticranger (Nov 3, 2017)

ryansdiydad said:


> So it goes
> 
> - Decking
> - Baffles to ensure I didn't push the foam boards up against the decking and cut off air flow..
> ...


Good info. That's pretty much what I will be doing, it's called cut and cobble. By "baffles" do you mean you have rafter vents in each bay or just spacers?

The greenbuilder article Bud9051 referenced does say you can't do cut and cobble in an unvented cathedral ceiling and that's my problem as I only have one soffit on each side of my fireplace. 

Is there's a way to vent rafter cavities that don't have a soffit available? Do all rafter pairs have to have soffits?


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## Bud9051 (Nov 11, 2015)

@*arcticranger,* "The greenbuilder article Bud9051 referenced does say you can't do cut and cobble in an unvented cathedral ceiling" To save me from having to re-read the entire article can you post a quote for that statement.

As far as i know piecing together sections of rigid insulation can be used for either vented or unvented assemblies as long as you follow all of the other guidelines.

Yes, there are ways to vent rafter cavities that do not have soffits, as in under shingle vents or edge vents (or other approaches).

Bud


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> To save me from having to re-read the entire article can you post a quote for that statement.
> 
> Bud



Here's the quote. 

---------------------------------------------------------------------

*What about the cut-and-cobble method? *


Cut-and-cobble is an insulation method used by some homeowners, but never by insulation contractors. It involves cutting rigid insulation into narrow rectangles, and inserting the rectangles between rafters or studs. In most cases, the perimeter of the each rectangle of rigid foam is sealed with canned spray foam or caulk.


When it comes to cathedral ceilings, here's the rule: the cut-and-cobble method can be used for vented cathedral ceilings, but not for unvented cathedral ceilings. (There have been several reports of moisture problems in unvented cut-and-cobble cathedral ceilings.)"
-----------------------------------------------------------------------


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## ryansdiydad (Aug 16, 2015)

Did you discover any issues or signs of issues when you opened up the ceiling? 

At some point someone, maybe Bud, provided a calculation for the amount of venting required for an addition on my house. You maybe have enough to leave it vented? Have you looked in to a calculation for your place?

The baffles are just spacers used to keep insulation away from the decking..


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## arcticranger (Nov 3, 2017)

Just FYI that same author Martin Holladay warns against cut and cobble for vented OR unvented ceilings and cites various disasters. He then provides instructions for doing both. In the same article!


http://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation


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## Bud9051 (Nov 11, 2015)

I don't have a specific example but the industry has been for years fitting rigid foam into rafter cavities on unvented assemblies. The "There have been several reports of moisture problems in unvented cut-and-cobble cathedral ceilings." just tells me those ceilings were not air sealed. "Cut and Cobble" makes it sound easy but the devil is in the detail and from doing a bit of cobbling I can say it takes patience.

One tip is to give the can foam you use enough space so you can get it in place to do its air sealing. Rigid foam cut to "almost fit" means a thin gap where the foam can only cover the outside and not get into where it can seal the entire space between the foam and the framing or other foam.

In fairness in my work I used as many full length pieces as possible and always staggered seams.

You will have to make your choice but my take on that statement was more of a caution as opposed to a definite never.

But I definitely prefer vented assemblies.

Bud


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## Nealtw (Jun 22, 2017)

arcticranger said:


> Just FYI that same author Martin Holladay warns against cut and cobble for vented OR unvented ceilings and cites various disasters. He then provides instructions for doing both. In the same article!
> 
> 
> http://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation


So have another look at venting.
What kind of roofing do you have and what does the roof look like behind the chimney.


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## arcticranger (Nov 3, 2017)

ryansdiydad said:


> Did you discover any issues or signs of issues when you opened up the ceiling?
> 
> At some point someone, maybe Bud, provided a calculation for the amount of venting required for an addition on my house. You maybe have enough to leave it vented? Have you looked in to a calculation for your place?
> 
> The baffles are just spacers used to keep insulation away from the decking..



My cathedral ceiling has a badly leaking skylight. It did cause damage to the ceiling and a bit of the sheathing which will get repaired this week. So I've stripped it and am considering my options.


Not sure what you mean by a calculation. My rafters are 11" so there's plenty of room, but spacing alone is not ventilation, is it?


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> But I definitely prefer vented assemblies.
> 
> Bud



I frankly don't trust myself or anyone around here to make it airtight. So maybe the best thing is to figure venting out first and then go with the assembly @ryansdiydad described. 



Here's a pic. The only soffit vents are on either side of the chimney outside. The dark space over the brick is just where I've not finished retiling the fireplace. And I haven't pulled out all the foil insulation yet.



Any ideas on getting airflow into those middle bays?


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## arcticranger (Nov 3, 2017)

Nealtw said:


> So have another look at venting.
> What kind of roofing do you have and what does the roof look like behind the chimney.



It's the Owens architectural stuff, Duration. I'll have to go look and see how the tile meets the chimney. My ladder was just stolen so...lol


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## Nealtw (Jun 22, 2017)

arcticranger said:


> It's the Owens architectural stuff, Duration. I'll have to go look and see how the tile meets the chimney. My ladder was just stolen so...lol


 Can you get much the same picture from the out side where the chimney reaches the roof.


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## arcticranger (Nov 3, 2017)

If you mean from ON the roof looking at the chimney I need to get a new ladder.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> If you mean from ON the roof looking at the chimney I need to get a new ladder.


 No from the ground for now.


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## arcticranger (Nov 3, 2017)




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## arcticranger (Nov 3, 2017)

Nealtw said:


> No from the ground for now.



I'm not seeing my own images let me know if you can. I could replace the left and right soffit vents with long continuous vents but the rafters behind the fireplace still wouldn't be vented.


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## Bud9051 (Nov 11, 2015)

No pictures as yet.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> I'm not seeing my own images let me know if you can. I could replace the left and right soffit vents with long continuous vents but the rafters behind the fireplace still wouldn't be vented.


Behind the chimney there should be cricket or saddle that looks like this.








A box vent or ridge vent might be able to be placed on that and then you would just cut into the cavity from below.
The other question is why did they put solid blocking above the window.
I doubt it was needed other than holding things in place during construction and a few 2" holes thru them would not hurt anything.


There are also vents much like the ridge vent that can go lower on the roof but that is disturbing the roof likely more than you would like.
I will down load that picture and post it.


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## arcticranger (Nov 3, 2017)

the insert from URL is not working so here are direct links

http://randomorbit.com/rafters.jpg

http://randomorbit.com/chimney.jpg


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## Nealtw (Jun 22, 2017)

................


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## ryansdiydad (Aug 16, 2015)

https://www.owenscorning.com/roofing/components/vent-calculator

I can’t see the pictures but the calculator should help you figure out how much venting you need at the soffits and ridge... 

Seeing the pictures might make this irrelevant so someone correct my mistake if this just doesn’t apply in this case


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## Bud9051 (Nov 11, 2015)

Those look like substantial rafters (perhaps larger than necessary) so one might consider many holes along the top edge to allow bay to bay air movement. This approach is common with a hip roof where rafter bays don't go all the way to the top. Not the best and should have local code approval, but it could get some air in there from each side. Also depends upon good venting in those side bays.

Lacking ventilation can be minimized by better air sealing and a good vapor barrier across the bottoms of those rafters.

Bud


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## arcticranger (Nov 3, 2017)

Nealtw said:


> Behind the chimney there should be cricket or saddle that looks like this.
> 
> 
> 
> ...



The whole ceiling was done by a DIY'er who refused all advice, including the whole corner window thing with the blocking above. That's a separate issue but anyway above that block I have access to a soffit and can put a regular rafter vent in, so there's no reason for holes. 

The two rightmost wood blocks above the chimney do block access to the soffit but it's possible I could cut a U at the top to get access. The three leftmost blocks have no soffit access. I don't know if those blocks serve any other purpose than keeping the rafters aligned. 

I will look into the box vent but I wonder if having 4 out of 7 bays vented would be enough.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> The whole ceiling was done by a DIY'er who refused all advice, including the whole corner window thing with the blocking above. That's a separate issue but anyway above that block I have access to a soffit and can put a regular rafter vent in, so there's no reason for holes.
> 
> The two rightmost wood blocks above the chimney do block access to the soffit but it's possible I could cut a U at the top to get access. The three leftmost blocks have no soffit access. I don't know if those blocks serve any other purpose than keeping the rafters aligned.
> 
> I will look into the box vent but I wonder if having 4 out of 7 bays vented would be enough.


 I would be nice to see the cricket if you have one. 
Worst case would be a couple box vents each straddling a rafter so they serve 2 bays each..


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> Those look like substantial rafters (perhaps larger than necessary) so one might consider many holes along the top edge to allow bay to bay air movement. This approach is common with a hip roof where rafter bays don't go all the way to the top. Not the best and should have local code approval, but it could get some air in there from each side. Also depends upon good venting in those side bays.
> 
> Lacking ventilation can be minimized by better air sealing and a good vapor barrier across the bottoms of those rafters.
> 
> Bud


I _was _considering using holes to get bay to bay transfer. But how does bay A transfer air to bay B if bay A is using a rafter vent? 

Or could the the 1" gap space between deck and solid foam perform as a rafter vent?


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## Nealtw (Jun 22, 2017)

arcticranger said:


> I _was _considering using holes to get bay to bay transfer. But how does bay A transfer air to bay B if bay A is using a rafter vent?
> 
> Or could the the 1" gap space between deck and solid foam perform as a rafter vent?


 Even with oversized rafters you are not allowed to drill holes other than the center third as far as I know. But cupping around hoes in the center would not displace much insulation.


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## Bud9051 (Nov 11, 2015)

Neal, not sure what you are describing as cupping.
First we would need to know what is up there 2x?. And how long they are.
As for not allowed we all know the final word comes from the local authority. Slim chance but may be worth asking.

As for how the air would move, poorly, but better than nothing.

Another option might be to spray foam the difficult bays and vent the rest.

Bud

Just to note, the expensive spray foam only needs to be thick enough to keep the inside surface above any possible dew poing, reference available. The remainder could be filled with Roxul, keeping in mind what you need for total r-value.


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## Nealtw (Jun 22, 2017)

Bud9051 said:


> Neal, not sure what you are describing as cupping.
> First we would need to know what is up there 2x?. And how long they are.
> As for not allowed we all know the final word comes from the local authority. Slim chance but may be worth asking.
> 
> ...


 I was looking at drilling joists and I imagine the rules would be the same.
If you drill a few holes down a few inches from the sheeting and then you would need a channel of some sort on both sides to leave room for the air to flow. Something like this cut in half and attach over the hole to hold insulation back.


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## Bud9051 (Nov 11, 2015)

rethinking my post and I like the no-vent spray foam for the difficult rafter bays. Saves inventing something that the inspectors would probably not like anyway.

Bud


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## ryansdiydad (Aug 16, 2015)

Going back to an earlier question. Besides the window leak was there any indication that you were having issues with the ventilation/insulation that existed before you opened the ceiling? 

Did you already mention that you have a ridge vent or some other type of high venting?


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> First we would need to know what is up there 2x?. And how long they are.
> 
> Another option might be to spray foam the difficult bays and vent the rest.
> 
> ...


The rafters are stacked 2x6's, length is only 12ft. I don't like the idea of drilling holes now that I consider these aren't 2x12's.

This is a 700 sq ft house with a normal sloped roof. It's an open floor plan with 9 ft ceilings in 3/4 of the space and a half-cathedral in 1/4 of the space. That is the space shown in the pic. 

So 3/4 of it has soffit vents. 4 of the rafter bays in the cathedral do not. I'm describing this because maybe the totality of it changes the venting requirements picture.

I like the idea of venting the bays with soffit vents and spraying the other 4. Any reason not to use cut and cobble for all the bays whether sprayed or vented? I'd just skip the air gap for the sprayed bays.


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## Nealtw (Jun 22, 2017)

If it is only 12 ft, one 2x6 would have carried the weight the extra only for insulation depth. If the lower one is doing the work notching the upper one might be OK?


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## arcticranger (Nov 3, 2017)

ryansdiydad said:


> Going back to an earlier question. Besides the window leak was there any indication that you were having issues with the ventilation/insulation that existed before you opened the ceiling?
> 
> Did you already mention that you have a ridge vent or some other type of high venting?


There is mold on the sheathing around the skylight but nowhere else. And I found a thriving ant colony in the same bay which is now history. 

Venting: I have no ridge vents at all and only a tiny gable vent. Which means most air coming in via soffit vents is going nowhere. The attic is crazy hot in the summer but I would expect that anyway.

Insulation: none of the rafters have insulation but the floor of the attic is R30 Roxul with sanded plywood over it to make for storage space. The open floor plan below has 4 baseboard electric heaters and the only room that can't be heated is the cathedral part. I had to partition it off with plastic last winter which sucks as it would be nice to sit by the fireplace. Also, there are five 6x3' Low-E windows across two of the walls giving it that Star Trek look. So that's a heating challenge as well.


ps I am a software developer and don't know *** about all this:vs_laugh:. But at least I ask questions!


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## arcticranger (Nov 3, 2017)

So update (sort of): I've removed the bad skylight and re-roofed it. It rained like crazy for 3 days afterward and not a drop got in. Done. I also discovered from the previous owner that the house already _*had *_an existing aluminum ridge vent. Sure enough in my attic I saw perfect ridge vent cuts in the sheathing. So the ridge vent was obviously removed and the 20 foot 3" aperture shingled over when my new roof was put on. Last week I had new ridge vents installed and capped. So done on that too. Lastly, soffits on the entire South side of the house were blocked by "decorative" rough cut lumber, I've torn that down and reopened those soffits. 

The evil that men do.


I got estimates on commercial closed cell for the cathedral ceiling bays and they are in the range 1200-1500. So I am going with cut and cobble as we discussed but I am still unclear on the correct type of rigid foam to use. It seems I need a layer to air seal the sheathing and additional layers to add R value while allowing drying to the inside.

Does any rigid foam actually create an air seal (with sprayed edges) or was @*Bud9051* referring to an initial layer of spray closed cell? 

Here's that article again which states that EPS or fiber-faced polyiso are better than XPS because they allow drying to the interior. 

https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation

I am very confused between the air sealing and drying elements of this project.


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## Bud9051 (Nov 11, 2015)

First, are those cathedral bays isolated so they do not have air low access to the ridge vent. I'll send this while I go back and read.

Bud


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> First, are those cathedral bays isolated so they do not have air low access to the ridge vent. I'll send this while I go back and read.
> 
> Bud



hi Bud, 

These bays have no soffit vents due to a chimney in the way, but they do terminate at the same 20 foot long ridge vent which services the rest of the house. They have no dedicated airflow.


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## Nealtw (Jun 22, 2017)

If you have the proper cricket behind the chimney you can put a box vent there and open a hole in the roof sheeting to supply venting behind the chimney


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## arcticranger (Nov 3, 2017)

Nealtw said:


> If you have the proper cricket behind the chimney you can put a box vent there and open a hole in the roof sheeting to supply venting behind the chimney


I do have the cricket but am not sure how a box vent there would work. Are you saying it would act as a soffit and suck in air that flows UP the rafter bays to the ridge vent?


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## Bud9051 (Nov 11, 2015)

To be honest I'm not making a lot of progress. Neal's cricket or roof vents across several bays would involve shingle work, is that an option?

Is there an inspection involved with this project or can we proceed with common sense? Issues are bay to bay air flow and total R-value.

Note, I see a small soffit vent on each side of the chimney, is that it for soffits on that side.

Bud

You just posted so maybe the cricket can be utilized.


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> To be honest I'm not making a lot of progress. Neal's cricket or roof vents across several bays would involve shingle work, is that an option?
> 
> Is there an inspection involved with this project or can we proceed with common sense? Issues are bay to bay air flow and total R-value.
> 
> ...



There's no inspection involved here. Let the common sense fly.

The cathedral ceiling has 7 bays total and the outer 2 bays have available soffit intakes. I can possibly add another outside soffit to reduce the unventilated soffits to 4.

Shingle work is an option but I'd like to hear others.

Am I overthinking this? I don't see a 1-1 soffit / rafter bay pairing on any house I know.


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## Bud9051 (Nov 11, 2015)

In many homes the soffit area is connected from rafter bay to rafter bay allowing a few soffit vents to provide air flow to all rafter cavities, especially important with a vaulted ceiling where those cavities do not share a common attic.

Here's my common sense solution. If my house I would take a ¾" drill and perforate the tops of those rafters (up close to the sheathing) for the first 2' or 3', one every 1.5". You have plenty of strength there and a dozen or so holes will not cause a risk.

Then create an air gap that will include those holes allowing air flow from bay to bay. We aren't looking for a lot of flow, just a path, natural pressure will do the rest very slowly. If you have 10" depth (it may be 11") add 2" spacers between those holes to hold the first layer of rigid away from the roof deck and create your air channel. Use whatever thickness are necessary to fill further down.

Completing the fill you could use all rigid or shift to Roxul as long as the rigid is ridiculously well air sealed. I actually prefer a compatible caulking over can foam, it is stable for a long time. You could also use some foil tape on seams or edges along with the caulking.

Review:
More soffit venting
Holes for rafter to rafter air flow.
Well air sealed cut and cobble to make your air channel and several inches of rigid (pink or blue)
Finish with more rigid or fill remainder with Roxul.

The result will be a ventilated air space above all cavities, lots of r-value, and no compromise in rafter strength.

IMO,
Bud


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## Nealtw (Jun 22, 2017)

arcticranger said:


> I do have the cricket but am not sure how a box vent there would work. Are you saying it would act as a soffit and suck in air that flows UP the rafter bays to the ridge vent?


 Yes. ..............


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> f my house I would take a ¾" drill and perforate the tops of those rafters (up close to the sheathing) for the first 2' or 3', one every 1.5". You have plenty of strength there and a dozen or so holes will not cause a risk.



That's very doable. But my 11" rafters are actually stacked 2x6's. Is it safe to drill 3/4" holes on the top end of those 2x6's?


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## Bud9051 (Nov 11, 2015)

In my opinion, yes. One 2x6 would probably be sufficient so whatever the resulting strength of the other just adds extra. By the book they may not find it, doubling up 2x6's vertically isn't a common approach. But from the common sense chapter in my book it will be fine. 

Bud


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> In my opinion, yes. One 2x6 would probably be sufficient so whatever the resulting strength of the other just adds extra. By the book they may not find it, doubling up 2x6's vertically isn't a common approach. But from the common sense chapter in my book it will be fine.
> 
> Bud



Great. I'm on it!

Should I use baffles just long enough to carry air from the soffits to the holes I drill? Or skip em?


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## Bud9051 (Nov 11, 2015)

With spacers to hold the first layer of rigid away from the roof deck that becomes your baffle. How to space it is not critical, blocks of wood of blocks of rigid. If you use blocks of wood that might give you a place to screw up the first sheet of rigid. Once you caulk or foam them in place they aren't going anywhere.

You have plenty of depth so allowing 2" for the air gap will provide clearance for those holes. 

For the cavities with soffits below you should have baffles all the way to the top to be sure no insulation comes in contact with the roof deck and guarantee air flow. Obviously the holes cannot be blocked.

Bud


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> For the cavities with soffits below you should have baffles all the way to the top to be sure no insulation comes in contact with the roof deck and guarantee air flow. Obviously the holes cannot be blocked.
> 
> Bud



Sorry I am still missing something. The outer 2 bays have soffit vents and need to direct a portion of their airflow to the holes for the inner bays, right? If the outer bays have baffles all the way to the top how will they supply air flow to the holes?


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## Bud9051 (Nov 11, 2015)

The adjacent rafter channels with soffit vents at the bottom and a ridge vent at the top have air pressure at every level. Same for the channels that are blocked by the chimney. Those pressures are not the same thus some air will flow from side to side. Not a lot, but better than no air flow.

Hard to calculate the actual flow, but with the holes we can reasonably assume it won't be zero and the long story is, no one knows how much is actually needed. Those Net Free Area calculations we often follow were a wild guess from 1942 and have remained in place and have been adopted by virtually all codes. Basically they have worked but they certainly are NOT supported by the usual exhaustive testing we would like to see. In reality there are many other variables that could be considered instead of this "one size fits all" requirement. Beyond by focus to rewrite it so I just repeat it, until we need to do something else.

I don't like zero ventilation and modifications to meet the standard approach would bust your budget, at least mine.

Connecting the adjacent channels will allow some air flow. And i do understand your confusion.

Bud


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> The adjacent rafter channels with soffit vents at the bottom and a ridge vent at the top have air pressure at every level. Same for the channels that are blocked by the chimney. Those pressures are not the same thus some air will flow from side to side. Not a lot, but better than no air flow.
> 
> Hard to calculate the actual flow, but with the holes we can reasonably assume it won't be zero and the long story is, no one knows how much is actually needed. Those Net Free Area calculations we often follow were a wild guess from 1942 and have remained in place and have been adopted by virtually all codes. Basically they have worked but they certainly are NOT supported by the usual exhaustive testing we would like to see. In reality there are many other variables that could be considered instead of this "one size fits all" requirement. Beyond by focus to rewrite it so I just repeat it, until we need to do something else.
> 
> ...



I think you are indicating that the baffles are attached below the drilled holes so as to enclose the holes and share their airflow with the neighboring cavity. I was thinking the baffles would cut off that sharing.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> I think you are indicating that the baffles are attached below the drilled holes so as to enclose the holes and share their airflow with the neighboring cavity. I was thinking the baffles would cut off that sharing.


 You adjust the baffles to allow the air flow.
Which 2x6 lands on the wall if it is just the top, you only drill holes in the center of it.


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## Bud9051 (Nov 11, 2015)

The baffles are supposed to provide a space between the top of the baffles and the bottom of the roof that is open to the soffit area and open to the ridge vent. Because those cavities that butt up against the chimney are not open to the soffits you would be drilling holes at the top of those cavities connecting the venting spaces from channel to channel.

Bud


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## arcticranger (Nov 3, 2017)

Quick update, I have started this project finally. Bud, I completely overlooked this statement by you:

"With spacers to hold the first layer of rigid away from the roof deck that becomes your baffle"

OK got it (bonk) the 2" foam IS the baffle. Should that XPS channel run all the way to the ridge vent?


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## Nealtw (Jun 22, 2017)

arcticranger said:


> Quick update, I have started this project finally. Bud, I completely overlooked this statement by you:
> 
> "With spacers to hold the first layer of rigid away from the roof deck that becomes your baffle"
> 
> OK got it (bonk) the 2" foam IS the baffle. Should that XPS channel run all the way to the ridge vent?


 Yes.............


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## Bud9051 (Nov 11, 2015)

The baffle is there to keep the insulation away from the the roof deck so if insulation goes all the way to the ridge so should the baffle.

Bud


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## arcticranger (Nov 3, 2017)

So I've got my 2" XPS foam with 2" air channel enclosing 3/4" holes, foamed tight. I should probably have gone with caulk but I lost that detail.

I was surprised it was only 13' from floor to peak.

Now I've got to figure out the best filler for the remaining 6.5 inches. 3 more layers of rigid will run $300 as the first layer took 3 full sheets of 4x8 XPS at $100. Filling it with R23 Roxul will be about $150, I could go R30 Roxul but I'd have to add an inch to the rafters. So with the 2" foam being R10 my options are:

air gap
4 layers of 2" XPS 
-----------------
R40 at $400


air gap
1 layer of 2" XPS 
5.5" of Roxul R23
-------------------
R33 at $250


air gap
1 layer of 2" XPS 
7.5" of Roxul R30
------------------
R40 at $280


I do have a good deal of 1", 1.5" and 2" XPS as well as a LOT of 1 and 2" foil faced polyiso sitting around but I am not sure if puzzling it all together is a good idea. Also I've been avoiding the foil faced rigid as I don't want to create a vapor barrier. 

I can also add a layer of 1" XPS over the inside joists, that would only be $60 or so.

And there will be a ceiling fan up there. 

Further thoughts?


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> The baffle is there to keep the insulation away from the the roof deck so if insulation goes all the way to the ridge so should the baffle.
> 
> Bud


and ps to Bud thank for the great advice, and to the others who chipped in. I do think this was the best solution to my tricky cathedral ceiling issue. 

(I can already see the real estate agent's eyes rolling as I wax poetic about my fancy air channel:vs_laugh


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## arcticranger (Nov 3, 2017)

A few questions if anyone is still out there. 

What's the purpose of air sealing the rigid foam that created my air channel? Is it about maintaining a certain pressure? My worker did it pretty well but not perfect. I may tape the seams tomorrow.

I used 2" of XPS for my baffle. Some guy on the greenbuilding site said 2" has a perm rating of .5 and will block drying of room moisture via the air channel. He said I should have used 1" rigid and a 1" channel. I didn't pursue that as those greenbuilder guys seem to go OCD on everything. But I would appreciate any feedback on it.


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## Bud9051 (Nov 11, 2015)

Doesn't make sense for that advice to come from GBA, maybe another site using the greenbuilding name. Send me a PM with the link to those comments so I can see who said that.

2" is the desired channel and 1" is considered absolute minimum. As for 2" rigid not having a high enough permeance to dry to the outside, it's not supposed to. The objective is to prevent moisture from reaching the cold roof deck and if any gets through the ventilation is supposed to carry it away. 2" is also thick enough that it's inside surface will remain above the dew point. 1" becomes questionable.

The benefit of air sealing those channels is to prevent air leakage from the house carrying humidity up where it can find a cold surface.

Bud


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## arcticranger (Nov 3, 2017)

My semi-vented cathedral ceiling has been done for a month, the layers from top to bottom are

roof sheathing
2" air channel with 3/4 holes as Bud recommended
2" solid foam (XPS)
6" Roxul

I've been holding off on closing it up to get some wiring done up there like a ceiling fan. Today that was completed.

I'd like to ask for a last bit of advice on closing this ceiling up. Should I go with sheetrock or is metal or some other material ok? The most important thing of course is maximizing the insulation I have done. I'm considering galvanized steel corrugated roof panels as an interior ceiling, mostly because they are light and go with the industrial look of the space. And maybe they can help reflect heat?


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## Bud9051 (Nov 11, 2015)

I've never worked with or read about using galvanized steel corrugated roof panels on an interior ceiling. The only concern that comes to mind would be a perfect air seal. Metal expands and contracts with temperature and screws will often end up in elongated holes. The screws with rubber washers might work but unknown how long.

Bud


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## Nealtw (Jun 22, 2017)

Drywall and wall paper for the look.
https://www.wayfair.ca/decor-pillow...CTJD-XzG6M3ONEhoFuER_2iDtvBL9kQRoCwPkQAvD_BwE


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> I've never worked with or read about using galvanized steel corrugated roof panels on an interior ceiling. The only concern that comes to mind would be a perfect air seal. Metal expands and contracts with temperature and screws will often end up in elongated holes. The screws with rubber washers might work but unknown how long.
> 
> Bud



Yes all true. Ever heard of using Tyvek on an interior wall? I'd have to cover it but it is 

a) an air barrier 
b) not a vapor barrier. 

If that was tight it might allow for more creative finish options than drywall. Just thinking.


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## Nealtw (Jun 22, 2017)

http://www.dupont.co.uk/products-an...-housewrap-breathable-membrane-for-walls.html
Highly permeable to water vapour – can be fixed directly to insulation (Sd-value: 0.01m)
That would not be what you want for a ceiling you want to keep dry.


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## Bud9051 (Nov 11, 2015)

Tyvek was specifically designed to not be a vapor barrier and it was not intended to serve as an air barrier. It provides a drain plane to shed water while allowing drying to the outside. As for the air barrier, it can help, but an air barrier also needs to be rigid. Tyvek alone would billow in and out with air pressure pumping air in and out as it does.

Bud


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## arcticranger (Nov 3, 2017)

The sheet metal ceiling is just an idea at this point. I found another guy on GBA who actually did it. Interestingly it was on a cathedral ceiling with the same vented + rigid foam + air channel gap setup that Bud gave me. The 3rd entry by Don J describes it

https://www.greenbuildingadvisor.com/question/corrugated-metal-indoors-as-ceiling-wall-panel

My idea was to staple the Tyvek to the rafters, flush against the Roxul, with the metal sheeting directly over it. The Tyvek would be my air barrier and my 2" XPS would be my moisture barrier. 

But aside from the difficulty of finding anyone who can do this well, I doubt I could get this assembly as tight as sheetrock. It's kind of amazing in 2018 that there isn't another technology lighter and easier to install than drywall.


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## arcticranger (Nov 3, 2017)

Nealtw said:


> http://www.dupont.co.uk/products-an...-housewrap-breathable-membrane-for-walls.html
> Highly permeable to water vapour – can be fixed directly to insulation (Sd-value: 0.01m)
> That would not be what you want for a ceiling you want to keep dry.



If I add sheetrock this would be my assembly. I assume some moisture would exit through via the XPS to the air channel and the rest I don't know, back out via the sheetrock? 

roof sheathing
2" air channel with 3/4 holes as Bud recommended
2" solid foam (XPS)
6" Roxul
sheetrock

Those wallpapers really are cool.


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## Nealtw (Jun 22, 2017)

arcticranger said:


> If I add sheetrock this would be my assembly. I assume some moisture would exit through via the XPS to the air channel and the rest I don't know, back out via the sheetrock?
> 
> roof sheathing
> 2" air channel with 3/4 holes as Bud recommended
> ...


 You don't want any moist air from the house getting in there. If it were here it would get a 6 mill poly before the drywall,


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## arcticranger (Nov 3, 2017)

Nealtw said:


> You don't want any moist air from the house getting in there. If it were here it would get a 6 mill poly before the drywall,



I know Bud's idea was to 1) use rigid foam to prevent room moisture from hitting the cold roof deck 2) have the air channel vent any moisture that did get through

Not sure about where the blocked moisture would naturally go. But I could see 6mm plastic trapping moisture behind the sheetrock, no?


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## Bud9051 (Nov 11, 2015)

If the channel doesn't have a lower inlet to work with the high vents then the ventilation basically isn't going to work.

I'm out of ideas.

Bud


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## Nealtw (Jun 22, 2017)

arcticranger said:


> I know Bud's idea was to 1) use rigid foam to prevent room moisture from hitting the cold roof deck 2) have the air channel vent any moisture that did get through
> 
> Not sure about where the blocked moisture would naturally go. But I could see 6mm plastic trapping moisture behind the sheetrock, no?


 There should be no moisture in there so it is a matter of keeping it out and there is a different theory for different areas and climates. mostly what you worry about is a leak that would allow house air to get to the vented air above. If that happens warm air would cool on the way and any moisture would condense somewhere, usually beside the rafters where the thermal bridge is and air might find a path. that's not to say air travels thru drywall but it is the holes mostly like wires thru the tops of the walls and light and fan fixtures.


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## arcticranger (Nov 3, 2017)

Bud9051 said:


> If the channel doesn't have a lower inlet to work with the high vents then the ventilation basically isn't going to work.





Bud9051 said:


> I'm out of ideas.
> 
> Bud





Yea the assembly will stay as it is with sheetrock for air seal, no plastic or Tyvek. I can do some cool decorative things like faux-beams on the sheetrock. 



Anyway you can't quit you're my architect :biggrin2:


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## Bud9051 (Nov 11, 2015)

LOL, I didn't say I quit, I said I was out of ideas, or at least conventional ideas.

If you leave a gap but have no lower intake there will still be a small amount of natural circulation, cold air wandering down into those dead end cavities and displacing the slightly warmer air and push to join the air above and reach those higher vents. In addition, any moisture that passes to the ait results in lighter air, just like we think about warm air being lighter. So between a small amount of heat and a small amount of moisture the air will circulate a bit.

A tip for all who bury their attics in lots of insulation, plan on a way to get back uo there without destroying the new insulation. In one large attic I created a raised walk down the center and it sure was handy. In many attics I have been in the kind framers left some 2x6's flat across the tops of the joists. Not perfect but better than trying to find each joist to step on.

Bud


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