[Bud9051]

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

 

[Bud9051]

@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

 

[Bud9051]

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

 

[Bud9051]

No pictures as yet.

 

[Bud9051]

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

 

[Bud9051]

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.

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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.

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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

 

[Bud9051]

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