I have read the archives, and other articles about insulating my flat and cathedral ceilings. I would like to validate my plans before moving ahead. House is in Florence, MT 59833 at 4000' altitude, so the colder end of zone 6. ICF walls with radiant hydronic slab. The flat portion of the ceilings have raised heel (14") trusses. Plan is to install a poly air & moisture barrier on the bottom of the trusses, 2x4 sleepers below the poly, then 1/2" Sheetrock below the sleepers. Insulation will be R50 blown cellulose on top of the poly sheeting. The center section of the house is a 4:12 cathedral ceiling, made up of 14" TJI. I plan to create an air space under the sheeting by attaching 3/8 ply to the flanges on top of the TJIs, the remaining 12-3/8" of the TJIs will be spray foam, and dense pack cellulose(recommendations on the ratio are welcome). In the end bays of the cathedral portion it gets more complicated. There are 2x6 lookouts across those bays to support the 24" overhangs. How do I vent those end bays? Due to the sheathing method there is a 5/8" gap between the top of the lookouts and the sheathing. If this is enough, wonderful, but I somehow doubt it. How do you vent those end bays? Also, any thoughts on insulating the walls that transition between the cathedral ceiling and the flat ceiling are welcome. I appreciate you guys sharing your experience and expertise.

Question, are ALL roofs involved vented? Or just the cathedral?

As to the tramsition walls.
Rigid foam outside (taped), plywood sheathing/zipwall/etc, spray foam to vapor barrier and dense-pack.

All the roof areas are vented. The sheathing is covered with Titanium then shingles. The Titanium is impermeable, so it requires venting under the sheathing for moisture management.

Strangely enough:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Posted By MTicf on 27 Nov 2017 02:45 AM
I have read the archives, and other articles about insulating my flat and cathedral ceilings. I would like to validate my plans before moving ahead. House is in Florence, MT 59833 at 4000' altitude, so the colder end of zone 6. ICF walls with radiant hydronic slab. The flat portion of the ceilings have raised heel (14") trusses. Plan is to install a poly air & moisture barrier on the bottom of the trusses, 2x4 sleepers below the poly, then 1/2" Sheetrock below the sleepers. Insulation will be R50 blown cellulose on top of the poly sheeting. The center section of the house is a 4:12 cathedral ceiling, made up of 14" TJI. I plan to create an air space under the sheeting by attaching 3/8 ply to the flanges on top of the TJIs, the remaining 12-3/8" of the TJIs will be spray foam, and dense pack cellulose(recommendations on the ratio are welcome). In the end bays of the cathedral portion it gets more complicated. There are 2x6 lookouts across those bays to support the 24" overhangs. How do I vent those end bays? Due to the sheathing method there is a 5/8" gap between the top of the lookouts and the sheathing. If this is enough, wonderful, but I somehow doubt it. How do you vent those end bays? Also, any thoughts on insulating the walls that transition between the cathedral ceiling and the flat ceiling are welcome. I appreciate you guys sharing your experience and expertise.

With 12-3/8" of space for insulation with for the cathedralized ceiling of the 14" TJIs, using all 0.7lb open cell foam (R4/inch) to hit R49-R50 and in interior side vapor barrier would be a lot greener than using closed cell foam + dense packed cellulose.

In zone 6 you'd need at least half of the R to be closed cell foam on the exterior side of the fluff for dew point control at the foam/fluff boundary. That would take about 5" of 2lb foam (R30-R32) to 7.375" of cellulose (R27-R28). Just in polymer alone the 12.375" of 0.7lb foam would be lower impact than 5" of 2lb foam, and when you stack the HFC blowing agents for 5" of 2lb foam against the water use for blowing agent it's a no-brainer. Yes, the center-cavity R-value of the closed cell foam + dense pack is ~20% higher, but the environmental impact is many times higher.

In climate zone 7 you'd need at least 60% of the total R to be closed cell foam to use a hybrid foam/cellulose approach, which would be even worse.

Thanks Dana! This is why I asked the question. There are several ways to skin this cat, and you always boil it down nicely. Can you comment on dealing with venting in the end bays, where perpendicular 2x6 lookouts block all but 5/8" of the ability to vent from eave to Ridge?

I was told 30 years ago by a insulation rep and read several articles over the years supporting the idea that cellulose absorbs too much moisture to be used eith a vapor barrier. All that moisture needs to mirgrate out unimpeaded.

Not sure if this has ever changed.

Also not sure why all the flat roof Iso sandwiches I've been looking at on the Internet show a vapor barrier on top of the metal deck or wood ceiling sheathing resting on timbers if Iso is such a vapor retarder. In addition isn't metal roof decking rather vapor impervious all on it's own?

I was told 30 years ago by a insulation rep and read several articles over the years supporting the idea that cellulose absorbs too much moisture to be used eith a vapor barrier. All that moisture needs to mirgrate out unimpeaded.

Not sure if this has ever changed.

Also not sure why all the flat roof Iso sandwiches I've been looking at on the Internet show a vapor barrier on top of the metal deck or wood ceiling sheathing resting on timbers if Iso is such a vapor retarder. In addition isn't metal roof decking rather vapor impervious all on it's own?

I was told 30 years ago by a insulation rep and read several articles over the years supporting the idea that cellulose absorbs too much moisture to be used eith a vapor barrier. All that moisture needs to mirgrate out unimpeaded.

Not sure if this has ever changed.

Also not sure why all the flat roof Iso sandwiches I've been looking at on the Internet show a vapor barrier on top of the metal deck or wood ceiling sheathing resting on timbers if Iso is such a vapor retarder. In addition isn't metal roof decking rather vapor impervious all on it's own?

The cellulose has to be able to dry either toward the interior or the exterior. So in an UNvented roof assembly it has to dry toward the interior, but must have suffcient R-value in air-impermeable insulation on the exterior side of it to not take on too much moisture when the interior side is moderately vapor open (say, 5 perm standard latex ceiling paint, no plastic.)

In a VENTED assembly in climate zone 7 it will need adequate venting above the insulation but also an interior side vapor barrier (or Class-II vapor retarder) to avoid taking on too much wintertime moisture.

Posted By Dana1 on 14 Dec 2017 11:00 PM
The cellulose has to be able to dry either toward the interior or the exterior. So in an UNvented roof assembly it has to dry toward the interior, but must have suffcient R-value in air-impermeable insulation on the exterior side of it to not take on too much moisture when the interior side is moderately vapor open (say, 5 perm standard latex ceiling paint, no plastic.)

In a VENTED assembly in climate zone 7 it will need adequate venting above the insulation but also an interior side vapor barrier (or Class-II vapor retarder) to avoid taking on too much wintertime moisture.

Good answer Dana1, the moisture in cellulose can be allowed to migrate in both directions, but may on;y have to dry out half as much if it's let into the assembly from one direction only.

Answering my own flat roof question; it looks like as vapor retarding as foil face Iso and roof membranes are, it is still best not to let the warm moist air of the interior enter the assembly in the first place.  Just live with the low rate moisture permeation to the outside verses drying to the inside.

I should note, that this is in contrast to the wall assemblies of a exterior siding, air gap, foil faced Iso over exterior sheathing and 2x studs filled with cellulose insulation with no vapor barrier, and drywall attached directly to studs.  That assembly I just described is proclaimed in articles to dry to the inside.  It is not one in moisture control concept to be similar to flat roofs.

I am still a little puzzled why roofs and walls are treated so differently moisture management wise, always have been.

Net-Zero Home

Just netting on the inside stud face to keep the blown-in cellulose in place prior to drywall attachment.

A roof is not just a tilted wall. Roofs get copious amounts of bulk water wetting, often stay wet for weeks/months at a time with a snow load, and roofing materials are of-necessity always very low vapor permeance. (A #30 felt + asphalt shingle layup runs about 0.1 perms, close to a true vapor barrier.) That doesn't happen with walls.

Roofs also undergo nighttime radiational cooling, and frequently cool to the dew point of the outdoor air making the surface wet, since on a clear night they are facing an very low radiation temperature (deep space, filtered by thin atmosphere). Walls are facing the landscape and horizon, with a much higher radiational temperature, with much less radiational cooling, and rarely (if ever) drop to the outdoor dew point.