Looking for advice on the best practice to air seal a vented cathedral ceiling. Framing is with 2X12 and will probably have either high density batt or blown fiber glass.

Thanks

Drew

Consider sealed drywall or MemBrain, taped rigid foam, stabilized cellulose and building wrap (draped into the gap).

I'm not sure how you'd be able to use a blown high density fiberglass or stablilzed cellulose in a vented cathedral ceiling without risk of compromizing the ventilation gap. Both work measurably better at higher than loose-fill densities, and low density fiberglass without air-barriers on BOTH sides has very low performance at the temperature extremes.

If batts, rock wool is priced comparably to high-density fiberglass batts, has better fire resistance & fewer indoor-air quality issues.

In most of the US it's possible to go unvented without interior side vapor retarders safely using 1-2" of closed cell foam against the roof deck and dense fiber filling the remainder, which is easier to make air tight than an air-tight gypsum approach. See:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

"In most of the US it's possible to go unvented without interior side vapor retarders safely using 1-2" of closed cell foam against the roof deck and dense fiber filling the remainder" in climate zone 6, the code requires 4" of closed cell foam against the roof sheathing

Posted By Bob I on 11 Dec 2012 05:15 PM
"In most of the US it's possible to go unvented without interior side vapor retarders safely using 1-2" of closed cell foam against the roof deck and dense fiber filling the remainder" in climate zone 6, the code requires 4" of closed cell foam against the roof sheathing

There's code, but there's also what actually works, and this is one where the IRC has it wrong(!).

The IRC requirement for dew point control using insulation above the roof deck has some real science behind it, but there's no comparable factual basis for specifying exactly those same R-values when putting low-perm foam on the underside of the roof deck. (This was the whole point of doing the WUFI simulations for RR-1001.) Not only is the IRC out to lunch on this, they're having lunch at the Ritz, at your expense, if you follow it to the letter.

Putting 4" of closed cell foam on the underside of the roof deck puts a ~0.25 perm vapor retarder under the deck, and a felt + shingle layup on the exterior is something like a 0.1 perm vapor retarder.  With that sort of stackup there the roof deck can't dry at a reasonable rate in any direction, which becomes a resiliance issue.  With 2" of foam  you have about a 0.5 perm vapor retarder, and the WUFI simulations show that the roof deck is reasonably protected from wintertime load even with light-colored metal roofing on a north-facing pitch, even in International Falls MN (zone7) as long as the interior humidity is reasonably below 40%RH @ 70F in the dead of winter. With composition shingles it's protected even in the higher-humidity profile used.

See the short-sheet of results on Table 3 p.12 (.pdf pagination). The X/Y numbers are simulations at high/normal humidity profiles, which are described earlier in the document.  The simulations used 1.2 perm @ 1" foam, so at 2" it would be running ~0.6 perms.  A "0" score means that the moisture content of the roof deck never exceeds 16%  at any point during the year.

Condensation at the interior surface of the 2" foam during hours when it's below the dew point of the interior air still doesn't reach liquid & dripping proportions in the fiber as long as the fiber is dense-packed and the interior gypsum is reasonably air tight.  If one uses cellulose rather than fiberglass it would be adsorbed and re-released by the cooler part of the cellulose even if it wasn't perfectly air-tight to the interior, but making the gypsum as air tight as possible is still a very good idea.

This isn't just trivial academic stuff- if you can use half the foam and achieve a more resiliant assembly for less cost and a lower net greenhouse gas hit from the HFC blowing agents, it's worth it. 

And unlike some garbage-in-garbage-out practitioners out there, I trust John Straube's facility with the use of WUFI.

I'm not sure how you'd be able to use a blown high density fiberglass or stablilzed cellulose in a vented cathedral ceiling without risk of compromizing the ventilation gap

Like a wall - damp sprayed in (from above) then scraped (or pressed?) down to the desired thickness. In this case, at ~2" below flush with the framing edge to create the gap.

An interesting case of "you don't need venting" not working is here. Note that this has to do with snow cover (as little as 4") and doesn't involve very cold weather or condensation. Best to vent where you get snow accumulation.

So you're saying the gypsum goes up before the roof decking to be able to compress the air gap?

While the MI ice damming case is of some interest, the author states:

 "... I estimated the overall effective R-value of the roof to be 30, ±4. While this is slightly lower than the Michigan code requirement of R-38, ..."

Even in a vented roof situation an R30 roof will non prevent ice dams in the MI-UP, and in a composite built-up roof (rather than a membrane) the saturated slush under the snowpack will also saturate the roof deck.  

Ice dams are more of a warmer-wetter winter phenomenon than a coldest weather issue, since that's when the roof deck can reach and sustain 32F for long periods of time- when it's -5F out even the R30 is fine. (The modeled and calculated tipping point with the given snowpack was +10F, which can easiliy be an early-January week's average temp in the UP.)  It's not surprising that the problem in the article exhibited itself.

In the on-the-temperature-margin maritime slush factory called "eastern Massachusetts" where the temperature ranges cited in the article persist for most of the winter, empirical observations indicate that it takes ~R50 or higher to reliably suppress ice dams, but at R50 it seems to work whether vented or unvented, heavy snow load or light, but I'm sure there will be the once every handful of decades condition where that too doesn't work completely.

Venting  roof decks does indeed add resiliance in less-than-ideal situations, and can significantly mitigate the formation of ice dams during ice-damming weather.  Still, R49 is IRC 2009 code-min in the MI-UP, which probably WOULD have cut it even in the unvented condition:  

http://energycode.pnl.gov/EnergyCodeReqs/?state=Michigan

And 2" of R6/inch closed cell + the remainder in R3.7/inch cellulose in a 2x12 cavity probably would too, despite only hitting around ~R45 @ center cavity instead of R49.

The proposed solution in that article is a reliable one that I've recommended on this forum multiple times- add exterior foam above the roof deck (bringing the total R up to code) with a furring-mounted vented nailer deck above it.  While it may not prevent the ice damning every time, it prevents it most of the time, and prevents the damage when it does occur. (It's what's going on my central-MA house when it's time to re-roof over the paltry 2x6 rafters.)   While it's more expensive than a membrane approach, it's more resilient.

BS writes here:

You can’t build unvented roofs where it snows a lot even with super insulated roof.

Then they go on to say maybe with R60. Ice dams are so damaging, I'd err on the side of venting.

So you're saying the gypsum goes up before the roof decking to be able to compress the air gap?

At least the rigid foam (under the rafters), gypsum could come later. I'll agree; gypsum, dense packed cellulose between the joists, rigid foam, furring, decking might be easier with the only extra cost being the 2x furring.

Posted By jonr on 12 Dec 2012 12:52 PM

BS writes here:

You can’t build unvented roofs where it snows a lot even with super insulated roof.

Then they go on to say maybe with R60. Ice dams are so damaging, I'd err on the side of venting.

And yet, the roof at Joe Lstiburek's own house in Westford MA, is a reasonably effective kludge: 

He started out with 5" of closed cell foam between the rafters when he finished out is attic because he wasn't ready to re-roof. He claims it never got ice dams even in that configuration, but he got embarrassed by the visible rafter-striping on frost days, so he added a membrane over the top and added a bunch of rigid foam, without benefit of a vented nailer deck- go figure! (He's a slow learner?)  

So far it's been working, at about R60, but again, by his recounting it had also worked with R30 between the rafters.

But with a fully lapped fully adhered membrane covering the whole thing even if it dammed once every quarter century, it wouldn't be a damaging event.  In his setup he has created a moisture trap at the roof deck though- nothing much dries through 5" of 2lb foam.

He also did his barn that way, but without the spray polyurethane, still unvented.

A case of "Do what I say, not what I do, unless I tell you something different later on?", mayhaps!?!

But it's clearly not as simple a model as Jeffrey Hoffman's analysis might lead you to believe.  I've yet to see an R50 roof with an ice damming problem in this VERY ice-dam prone region, but it wouldn't surprise me if there is at least one counterexample out there to prove the rule. 

I've also seen a number of houses effectively "cured" of problems with 2" of XPS over the roof deck at total R values still a bit under R50, without benefit of venting. In my own house I have have a soffit-to-ridge vented R38 addition on one corner that still gets ice dams occasionally, though they're pretty small compared to the thinner cathedralized ceilings in the original 1923 house (I'll be dense-packing over all vented fiber-insulated rafter cavities before adding the exterior foam.)

As I said, cost notwithstanding I'm going to vent the nailer deck when I do the foam-over on my house, whether it would really need it or not. (I've got some roof valleys practically designed to collect a lot of snow when the wind is out of the northeast during a storm, which is a common heavy-snow event in this region. One is even impeded at the bottom with poorly placed masonry chimney.)  But whether those precautions are really necessary in our original poster's case isn't entirely clear.