What is the best way to insulate a cathedral ceiling? I don't want to use sips but would like to get to at least R49 and form some type of thermal bridge. Asphalt shingles would be preferred. Should I vent it with baffles and spray foam? Should it be unvented?(possible problems later). I am in the stages of planning a new house and would appreciate any advice.

At least consider "AtticWrap" with the primary purpose being to form a baffle/vent under the sheathing for better moisture and ice dam performance. Plus some rigid foam on the interior side to address thermal bridging.

Depend on where you are. I have a 2# foamed hot roof, here in Minneapolis and plan to replace the new shingles in 25 yrs. or so.

Pete, climate is very important in deciding what's "best." Where is the house going to go? Also, what "look" are you intending for that cathedral ceiling - just painted sheetrock, or wood planks over exposed beams? You'll see plenty of opinion as to whether a "hot" (unvented) or vented roof is best, and either can work well if the details are worked out. Here are some thoughts, and I'm sure others will have plenty to say about all of them. For now, I've assumed that in wanting R49 you are in climate zone 5 or 6, and that your ceiling will be painted sheetrock. I'm also assuming that you won't have recessed lighting up there, something to avoid in a heating climate.

To insulate to R49, you need 7-8 inches of closed cell foam insulation (depending on type), perhaps 12-13 inches of open cell foam or dense-packed fibrous material (cellulose or fiberglass fibers). Open cell foam or a dense-packed blown insulation is quite permeable to water vapor, so you would want a vapor retarder film such as MemBrain under the sheetrock or vapor retarder primer paint on the sheetrock, but polyethylene sheet probably should be avoided, especially if the sheathing is not vented. If you want a vented roof deck above the insulation, with soffit and ridge venting, you can install insulation baffles (eg. Accuvent) to keep the insulation off the underside of the roof sheathing and to keep air flow from washing through the top layers of insulation (if fibrous). Another way is to use sections of rigid insulation board, held down from the roof sheathing by narrow rips of insulation glued or stapled to the rafters.

Providing the vertical space needed for the amount of insulation required to get R49, if the insulation is to go under the sheathing, requires tall enough framing members, which could be either sawn lumber or I-joists used as rafters. Another way to get the space is to use scissor trusses, which would give you increased spans and plenty of space for loose-blown fibrous insulation with insulation baffles under the sheathing. If you use sawn lumber for rafters and want to address the thermal bridging through the wood, you can apply a layer of rigid insulation under the framing and screw the sheetrock through that into the framing. In this case, I wouldn't recommend a foil-faced foam layer unless you have venting under the roof sheathing.

If you use closed cell foam, you can apply spray foam underneath the roof sheathing. Another way is to layer rigid foam sheets over a lower layer of sheathing, with all seams taped, screw 2x4 sleepers over the foam and into the rafters, then apply another layer of sheathing with asphalt paper and finally roofing material. The upper layer of sheathing would be vented from soffit to ridge by that 1.5" space provided by the sleepers. With layers of foam over the framing, you have addressed the thermal bridging through that framing.

I haven't covered everything, and opinions will vary, but it's a start.

Thanks for the reply. Dick, What would vary if doing the ceiling with wood planks? I am in zone 5. I also would like some lighting cans in the ceiling if there is a way to do it without any issues. Not sure if I am a big fan of an unvented roof because of possible moisture issues.

The problem with wood planks for a ceiling, rather than sheetrock, is that they don't provide a good air barrier to keep moisture-carrying air leakage out of the insulation cavity; the joints between the planks aren't all that tight, and they open too much as the moisture content of the wood changes. If you want this surface, then you need something above it to be the air barrier. That could be a vapor retardant film material, such as MemBrain. If closed cell foam is used to fill the cavity, that is both the air barrier and vapor retarder. The reason I mention MemBrain (from Certainteed is that it is what they call a "smart" vapor retarder. Its permeability to diffusion of water vapor is low, about 1 perm, when the air next to it is dry, but that increases considerably with humidity, to allow drying to the interior as necessary. It's also a fairly tough film material, and you can seal overlaps together well with the right tape. It is expensive, though. I used it on my upper level ceiling and on exterior walls, and the total cost was I believe around $1,300.

As to can lights, even if you use "air tight, insulation contact" (AT/IC) cans,they still leak some. Also, they are thermal holes in your insulation layer, giving rise to hot spots and considerable heat loss. Most advice on their use is to build boxes of insulation around them in the attic out of rigid foam board, then heap loose insulation around them. A cathedral ceiling makes this harder to do, unless you have scissor trusses and locate the cans away from the walls where there is more depth for insulation around the cans. There also are shallower cans, for CFLs and LED lighting.

I agree with you on preference for venting under the roof sheathing, although there are many who argue that a well built and detailed "hot" roof works as well.

When I spoke about using foam above the lower layer of sheathing and taping the seams on those sheets, I was thinking of preventing leakage from any incidental liquid water getting through the roofing above it, and air leakage from below if this is the air barrier layer and provides most of the insulation for the assembly. If neither is a concern, then the taping of foam seams isn't necessary. A good reference for more information on foam thickness above the roof sheathing vs cavity insulation below it is: http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing.

Lighting cans in a cathedral ceiling are pure misery on a couple of accounts. The light source is further from the areas you're trying to light up, so you end up going higher luminosity, which only introduces more glare (the contrast between the ceiling brightness and the luminaire brightness), only made worse by a low-luster non-white wood ceiling. A far better approach to ambient lighting in cathedral ceilings is use linear fluorescent up-lighting in coves (dimmable or not), lighting up the ceiling evenly, and use lamps/sconces etc for local task & accent lighting . This no-glare approach offers much better visual acuity, even at lower light levels.

In zone 5 a lighting can is also a thin spot in the insulation creating a localized heat leak. At R49 center-cavity R you'll probably be OK, but at R38 you could end up enhancing ice-damming potential with spots thinned out to R15 or less over the cans.

Unvented designs work well in zone 5 from a moisture point of view if you do it right (and Dick Russell has it right). The material of greatest risk is the roof deck, followed by the cold edge of the rafters or truss elements. If you put sufficient R value outside the roof deck it'll stay above the dew point of the interior air most of the winter, and it can be set up to dry toward the interior. For an R49 stackup in zone 5 putting 3.5" of exterior rigid iso (~R20), and 8" of dense-packed cellulose (R29) in direct contact with the underside of the roof deck works, and meets code and you could use a MemBrain type interior vapor retarder to allow it to dry toward the interior (but not poly.)

Putting 2" (R12) of closed cell directly on the underside of the roof deck and dense packing 10-11" of cellulose below it also works, but doesn't meet code. By making the condensing surface the foam, but using foam that is between 0.5 and 1.0 perms @ 2" the roof deck can still dry toward the interior, but the amount of moisture that gets to the roof deck in winter via vapor permeation isn't enough to cause a problem. With a plank interior this too would need a MemBrain type vapor retarder to allow drying toward the interior. See:

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

Last (and not least), using 2x furring through-screwed to the rafters and a separate nailer-deck for the shingles on the exterior to create and EXTERIOR ventilation cavity with a soffit/ridge approach (better if the furring isn't continuous to allow cross ventilation as well) allows you to simply dense-pack the rafter bays and be done with it. Using this approach also works with semi-permeable exterior foam as well (fiber faced-iso, up to 2" of XPS, or up to 4-5" of EPS), since the roof deck can still dry toward the cavity through the foam, as well as toward the interior through the MemBrain. You may want to price out the furring + nailer deck approach using thicker cellulose vs. a 2" of cc foam + cellulose approach. The vented-above approach is far more resilient to roof leaks and the ability for the roof deck to dry toward the exterior even in winter, and purging any winter moisture accumulation very QUICKLY going into early spring is huge.

Dana,

In the over deck vented assembly, is there is any issue with taping the seams on the foam panels when it comes to its ability to dry to the vented surface?

Is there a limit on the PolyIso layer thickness in this case if the seams are not taped?

Okay, just to recap for the vapor-challenged among us (me); When you have an UNVENTED roof deck, use of the MemBrain product above the ceiling covering is indicated where the covering (say wood planking) can't provide a continuous VAPOR RETARDER, such as normally might be supplied by the interior paint primer with sheet rock. If you use entirely closed cell foam to fill the cavity, this isn't a problem as the closed cell foam functions as the vapor retarder. HOWEVER, if you have a combination of insulation up in the bays, such as 2" of sprayed closed cell foam and something loose fill or batted below, you are back to needing the VAPOR RETARDER to prevent interior moisture from accumulating in the loose fill insulation.

So far, so good?

You don't want a double vapor retarded in that case because you will create a trapped moisture issue with no area to dry to.

The planking is obviously loose and will not be an air/vapor tight assembly. The MemBrain is, I suspect, a vapor permeable membrane but is designed to be air tight (i.e. like an interior Tyvek).

The MemBrain and loose fill insulation combination will allow the moisture to move into the insulation space but not hit dew point on the roof deck surface with the 2" of closed cell foam.

I do agree that the over deck venting option is preferable if possible.

Posted By ICFHybrid on 07 Nov 2011 01:23 PM
Okay, just to recap for the vapor-challenged among us (me); When you have an UNVENTED roof deck, use of the MemBrain product above the ceiling covering is indicated where the covering (say wood planking) can't provide a continuous VAPOR RETARDER, such as normally might be supplied by the interior paint primer with sheet rock. If you use entirely closed cell foam to fill the cavity, this isn't a problem as the closed cell foam functions as the vapor retarder. HOWEVER, if you have a combination of insulation up in the bays, such as 2" of sprayed closed cell foam and something loose fill or batted below, you are back to needing the VAPOR RETARDER to prevent interior moisture from accumulating in the loose fill insulation.

So far, so good?

It's the lack of continuous AIR BARRIER of the plank ceiling that's most critical, independent of it's vapor retardency.  MemBrain provides a continous air barrier, but allows sufficient vapor diffusion that the assembly can dry toward the interior during the 35-40 weeks of the year when the average temp at the roof deck at or above the dew point of the conditioned space. 

Vapor barrier paint is lower perm than  MemBrain, but still sufficiently permeable to allow seasonal drying toward the interior.  MemBrain's permeance goes way up during high humidity events, like when the sun beating on the roof deck is cooking the water out of the wood, then drops to a perm or less when it's very dry.  MemBrain will be over 2-perms in a high humidity environment (which can be good  or bad), but at least lets the moisture out quickly when the insulation-air is moist.  In a high-humidity interior (like an unventilated steam bath/shower) it can be too permeable, allowing  too much moisture into the insulation cavity to collect in the roof deck over a winter.  At "typical" indoor humidity levels MemBrain it may leave the roof deck at slightly higher peak moisture levels in high-humidity homes than VB paint, but it also allows the assembly to dry more quickly than VB paint when conditions change.

Closed cell foam is a vapor retarder at 2" or less, but at more than 2" may be TOO impermeable, trapping the roof deck between impermeable roofing materials on the exterior and impermeable foam on the interior.  At 1-2" depth condensation will occur on the surface of the foam when it's cold enough out, but isn't absorbed by the wood- it's on the surface of the foam, and when the average temp at the roof deck warms up in late winter/spring, any moisture that did find it's way in via diffusion through the foam can leave (over several weeks of warmer weather) through the foam.  If the wood were exposed directly to the insulation-cavity air, the moisture is immediately absorbed into the wood, absorbing enough moisture into the wood over a winter that it could take a few months to be rid of it through vapor retardent paint (or a few years through a poly vapor barrier.)

In the no-foam configuration it's the roof sheathing wood, not the insulation that gets wet in a wintertime condensing conditions (at least until the wood is fully saturated). The wood is the coldest surface in the insulation bay, and as soon as it's below the dew point of the air in the insulation, it begins absorbing moisture, essentially regulating the dew point within the cavity.  The bulk of the insulation isn't in direct contact with the roof deck, and thus remains above the dew point. No condensation occurs on the fiber if the fiber is above the dew point.  Cellulose in contact with the condensing surface will wick moisture away from the condensing surface, and store it until conditions warm up, so even when the condensing surface is your 1-2" of closed cell foam, no liquid appears to form, the R value of the cellulose isn't much change and the moisture is redistributed just fine until the moisture level in the cellulose reaches 20% or more by weight.  If a lot of room air is allowed to convect into the cavity, that can actually happen, but with an interior air barrier and dense-packing, it mostly doesn't.   If one used fiberglass there is a temporary reduction in R in the wet fiber layers during condensing conditions, but that is quickly reversed as soon as the surface of the spray foam rises above the dew point of the air between the fibers.

In degree of importance for a zone-5 stackup with no insulation above the roof deck:

1. air-barriers are more important than vapor retardency at the interior layer, but uses something that is at least somewhat vapor retardent as well.

2. making the condensing surface a non-wicking moisture impervious put at least partially (but not too) vapor permeable material slows absorption by the wood without blocking drying completely

3. Use fiber insulation that can buffer the moisture of condensing events without damage or loss of performance, but at an air-retardent density to limit moisture transport via convection within the layer.

It's the lack of continuous AIR BARRIER of the plank ceiling that's most critical, independent of it's vapor retardency.

Okay, good, that's how I understand it.

Vapor barrier paint is lower perm than MemBrain

Here's an issue that has been a roadbump for me. It helps to understand that it actually is permeable despite the term "vapor barrier" commonly applied to it. Can one make a mistake here? For example, does two coats (or more) of paint applied without thinking about the vapor consequences change it from permeable enough to not quite permeable? The "wrong" brand of primer, etc? These kind of things can happen without the GC's knowledge....

Closed cell foam is a vapor retarder at 2" or less, but at more than 2" may be TOO impermeable,

Yeah, another example of "some" being good and "more" potentially being bad.

By poking around in roof structure, I've seen enough stains that are evidence of something occurring, often just once, for whatever reason, that didn't do damage or become apparent on the interior. of the home Just hoping that an unvented assembly has enough capacity to buffer things like that out.......

Thanks, once again for your explanations of these...things. :-)

Vapor barrier primers run about 0.4-0.5 perms on one decent coat. Double that up with 2 heavy coats and you could be looking at 0.2 perms (probably worst-case, but a potential problem.) Two coats of finish latex is typically 3-5 perms, you can go hog wild on it changing colors once/decade for 50 years without making it too vapor-tight. Most oil paints are pretty low-perm though- don't go there.

MemBrain is more forgiving than VB paint on the low-perm end, and since it's generally drier indoors in winter (unless you actively humidify- a BAD idea in mid winter if taking it over 35% RH @ 70F) it slows the transmission of moisture heading out from conditioned space at precisely the season where the moisture might accumulate. Then when the sun raises the humidity in the rafter bay by cooking the moisture out of the roof deck, the it opens up, and releases moisture out of the rafter bay and into the conditioned space at a rate 2-5x faster than it was letting it during mid-winter. It's really not a bad product if you keep the wintertime interior RH reasonably dry- not bone-dry, but comfortable, healthy dry. Above 30% is good for humans, OK for the building. Above 40% you're relying on your stack-up to be protective. 50% RH interiors in mid-winter s too high for most timber-framed buildings, and when you have to start looking at true vapor barriers like 6-mil poly or foil, (<<0.1 perms) and design the stack up to be outward-drying

it opens up, and releases moisture out of the rafter bay and into the conditioned space at a rate 2-5x faster than it was letting it during mid-winter.

And, does good old gypsum drywall tolerate that kind of treatment?

If you are using a concrete wall system, another idea is an ICF (insulating concrete form) for roofs. For an example, see the last three photos on http://www.quadlock.com/insulated_concrete_forms/floors_tilt-up.htm

Gypsum is quite tolerant moisture, and is quite vapor permeable. If using MemBrain the gypsum will always be within the conditioned space. If VB primer, it'll be at the very warmest (in winter) edge of the assembly, and not prone to accumulating moisture. In the VB paint case if there were an extreme summertime moisture drive from the exterior (say you had a small undetected roof leak), and the room is air conditioned there's some potential for condensation on the gypsum leading to saturation and a bubbling of the paint, but the wallboard is still mostly unaffected. With a similar leak in the MemBrain case the summertime interior condensing surface would be the MemBrain, and the gypsum would stay dry.

Does code recognize the substitution of the MemBrain product for vapor barrier primer or paint?

Posted By ICFHybrid on 08 Nov 2011 11:24 AM
Does code recognize the substitution of the MemBrain product for vapor barrier primer or paint?

Codes treat MemBrain as a class-II vapor retarder, since it averages under 1 perm during the cold weather, even though it dries like a class-III vapor retarder during the warmer more humid months.  It's ASTM tested permeance varies by the type of test, and more than an order of magnitude depending on conditions!

http://www.certainteed.com/prod...ion/317391 (click on the technical information tab)

See the curve of permeance vs. RH at the top of p.2 in this document:

http://www.certainteed.com/resources/3028082.pdf

At 50% RH (a healthy summertime interior RH) it's running about 3 perms, and if it's more humid than that in the cavity it'll be even higher.  At 70% RH where mold & rot starts to take off it's a whopping 9 perms (your interior latex paint is more vapor restrictive than that.)  But at a wintertime healthy-interior 30-35% RH it's about 1 perm.

VB paint is also a class-II vapor retarder, but it's permeance is constant rather than variable.  It may let less moisture in during winter, but it dries at less than 1/4 the rate of a MemBrain + latex-painted wall.

I have question about cathedral ceiling design. The house is in Eureka, Northern CA on the coast. When I acquired the house the previous owner was in the midst of a remodel, the cathedral ceiling is open from the interior. The house rafters are 2x12. There is a 2 inch vent channel under the roof sheathing created by attaching rigid foam pieces to the sides of rafters in contact with the underside of the sheathing. Foil faced poly iso was the fit into the rafter bay, shiny foil toward the exterior, paper side on interior. This leaves 9 1/4 inches underneath the poly iso. Is this acceptable practice? If so, what would be the ideal way to finish off this assembly. I worry about condensation of vapor on the interior side of the polyiso. I believe code requires R38 for ceilings. Thanks

Posted By stinkyp1 on 08 Aug 2014 08:05 PM
I have question about cathedral ceiling design. The house is in Eureka, Northern CA on the coast. When I acquired the house the previous owner was in the midst of a remodel, the cathedral ceiling is open from the interior. The house rafters are 2x12. There is a 2 inch vent channel under the roof sheathing created by attaching rigid foam pieces to the sides of rafters in contact with the underside of the sheathing. Foil faced poly iso was the fit into the rafter bay, shiny foil toward the exterior, paper side on interior. This leaves 9 1/4 inches underneath the poly iso. Is this acceptable practice? If so, what would be the ideal way to finish off this assembly. I worry about condensation of vapor on the interior side of the polyiso. I believe code requires R38 for ceilings. Thanks

The thickness of the polyiso matters, and will be the limiting factor on how much fiber insulation you can put on the interior side. Eureka is in US marine zone 4C.  Per IRC 2012 the code-min roof R would be R49, but in that climate zone in unvented assemblies with foam on the exterior side of fiber insulation, code also prescribes that the foam be a minimum of R10 in zone 4C for dew point control at the foam/fiber boundary.

The ratio of foam-R/total-R is what determines the average temperature at the foam/fiber boundary,  so at whatever total R, you need about an R10/R49, or about 20% of the total R as foam.  If you have say only an inch of polyiso, that's about R6, and you can put up to R24 on the interior without creating mold/moisture issues in the fiber layer.  If you have 1.5" (R10) you can put in as much as R40.  

Batts designed for 2x10 framing are manufactured at about 9.5" thickness, and would be ideal for your 9.25" space if the polyiso is thick enough to allow at least R30. But it pretty clearly ISN'T thick enough for dew point control if you truly have 2" of air between the foil facer and the roof deck, given that 2x12s are nominally 11.5" deep.  That 11.5" less 2" of air gap is 9.5", which means if you have 9.25" of space to fill the polyiso is only 1/4" thick(?) which would have an R-value of about R1.5.

If the polyiso is only 1/4" thick it's probably worth yarding it out, then installing 2" of close cell spray foam on the roof deck (R12-ish), which would then allow you to put as much as R36 in the remaining 9.5". 

Alternatively you could spray an inch of of closed cell foam on the existing polyiso and compress a standard density unfaced R30 into the remaining 8.5" of space.  

A third choice would be to install high-density "cathedral ceilng" R38s in there, and use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus between the fiberglass and ceiling gypsum (or skip the vapor retarder, make the gypsum air tight, and use half-perm "vapor barrier latex" primer on the ceiling gypsum.)