Dave - 9.19.1.1. is the same as quoted in the Ontario case except that last part. •••permit the transfer of moisture from the ••• Note that the ABC speaks of moisture while the OBC speaks of air movement. This is significant.
Our code (ABC) also adds an explanation (See Appendix A)
•••recent research indicates that venting of attic or roof spaces is generally still required.The exception provided in Article 9.19.1.1. recognizes that some specialized ceiling-roof assemblies, such as those used in some factory-built buildings, have, over time, demonstrated that their construction is sufficiently tight to prevent excessive moisture accumulation. In these cases, ventilation would not be required.•••

Note that the language "generally" is not exclusive so there would be room to argue that spray foam might make the cut.

In the Ontario case, he wins but he losses. He can use the foam but he much use a 6 mil vapour barrier on the warm side. I don't see this as a good thing.

•••So there are no channels that I could vent even if I wanted to••• An SCO will probably argue that it is up to you to design the roof such that you can meet the code. In other words, you cannot introduce valleys and you need to increase your roof assembly thickness to meet the Code. Just because you want to put in living space does not mean the code allows it.

If you put in collar ties at the top about 16 inches below the ridge to form a small attic in the peak, and then vent this, you might be able to argue that the moisture can migrate up through the insulation to this attic and then vent to the outside.

However if you read Division "A" of the code the Functional Statements for this section also include things like F51 To maintain appropriate air and surface temperature which is coupled with OS2.3 -damage to or deterioration of building or facility elements. This would indicate that the code is still concerned with the life span of roofing materials on an unvented deck.

If you go and talk to the local SCO, you might be able to convince him (her) but you might also open a can of worms. If you do go to talk to them, take copies of the appropriate sections with you, so you can show them how you believe you are meeting the intent of the code.
Good luck and take a couple extra patience pills!

Actually you CAN vent a hipped gable roof, by dropping the valley rafters to leave a vent channel, and installing vents on the ridge. It could also be done by dropping the whole roof and strapping it with 2xs to create vent channels.

Yes, you can but it depends on what you call a hip roof. In this case OP has stated he cannot raise the outside and will not lower the inside.

I cant change the rafter sizes at this stage

The point was that 10" engineered wood I beams are the same size as 2x10s while providing less thermal bridging.

Posted By TLP on 21 Apr 2014 07:38 PM
Dana, actually the phenolic resign in OSB/plywood does not need dew point control, it is a vapor barrier.

Sure it needs dew point control- if not to save the OSB, to keep the insulation functional.

Having an exterior side vapor barrier on an unvented roof without at least some amount of air-impermeable insulation at the roof deck risks saturating the outer layers of insulation over the winter.  It probably doesn't take the full IRC prescriptive R ratio though.   With an unvented assembly you can't put a true vapor barrier on the interior side without creating a moisture trap, and you still need to control where that moisture goes.

Sure it needs dew point control- if not to save the OSB, to keep the insulation functional.

Having an exterior side vapor barrier on an unvented roof without at least some amount of air-impermeable insulation at the roof deck risks saturating the outer layers of insulation over the winter. It probably doesn't take the full IRC prescriptive R ratio though. With an unvented assembly you can't put a true vapor barrier on the interior side without creating a moisture trap, and you still need to

That depends on material properties. There are toxic materials such as processed foams I would not want to inhale, completely sealed for health hazard reasons and if code prohibited that I would not use them. Venting to a very hot, humid, low wind environment can cause ALOT of harm to certain products that are not design for it.

Roxul is made of lava rock wicks. Takes up to 2250 F, is non-combustible, no smoke (try burning stone), and is a naturally water repellent, insoluble, and enhanced with oil additives in the binders. Stone wool allows trapped vapors in a roof assembly to disperse throughout the insulation layer and dry out, effectively maintaining moisture control in sealed assemblies. No r-knock-down-factors from sag (as in processed foams and installations r-values that peak @ 75 F, drops in cold/heat due to blowing agents and binder out-gassing, or that are not dimensionally stable causing mating gaps from thermal expansion) as high as 35% loss with glass fibers government test show. The rigid boards for commercial roofs are strong enough to walk on hot roof ready, are inert. Comfortboard IS residential still needs sheathing, won’t toxic chemical outgas and lose r-value over 20-30 years like XPS/poliso(as high as 10-20% loss) and is made of air only. Test show r-value increases in cold. Low emissivity Greenguard certified for schools) low eura-formaldehyde for IAQ or cancer causing C or BFRs, uses low emmitivity phenol plastic binder compatible with OSB/plywood(Avantech’s fiberglass/phenol/pulp, moisture isolation ply compatibility) . Fire resistance mates well to OSB/plywoods with no fire retardants. Rigid fits tight to minimize air flow. It has been fungi tested no spore attraction or growth, hail impact resistance, and sound resistant. This assemble does not need ventilation and is great for cathedral ceilings with an r-value of 40 in 2 x 10 rafters. In my area cost around $.20/sq-ft more, easy ROI with less labor, better IAQ no potential law suits, less maintenance, lower long lasting energy bills, etc...

Sealant:Knauf EcoSeal (need high pressure paint sprayer) . Good, tapes and mastics too. MUCH better than spray foam.

One could easily build a small test sample. Drill water holes, section cut through them see the water migration and damage if any.

Dana

Was that last part of you last message for me? Putting an inch of EPS foam on the outside of the sheathing with 2" of ccSPF on the inside it would create a moisture trap? If so ill skip the EPS and just go OSB-->2 or 3" of ccSPF-->cellulose-->membrain-->drywall.

FBBP
Im pretty sure there are unvented roofs being built in Alberta with just ccSPF. I wouldn't use poly but rather the membrain stuff Bob and Dana are recommending. For what its worth I have my permits already and construction will begin in two weeks.

Jonr
Sorry it was the other poster(TLP?) who suggested the smaller rafters. I originally designed the house with 10" TJI joists but the framer, joist supplier and project manager all recommended dimensional lumber in this specific case due to the complexity of the roof design. Not that it cant be done with TJI..just that it would be more work and a PITA

Bob
My valley beams were sized by an engineer so I'm not sure I can make them smaller in order to create a channel. If I can make them smaller the channel would only be about an inch high and 3 inches wide at the 4 valley beam ends. So very approximately 12 sq inches total for the whole roof at 4 entry points. Would that be enough?

Your 2nd idea is also possible. The roof is designed with 1/2" OSB. I could drop it to 1/4" put the sleepers on that and then another1/4" OSB. If I only put a 1 inch gap between the two layers of sheathing then I might be able to get away with it. I dont know if the engineer would go with the 1/4" osb then sleepers then another 1/4" osb. He originally designed it with 5/8" OSB but was ok with 1/2". Im guessing 1/4 + sleepers+1/4 might give it the same strength(or more). With this setup I'd only be drying the OSB to the outside correct?(id still have ccSPF on the inside)

If I went this route Id still treat it as an unvented roof with 2" of ccSPF but add the small gap for safety. How small can I make the gap if I make the assembly airtight(ccSPF, etc membrain...ect)? The pitch is 8:12. It's not a gabled hip. The best way I can describe it is you take two gabled roofs and intersect them perpendicularly. 4 ridge beams and 4 valley beams all meeting at the middle. (its a little more complex than that as one of the gabled ends is actually 3 gables of different sizes and setbacks) . So if you look straight down on the roof, 4 rafters form larger and larger squares until you get to the end rafters.

btw Thanks to everyone for the replies.

DaveJJ:  A mere 2" of closed cell foam is not a moisture trap, since it's about 0.6 perms, a class-II vapor retarder to be sure, but still about 100x as vapor permeable as 6 mil polyethylene.

TLP: I can appreciate you're wanting to avoid the outgassing of foam, but that has no bearing on whether it's safe to go unvented with ANY OSB product without at least a class-II vapor retarder on the interior.  I'm a big fan of rock wool too, but it's still air permeable, and it doesn't matter that it doesn't wick liquid water, it will still get performance robbing condensation or frost in the outer layers of the material if it has an exterior side vapor retarder as low-permeance as OSB + #30 felt.

Perhaps just keep the roof deck breathable with plywood and this.

Apparently (see Building Science Corp) gaps as small as 1/8" (or even double drain wrap) can work, but 3/8" is a common minimum gap.

A 4 sided gable roof makes it less clear that there is a straight upward path for air to flow and so more might be needed.

Jonr

At 1/8" I wonder if one could get away with heating up a garden rake and running it across rigid foam to create channels or grooves and then place that side against the OSB.

Various companies make grooved EPS and other things (like mesh) to create drain channels/vents against a foundation or wall. Exactly how much is needed to allow sufficient air flow - I don't know. But some ability for moisture to escape to the exterior (cold climate) is better than none. Especially in a roof, where leaks are a question of when, not if.

As Straube says "when you add the three components: ventilated cladding, plywood, and dense-packed cellulose, you have reduced risk tremendously".

Dana 1: “TLP: I can appreciate you're wanting to avoid the outgassing of foam, but that has no bearing on whether it's safe to go unvented with ANY OSB product without at least a class-II vapor retarder on the interior.”

You missed the main point of my last post. You design to code and perm rating which is short sited. There are many other material and design parameters to consider along with test/field data, no WUFI here. Avoiding health risk from outgassing is one parameter of many. The main issue is fungi that can rot wood and potentially collapse a roof, that and keeping certain insulation with fungi additives dry. Again, the products that do not list in the spec or MSDS sheets as being “anti-fungi or promoting” or hold a certification such as Greenguard are at risk for rotting major structure, especially in enclosed cavities.

The taped ZIP roof sheathing deck (exterior, OML) is an air barrier (per ASTM E2357) AND CL 1 moisture barrier (passed ASTM E331(no “Water Penetration”), zero less than 1 perm). The CL D bonded WRB has a 12-16 perm rating per ASTM E96-B “Vapor Transmission” to protect cladding to the deck(OML).

Note the spec difference between “Water penetration” or “barrier”, “Vapor Transmission” or perm(where the perm applies to “WRB overlay” only not through the OSB by vapor diffusion or any related theory that has to do with “solar vapor diffusion or penetration” , not going to happen per test data. Another is Moisture Capacity (MC), storage, or absorption %/volume. When MC capacity is exceeded it passes moisture to the substrate materials. Modern assemblies put faith in wetting control using many hydrophobic materials incapable of storage, which can limit drying potential. MC threshold before fungi can begin to develop in wood is 28% MC or 95% RH. Roxul, ASTM C 1104 Moisture Sorption 0.03%

Two entirely different mating material parameters above and below the OSB air/moisture barriers perm rating alone or code does not always account for, since it does not understand the specific performance of this assemble. Below OSB, IML ,

Dana 1: “I'm a big fan of rock wool too, but it's still air permeable, and it doesn't matter that it doesn't wick liquid water, it will still get performance robbing condensation or frost in the outer layers of the material if it has an exterior side vapor retarder as low-permeance as OSB + #30 felt.”

Actually, it does wick and repel water by capillary action( see definition below) that promotes vapor drying over a larger area, it won’t bead up or accumulate water or condense as easy. It is moisture vapor 30 permeable, meaning that, “it will not absorb (.03 rate), but it if does get wet, it will dry out (evaporate, not freeze) and maintain its R-value” (from website). MSDS: Insoluble (H20) (therefore vapor pressure, density, evaporation rate does not apply (nor does a r-value knock down or perm rating) and is not required to be tested by government and Greenguard standards): chemically not-reactive, will not promote fungi, no tested health effects. If you have data to prove all this wrong please post it?

With those material properties and interfaces I’d vent this assemble to the interior where there is humidity and temperature control, install Roxul batt and loose fill to get R40. The dimensional lumber is the risk for fungi/rot, it can be borate or pressure treated. Venting this assembly to interior poses little health risk. I’d leave a ¼ inch air gap for drywall under the rafters and install vents that act as inspection holes too. To pass 2012 IC, which makes no sense for this assemble, you may need rigid board to the exterior of the OSB. Venting to the interior will keep the assemblies low cavity pressure flowing to conditioned air. Gravity and capillary action of any moisture will dry out and vaporize to the interior side chase from each bay, then to perimeter vents and never freeze insulation. I’d seal the upper rafters to the OSB with eco seal or safe product, not SPF.

If you want to use toxic foams and fluffy fiber, you could put holes in the rafters to design an external ventilation system. You can create channels in foam with a torch, ICF and SCIP do it often. Air flow depends on the size of the channels (holes) in the rafters and foam, would best diverge the channel hole size along the slope/assembly to maximize exhaust velocity and pressure differentials, hole diameters are a guess and experimental. You’d have to replace the wood rafter holes volume with a doubler of the same volume and fasten according to PE requirements. Now you are more susceptible to moisture, dew point, fungi/rot, freezing and r-value knock down.

______________________________________________________________________________________________________________________________________________________________________________________
ROXUL COMFORTBATT® will not absorb or retain water in the event that moisture does get into the wall assembly.

Capillary action (sometimes capillarity, capillary motion, or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquified carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container act to lift the liquid. In short, the capillary action is due to the pressure of cohesion and adhesion which cause the liquid to work against gravity.

Water can and will condense onto rock wool fiber when the fiber temp hits the dew point of the entrained air. If the air does not have a sufficient vapor retarder between the rock wool and interior, sufficient water can and will end up in the coldest layers of the rock wool to impact thermal performance. If it's not vented to the exterior side you'll end up with an ice/frost layer at the rock-wool/OSB boundary that will become liquid when temps rise- that water is going somewhere as long as gravity still works, no matter how non-wicking the rock wool might be.

OP lives in Edmonton. Edmonton's outdoor 1% design temp is -34ºC. Any moisture/vapour entering the assemble will freeze when it hits the freezing point in its travel through the wool. No, it won't start rotting any of the structure above that point at that time because it probably will remain frozen till April! However the area below this point can be susceptible to damage and the whole assemble will be in trouble if there is no venting to let this moisture out when it thaws.

We are dealing with a very high delta t and it will cause different behaviours of the vapours then many of you are use to working with. This is why the Canadian codes say that it MUST be vented. It also calls for the 6 mil vapour barrier because any moisture traveling through the wool will freeze prior to hitting the air space. This, as Dana so correctly puts it, will "impact thermal performance". If there is a high moisture load all winter, the frozen portion will move lower and lower through the wool as the thermal performance above it is impacted. We are often lucky in that the dry air in Alberta will forgive a multitude of sealing sins.

In older houses, (prior to 6 mil poly), I still get calls saying their roof is leaking whenever a chinook blows into town. Well, no, your roof is not leaking, it just the stalactites in your attic melting!

Of course having an asphalt roof and a 6mil plastic vapour barrier is going to cause a major problem in the long run if the space is not vented.

Dana1: Water can and will condense onto rock wool fiber when the fiber temp hits the dew point of the entrained air.

"Dew Point" is often misused, it is the temperature at which vapor pressure in the air at constant ‘barometric pressure’ condenses into liquid water at the same rate it evaporates. In a vented vaulted assemble, we are not dealing with barometric pressure or dew point, we are dealing with dynamic pressures. At a given temperature, but independent of barometric pressure, the dew point is a consequence of the absolute humidity. We are concerned with the absolute humidity or MC capacity of air and the material properties in contact with air (EG: MW=.06/volume). When both are 100% condensation and "dew" forms that can freeze on the material or in the material that is a function of air pressure and temp, material temp and properties, as well as absorption rate.

Dana 1; If the air does not have a sufficient vapor retarder between the rock wool and interior, sufficient water can and will end up in the coldest layers of the rock wool to impact thermal performance.

Rock wool is not thermal conductive or has “cold or hot spots”, as a matter of fact it has a flame spread of zero(see MSDS). It will not freeze since it cannot absorb water (see MDSA freeze point is “N/A”).

Dana 1: If it's not vented to the exterior side you'll end up with an ice/frost layer at the rock-wool/OSB boundary that will become liquid when temps rise- that water is going somewhere as long as gravity still works, no matter how non-wicking the rock wool might be.
Rock wool is made of lava rock or iron slag neither absorb significant amounts of water(.03 sorption MSDS) or condensation to freeze. I’m not sure why you think it will “end up with an ice/frost layer at the rock-wool/OSB boundary” I did ask for some data to back your theory in my last post I noticed you ignored. Think of rock, as in a rubble-trench(clean compacted stone) footing below frost lines used for centuries vs. concrete that needs moisture protection. For example, compacted stone in the same manner addresses the issue of frost protection on footing walls by ensuring that no water can collect below the structural bearing points of the building therefore, there is no water to freeze and heave. Water collects below frost lines to a perforated drain pipe drainage system. Mineral wool also can be used as drain board on concrete and pipe to keep them from freezing.

I agree if you are seeing this much freezing in the wall assemble draining it to 6 ML poly where it will accumulate, leak to interior space, rot lumber, requires a detailed drainage system(holes in rafters, broken insulation, 2+” perforated piping, etc, to a gutter system.) I think it is safe to assume the exterior ventilation system is at risk too needing an expansion chamber.

If the wall cavity(s) are freezing to a point of significant accumulations the design has more issues than R-value loss mainly from convective loop loss. Since frozen water expands 9% by volume, the amount of force that would be exerted on structure is 150 tons psi, enough to move an entire building at the foundation. The cavity bays should be capable of managing burst pressures or the assembly will fall apart, burst at the seams!!

FBBP: Any moisture/vapour entering the assemble will freeze when it hits the freezing point in its travel through the wool.

There is no condensation traveling through the wool. Please post some data showing the direction of travel, freeze point?

FTTB: No, it won't start rotting any of the structure above that point at that time because it probably will remain frozen till April! However the area below this point can be susceptible to damage and the whole assemble will be in trouble if there is no venting to let this moisture out when it thaws. It also calls for the 6 mil vapour barrier because any moisture traveling through the wool will freeze prior to hitting the air space.

The design I described vented to interior is that not allowed in CAN? Read above if you are freezing the wall cavities your need an expansion chamber and drainage system. That is a bad design. I’m not sure where you are draining it but, I hope it is more than the bottom of the bays where water puddles will completely destroy the drywall screws in weight, untreated lumber, insulation, etc, in addition to destruction by massive burst pressures from ice.

Can you please post some data of Roxul freezing in this application? At best it will have some surface freezing relative to its low sorption .03 rate with negligible loss to r-value as I already stated in my last post. Roxul has A LOT of data on their website by third party testing that contradicts your “theories” .

I would not design an assemble subjected to ice bursting, and I would not add insulation cost to compensate for an r-value knockdown that does not exist. I don’t know where these knockdown values come from that have no credible source, other than people pulling them out of their hat. They would have to be tested to the many parameters that exist in an specific assemble, come from the manufactures third party testing per a ASTM/ISO, not from a bunch of people’s opinions or gut feeling.

I'm glad you guys are discussing this. Thanks to all of you for affirming my plan to install venting in my cathedral ceiling (Zone5)...Should be a good roof with R45 below the venting and air sealed as good as possible...

Are you going to use plywood or OSB between the ceiling and vent?

Yes, the stackup is as follows:

metal roof
#15 felt
wood sheathing
TJI 14 rafters
wood sheathing (underside of top "I" between TJIs) creating 1-3/8"
cellulose
intelloplus smart vapor retarder
drywall

TLP - where do you live?

FBBP – Currently in MI, I have building’s in the Midwest and SW and have lived all over the USA. Not that it matters or changes the design parameters or material properties.

http://www.applegateinsulation.com/Product-Info/Technical-Pages/249234.aspx

"Our recommendation for the installation of (cellulose insulation) is not to have an air space above the material in a cathedral ceiling application. This is based upon 36 years of experience and approximately 16,000 homes insulated in the Mid-Michigan area (with a separate contracting entity)."New research in building science has confirmed that the tight blown, unventilated cathedral ceiling method is best."

I found yesterday on Surfsup thread a good read and validates my intuition. Let’s look at the 3 main statements in more depth,

1.There is the traditional moisture control method in which insulation is installed on the bottom of the cavity and a ventilation area is left on top of the insulation so that moisture moving through the insulation enters the ventilation area and is transported out of the cavity. The traditional control method was developed in an attempt to deal with moisture because vast amounts of air move easily through what has been the most commonly used insulation material – fiberglass. This warm moisture-laden air is drawn through the fiberglass and is cooled until it must "dump" the moisture it can no longer hold. This is usually on the strands of fiberglass as beads of water form on the insulation, similar to the effect seen on a glass of iced tea on a pleasant summer day. This propensity of fiberglass to "bead up" moisture that has condensed.

TLP – This method places a lot faith and assumes complex air is moving moisture in and out of a cavity in equal and/or larger amounts. The ventilated air stream causes a pressure drop above the insulation, or when air velocity goes up, pressure goes down, so a suction is created to pull moisture out of the insulation but, how effective is the question. The cavity and ventilation is experiencing turbulent flow, it is not clear all the moisture entering will be expelled to the ventilation flow. If the ventilation system size cannot overcome turbulence, the laminar flow decreases stops moving moisture out, vent path pressure increases, and suction decreases (as I stated above, a divergent ventilation duct would keep velocity high but, the taper would be labor intensive). The cavity is now experiencing conductive heat loss, “r-value knock down” as I said, and that occurs all year not just in winter. If the ventilation system malfunctions due to improper sizing, the glass beads up, or freezes, drops to the drywall, when thawed, and is in need of a drainage system as I said. At least the loose fill FG allows for an expansion chamber. If your ventilation system includes venting through OSB/Plywood’s phenol resign, think again, it won’t. You need furring strips on rafters so air is in direct contact with insulation and directed to a high flow duct, and may need to be large to get the air flow, at the same time may compromise structure by bending moments, and allow to much moisture at the intake and exhaust that can freeze and create dams. If air is not moving moisture from one point to another to prevent condensation, or water is not flowing, it will freeze. Mineral wool drain board, as in a compacted rock footing’s or wall cavities, keeps water moving if there is any. In a well sealed cavity, small amounts of water would be evaporated by conditioned indoor air with no air barrier on the ceiling covering. You can see there is no constant barometric pressure driving dew point, rather dynamic pressure and temperatures, absolute humidly, in the cold upper uninsulated cavity where likelihood of condensation would freeze, further obstructing ventilation efficiency.

2. So that very little moisture enters the cavity to start with, and eliminates the need for ventilation. This method stops most harmful moisture from ever penetrating the ceiling cavity, as opposed to simply trying to control it once it is already there. Although many view this as a new method, as stated previously, many contractors have been doing this for years. In fact, as far back as 1975 NCIMA standard #N-301-75 stated: Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage, it may not need a vapor barrier, as defined.

TLP – I already explained this in depth. I prefer mineral wool over cellulose, not only for its better performance, but the fire retardants such as bromine vented to interior space has health issues and can cause rot of the rafters. It is easier to work with and obtain a tight fit in the cavity that relies less on skilled labor or filling to spec density that is manufactured by the contractor, no settling or sag worries. I’d borate spray the rafters.

3. “Contrary to common belief, attic or roof ventilation in cathedral ceilings appears to have little or no effect on shingle or sheathing temperature.”
“tests at the Florida Solar Energy Center showed no more than three- to five- degree difference in sheathing temperature between vented and unvented roofs…" and "although ventilation appears to have little effect on roof surface temperature, it may have a significant effect on attic air temperature and heat gain into the house.”

TLP: The phenol resign in the sheathing is acting as a thermal bridge to the OSB wood chips or plywood veneers. As I said, if your plan is to air, moisture, venting to sheathing in your design…#1 most OSB has a class 1 exposure rating and cannot take it, Georgia Pacific has an “Exterior’ rated plywood that can, or use Zip it has an air and moisture barrier rating when taped correctly. None the less, #2 the phenol is not air or vapor permeable check the manufacture spec sheet. All designs should be to the manufactures spec data.

I need to take a close look at SIP/SCIP next