Insulating a cathedral ceiling in zone6/7
Last Post 06 Apr 2015 07:51 AM by David_lee. 84 Replies.
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DaveJJUser is Offline
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19 Apr 2014 03:48 AM
Hi

Im building a new house in a zone 6 or 7 climate(not sure...Edmonton AB, CAN) that will have a somewhat complex cathedral ceiling. There are a couple restrictions/limitations: I can't insulate to the exterior and the rafters are only 2 x10s . Im going with unvented due to the limited space and due to the design not lending itself to venting (its basically 4 gable ends...double gable?)

I could just fill the bays with cc_spray foam and be done with it but that is very expensive.  Are there any alternatives to insulate an unvented cathedral ceiling in a cold climate with 10" rafters and no exterior ridged foam?

Could I cut EPS boards to fit between the rafters and fill the gaps with Great Stuff and then cover with latex painted drywall? 10" of EPS would give me R40 along with an air tight seal with the great stuff.

Are there any dangers or drawbacks with this approach?

I have read I cant do flash and batt  because Id need a ton of foam to prevent condensation on the surface of the foam... although Ive never understood why that is an issue. I would think that as long as you dont use a vapour barrier it would just dry to the interior and the foam would prevent the osb from getting wet....but maybe I'm missing something

Any suggestions would be appreciated.

Thanks
Dave



Bob IUser is Offline
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19 Apr 2014 07:46 AM
hang 2x4s below each rafter to create 24" + deep bays and use cellulose or Roxul insulation. The lack of venting is problematic, though and I'd suggest either adding 2xs and a second layer of sheathing about the roof, or at least leaving a sealed cavity above the insulation so the cellulose is not in contact with the roof sheathing.

The best way to insulate a new home is to figure out the insulation methods while you are designing the house; seems pretty common - and backwards - that many people see air sealing and insulation as an "add-on" to whatever you've already decided to build, and are therefore forced to settle for a poor substitute that may or may not work.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
DaveJJUser is Offline
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19 Apr 2014 12:18 PM
Thanks for the reply Bob

Unfortunately I cant increase the size of the bays either. If I could Id just use bigger rafters. Its a bit of a long story but Im up against some very restrictive development zoning bylaws in my neighbourhood. I cant increase the height of the house and I cant reduce the headroom. Basically the laws forbid a 3rd storey but you can get around it by creating living space in the gable. 10" deep rafter bays allow me to create this living space without violating height and headroom bylaws. Some may wonder then why bother with this "third storey" at all? I am also restricted to building a house with a maximum footprint of 400 sqft on this particular lot so in order to get enough living space I have to build up instead of out. So creating cathedral ceiling with very large dormers has allowed me to do this. This is a long story short.

Anyway...I understand that cc spray foam may be my only option in this case but I was hoping someone here had a creative alternative like the one I asked about.

The building code here allows me to build a roof that is RSI 6.0 or R34 so i can get away with maybe the two ideas I mentioned above. I just don't know if they are sound. I realize building to minimum code isnt very green but it is what I have to deal with and Ill be using plenty of other green practices throughout the project....(lots of external foam on walls...passive solar...etc)

Thanks again




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19 Apr 2014 01:35 PM
I am doing similar to what Bob is saying. I am using TJI 14" engineered rafters though. I plan to affix a 1/2" plywood ripped to the width to the top of the webbing creating an airgap to allow drying (the thought is no matter what, one day water will get in there - so future repair, etc is easier). I plan to go with Applegate cellulose. You might be able to do it right to the sheathing with no air gap. The cellulose is much less expensive than blown in.

Also, I have read the blown in insulation could shrink over 5-10 years and create gaps along the edges. Not sure if this is internet "lore" or what, but the cellulose is supposedly blown in with a positive pressure - so it wants to expand rather than shrink.

You could do a layer of ccSPF then batt insulation on the inside toward the house? I'm not sure what R value you need for the dew point control on roof Z6/7 though.

For condensation, it is the concern of getting dew on the inside of the assembly and not being able to dry adequately...
Bob IUser is Offline
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19 Apr 2014 01:56 PM
however you do the roof insulation, it MUST be airtight, so NO wiring or canned lights in the ceiling! We use a fabric membrane on the inside and strap below that, with all our lights in shallow boxes so nothing penatrates the air barrier.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
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20 Apr 2014 03:58 AM
Surfsup

Im don't think is advised to use cellulose right up to the sheathing in an unvented ceiling. As for flashing with ccSPF in my zone I think it as to be 50% of the R value...so at code minimum it would be 3 inches...if I were to go for R50 it would be about 4 inches... that would only leave me with a little more than 5 inches for fluffy stuff which wouldn't be able to get to the other 50% . I know its not real green but Ill settle for R40 for this part of the house.

Bob

There will be no canned lights or wiring. Does it have to be airtight all the way through? If I were to do some sort of hybrid of fluffy and ccSPF it wont be airtight until the ccSPF layer. Is that a problem with an unvented roof?

If I can sneak 1" of rigid foam on top of the roof could I go with 1" rigid then sheathing then 2" of ccSPF and then the rest with fluffy stuff or will the ridged board and ccSPF create a vapour sandwhich around the OSB? It might not be worth the risk to sneak it but an inch might go unnoticed.

Does anybody know of anyone trying my original idea of stuffing EPS foam boards between the rafters and sealing it with great stuff or applying an inch or 2 of ccSPF over top of the EPS? I think there is some discussion on another forum but I dont have access to it.

Thanks
Bob IUser is Offline
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20 Apr 2014 07:45 AM
If you're using a sufficient layer of ccsf, that will probably work for as air seal, but it is not perfect. With the rigid above the sheathing, any air leaks through the ccsf (not uncommon) and less foam than recommended could result in the osb layer rotting. I'd strongly recommend using Advantech as your roof sheathing - it has some resistance to moisture.

There are lots of folks on this and other forums that like the idea of cobbling foam inside stud walls or rafters. I tried it on a small section of roof which I then left unfinished for two years. When I checked it then, the sheathing was wet, so I removed it all, let it dry and installed ccsf. So I do not recommend it.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
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20 Apr 2014 06:13 PM
Applegate recommended to me to blow cellulose to the sheathing. So they feel it is ok. I didn't want to do that - I wanted to mitigate any future issues and with 14" TJIs I have plenty of room so I will sacrifice a couple inches for an air gap. I have a lot more room than you, I understand that, but they did say you can blow up to the sheathing. You can call them if you want. I don't think putting in hard foam is economical (I researched that too).

I plan to just do the 2nd ply layer with airgap, then blow in cellulose and use intelloplus on the inside and be done with it...

The only thing I had a concern with ccSPF is (1) cost and (2) eventual shrinkage that might cause it to separate from the rafters and promote air leakage. Not sure if (2) is a real concern or internet "lore"...
Bob IUser is Offline
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20 Apr 2014 08:18 PM
I believe unvented cellulose against the roof sheathing does not meet the building codes now.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
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21 Apr 2014 11:43 AM
400 SF house what loads are you using for 2 x 10's @ what span and pitch? Did you check the TJI to see if you can use a shallower depth? Or, pitch 2x8's less? If so you can add rigid foam and a thermal brake, have more space design options.
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21 Apr 2014 01:12 PM
To meet the letter of IRC 2012 code in zone 6 or 7 requires R49 minimum center-cavity.  If doing it as an unvented assembly with a combination of foam & fiber without at least a class-II interior side vapor retarder, IRC 2012 requires the foam to be tight to the roof deck, with either R30 (zone 6) or R35 (zone 7).

That would be a ratio of 60% foam/40% fiber for zone 6, or 70% foam/30% fiber for zone 7.  That ratio is good for dew point control at any given total R that you install. If you go better than code you'll need to bump up the foam-R to suit, but at lower R it can be relaxed.

The science behind the code requirement ratio is well established for assemblies with insulation above the roof deck, but it ignores the vapor retardency of foam and hyrgic buffering capacities of cellulose when using foam & cellulose in the stackup.  WUFI simulations by competent building scientists show that even 2" of closed cell foam against the roof deck with the rest of the R as blown fiber is protective in a zone 6 or 7 climate, even with high density fiberglass as the interior side fiber.  See the rows for Minneapolis (zone 6a) and International Falls (zone 7A) in Table 3 on page 11.  Only if you are using a "cool roof" finish material on the north facing pitch would there be a moisture accumulation issue of even remote consequence to an OSB roof deck if you have 2" of 2lb closed cell polyurethane (about 0.6 perms, as simulated.)

EPS is considerably more vapor permeable than closed cell polyurethane.  If you use the cut'n'cobble + seal approach, to be comparable to 2" of ccSPF you need to put in sufficient EPS thickness to get the permeance down to the ~0.6-0.75 perm range.  For Type-I EPS (1.0lbs per cubic foot nominal density)  that could be as much as 7- 8" (R27-R31 which is roughly code compliant on R-ratio too), but for Type-II (1.5lbs nominal density) you'd be good at 4-5" (R17-R21, and non-complaint on R-ratio).
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21 Apr 2014 05:19 PM
Dana, have you read this blog on oc on roofs...... http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/75042/Will-Open-Cell-Spray-Foam-Insulation-Really-Rot-Your-Roof "The two papers were based on computer simulations. In the first paper, Roof and Attic Design Guidelines for New and Retrofit Construction of Homes in Hot and Cold Climates, the authors used HERS BESTEST and AtticSim. In the second paper, A Hygrothermal Risk Analysis Applied to Residential Unvented Attics, the authors used MATLAB and WUFI."

"The conclusion to draw from all this is that it's best to be skeptical of modeling studies not backed up by field studies, especially when they seem to contradict experience. Yes, some homes with open-cell spray foam have had moisture problems. Many more have performed perfectly well."

I "Terry Lee" put some comments on it and I doubt he has data to support "Many more have performed perfectly well" or has a database of assemblies that have been torn apart and inspected. I've talked to Georgia Pacific and Huber Engineers about there products and mating foams to them. If we switched just the current OSB alone and mated it to the same foam we'd have drastically different results. In the coming months both are working on adding fire retardants that will change what they interface to again. Hurber has fire code data on their site, GP recommends a third party spray, I'd recommend Borate. It's not all about perm rating's and code, nor inaccurate models, it is about alot of variations of material property interfaces and chemistry that are not quantified in any model by empirical data.

Bob said it best, take a look at your design constraints, code data Dana posted read it carefully. Try and use foams with no additives other than anti-fungi, and moisture resistance. Huber's product has an outer face sheet of fiberglass, resign, wax, wood, the MDI resign is somewhat fire resistant...should be better than Georgia Pacific's. Huber also has R-Sheathing with polyiso bonded to in if you don't mind rumors of outgasing, at least there the mate is Engineered at the manufacturer by Material Tech Engineers (in white lab coats). Matting that to similar materials such as cellulose (wood based) without the petro, oil, color, plasticizes, bromine, certain foam has will yield better compatibility. If you don't understand that and how to adjust your structure to take out the loads hire an Engineer. A good one should be able to find a solution.



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21 Apr 2014 05:29 PM
Consider engineered I joists vs 2x10s for less heat loss. Some EPS on either side if at all possible and fill with cellulose . Use as little as a 1/4" air gap (far better than none). Or even consider double drain wrap. Keep the interior side well air sealed and breathable.
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21 Apr 2014 06:37 PM
TLP: There is literally an order of magnitude difference in the vapor permeability of open cell foam as discussed by my friend Allison on the Energy Vanguard blog) and 2lb closed cell stuff. There are many existence proofs of 2" closed cell foam being protective in zones 6 & 7, when applied to already dry roof decks. (Pond soaked OSB should be allowed to dry before applying that much foam.)

There are also many existence proofs of plywood (but not OSB) surviving zone 6 climates with R40+ of open-cell only.

The vapor permeance of open cell foam can be reduced to about 5 perms by spraying on "vapor barrier latex" (that performs at about 0.5 perms when applied to paper faced gypsum, but only ~5 perms when applied to open cell foam), which is sufficiently protective for R50 open cell foam and OSB decks in zone 6.

My strong preference would be to use exterior EPS at or above the IRC prescriptive value, combined with cellulose cavity fill (for the hygric buffering), and a smart vapor retarder (Intello Plus, Certainteed MemBrain, etc.) But short of that a couple inches of closed cell foam on the interior, cellulose fill and a smart vapor retarder actually works. Since both the foam and the smart vapor retarder are class-II vapor retarders in winter, the adsorption of moisture by the roof deck in winter is quite slow, but when the roof deck warms seasonally and releases it's load it dries at whatever rate the closed cell foam layer allows.
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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. If manufactured correctly, and I mean IF! The same resign is all over the 787-Dreamliner, and a handful of other business jets outer skins doing fine, and you will not see foam on the outside of them that fly in far more harsh climate zones than the USA, the difference being better control of the IML/OML resign bleed out under high temp and pressure manufacturing. The reason GP and huber are not using a fire retardant is because it causes the wood to swell to the IML/OML and I believe their automated bonding processes produces some wood to the OML/IML, they say the men and women in white coats are working on it. Huber soaks their chips prior to bond which helps. The same resign, wax, cat, is used for both but OSB has less resign and a lower moisture exposure rating than plywood. Once mated with foam fungi additives causes rot and decay of the wood that is resign shy (GP and others without the chip soaking and over fiberglass overlays Huber has). Graphite on the other hand is as much of a moisture barrier and phenol is fungi resistant so you don't see the rot and fungi issues. Phenolic core or foam mates well with phenolic resign. If foam were really needed for OSB/Plywood manufactures would change their resign systems which "some depending on quality" are already not affected by moisture. Use foam to insulation not to protect OSB, well if it is made correctly. Reading the 2012 code is interesting since they don't call out a specific roof sheathing and just make general requirements to them all given the fact the properties vary quite drastically between two GP and Hubard and there are more, that should tell you something. Jonr, agree due to the decreased mass of the I webbing it should bridge less.
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21 Apr 2014 09:09 PM
So all this discussion assumes an asphalt roof? What is concrete tile or something like that is used over a synthetic felt, could the osb dry to the exterior? Would it help?
DaveJJUser is Offline
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22 Apr 2014 12:14 AM
Thanks for the reply Dana
The Alberta Building Code(where I live) states the R for roof assemblies is Rsi6.0 or if I do the conversion correctly R34. The code book isn't online but I took a picture of the relevant page. Here is a link.

I actually read that paper you posted before posting here and was the basis for my comment about wondering if 50% or 60% of R value in foam was really necessary. So are you saying I can get away with 2" of ccSPF and blown in cellulose despite the dew point being a couple inches into the cellulose? It will dry to the interior as I suspected? The other thing to consider is that I am in an extremely dry climate unlike Minnesota or Alaska.

If I can sneak an inch of EPS on top of the OSB is it wise in an unvented assembly to have 1" of EPS on the outside  of the OSB and 2" ccSPF on the inside or should I just skip the EPS? I realize the preferred method is to put a lot of EPS on the top and cellulose inside but I cant make that happen.

Thanks

DaveJJUser is Offline
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22 Apr 2014 12:16 AM
Jonr & TLP

I cant change the rafter sizes at this stage

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22 Apr 2014 01:08 AM
Dave - read also ABC 9.19.1.1
If you use spray foam you might argue as per Appendix A. In all other cases the SCO will probably say no, you need to vent.
See also Zad's last post http://greenbuildingtalk.com/Forums/tabid/53/aff/14/aft/82360/afv/topic/Default.aspx
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22 Apr 2014 03:00 AM
FBBP

I dont have the code book handy but if memory serves me appendix A states that unvented is allowed if the assembly is airtight. Could you quote it for me? Sounds like ZAD had some bad luck. I don't know where Zad is but someone in Ontario took it to court back in 2008 and won. http://www.mah.gov.on.ca/Asset7544.aspx?method=1

Each rafter in this roof terminates at a ridge beam on one end and a valley beam on the other. So there are no channels that I could vent even if I wanted to. That's why I chose the unvented insulation route. It has 4 gable ends. I could leave a space between the insulation and the sheathing but it wouldnt do any good since there is no way of getting any exterior airflow through that gap.



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