Hello: This spring we will be insulating a 2.75" thick T&G cedar roof. There is tar paper & shingles on the outside. The inside is the unfinished cedar cathedral ceiling and is to be left untouched - all work is to be done to the outside surface. What would be the recommended approach to insulating this roof? The house is located north of montreal, Quebec. Note: this is an old (1965), single-story Panabode house (38'x20') over a 4' heated crawl space) (http://panabode.com/) so the whole house is constructed on 4"x2.75" T&G milled cedar - there is no insulation in the walls and none is planned. Only the windows, doors and roof are to be upgraded. Any suggestions would be appreciated.

best way is to insulate on the exterior - minimum of 8" of 2" polyiso insulation, with 2x4 strapping above and roof sheathing above that. The insulation is available used to keep the cost down. The thick cedar will be perfect to fasten the 2x4's to, through the foam. Since the cedar's continuous, it'll be a snap. Air tightness is critical, so take the joints and caulk the outside edges of the cedar.

Thanks Bob. :) Are you saying that it is critical to keep air from getting between the foam and the cedar? Thanks, Rob.

if cold air gets between the insulation foam and the cedar, that will negate your insulation, so yes.

One other issue sometimes overlooked - the joints between the cedar boards are conduits for air, so be sure to seal them at the exterior walls. Ideally there will be solid insulation or an air barrier where the boards terminate.

OFF THE SITE:

Western Red Cedar contains an abundance of natural preservatives that are toxic to many wood-rotting fungi, making it impervious to weather and pestilence. These innate characteristics make Cedar ideal for any climate, as mildew and insect infestation are easily prevented without introducing man-made preservatives.

the story of western red cedar
Western Red Cedar is also one of the best thermal insulators among softwood species used today. This is because Cedar is an exceptionally light wood with very low density, containing a high proportion of air filled cell cavities that act as extremely effective insulating units. As all Pan-Abode homes are made from 100% Western Red Cedar, they are cool in the summer while significantly reducing heating costs in the winter.

ONE SURE WAY TO RUIN THESE PROPERTIES IS SURROUND IT BY man made unnatural FOAM & TRADITIONAL CAULKS, ESPECIALLY POLISO!

Posted By PARAHOMES on 25 Jan 2017 05:31 AM
OFF THE SITE:

Western Red Cedar is also one of the best thermal insulators among softwood species used today. As all Pan-Abode homes are made from 100% Western Red Cedar, they are cool in the summer while significantly reducing heating costs in the winter. ONE SURE WAY TO RUIN THESE PROPERTIES IS SURROUND IT BY man made unnatural FOAM & TRADITIONAL CAULKS, ESPECIALLY POLISO!

I am glad you responded as you have got to be the most knowledgeable of everyone here. Thanks for speaking up! It is always a question - how much R-Value per inch is Western Red Cedar (WRC)? Can you please tell me, what is the actual R-Value per inch of thickness for WRC? I was thinking that WRC has an R-Value of 1.35-R per inch of wood thickness. Is that correct? I 'think' you folks typically use a BTU type factor/formula instead of R-Value, but if possible please translate all that into R-Value for me, as it's easier for me to understand R-Value. Thanks!

Even if the cedar had an r-value of 2.0, the roof would only have an r-value of 2.75 * 2 = 5.5. We need to do something...

Robert, I am very interested in what the rep from Pan Abode will have to say, regarding a fix for you, as I am pretty sure R-2.75 is way high for WRC. They deal with this point everyday, so I hope he is going to reply!

WRC R-Value ~ 1.66/in.....varies linearly w/temp...gets up to ~ 2 perm-in @ max moisture content between 60-100% RH. It has a high ability to absorb/release liquid water but peaks earlier @ .6-.8 normalized max content compared to EWC and so it is important to let it breath or it will rot.

I'm not a rep but thats whom I'd be asking these questions to if I were u.

Good luck!

Frankly, I don't see any reason why you can't enclose the exterior in 6" of foam, cover it with plywood and shingle it for that "cabin" look. Same over the roof. Or cover the roof with SIP's & shingles or metal.

Rob - at that vintage, I suspect the cedar is laid top to bottom over fir lintels.
That being the case, install 6 mil poly over the cedar deck (as per Canadian Codes) and seal every joint and intersection with acoustic caulk.
As BobI suggests, if the tails and tops of the cedar t&g are not sealed, you will get infiltration.
Any infiltration will cause condensate damage in your climate. When the whole house was leaky, it would not have been so apparent but if you channel the leaks to a certain area because the rest is air tight, it will be more of an issue.

Thanks Bob I, PARAHOMES, Roger R & FBBP.

Given that the walls are only 2.75" thick, I am wondering about the value of insulating the roof to R-31 with 'proper venting' to eliminate ice dams, which is what the owner is considering.

Is it really worth taking the roof to R-31 on this old cottage?

Thanks,

Rob.

<!--[if gte mso 9]> As BobI suggests, if the tails and tops of the cedar t&g are not sealed, you will get infiltration. 
Any infiltration will cause condensate damage in your climate. When the whole house was leaky, it would not have been so apparent but if you channel the leaks to a certain area because the rest is air tight, it will be more of an issue.

This couldn’t be further from the truth….vapor pressure and air flow are essential independent of one another, in building’s they are often opposite directions. Further, air has nothing to do with the freezing or boiling points of water nor the dew point of the cedar in this case. Water vapor content is driven by “vapor pressure” or partial pressure differentials, it is only dependent on water temperature of the cedar, not air.

One cannot design to conditions they don’t understand…..

Foam has practically no equilibrium moisture content like cedar but cedar unlike foam can dry fast, cedar vapor pressures will not dry well to foam rather condenses at the barrier once the cedar reached max content which is low too .6-.8 lbft3 , again air sealing has no affect nor on how fast cedar dries but what is matted to it will if it increases vapor pressures and cannot dry. If there is a fungi food rot occurs within 24-48 hrs.

I’d stick with a high perm natural mineral insulation like Roxul, stabilized or bound cellulose, flax, mated directly like IS sheathing , not far from you. They have low water contents like cedar but with high drying rates @ low max water contents. Follow their installation guides for cold/wet climates.

Thats close I'd need to run WUFI in your climate to be more specific.

With R-7 ish walls/ R31 roof about a 10 kBTU/hr capacity saving’s over R7/R7. R21/R31 15kbtu/hr.

R21/31 @ ACH@50

5 to 4 = 5kBtu/hr

4-3 = 6kBtu/hr

3-2 = 7kBtu/hr

2-1 = 9.5kBtu/hr

1-.5 = 11kBtu/hr

 You can figure cost per BTU from there to determine if it’s worth it.

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One cannot design to conditions they don’t understand…..

Absolute truth!!

It does not matter wither foam or cedar hold moisture. The point is, we are talking cold climate construction.

Tongue and grove cedar, regardless of its thickness is not vapour or even air tight.

If we put foam insulation over the cedar without an air barrier (vapour barrier in this case) the moist air from the house WILL work its way through the cedar and the joints in the foam until it gets to the point were it will freeze.
Montreal's 99% design temp will be somewhere around -23ºC. Not as cold as our design temp -32ºC but much more humid so less forgiving. Regardless of if it is vented or not. The top inches of foam will still be frozen at design temps so the vapours will condense and freeze. With foam, it is possible that during a prolonged cold spell, enough vapour will freeze to form an air barrier and stop the vapour transmission.
When a warming trend arrives, these frozen vapours with turn to water and find their way inside, staining the ceiling at best, or possibly causing dry rot.

Using Roxul would be many times worse. The vapours are not restrained to the cracks but will infuse the whole mat. As the vapours condense and freeze in the upper layers of the bat, the frost line draws downward and the condensation and freezing pattern happens lower in the batt until during a prolonged cold spell, the frost happens right in the cracks of the T&G cedar. And yes, I have seen this happen.
When the warming trend hits, the house is in for a indoor shower as all the water melts. Several cycles of this and you have some serious mould issue. And yes, mould will set up on both rockwool and cedar under the right (wrong) situations.

If there is a vapour barrier above the cedar and there is insulation above the vapour barrier, the cedar will not pick up more moisture than the ambient air it is exposed to. It will not condense at the vapour barrier as the vapour barrier is at the same temp as the cedar. This is why we insulate ABOVE a roof deck to protect the sheathing if it needs to dry to the inside.

Regardless of what anyone thinks, Rob is going to have to abide by the Canadian building codes. The Canadian Building Codes do not always get it right, but they are recognized world wide as the leading model for cold weather construction.

I'm not going to debate with someone that thinks air manages vapor drive.

All the BTUs/hr posted in my last post are using proven Montreal/Montreal TMY3 weather files. I'm looking at very accurate empirical physical, chemical, & a VERY large material property, databases none of which line up with the above, I fail to see above so I won't debate opinion any further.

"Using Roxul would be many times worse."

You might send the Roxul engineers in Ontario that design to code and give them your fault analysis installation instructions over wood T&G, doubt they listen either. I'd suggest to any reader to listen to them, they have plenty of empirical evidence there.

Anyway Robert, good luck hope those numbers help, they are accurate within +/- 2kBTU/hr. Had I more info on the job site I could get closer and accurately predict your utility bill, pay back periods, and mold growth w/WUFI_BIO globally proven for over three decades now. I do it all the time. Getting ready to build Roxul over unsealed T&G, with cladding ventilation with the correct vapor flows in a mixed wet cold climate breaking ground in a few weeks for a client, net zero PHIUS Certified using WUFI-Passive.

From the Alberta Building Code which is substantially similair the the NBC.

9.25.1.1 Scope and Application
1) This Section is concerned with heat, air and water vapour transfer and measures to control condensation.

9.25.4.1 1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a VAPOUR BARRIER so as to provide a barrier to diffusion of WATER VAPOUR from the interior into the wall spaces, floor spaces or attic or roof spaces.

9.25.4.2 goes on to list what is an accepted vapour barrier. Foam is accept if properly installed. Any fiber materials are not.

9.25.4.3 1) Products installed to function as the vapour barrier shall protect the WARM SIDE of the wall, ceiling and floor assemblies.

So you see, I won't have to contact Roxul after all. They already know the Code. And they actually understand it!

Para - do yourself a favour and spend a couple of hours (or months) with the Canadian Codes. You will find that all the things you think of as some kind of mystery are already laid out in the Prescriptive side of the Code so that any layman can understand them. Thousands of homes are built successfully in Canada every year without the involvement of any engineer because the NRC has already worked out all these numbers.

Canada has been at this vapour/condensate thing for many decades. We got serious about it back in the seventies with a program call R2000. Most of what is now called passive house standard originated in that program. It was the R2000 Program that first introduced HRV's when they figured out that that a R2000 house was so tight that the poor occupant could not get back in their house on a cold winter day because WATER VAPOURS froze in the keyway and the key would not go in. The term seal tight and ventilate right originated with the R2000 Program.

You might be surprised to find that one of our favourite writers from Building Sciences started his career with the R2000 research team.

A little story. Empirical evidence, you might say.
In the seventies and eights, every time a chinook blew into town, we would get calls about roof leaks. The water is pour down EVERYWHERE.
The answer was always the same. No Dear Customer, your roof is not leaking. All the water vapours that moved through your plaster ceiling condensed and froze to the underside of your roof deck. (Talk about stalactites!) When we went from minus twenty last night to plus ten (C), those stalactites melted and that is what is causing your flood.

With the R2000 Program came the introduction of properly applied vapour barriers. Those first years, we wrapped each electrical box in poly and than sealed that square of poly to the poly vapour barrier on the warm side of the insulation. Today we no longer have desperate calls when when chinooks happen. And no, contrary to Mr. DiCaprio's great knowledge, a chinook is not a sign of instant climate change.

FBBP

Now that I see your an aviation buff you may not be as dumb as I thought, Just Kidding! I was just on an mechanical design(HVAC/Hydraulics) team engineering contract/consultant mid last year until I quit to go to Airbus A330 propulsion turbine engine nacelle structures design. It's way more fun & challenging than this but, I don't like corporate and some of their stupidity like some AEC code. I don't blame Trump for challenging the F-35..If tax payers only knew what goes on in these companies they be outraged! Just when I thought they retire the old dog, it's being reconfigured for even Germany kings of PHPP. They are wanting to know if I want to come back for a bit just yesterday.

http://www.reuters.com/article/us-boeing-helicopters-idUSKCN0S629S20151012

It's pretty much a configuration nightmare with that many wanting custom designs. The turbo shaft engines drive some amazing gearing, awesome stuff!

I'm always impressed with Boeing factories, you be amazed if you like high tech. I also did structures design back in 1990 for the prototype V22 Osprey. Rumor has it Boeing is looking to do a Quadcopter with the highest lift capacities ever. You get familiar with air/atmosphere and the importance of material properties after working in Aero for over three decades but, the Aerodynamics & Material Tech/Chemist Scientist are the experts I work closely with AEC lacks BIG time code included, as a Designer. In AEC I wear many hats and my energy and moisture/WUFI models and my "expert" Building Science, tools, not words, code, or anything else including some of these so called "Building Scientist" or "Green Building Advisers" internet seat designers offer......

This is nothing new, lots of same empirical data that you find in WUFI with decades of tested proven builds throughout the world is also in Computation Fluid Dynamic (CFD) Models like Flowmaster, CATIA FLUENT I use in Aero. The difference is we can't FAA/JAR certify without the models being ran by professionals. As a matter of fact, you cannot design without being a degreed engineer for starters, what needs to happen in AEC including residential that is a mess from being designed by builders that do not understand basic physics & chemistry. WUFI knows this thats why they want Engineers running their models. I communicate with them from time to time and know. Talked to many factory engineers, Roxul CAN has my respect, although I think often all of them are out to protect their warranty and could careless what happens to mating materials as longs as theirs survives within the warranty period. I've worked factory engineering long enough to know this can be engineered.

I see PHIUS is requiring WUFI PASSIVE to certify under that name so it will be harder to ruin like "green" is, which is a step in the right direction. No more code, no more opinion, and by the way I don't think there are other world wide test code databases like WUFI that has fully instrumented it well enough to verify it.....If there is please post a link to such as database you are referring to? On the contrary, one does not need to look far to find field test IAQ experts are uncovering more and more mold issues with designs and it is growing with the air seal movements. You should see the data base in WUFI plus and it's capabilities, Im qualified to say I am impressed. In a short time looking at Germany and other databases other than USA, you'd see how much further ahead they are.

I did recommend following Roxuls installation instructions. It is made from an inert rock mineral slag, iron oxide process, is chemically stable, has no fungi food like blowing agents and fire retardants found in foam. It also has over 10 times the deflection(which T&G has alot of and will probably crack it on the hot/humid side in tension), 700+ PSI compression @ 10% max deflection, & much higher creep properties then foam other than FOAMGLAS that is very high priced. Also, if you designed to rely on caulk air seals for long life cycles you don't know what you are doing, they hygrothermally fatigue in short lifes and need R&R.

Not sure about CAN, here in the US not all jurisdictions adopt the same energy code or at all like where I'm about ready to build, so I did not see any code requirements posted. If any OP does not understand them they are wasting time obtaining opinions. Even if I did spend time in CAN as you suggest it would not make me a CAN expert without the proper design tools, nor does it you or anyone else. Here in the US, the AHJ has ultimate authority to determine code and vapor barriers a good designer could provide the tech data. In most cases, if not placed right interstitially it can ruin mating properties. I know it is a sin to place reactive plastics and foams against hygrothermal wood & if you check their data sheets they are not "intert" have fungi foods. I can guarantee there are better ways. There are all kinds of places for it if you know what you are doing to comply with code & "protect the warm side". Yes, the average laymen should follow code and make sure they don't misinterpret it & the mfg installation procedures for warranty. Typically, like for stabilized/bonded cellulose I suggested there are alternate material code paths with supporting data comply, I'm sure PHPP/PHIUS/DOE/ETC satisfy's in most cases. I can guarantee the warm side is not the only place for a vapor barrier and there are plenty of mold free cold/hot/wet roof designs out there with none. Alot of this also depends on the HVAC vapor partial pressures are, which is part of the CFD model and too complex to be comprehended by code in most cases or laymen on forums...

Hello:

How about entirely covering the roof with Ice & Watershield and then 2" of foam (R-10); then 2 x 4 strapping, plywood and shingles. Vent entire soffit to the 1.5" channels and use a ridge vent system.?

I know R-10 is not much but it should be better than nothing and the venting should eliminate the ice dams, right?

Water vapour shouldn't be a problem because the cedar ceiling will "dry to the inside", right?

About "ridge vent systems", do they work when covered with 2 feet of snow?

Thanks,

Rob.

R10 roof only will save ~ $120/yr utility bills according to my quick model.

I added some insulation properties here: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/14/aft/84342/afv/topic/Default.aspx

One should conclude from the reliable data in WUFI MWB has better saturation properties and is less likely to freeze than any of the foams due to Dww/Dws. Poliso does not have much higher properties than XPS/EPS I doubt .02(XPS) .1(EPS) u-values @ MMC would make alot of difference for the higher cost and it is not inert, only that it's perm-inch is higher/faster in free saturation to MMC then it like the others it's pores don't distribute all all, and only IF it's not faced w/ W&I included or poly.

So your design is ok, should code comply depending on AHJ, typically better if you leave the hot sides ability to dry inward(no vapor barrier) but, that depends on partially pressure being lower in the room. It is complex, how the material state vapor dries, so often complex math/sim models try and simplify it by assuming lower RH causes inward drying and outward but, that is not entirely true and depends on the state of the materials in contact. A good enough assumption other than outward is more about partial pressures being lower. that is what a ventilation gap tries to do by increasing CFM/lowering pressure. I'm still waiting for a database to prove R2000 right like WUFI? And poli on T&G w/a hot poly chemically reative vapor barrier does not produce fungi? One cannot sit back look at code interpret it in a way to be accurate in all of CANs designs w/o instrumented/tested databases.

Read this for a BASIC explanation. Use 3/16 furring, 2xs are way to large and will produce low flow to get the proper CFM/ACH that will produce surface friction heat to keep dry & from MMC/freezing. Yes vent to ridge and soffits but you have to size/balance them right to get the proper CFMs/ACH depending on winds. The gaps need to be commissioned like any other duct system.

https://buildingscience.com/documents/building-science-insights/bsi-089-wufi%E2%80%94barking-wrong-tree

As far as an air sealing a 10-15 year caulk like latex or poly will need R&R 5-10 years, not good to rip out all the T&G. Silicone is 20+ yrs...Prosoco at the sheathing is better since it is made out of limestone/silane although it oxidizes over time too and does not like ozone or acidic boundary conditions. So perhaps air tight home sustainability depends on re-sealing sheathing roof replacement when the ACH @50 increases or utility bills increase. The "Air Planes" need to be continuous...

Don't know why I am debating r-10 roofs, principles remain the same :) Good luck, I'm sure the cedar is nice! I'm currently doing it too w/pine with decorative trusses on a all metal building design client wants, although I have a better design not willing to disclose for free :)