I see, i did see something somewhere about a SIP acting like an I beam, with the foam being the web and the OSB being the flanges. That is interesting. Although, it seemed odd to me that a SIP would be able to clear span 15 feet or so with nothing supporting it and still be plenty strong, it just didn't sound feasible to me, maybe it is I was just assuming based on the materials that say a 12" thick SIP would not be as strong clear spanning 15 ft as 2x16 @ 16" or 24" o.c. would be, it just didn't seem logical to me, but maybe I'm wrong.

Yeah, price is definitely the thing, and I really haven't had anyone be able to tell me relatively how much they are with all the materials included that I would need, and I didn't really understand how they would work for a roof if I didn't have at least a ridge beam or something, unless you had some mighty fasteners for the ridge and overlapped one over a slightly shorter one and there's your roof? I couldn't fathom how that could work, especially since I couldn't see how you would attach collar ties to them, seems like they would want to spread if you didn't have ceiling joists directly attached to the bottoms.

I guess I don't get SIPs but I'm ok with that, unless by some act of divinity I was graced with riches, I'm pretty sure they are beyond what I can afford anyway, and even if I could, if I could build is very similarly for cheaper, why not? And i like modularity, I can fix a problem in a regular built roof assembly myself. And also, while I'm sure you can do special things to make a SIP more fire resistant, then it would just cost even more... I just don't see the pros outweighing the cons personally.

But Anyway! Highly likely I'm going to stick to sticks as it is. So then, back to perfecting the design. I'm thinking at least 4" or rigid would be wise in the zone I am in, and the choice is XPS for sure over ISO, I decided that XPS is more resilient, and responds to repeated saturation better, and also it's likely to be punctured by a million nails, so any foil facing would be largely moot, I don't know why I didn't think of that sooner...

Now if I used 4" of rigid xps, I would have to use purlins or battens or something at some point because I thought about how to fasten it all together.

If my layers are like so;

1. shingles
2. impermeable layer (what to use here? titanium, gorilla wrap, grace ice & water?)
(I'm not sure what the best underlayment here would be, but I do need one here, otherwise, I'm going to get vapor drive into the 1st layer of sheathing and onward when the sun hits rain etc, whatever I use should be able to protect against that vapor drive)
3. Exterior sheathing
4. Purlins w/ 2" xps (seal in the foam? or wrap the foam? or do nothing?)
5. 2" xps
6. Interior sheathing (underlayment between this and 1st layer of foam?)

A few questions still remain. If I used long fasteners and went all the way through from exterior sheathing to interior sheathing, I would create a way for water to potentially get all the way through to the interior running down a corroded fastener. Don't want that. So that is why I thought about using some purlins with the 2nd layer of foam, doing that I could fasten those down through to the interior sheathing, then the exterior sheathing screws into the purlins. The remaining questions are, is that the best way to fasten it all together? Where should I put any underlayments and what would perform best there? Also, any underlayment I put directly under the shingles must be able to withstand 170+ degrees continuously and seal penetrations.

No one wants to touch this huh?

Sips are tested and proven to be 3 x stronger than frame with superior insulation, some will easily exceed a 15 ft. span depending on load conditions.

If you have made up your mind that you will build an innovative un-proven, untested sytem that will out perform a sips system, then I hope you also report back when its built and let us know how that worked for you.

Good Luck

I would definitely check out the SIPs and do a cost analysis to see if they are competitive as they are a proven building system with superior insulative capabilities.  I don't know if you read the introductory information regarding the use of SIPs as cathedral ceiling/roof assemblies in cold weather but there is some very good information regarding the stack effect and how important it is to make sure no vapor can escape and information regarding roof decks with venting over the SIP roof deck.

There was an article within the last couple of months in the Journal of Light Construction (no, I'm not one of their salesman but the type of construction I do is similar to what they write about so I suscribe to the magazine)  and it was about a house with a cathedral ceiling being built in Maine and how they insulated it.  It looked like a lot of work but it also looked like it might be a system that you could employ.  It was a stick built house.  Check out their archives or maybe a library in your area has back articles.  The article was within the last few months.

Good luck on your project.

Rio, i was trying to do some more research toward the things you pointed out, howerver you mentioned "did you read the introductory information regarding SIPS" I'm not sure what and where you are referring to? Also I couldn't seem to find that article in the JLC?

Hi.  I just found the article.  It's in the June 2009 edition of The Journal of Light Construction and it's called Double wall framing.  If you have trouble getting the article give me your address and I'll send the issue to you.  I've read the magazine and don't need it hanging around the office.  Just let me know if you want it.  It's got a lot of good information on building the whole shell for a cold climate.  the way they do it the insulation in the roof is R-52!
Good luck on your project,
Rio

Mmm, I got your address on my email but I'm surprised that your post is not here.  In any case I mailed off the magazine to you.  I hope you get some good information out of the article.  On the vapor escaping and the water condensing out thereby causing a leak and rotting the SIP I think I read it on this forum under the heading "Read this before anything else" but I'm not sure as I do a lot of reading on various building technologies.  If I find the article I'll make a comment on where I found it.  I just did a test google and googled this  SIP roof failures in Juneau, Alaska and a bunch of articles popped up.  There should be some good information there.  The problem was with not having a really tight shell and when the moist warm air in the house pushed out due to moving from warmer to cooler and the stack effect, in that cold climate the water got wrung out of the air like squeezing a wet sponge.  The lesson seemed to be build it tight and also to consider a vented roof deck over the roof deck.

Yup.... I keep getting knocked down every time i bring this up....
it's the OSB stupid. OSB will rot, if not now but eventually.

Why build with inferior materials, when better quality one are available?
Just my opinion.

Any type of affordable wood building material will rot if it stays wet.  The key is to keep it dry (pretty self evident but I thought I'd mention it).

That's OK, Rio, any reasonable, thinking, person would understand exactly what you were saying, I've alluded to the "Alaskan Fiasco" myself in the past.

For egodesk "its the INSTALLATION............stupid!"

well, i guess i try to count myself as a reasonable person, however I'm not the best when it comes to building things you know... I've only been learing for a few months now without having any knowledge whatsoever previously, it doesn't exactly come naturally for me.

I'm surprised that 7/16 osb and 6 to 10 inches of eps foam are so strong, but apparently they must be, still don't understand much about sips but I have a better idea now. the price is still a bit beyond my budget though. The design that I have tried to come up with here should come out cheaper, which whatever I do it has to, I can't just magically make money, wish I could or this would be done by now! lol.

At this point I think I understand the assembly better than I did before about how I could feasibly do the design I've been trying to plan for, and I'm almost there as far as how to do the layers and fastening. The biggest thing I'm still not sure of at the moment is the whole vapor barrier thing. I'd rather not have one, but I'm not sure if it would be an absolute necessity, I guess I could probably say that even if someone said it was I'm still not going to put one in, lol, it's just not possible to seal up the whole blasted thing. What I do plan to do though is make sure that there are no big air gaps, I was going to go around the whole inside with caulk or spray foam or both and try my best to fill every single crack.

There are still details I haven't worked out as far as the design, I'm not entirely satisfied with a simple, stack this on this plan. What to do around the edges, the ridge, what exact materials to use where, orientation, etc are important to know for when it comes time to actually do it.

I've refined it a bit more to look like this;

1. 2x12 Rafters @ 24" OC
2. Lower layer of OSB, (1/2 inch for strength?) *sheathing attached to rafters as normal, horizontal or vertical*
(Go around all the edges with 4" high wood to give it all a solid edge? do they make anything cheap in true 4" dimension?)
3. 1st layer of 2" foam, loose laid with a few cap nails just to hold it on
(Add in strips of 2" thick (something?) these will have to attach to the rafters, which necessitates them to run vertically also at 2' centers, which is going to force me to do a lot of cutting of my foam, and interrupt the top layer's continuity, but it's the only way I can see creating something for the upper deck to attach to that will be solid and connect into the rafters to resist uplift without going straight on through with the fasteners. This way I can at least fasten these verticals to the rafters, use something like grace ice and water over all of these verticals and overlapping the edges of each strip of foam a bit, then place 2x on the flat right over them, nail those to the 2" material underneath that has the grace ice and water over it, to nail the upper deck to the 2x material. I figured that this build would allow me to keep the foam in good condition and not have to puncture the snot out of it every time a reroof happened, and not have to worry as much about fasteners letting water deep into the assembly.)
4. 2nd layer of foam 2" - Again what to use for 2" thick material, place that material vertical at 24" O.C. & fasten to the rafters.
5. Grace ice and water over the vertical wood members & lapped over the edges of the foam a bit to create a drainage plane.
6. 2x on the flat over the 2" thick material to create air space
7. Upper deck (7/16 osb)
8. tar paper or maybe something better if necessary, not sure if it would be necessary
9. Shingles or metal roofing.

aeridyne,

check out this webpage I found at green building advisor. This might help your situation.

http://www.greenbuildingadvisor.com/cad/detail/unvented-roof-cold-climate-metal-roofing-over-exterior-rigid-foam

late to the party - sorry but I just found this website. I'm a spray foam contractor so of course my only advice is to scrap everything above and 'just foam it'...but in the meanwhile, have you actually done a heat loss calculation with various combinations of your roof and also a temperature gradient of the roof (with design winter temps) and estimated snow loads?

I ask because you mention 35,000 price tag for foam which seems like ten degrees of overkill.

Despite all of the rules of thumb out there, the science will always be accurate for your situation. You might be able to prove, to yourself, your building inspector, this forum, that "X" inches of "Y" insulation will eliminate the need for venting. And ice damming. Or not. I like unvented personally, but I also defer to the numbers.

I know about the heat loss and manual J.  However, how do you do a  temperature gradient of the roof?

Thanks,
Bruce

I think he's talking about the gradient through the insulation layers to ensure the average dew point stays within a foam layer (or on the drying-side of any vapor retarders) rather than a fiber layer.

The simple model is to use fraction R values against and internal temp & exterior temp assuming temp is linear with R.  That will get you "close 'nuff", but it can be off by quite a bit in periods of large diurnal temp swings due to thermal mass effects, relative opacity to infra-red, internal convection within fiber insulation layers, and other non-linearities. 

To be sure the vapor & temp gradient model is a lot simpler with just huge slug of 2lb foam, but that can be a lot to pay for simplicity in high-R layups, eh? ;-)

Yes it can, the cost of 12 inches thick of spray foam open or closed cell for a cathedral roof about 1400 ft square is a wallet obliterating number. And yeah i could do flash and batt, or flash and ... blow? (hahaha)

I like the dual drainage plane that this roof assembly will provide though, so I'm sticking with this design.

Dana is good. I was just reading the book by Joe Lstiburek (or however you spell his name) titled Building in Cold Climates, great book, was surprised to see a lot of the articles from building science in his book. anyway something right along those lines for a calculation of temperature delta, which is difference of temp inside and outside and the relative humidity, coupled with the r value of the insulation on the underside of the lower deck and the R value of insul. above the lower deck will give you two numbers, the temp and RH of where the dewpoint is for that lower layer of sheathing between the two insulation layers. Basically, the higher the R on the outside, the higher the temp is going to be for that lower layer of osb or plywood. Pretty much want to keep it above 45 degrees at 45% Relative humidity. Generally this can be done observing a 1/3 to 2/3 rule for outside to inside R of insulation, and key to this is air sealing. I think I explained all of that already, maybe not in this same thread, and I've seen Dana mention much of that in other threads as well, at this point I've read all of those things in at least three different places (with much more detail of course). In other areas where it is warmer, you don't need as much on the outside, areas that are colder, you need more, for example in alaska, the rule is reversed, 2/3 outside and 1/3 inside. By the way, the 35k was sort of a musing number, but I wouldn't be surprised were I to do the whole house, back room, the basement ceiling, crawlspaces, foyer and garage in spray foam to be close to that number... which is all the areas that I need to insulate. Which means I need some creativity to maintain feasibility... as much cellulose as possible is the answer as far as I can see, and rigid on the main roof so I can get an enclosed attic, while also greatly increasing the durability of my roof (i don't ever want to fix a leak on that roof, ever, it'll be way too high up there) Only thing I haven't been able to decide yet, is whether I think i can get away with 3" to make installation easier so I can use simple 2x4s rather than having to find a way to make true 2" material to use 4".

At this point, I think I'd like to have coffee with Dana sometime... lol

Can we make it beer instead? :-D

Works for me! lol

I think your getting hosed on the foam prices you are saying @ $1.50/bdft, here in New England the prices have come down to the .75/bdft range. I am building a house now that will be insulated within the month and the foam price was lower then that. You might want to talk to a couple more foam applicators.

 Really?  Damn!

It's been well over a year since I had any 2lb foam quoted here in MA, that's quite a bargain compared the $1.10-1.25 I was getting last time around!  (It sure makes those 600 board foot kits look expensive!)

But markets are very local-  without local competitors a foam installer could just quote a take-it-or-leave it number as long as they're staying busy.  It WILL vary regionally, but a factor of 2 seems extreme for non-remote areas. (I'm sure it's more expensive than that in Nome or Antarctica.)