Cellulose/rockwool roof assembly - thoughts?

I'm designing a small (tiny?) house to build this summer. Roughly 300 square feet on the first floor with about half of that again as a bedroom loft. Nothing new. Below is a sketch of a roof assembly I am wondering about. It has 2x10 rafters (24" OC) filled with cellulose. Moving up from there, the rafters are "sheathed" with 1.5" rigid rock wool, which is held down by vertical 2x4 straps. Bear with me here. Then there is a layer of felt over the 2x4's but wrapped into the troughs between them. That is held down by 2x4 purlins which support a metal roof. Below the rafters, there is another layer of rock wool boards held up by 1x3 strapping. Attached to the strapping is wire mesh and probably a clay plaster.
Some reasoning for this assembly. I need a plane above and below the rafters to blow the insulation into. I want it built and insulated before bothering with interior finishes. I could use wood, or some kind of fabric, but I figured rock wool would: contain the cellulose, provide some extra insulation (R-12 for 3 inches), and provide a thermal break from the rafters. The idea behind "wrapping" the felt between the 2x4's is that there is now a chute running between each rafter that has no penetrations. I thought this would be great protection against leaks, especially for exposed fastener roofing.
What I'm curious about is how this will perform with regards to moisture and condensation. With the plaster, two layers of rock wool, and 10" of cellulose, I think it should be pretty airtight, but there will always be moisture migrating through. Where is the moisture most likely to condense? Between the cellulose and rock wool, below the felt, or on the metal? I know rockwool and felt are both somewhat permeable, but I don't know how much.
Thoughts? Improvements? FYI one goal in this project is to use absolutely no foam or plywood.

This cannot be answered without having a complete description of the design, put more effort into it if you want a good answer. As a stand alone assemble I'd say it does not look good and advise getting some professional help.

I am assuming that you are trying to avoid plastic as a moisture barrier as well?

You will need some airtight membrane or air will move through the layers, even though you have designed the layers to be perpendicular to eachother, ( that does help).

Thinking natural are you against a raw hide membrane, goat sheep or deer could make a good seal for the air tightness factor, especially when saddle stitched together.

If you are not into the hide solution, canvas coated in pitch like the old Irish boats could serve well also....

Terry: What more information would you like? I described every component of the assembly. Put more effort into it? OK, how about you do the same and explain why it "does not look good." I think you might also need some "professional help."

Chadwick: First, thanks for the thoughtful, constructive response! Yeah avoiding plastic would be great. I'll probably put a sheet under my earthen floor, but you gotta pick your battles. Actually plaster, when done well, is a pretty good air barrier, so the ceiling should perform that function. Air barrier, not moisture barrier. I'm expecting everything in this house to be pretty airtight but breathable. I think as long as the inner surface is less permeable than the outer, then moisture should not get stuck within the wall. That's why I'm curious about where the condensation would occur. I like the animal hide idea, never thought of that. I also don't know the first thing about working with hide. Canvas sealed in some way is another good thought. I wonder how canvas would take a clay paint...

You are right, I missed the plaster, I still think like a standard builder a lot of the time, I was looking for the ecpxterior sheathing barriers but ignored the interior one!

If you are in a ver humid area an exterior vapor barrier will still serve well for mold issues.....but if you are in a dry hot climate not as much....

Ben, what will happen is moisture will work through your plaster ceiling and condense on your cellulose insulation and start a huge mold farm.

In cool climates you must have a single vapor barrier on the interior side of any insulation. 4 or 6 mil plastic is usually used, but if you can make the leather into a moisture barrier, that should work. You might be able to get away with a water proof paint, like is used for bathrooms or spas, but I wouldn't trust it. Since this is a tiny house, the humidity level from respiration and cooking will be very high. Seal all ceiling penitrations as well.

Also, you need an air space between your insulation and roof sheathing, and it needs to be vented. Otherwise you will cook your rafters in the summer and they will eventually fail.

I think I would eliminate the blown cellulose. It wont stay where you put it if the house is mobile, it compacts over time, harbors mold, and I think it is also flammable.

I would stick with all rock wool, if you like that. It is fire proof, mold and bugs don't like it, it will not compress much over time so it retains its insulative ability. Sheeps wool insulation is another alternative (batts or loose fill), but is more expensive than rock wool and you need to order online.

Best of luck!

Brett, thanks for your thoughts. You don't seem to be a fan of cellulose but I think it is a great option if done right. Maybe you should give it a second chance. If it is dense pack it will not settle. And it's actually considered to be mold resistant as well as fire resistant. Of course, that doesn't mean I want it getting wet in my ceiling. That's why I'm trying to make sure any condensation happens above it and can dry to the outside. I like the rock wool boards on top of the rafters since that makes the surface just above the cellulose "warmer." It also gives the rafters a thermal buffer from extreme temps. I don't think that moisture would condense on the underside of the rock wool since it's so permeable and hydrophobic. I guess I'm just worried about it condensing above the rock wool, getting trapped, and finding it's way down once it accumulates. Since I first wrote this, I've been thinking it might make sense to have the vent channel run directly over the surface of the rock wool, wicking away whatever moisture gets through. It will be in western Colorado, a pretty dry climate so that makes everything a little easier.
Also, no, this house is not mobile. I find myself avoiding the term "tiny house" since everyone associates that with "tiny house on wheels" these days. 300 sq ft would be a big one to have on wheels.

Ben,
You are correct. I hate cellulose insulation. I hate breathing those very fine dust particles every time I work in someone's attic, and having to wear a dust mask just to visit your attic . Even if you don't sstir it up, they are airborn. Think of all those tiny particles that are finding their way into living spaces via tiny cracks in duct work, electrical and plumbing openings. Mark my words, in a few years someone is going to announce it ccauses cancer and have to be removed. Ok, maybe I exxagerate, but that stuff is absolutely horrible to work around. I would much rather work with fiberglass. Tiraid over.

True cellulose, is the perfect medium for mold. So if what you are using says it will not support mold, and won't burn, then it is treated with chemicals to make it that way.

That said, mold will grow on anything if it is the right temp and humidity,! including metal, but especially wood, provided it can find a tiny bit of food too (like cellulose airborn dust).

Regarding condensation, you are frequently going to have very high humidity in your small living space (90%+). Warm air can hold much more moisture per cubic foot, than cold. So when that moisture ffrom inside, works its way through your attic insulation, if it cold enough outside, it will condense.

On days where there is not much of a temperature difference, you might not get any condensation. I may pass right through your breathable roof. On other days when it is a little colder, it may condense on the underside of your metal roofing at night. On other days when it is very cold, and the cold air cannot hold hardley any moisture per cubic foot before going from 0 to 100% humidity, it will condense in the middle of your insulation as it reaches condensation temperaure. It doesn't matter how permiable your insulation is, it has to do with temperature. It will condense, and when it does you will get mold.

I suspect every jurisdiction that has cooler temps mandates a vapor barrier now on the inside of any insulation now because the mold can be deadly. It may be so bad when you find it, that it may have spread to your walls and have to tear the structure down. Don't take my word for it. Google it and you will see tons of stories of mold. I see it in most old houses I work on. I wouldn't want it in my house.

Best of luck!

Hi Ben,

My apologies if my last post squashed your discussion thread. I'll post this comment to bump your discussion thread back to the top to solicit more input.

It's just that I have seen so much mold, in so many houses when I open a wall or ceiling for a home improvement or repair, that the owner had no idea was there, that makes me very cautious. I understand your desire for breathable surfaces that absorb and release moisture to help regulate humidity, or even let it pass through. The two biggest factors to overcome in your application is that you have wood in the roof, which can harbor mold, and being a small house you will have instant very high humidity levels inside when you cook and bathe. Perhaps heat-exchanging ventilation would be good here. With exhaust fans in your kitchen and bath. Theory and actual practice are very different. So....

Does anyone have experience successfully building a roof structure containing wood and insulation, without a vapor barrier, in a cold climate, for a small building (approx 300 sqft) with a shower/bath and kitchen, that has been in place at least 5 or 10 years without any mold? If so, how did you do it? How do you know you have no mold?

Best of luck.

Brett, no you didn't kill the discussion. I just hadn't revisited it in a while.
I too am very cautious of mold and want to make sure to steer clear of it. It's not that I'm being cavalier about it. I just have a different understanding of the building science. And I am certainly not claiming to be an expert on building science. That said, there's no shortage of "experts" who have completely contrary beliefs on things like this.
Regarding cellulose as a growth medium for mold. Of course paper or wood or any cellulose product is a tasty treat for mold. But I believe most blown in cellulose insulation is treated with borate. Or borax. Or boric acid. Whatever. That is supposed to make it resistant to mold growth (as well as pests). I know that this is pretty standard for the industry now. Whether the borate poses any kind of health hazard, I don't know. Most of the "natural builders" that I have followed and read consider cellulose to be one of the best all around insulations, in terms of being effective and earth friendly, human friendly, and pocketbook friendly. So until I find the time to do my own independent research, their word is good enough for me.
My understanding is that as long as the "drying force" is more powerful than the "wetting force," you should be good. As in moisture can get in, as long as it can get out as fast or faster. I also know that more moisture, by orders of magnitude, is transported through a building assembly by air movement than by vapor diffusion. So my guess is that with a good airtight ceiling, and plenty of drying potential to the top... it should be safe. "Installations have shown that even several months of water saturation and improper installation did not result in mold." That's from Wikipedia and the source link is broken, but hey.
We'll definitely have exhaust in the kitchen and bath, and I might do an HRV as well. Thinking about trying to build my own air-air heat exchanger, but that's for another thread!
I also just read that Roxul is made with urea formaldehyde as a binder. I don't know enough about the associated health risks to make a judgment on that one. Do you have any thoughts on that? I mean, there's only so far you can go in eliminating toxins before you just get paranoid...

I think a spy in an old war movie once said, "I have survived this long because I am afraid of everything, and assume nothing.", or something to that effect. So there is something to be said for a certain amount of paranoia in life.

I can't imagine untreated wood could be damp for months and not get moldy, unless it is totally submerged in water, so I am curious about your wiki quote. Around here (MD) in the summer under perfect temp and humidity, damp wood and drywall starts showing visible signs of mold in 24 to 48 hours and can be completely covered in 3 or 4 days. Mold can survive for weeks with no food or water so an occasional dampening is just fine for mold. Also, insulation that states it does not support mold, means that mold doesn't use it for food. But mold can still grow in it, provided food is coming from somewhere else (like adjacent untreated wood). I have read mold will not grow on a surface with a ph above 10, but I do not know any insulation that meets that requirement. Maybe some type of cement based insulation. Lime plaster will keep it off the interior surfaces.

Per the Roxul web site, under FAQ:
"The organic binders that are used in the manufacturing process are introduced at a high temperature curing phase, virtually eliminating volatile components." i.e. There is no off-gassing.

Take that with a grain of salt, since it comes from the manufacturer. If you know of any independent labs that have tested insulation for VOCs and toxins, please let me know.

Here is a good link comparing various types of insulation. They indicate rock wool does not contain formaldehyde, but fiberglass does contain the safe version (PF.) The EPA states PF has no adverse affect on air quality, since the amount off-gassed by the fiberglass insulation is less than the amount of formaldehyde that is found in outside air naturally. Apparently pressed wood and flooring products are the main culprits of dangerous levels of formaldehyde.

http://greenhomeguide.com/know-how/article/buyers-guide-to-green-insulation

Regarding air exchanging, here's an interesting link from the EPA with recommended rates, although my local building code requires much more based on the size of bathroom, shower, etc.:

https://iaq.zendesk.com/hc/en-us/articles/211434788-How-much-ventilation-do-I-need-

I did a lot of research on liquid heat exchangers a while ago, and found the most efficient are very good conducting (e.g. copper) tube within a tube, where the flows were in opposite directions. Pretty simple to make with copper pipes. I don't know if a similar design (tube in tube) would work well for air to air. I did some research on air exchangers a while ago, and they are far more expensive than I would pay, and too complicated to make the ones I saw (many little tiny passages), so if you come across a simple efficient cheap homemade design, please post. I'm sure a lot of people would be interested.

Per the EPA link above, to maintain healthy air you would need to exchange all the air in your size building about every 24 hours per person (for 2 people, every 12 hrs, etc.), in addition to using bath and kitchen exhaust fans. And if you have any appliances requiring any venting (e.g. woodstove, furnace, gas hot water heater), you know you need some way of maintaining positive air pressure in your building while the kitchen/bath exhaust fans are running so the CO from your appliance doesn't get sucked into your house and kill you. In older homes with lots of cracks and gaps this is not an issue, but is a problem for modern air-tight homes like you are building.

Sorry I can't be more help, but enjoy learning. Still hoping someone has done this before, and has a roof system for you that is proven to work (per my earlier post) rather than just theory.

Best of luck.