Because of the design of the home I am planning, the interior shell and exterior shell of the home will be airtight. In otherwords the insulation will be contained inside an airtight cavity... I can find plenty of information on sealing the exterior, and plenty on sealing the interior, but nothing on doing both and if it will cause any problems. The insulation will be blown in cellulose with tight areas using spray foam. All walls and ceiling will be approx R40+. I'm in central Florida and it gets pretty humid here. Any thoughts? Thank you!

Nothing wrong with airtight on both sides as long as one  or preferably both sides are vapor permeable. 

Cheers,
Eric

The vast majority of data/experts/codes support the use of a vapor barrier on the inside for almost all of the USA.

Cellulose in Florida... no just painted drywall on inside, dense pack it. Thet is the Building Science and Cellulose industry design.

Needs to be able to dry to the inside. Need to stop exterior vapor.

http://www.buildingscience.com/documents/insights/bsi-021-thermodynamics-its-not-rocket-science/?topic=/doctypes/building-science-insights

http://www.buildingscience.com/documents/insights/bsi-015-top-ten-dumb-things-to-do-in-the-south

This PDF is more info than you could ever ask for....
http://www.buildingscience.com/documents/reports/rr-9302-humidity-control-in-the-humid-south

Map of the USA... and much more reading... This profile includes Cellulose as the insulation.
http://www.buildingscience.com/documents/profiles/etw-houston-profile?topic=doctypes/enclosures-that-work/etw-building-profiles

This is a clip from the above:

"Cavity insulation - Any type of cavity insulation would be acceptable in this application—spray foam (open cell and closed cell), cellulose, fiberglass, (as long as air sealing is accomplished by a separate component or system when cellulose or fiberglass is used). The only exception is with high permeability sheathings (such as fiberboard and gypsum), where a vapor permeable cavity insulation should be used. Since this wall assembly is designed to dry exclusively to the interior, do not use any layers at the interior surface that have a low vapor permeability (polyethylene or vinyl wall covering). Note that when foam insulation is left exposed in an assembly, a "thermal barrier" or "protection against ignition" may be required. Code implementation/interpretation have proven to be particularly troublesome for "gray" areas, such as spaces that are conditioned but not occupied (conditioned attics and crawlspaces)."

I bookmarked the links you gave me and will be checking them out shortly. One thing I didn't mention, mainly because people go cross-eyed, Is that this will be built from shipping containers. So the exterior will be steel. On the inside there are no studs attached to the exterior wall directly. The wall studs will be 2x3 and attached to steel welded to the ceiling and floor. The ceiling studs will be attached to steel hangers welded to the walls. The interior walls and ceiling will contain a 3/4" layer of foam board which I will air seal, then stagger the drywall, which I will also air seal. What this leaves me with is a large continuous cavity to fill on the ceiling and wall. I'm planning the cavity to be about 11" which should give me approx R35 to R40 with cellulose. The foam board is listed as R5, so I'm hoping for total R40 on the low end, but it should be higher.

Neat project... if you look at Joe L's analysis of the Space Shuttle tank foam... he would like your shipping container idea.

You should seriously consider closed cell spray foam to inside of steel after the studs are up.... then cells, and no vapor barriers! Regular paint on drywall... use paperless drywall if you really want to be safe and blue wood to frame. Seal your electric outlets and minimise putting them in the outside walls. The home will be tight as all get out. I am not into ERV systems but you may need something, a little bit. Not easy to do right if you read all of the Building Science pages.

Old loosely built homes have less problems than new tightly built. Enjoy the reading.

The reason I say no sheet foam is you will be making an air space next to the steel that who knows what will happen in there. Joe L may know...

The foam sheets would be on the interior side of the wall between the drywall and cellulose, the only thing up against the steel would be the cellulose. The reason I'm not doing spray foam is that I plan on doing everything I can on my own. Also people mainly use spray foam for sealing, but since my exterior shell is already airtight it loses some appeal. I will use it in hard to reach areas though. I have some 3d models I can take screenshots of and post up to better illustrate the wall structure.

NO FOAM BETWEEN CELLS AND DRYWALL. ABSOLUTELY NO.

Your inner wall and cellulose can only receive and give back moisture to the inside through latex painted drywall.

Do not use any vapor barrier. The sheet foam is a vapor barrier.

Like I said... you can put spray foam on the steel and you should. That steel is going to get hot and cold like you can't imagine.

Read up on others who have converted shipping containers.

Read the info I posted.

You do not want that foam where you are saying... no no and no.

Dana1 is the resident insulation master... Check back and hopefully he will post here for you.

Keep in mind that these guys have a vested interest in differing opinions - interesting to read but poorly supported.

http://www.buildingscience.com

Which guys have what vested interests?

I am a builder and homeowner.. with interest in improving the homes I work on for the best value and lowest cost to that end.

Building science... fantastic people with no dog in the hunt that I know of...

Dana seems pretty non vested in his postings.

The authors of the buildingscience.com website. They are consultants - and consultants make more money when they have something different to say. Check the footnotes on what they write (usually none) - no references, just chatter.

OK.... meet Joe L in person... then tell me what you think. I here your concern and just have to agree to feel different.

OK, so on the interior side of the wall I can have an air barrier but not a vapor barrier. The exterior can and should be an air and vapor barrier. I see what your saying about the spray foam. I was wondering what to do about condensation on the steel, but wasn't even sure if it would produce any condensation. I'm wondering now if a thin layer of spray foam over the entire interior steel shell would eliminate the problem. I was trying to keep the use of spray foam low because I will either need to buy the equipment ($$$) or pay someone to do it ($$$). That and with the large amount of interior steel it will start to add up quickly. Quick calculation shows I would need to cover approx 8,240 sq feet of steel with spray foam. I haven't been able to find any decent idea on pricing in my area, but it just sounds expensive at that amount of coverage. Although the labor would be fairly simple, just come in and spray all the walls and roof, there wouldn't be anything in the way. I wouldn't have a problem doing it myself except for the equipment, which I could hopefully sell after-words. Any idea how thin/thick I would need to go to control the condensation? I'm not worried about using it as insulation so much as condensation control. Also would help keep it slightly more bearable while building out the interior...

Also as a note the exterior will be painted with ceramic insulating paint (blocks IR light) and the lot it will be built on is heavily shaded so the house should see very minimal direct sunlight.

Here is an idea of how the wall/ceiling will be constructed. The transparent gray is the exterior steel wall/roof. (ignore the RMax foam sheet) As you can see the interior of the wall/ceiling will literally be a large open cavity. There will be almost no thermal bridging to the studs. So I could weld in the stud hangers and pop in the studs. Spray foam the steel shell. Put up netting and blow in the cellulose, then put up and seal/tape the drywall. Pretty straight forward...

Posted By adkjacUpstateNY on 24 Jun 2010 02:54 PM
NO FOAM BETWEEN CELLS AND DRYWALL. ABSOLUTELY NO.

Your inner wall and cellulose can only receive and give back moisture to the inside through latex painted drywall.

Do not use any vapor barrier. The sheet foam is a vapor barrier.

Like I said... you can put spray foam on the steel and you should. That steel is going to get hot and cold like you can't imagine.

Read up on others who have converted shipping containers.

Read the info I posted.

You do not want that foam where you are saying... no no and no.

Sheet foam is not a vapor barrier, unless it has vinyl, poly, or foil facers.  Above 2" thickness XPS becomes somewhat impermeable, but still sufficiently permeable for the cellulose to dry over time.  EPS is significantly more permeable at any given R-value or thickness. 

As long as the interior-side assembly is ~0.5perms or higher it can pass seasonal moisture into & out of the cellulose, but steel won't.  An inch of XPS or EPS underneath latex-painted gypsum won't form a moisture trap. The key is to keep the interior vapor retardency such that it won't overload the hygric capacity of the cellulose during the winter, yet still be permeable to dry sufficiently during the warmer seasons.  Cellulose can handle quite a bit of moisture cycling, so it's not a bad choice for this app, and there's quite a range of permeability that might work, but it's local climate-driven. (It's probably difficult to make it work well in the arctic zones.)

In some climates the temperature of the sun-exposed sides of the steel would exceed the operating temp of SPF or styrene rigid board, but not above that of cellulose.  If sprayed with CA Title 24 complaint "cool roof" coatings the peak temps might stay within the temp specs for SPF in most climates, but not all. (The biggest concern would be the temp at the roof in a low-latitude during the summer-solstice periods.)  What climate zone is this crate going to live in?

Whatever buildup you use, I suggest using rot/moisture/termite proof materials in extreme or questionable conditions - but I just came back from a place where they don't use any wood in construction. I agree with painting it a light color although I've never seen any credible support for "insulating ceramic paint".