Wanted to get comments on my plans for insulating the "attic" in the garage I'm building soon. The most important point to consider is that this garage will be making a lot of changes over the next 3-5 years. I'm still young enough that I'm not fully established in life; so there are things I want to put in the garage eventually that will have to wait until the budget allows. I want to insulate the garage while still allowing for modifications down the road. My original plan was to put 12" of cellulose in the attic. Good insulating value for a relatively cheap price. The problem with this in my case is that it would make running new electrical/air/propane/exhaust lines all but impossible once it's all closed up. It's going to have a trussed roof with a 3/12 pitch and it's only 26' wide; so good luck getting into and navigating around the attic afterward. Even just installing the cellulose would involve hanging a 4' deep section of the ceiling, blowing in insulation, hanging the next 4', rinse and repeat. So with that said, here is my current thought: Get sheets of that 3/4" foam board with the radiant barrier on each side and put it between the top chords of the trusses about 2" below the decking. Use the same board to go down from the first board to the top plate of the walls. This would form a 2" cavity between the "attic" and the decking to still allow air flow. Next, have 3-5" of closed-cell foam sprayed to the underside ("attic" side) of the foam boards to provide additional insulating value and form a total air barrier. This would essentially make the "attic" shared air space with the garage that is independent of the roof decking while still allowing air to get to the underside of the decking. It would also give me the option of running whatever I need to through the attic area down the road with no more hassle than just removing ceiling panels. I do realize the cost is going to be substantially more than the cellulose and I am comfortable with that as long as this is going to work in the way I want it to. Thoughts/opinions? I've chosen closed-cell foam due to it's higher r-value/inch; but is the low permeability going to cause issues with the wood for the rafter chords not being able to "breath"? Any suggestions as to how thick to have the foam put on? I know there is probably a point where the additional cost is not met with equal benefit? Thanks in advance for the help!

Going with 1-2" of closed cell foam does a pretty good job of protecting the roof deck when sprayed right on the roof deck, but over 2" it's lifecycle greenhouse gas potential is greater than that of the energy use it's offsetting. With 1-2" of foam at the roof deck you can then add some amount of fiber insulation by building a truss configuration with the rafter-elements. The amount of total fiber insulation you can safely add depends on your climate.

SFAIK the lone exception to the greenhouse gas equation with closed cell foam is an Icynene product, MD-R-200, which runs ~R5/inch, and uses water as the blowing agent instead of HFC245fa. Don't confuse it with their MD-C-200 product, which is higher R/inch, but blown with HFCs. The water-blown stuff is also more vapor permeable, something like 1.5 perms at 3", so even a 9" thickness it's as permeable as some kraft facers. Even at 9" it would limit the wintertime moisture accumulation, but would have adequate summertime drying capacity in most US climate zones.

I know it's been done plenty of times; but I am going to extra efforts to avoid spraying directly on the roof decking. I've helped family/friends replace enough roofs to know that it's not uncommon for sections of decking to need replaced. Seems like it'd be a real pain if the insulation is sprayed directly to it. Thanks for the info on the different spray foams. My crawl was encapsulated with the Icynene; but it was the 1/2# open celled stuff. BTW, how did you get separate paragraphs in your reply? My initial post was actually three separate paragraphs; but when I hit "submit", it posted as one big glob. Even tried to edit it to no avail.

With closed cell foam there's no need to vent the deck (really!) Most roof deck issues with other insulation types are from moisture drives from the interior side collecting in the wood, and closed cell foam won't wick condensation, and is fairly vapor retardent, but not so vapor retardent that it prevents the roof deck from drying toward the interior during warmer weather.

Download and really read this:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems/view

Read also:

http://www.buildingscience.com/documents/insights/bsi-063-over-roofing

Given he went a coupla decades with several inches of even LESS BREATHABLE closed cell foam on the underside of the deck without needing to replace decking, you won't have an issue either. At 3" closed cell foam is structural, and at 5" it might as well BE the roof deck.

The formatting issues is a function of the web-browser you're using (I'm using Firefox on this machine) and the compatibility of the formatting characters that get transferred. I'm not sure which browsers have the issue, but you're not alone here on that.

What's your location/climate zone? It makes a difference.

I'm not worried worried about the SF insulation rotting the deck, I'm worried about the deck taking on moisture from something like a damaged shingle or minor leak around the flashing of a roof penetration (ie. exhaust or vent line) that slowly rots the decking away over a 15-20 year period before the roof gets replaced again. We just re-roofed my grandpa's garage this past weekend and had to cut out a bunch of bad decking due to "small" rotten areas all over the place. Several of the shingles had gotten damaged at some point, who knows when since no one had gotten on that roof in a decade. Wasn't a big deal, just time consuming... but if his decking was spray foamed, the foam would have gotten torn up in the process. My plan is to essentially create a faux "sub-deck" below the existing deck to attach the spray foam to that still allows the true deck to get good airflow and be easily replaced should the need unfortunately arise. I'm in central Indiana just north of center, Climate Zone 5 according to the map google search found for me.

You don't think Joe Lstiburek had minor nail leaks in his shingles over 20 years? From a structural point of view those punky areas become irrelevant with even 2" of closed cell foam on the underside. (Westford MA where the house in the "Over-roof" article is located is also in mid zone-5.)

I completely understand what you're saying: basically what I'm proposing is not necessary and just added cost/time when the same thing could just be done directly to the bottom of the decking itself.  With that said; let me ask an associated question as a compromise.  Though unnecessary, is there anything functionally "wrong" with the proposed approach in the sense it would create a problem?  Is the 2" gap I'm proposing adequate for air flow to the underside of the true decking?  Is 3" of closed-cell foam sprayed onto 3/4" rigid foam panels (~R19 total) in a completely sealed ceiling going to be adequate in a garage; or is there benefit to the added cost of 5" to justify the investment?  Thanks!

A 2" air gap is fine for drying capacity on the roof deck. (1.5" would be code-min on the air space.)

R19 of foam would be sufficient exterior R to allow another R19-R21 of fiber on the interior side with only a class-III vapor retarder between the fiber and the interior space. The IRC calls out R24 as minimum foam on the exterior in zone 5 to be able to bring the total R up to code levels on the interior side of the roof deck, but since you don't have a susceptible roof deck between foam & fiber.

If you're going to cut'n'cobble the rigid foam you'll have a significant waste fraction, so there's no point to going with perfect new-stuff. It's cheaper/better to buy some reclaimed roofing iso (or EPS, or XPS) and put in 6" or more of iso, sealing the edges with 1-part gun foam (or buy a couple 12 board foot FrothPak 2-part foam kits down at the box-store.) It's available from multiple sources near me @ 25-35% the cost of virgin stock (eg: http://boston.craigslist.org/gbs/mat/3242658674.html or http://www.insulationdepot.com/ ). Rigid foam purchased that way is even cheaper per unit R than low-density batts. If you cut them with a sloppy fit to the rafters with a half-inch on each side so that you can slip the tip of the foam gun easily it'll be super air-tight, and needs on interior vapor retarder (the fiber facers are retardent enough, whether they're the fiberglass or the asphalted paper/felt type.)