Roof/insulation considerations in Zone 7

[Myrtleboone]

I have several related questions within this post that I feel need to be addressed at the same time.

I live in northern Maine (~8300 HDD) and am attempting to build a passive solar house next summer (2-story, slab on grade ~2000 sqft). I have been reading a lot of forums regarding attic/roof insulation and air sealing and am still left with some questions.

The following is what I know (or think I know) and planning presently: Standing seam metal roofing. Without crunching any numbers, I am estimating that I will require ~R-75 in the attic. Unvented cathedral trusses with 20-24" heel height (not sure if that's a realistic depth) manufactured without overhangs (allowing for wall sheathing to be run up to the roof sheathing, covering the outside face of the truss-tape the roof/wall joint). Overhangs to be fastened to trusses, over the wall sheathing. Blown-in cellulose to fill the entire truss volume (assuming 1 inch=~R-3.5, 22"x3.5=R-77).

This is where I become unsure about my materials and the order in which they are installed. Zip System roof sheathing (taped). If Zips are used, I will not require felt covering as I would with Advantech, but not sure if the savings from avoiding the felt covers the increase in Zip panel cost. Both types will provide me with a water tight barrier as all seams will be taped.

One major question: To create an air tight envelope that resists condensation at the roof sheathing (or any other level), is this best accomplished with CC spray foam on the underside of the roof sheathing which then envelops the upper chord framing down to the top sill plate? I have been told from a passive home consultant that good taping is a better air sealer than spray foam and less expensive. If this is true, have I then achieved it my simply installing Zip panels that are taped? Other people have said why air seal the roof and create a conditioned space in the truss, when you should air seal the ceiling? Would seem to me that the chances of developing condensation in the attic recesses is dramatically reduced if air is not allowed to pass through the ceiling into the insulated truss. Maybe a sheet of air barrier plastic does the trick in the ceiling above the OSB and sheetrock.

My priority is of course creating an air tight envelop to maintain a passive solar house characteristic while eliminating any chance of developing moisture in areas that are hidden. Any ideas would be much appreciated. Thanks.

[foursevenfive]

Yes taping is more reliable and much easier to verify and sprayfoam. So if your goal is to build airtight, economically and rather spend the money on tape that works compated to sprayfoam that might work, the solution I propose will work for you.

Since you are using a metal roof, venting your roof would be very easy and the best way to prevent condensation issues. You can use a membrane on top of your trusses, shiplapped to make this vented space that is both a waterproof temporary roof and a vent space.

You are right that preventing any air moving into the insulation is important to prevent condensation issues in your trusses. You can use a (intelligent vapor variable) membrane on the interior/ceiling to accomplish this (or use plywood/osb and tape that).

More info can be found in this blogpost: http://www.foursevenfive.com/index.p...e&id=15&p=1116

[Martin Holladay]

Myrtle Boone,
Many of your questions are answered in this article:
http://www.greenbuildingadvisor.com/...hedral-ceiling