As we turn our work to the inside of the house, I'm thinking about insulation (while I'm working on plumbing & electrical). ;-)
Some photos and then Qs below if you have time. Thanks.

Zone 6: new construction, taped sheeting, 2x6 walls, blown in cellulose planned

For reference the two areas in question; 'little attic' & the wall between 1st story attic storage & 2nd floor living area.
 



Inside of the 'little attic' structure (circled in photo above) 


To insulate the 'little attic area'; drywall on the living area ceiling & then blow insulation into this 'little attic'. But, is it also worth it to:
- put rigid foam on the portion of the exterior wall I have access to? (in addition to the planned blown-in insulation in this area)
- before ceiling drywall, put rigid foam along the plane of the ceiling (pink outline in photo if you can see it), i.e. create change in ceiling height for alcove near windows (aka Not So Big House) 




In 1st floor attic storage, looking toward the 2nd story 'exterior' wall still under roof line
Also note gap in framing between 1st & 2nd story at bottom of photo


To insulate the WALL and GAP between attic storage area & 2nd story, could I:
- put rigid foam on the portion of the exterior wall there? (in addition to the planned insulation in the stud cavities on the interior side of the 2nd story wall)
- how many inches of rigid foam enough to cover the GAP, or should it have sheeting, then rigid foam?



And finally (no photo on this one), to insulate the back wall of the attached garage with master bedrm & mud/laundry room on the other side of it.
Currently planned (from garage side): 5/8 dry wall, 2x6 stud wall (existing already), blown-in cellulose, interior drywall
DIY upgrade? (from garage side): 5/8 drywall, 2" taped rigid foam, zip sheeting, 2x6 stud wall, blown-in cellulose, interior drywall

I have rigid foam and zip sheeting materials available. The rigid foam adds insulation. The sheeting is for added strength - the orientation of the garage, it's doors, the prevailing winds (it is a windy site) and other things make me interested in added strenght here. I'm not asking you to comment on the strength issue, but rather whether or not I'm getting into trouble area with this stack up due to moisture issues or other things.

We add XPS to walls to add r and provide a backer for blown in. So for what looks like a 2x4 vertical web in your little atic that provides wall framing we might put xps on the attic side of that wall; net the other side and blow in dense pack. For ceiling we usually dont add xps because we can easily get the r we need UNLESS air sealing is difficult. XPS chunks are great for simplifying air sealing in areas of complex framing then using 2 part or 1 part to seal the perimeter.

If it were me and based on your pictures I would have the horizontal ceiling hung, air seal it, apply xps to the exterior of the vertical webs, net the interior of the webs. I would look for a place where I could access the attic cavity, usually from a side and fill the cavity backing out. As we get close to full I add netting and blow the last bit through that.

Attic area: Do you need thermax in your attic area? That stuff costs serious money so use sparingly. Again if I can adequate r behind the area in question use the foam as a block and add the "cheap" r behind it if possible. If you need thermax and thickness in foam, use XPS or eps and then laminate 3/4" thermax over it, much cheaper.

Think about your service door and thickness of wall before you go wild adding thickness to your fire wall, garage areas are somewhat tempered from the outdoors so as far as exterior wall insulation what do you gain by doing it in the overall scheme of your insulation? We usually sheet the firwall in osb, mainly for ease of installing the service door and ease of defining the fire wall above the top plate for the drywallers. Your gypsum will add shear value even though your code may not support it, in WI a double sided drywalled wall with a specific screw pattern will qualify us as a braced wall.

Thats an installers perspective, Im not going to figure out dew point calcs for you to answer what seems to be your moisture concerns, Ill leave that for the desk nerds, just letting you know what we do in the field for those areas.

greentree, thanks for the detailed info, it is great to get an installer's perspective. I understand your explanation and it makes a lot of sense, especially being mindful of doing less costly R-value material if possible and saving the big bucks for where they're really needed. Prior to seeing the framing in real life, I had thought that hubby and I might tackle the blown-in cellulose task. But after looking over the framing, and investigating rental blowers, and hearing tips/tricks that experienced installers use (from your post here, and from other sources) ... I'm thinking I'm going to get quotes and hope to hire out this work. I think we could blow the attic with not too much of a problem, but the exterior wall framing is maybe a bit trickier.

With rough plumbing done and hubby working on electrical, I'm caulking the heck out of the place - basically all the stud cavities, joint between double top plates, and the bottom plate to sub floor. Then I'll tackle all plumbing and electrical penetrations. After that, I plan to put up ripped 2x4s as interior horizontal strapping to give us a bit thicker wall cavity on exterior walls.

- Do you think the horizontal strapping (Mooney wall, as some call it) will cause problems with getting quotes from blown-in cellulose installers?

- Do you have any comments on that on how customers can/should evaluate the actual cellulose material that is proposed by an installer? Are all name brands pretty much the same and borate treated? Or is there a lot of variability and this should be an area of caution in selecting an installer based in part on the material he/she uses?

- And one more odd-ball Q: Regarding the use of the foam 'cord' material that one stuffs into gaps in the framing (like between the window and the framing), I'm not sure what to do on a narrow gap where I could stuff this material, but the gap is some 5 1/2 inches deep (2x6 framing).  Do people stuff insulation material into the back of the gap, and then the foam rod on the interior side? You don't just leave an air gap back there, do you?

Thanks!

Mooney wall shouldnt affect quotes.

Use an installer you feel you can trust, if you aren't sure have several people come over. Insulators can be a shady bunch full of shoddy work, that is why we bought our own equipment and do it in-house.

Foam backer rod (the cord material) is for caulk joints. If you are sealing windows with a caulk joint most window manufacturers instructions will read to stuff the gap with fiberglass or leave empty and then seal with backer and a caulk joint. Most approve low expansion foam, that is what we do. It penetrates deeper into the frame and provides a good bond. You need to know what you're doing when making long lasting caulk joints, most don't and they fail fast.