I have an 1100SF home in East TN (mixed humid) that was built in 1954.  There were 1" rock wool batts installed in the walls and at some point in the last 15 years about 6" of blown in something installed in the attic floor.

I am planning on replacing the exterior wood siding and the black fiberboard sheathing underneath with Hardie plank and MDO sheathing.  I would like to reinsulate at that time and obviously sprayfoam would appear to be the best but most expensive.  One option I am considering is removing all of the exterior siding and sheathing and placing fiberglass batts in the cavities and covering that with rigid foam baords to prevent thermal bridging and air infiltration.  I would cover that with MDO and then the Hardie plank.  Does that sound like a good option?  Any problems with the rigid board under the MDO?  BuildingScience recommends the exterior sheathing go under the foam board, so that's why I'm unsure.

Also, I am going to add R30 batts in the attic in January.  I can leave the existing blown in there and just cover it or I was thinking of removing it and leaving the cavity between the joist open under the fibergalss batts to aid in any lighting or electrical work I may do.  Does this sound like a bad idea?  Would the cavity cause me any problems?

Thanks for any advice.

Educate yourself about the benefits of radiant barriers first. studies prove that adding radiant barrier is superior to adding more mass insulation esp. in the south

Sounds good, but for structural reasons, you can't put foam board on all the walls, leaving no diagonal bracing.
Here is what I would suggest to do. Do not remove all the siding at once, work one wall at a time so that you don’t weaken more than one wall at a time. After you remove all the siding and install your batts, on the corners, first install one sheet of some plywood or osb. Use about ½ inch thickness. Cover the rest with foam and the plywood parts with thinner foam. For example, use 1” for most of the wall and ½” foam over the osb. This will still give you a thermal break, without compromising structure.

As for your attic, getting more bulk insulation up there is always good especially in the winter. I would just put your fiberglass batts over the existing cellulose, no need to reduce the R value before you increase it. Also, be sure to roll your batts opposite of how the joists run, and be sure to get unfaced. If you want to address radiant, the best solution is once you have added your fiberglass, get some of those cheap foil faced ½ inch foam boards, and just lay them on top of your fiberglass, reflective side up. Take care not to compress the fiberglass. Do not tape the joints, but leave a small (1/2”) gap, to let them breathe.

Why are you using MDO plywood?

John

Thanks for the posting the help.

I am using MDO (medium density overlay) because of its cost, lower than plywood, and its durability, weatherproof and insect resistant.  This would add quite a bit of structural reinforcing to the house.  I think I was unclear on what exactly I was doing.  The MDO would be the "OSB or plywood" sheathing, the added structural support for the walls and the foam board would be under that.  Most people recommend the foam to go on over the sheathing, that's why I was curious about reversing them.

I know reducing the attic r-value needlessly would be less than ideal, I just had an idea for dealing with any future elcetrical work by having the completely open joist cavity.  I like to come up with weird ideas sometimes just to solve simple problems.

MDO is a specialty plywood with a paintable surface. It is used for signs and concrete forming. I have never heard of it being used for sheathing. I priced a piece of three quarters last month and it was about $80. Advantech, which is an osb product with waterproofing resins added would be a more reasonable choice. I am on the west coast, perhaps you have a different product or usage back east.

John