"The house will be completely wrapped in Tyvek and we are also using an insulated backed siding from Alside which provides another R-5 of insulation behind the siding. Our attic space is free of venting and HVAC equipment. Our roof has a ridge vent for breathing." we put our reflective sided closed cell foam sheathing on the exterior of the walls. foam board caulked at top and bottom, all holes sealed prior to cladding install. this in conjunction with caulking the sole plates begins and insulated and air sealed wall from the exterior. in my climate we use 1' closed cell foam sheathing. this reduces thermal bridging..2x's are approx 1R-value per inch..R value of 2x6=5.5 a big reduction of insulation value.2x6 construction allows for R-19 of conventinal insulation add the closed cell foam sheathing and your wall value increases to R-25 combine this with air tight drwyall approach to interior of walls after insulating with conventional insulation. this I've found is a better use of $$ for foam in walls. we don't see many houses who foam seal the attic floor..since we so stupidly put our ducts and mechanicals in the attic..we mostly foam insulate the roofline..costly dependint on roof pitch. but by foaming the attic floor to air seal and then installing conventional insulation over it..as in your specific case, this is a good idea. the goal is to stop air leakage through the insulation so that it can perform as designed. my goal as an energy rater is to determine the most cost effective way to do this. foam in the walls is costly, and does nothing to reduce thermal bridging of framing members..think about it corners and tees..window framing..overall the thermal bridging reduces walls R-value by approx 20% you can still add the foam sheathing over the houswrap. and since the foam is continous it will perform better and make the wall tighter than an insulated siding. best of luck

I live in St Petersburg FL and my attic used to be like an oven. I used the cheap barrier long ago and it helped a little. I replaced it with a bubble wrap radiant barrier stapled to my roof rafters. That gave it a R value of about 12 and I installed two solar attic vents on the house to help remove excessive heat within this barrier. I attached 2x4 on the peak area of the roof in the attic and made it so all the hot air rises into this space. Then added an electric fan blowing outside air into the attic and the peak area. I added another fan on the other side of the house to exhaust the air out. My attic is cooler than ever. This makes my a/c unit that is in the attic to run less and keeps the house cooler. One more thing add another layer of insulation on top of anything that is there alreay. have it run the oposite way of what is up there now. Do you have an attic set of stairs or a hole with a lid. I installed a hatch cover with hinges and added a layer of insulation on top of it. This way when I leave the attic the hatch is closed and keeps out any hot air into the house and cold air in the winter.

Hi Solarguy, Thanks for the chock full of info. For the ornl.gov site, I'm not getting the same numbers. Did you assume polyurethane or something else for insulation? Either way, I think you are right that I should aim for a much higher R-value. Owens Corning recommends R7 rigid foam exterior sheathing and R21 for the wall cavities for R28 total. Right now for walls, I have cedar shingle siding, standard OSB sheathing, R-15 fiberglass batts, 6mm vapor barrier, then finally drywall. The air infiltration was below min standards so they added a heated air-exchange. I like the idea stopping air before it enters the wall cavity, so rigid foam sheathing and foam spraying the cavity perimeter makes more sense. I mean, I'm stopping the air from passing thru the houses interior, but I'm sure I've got air movement in the wall cavity which kills my fiberglass R-value. Gary Reif, If you could provide a link for the 8.5" wall contruction you are mentioning, that would be great. This along with solargary's insulation upgrade calculator should arm me with enough info to calculate various ROI. Thanks guys.

The 2x10" joists are fully sealed - bottom by the garage drywall, the top by the bonus room subfloor. It's only via the attics on each side of the room that I can access the floor space and the extra 3-4" between the existing insulation and the subfloor. My plan was to use the 3 1/2" R13, my fish tape, and essentially pull the insulation through from one side to the other, on top of the existing insulation. In order to even think about doing this, I'm sure I need a paper backing (i.e. I'll duct tape to the fish line). I'm hopeful I can pull it with the paper backing/barrier top side up. I currently have access now to both sides (my poor drywall, but this room needs lots of repairs), I have my insulation - and it's time to fish the tape and then get my wife involved to feed the insulation through as I pull! What a fun holiday!