Victoria's Cabin: beefing up the R value

[keyholefarmhouse]

I'm starting my material list and am buying product when on sale.  Right now Dow blue board is on sale.  I've been wondering how to add a little more R value and am aware of the benefits of redundant applications.  Although, I get a little confused over all the different types of foam, and whether to put it inside or outside.  

My wall is projected as follows:
                                             board and batton
                                              1/2" air gap                                                
                                                tar paper (30# felt)
                                                 "sheathing"   7/16 OSB for whole wall
                                                   2X6 with fiberglass rolls as per plan
                                                     1'' blueboard with reflective side facing in (warm side)
                                                      sheet rock
My roof as follows:
                             Roll roofing (reflective galvanized steel to be added later)
                                   OSB (7/16)  
                                    2X12 with fiberglass (as per plan with 1'' air gap)
                                     1'' blue board with reflective side facing in (R-6.5)
                                      sheet rock

I live in NE WY with a hot sumner and cold winter.  Cold season is longer.  Somwhere I read our walls should be R-30 and ceiling R-50.  I'm not to concerned about getting that high of of an R-value but would like to do better than what the plan shows.

We are not in a earth quake or high wind location.

[glenn-k]

I would suggest not considering using the Ondura --while not all bad there were quite a few bad experiences of others.

http://countryplans.master.com/texis/master/search/?q=ondura&s=SS

John will be best for the rest of this.

If you wanted red, you could go to red high rib roofing of some sort.

[jraabe]

I think you would find it easiest to place foam insulation on the interior side of the wall and ceiling systems. Especially if you use extruded polystyrene (blueboard or pink board). If cutting out for outlets, etc. seems like too much work you could do the wall foam on the outside over the sheathing but under the tar paper. Keep the foam to the interior of the rafters and eliminate can lights in the ceilings if you can. You will want to keep the roof ventilation air gap.

You can likely use 7/16" sheathing for both roofs and walls. Check with your supplier as the panels will be rated for the stud (wall) or rafter (roof) spacing. If you don't have strong winds or earthquake issues you can probably just use structural sheathing at the corners (4' each side) and can then save a bit by using non-structural sheathing (composite board or foam) for the rest of the wall.

I show a fully sheathed wall as I think it is cheap insurance, especially for new builders (see this overview of the APA's argument).

Your application of foam will add more to the efficiency of the house than the R-value of the insulation as it will insulate the framing and any gaps in the batt insulation. This greatly decreases these heat loss corridors.

You will not be sorry you superinsulated your home. The costs savings in your climate will pay it off fairly quickly and the quieter and more even tempered interior will also be appreciated.

PS - note on spacers for the siding. You only need 1/2" to break the capillary action and if you use 30# felt the wrinkles in the tarpaper will do the job without spacers.

[keyholefarmhouse]

I modified my above posting according to your suggestions.
The walls look great on insulation value.  The ceilng is @ R-37, should I do a second sheet of 1'' blueboard to it?  It's another $450-.  That would bring it up to @ R-44.

Thank you very much

[jraabe]

I would guess (it's not a trivial calculation) that a second layer of insulation in the roof would not be cost effective. You have a very good insulation system there.

For every doubling of insulation you cut the remaining heat loss by half. At some point you are spending thousands of dollars to retain a few escaping BTUs. This is the law of diminishing returns.

PS - I don't think blueboard comes with a foil face. If it does give me a link as it will be a new product to me. Isocyanurate (Thermax) insulation does come with a foil face (and a higher R-value per inch).