Hi,

Will try to be as brief as possible but dealing with multiple options and want to ask them without being too repetitive. I have read quite a lot of QA in the last year or so and it helped a lot but still – there are a lot to learn. I added details for clarity if needed below. So…

What Do I Want to Do? (TLDR Version)
– Insulate and tightly air seal my attic & external wall cavities with rigid foam and reach R49+ on attic and R20+ on external walls with continuous insulation.
– Don’t want to use can foam / PU sealant etc to air seal joist lines & penetrations and then cover with blown in insulation (avoiding reasons here to be concise), even though that’s the cheaper/most common way.
– Also I want to get rid of the 1/2″ fiberboard on the exterior walls but don’t want to replace it with something thicker, either – as I don’t want to deal with the adjustments it may require when re-installing siding back. Dealing with enough complexity as it is 🙁 .

Questions:
– Any concerns using polyiso boards that have previous mold or moisture damages? Even if I were to cut the sections and/or treat it with mold control spray (boracare and/or concrobium etc)? For example, if a moisture/mold damage is present in a polyiso board but placed between good polyiso boards in a tight manner – would it be still and issue – even if it’s treated that way?

– If external walls are filled with foam boards do I need any gaps between vinyl siding and foam boards?

– Since I want to replace existing 1/2 fiberboard with foam board, should I be concerned of any structural issues – even though existing fiberboard doesn’t seem to provide on already? Or, would it better to use 1/2 plywood instead as the replacement, to provide structural support – even that means I won’t have a continuous insulation/thermal barrier for studs.

– Which one of the following approaches would have been better to fill in the remaining space of 3.5″ external wall cavities that has 2″ polyiso board as the first layer? All foam boards will be cut-and-cobbled with can spray foam. Entire cavity will be pre-treated mold control + boracare. Exterior seams will be covered with vapor barrier tape.
— And additional 2″ polyiso board sticking 1/2″ outside and studs are covered with 1/2″ XPS to provide continuous insulation/thermal barrier. Second layer of polyiso will lose some R value on the winter this way but better summer performance.
— Same with above but second layer will be 2″ XPS instead. First layer of polyiso will keep its R value but less overall R value – even though not much difference.
— 1″ XPS that leaves an 1/2″ airgap but entire exterior will be covered with 1/2 XPS board. This will have even lesser R value but if an air gap is needed, with somewhat semi-permeability rating of 1/2 XPS on the outer layer, any moisture in the gap is allowed to escape, may be?
— Existing fiberglass batt squeezed into remaining 1.5″ space.   Again, exterior wall will be covered 1/2 XPS. Will provide additional R value for the first layer but any possible moisture is concerning.

– Any experience using joint and seam filler sealants instead of using vapor barrier tapes, like, PROSOCO R-Guard, for sealing exterior foam boards seams. Should I stick to vapor barriers tapes, instead? Or use vapor barrier tape on top of it regardless – if I am going to use it to fill the seams?

Additional Details:
What Do I Want to Do? (Precise Version)
Attic:
– Don’t want to cut-and-cobble rafter nor floor spaces, I have been there once and yep, not pretty, but don’t want to cover over top of the rafters, either, as I don’t need a conditioned attic, space is already not usable as it is.
– So, for easy to work sections, I am planning to lay down polyiso boards over attic floor joists. First layer will be glued to joists using Great Stuff Pro Construction Adhesive and/or Gaps & Cracks and in between. Seams will be taped with 3M 8067CW.  For not-so-much-easy-to-work-with sections, like towards to end of the gable, I will cover the sections and will fill with slow rise spray foam.
– Consequent layers will be added in a staggered fashion and again glued to previous layer but lightly – just to hold the pieces. And wide seams will be filled with can foam but won’t be taped.
– Existing fiberglass batts in the attic and external walls will be the final layer on the top of the polyiso boards for additional R value and keeping polyiso boards warm enough to prevent them losing their R values.

Depending on my patience, I am planning to have 6-8″ thick foam board this way and R49+ insulation in the end.

External Walls:
– Unfortunately this is the place I need to use cut-and-cobble due to limited space. Take out existing fiberglass batts and put 2x 2″ polyiso boards cut to 12″ length – as there is about ~14.5″ wide sections and I don’t want to cut to it to the size.
– First board will be glued to drywall on the center and second board will be glued to the first. ~1″ Perimeter will be filled with can foam. This will stick out about 1/2″ but as I mentioned above, I will cut 1/2 XPS boards strips to fill remaining sections and on top of the studs.

What Do I Have?
Home: Split Foyer house that was made in 1976 that is located at Zone 4.
– Attic: ~1000 sq ft attic floor that is under 5/12 roof pitch with 25 ft wide and ~5-6 ft high. Joist cavities are currently filled with fiberglass batts but not higher than joists. Have multiple penetrations due to recessed cans used and two ducts (1 kitchen hood & 1 bathroom fan) terminated at the attic. It has gable vents and soffits along with an attic fan.
– External Walls: 2×4 16″ OC walls with R11 fiberglass batts in it. On the outer layer it has asphalt coated black fiberboard insulating sheathing* with R1.3 insulating value with vinyl siding as the final layer. All the walls are 1/2 drywall covered in the inside on both levels.
*:Researched a bit with limited information on this but has AIMA certified? TEMPLE model)

Materials:
– Polyiso: ~240 pieces of 2″ 4×4 and 10 pieces of 4″ 4×8 glass faced reclaimed polyiso boards. Majority of them are in very good shape but certain mold/physical damages on the perimeter. Possibly removed from a commercial roof that has a water leakage.
– XPS: ~16 pieces of 1″ 4×8 & ~4 pieces of 1/2″ 4×8 Foamular XPS. Brand new but 1″ pieces were on outside for about a year. Will be getting more as needed.
– Tape: 3M 8067CW tape. I might use a mixture of Sika and Blue Tuck Tape or any vapor barrier tape as needed.
– Sealants: Will be trying Prosoco Joint and Seam filler caulk and/or similar as experiment.
– Spray Can Foam: Mostly Great Stuff Pro Gaps & Cracks will be used as it has a firebarrier rating, but I have pest control versions as well as some single use cans from other brands that I am planning to use also.
– Wood Treatment: Boracare, mold control and pesticide mixtures.

Equipment:
– Jigsaw with 6″ Bosch Serrated Jigsaw blade that seems to be working well. I have also bought a special circular saw blade that is advertised as dust free but not yet tried to cut the foam boards. Have also 200T 10″ table saw blade that I will try to see if it will cause too much of a dust. I am planning to use bandsaw or radial arm saw to cut if those won’t work well.
– I don’t want to use utility knife or sharpened putty knife – even though I haven’t tried the latter – despite the recommendations I read before. Manual cutting methods were taking too much of time even when I was cutting 1/2 foam boards before – to a point I despise them.