Ok, so we closed on our home last March and budgeted a new addition when we did. First off we wanted an attached 2 car garage with office space over top. So we got an estimate. The estimate was all but lost when we found out that we had a 15foot setback for attached additions on the side of the house with a driveway. With the setback we could only attach a 18.5ft structure. The other side of the house was out of the question because the need to adjust the driveway to the other side through woods. The cost to move the driveway and do the work to the trees would be insane. Our contractor has been extremely flexible. Anyway, our entire house is shaped like a P with the small end of the P on the side we wanted the addition. I got the idea to add the garage on the front and side of the structure. Thus extending the side only 14feet on the side with the setback. The structure is now shaped like an L instead of a P with the garage exceeding something like 15feet in front of the front porch. Sounds crazy but the initial plans looked awesome. However, the addition of the 25x25 garage with the 20x14 foot attachment added an extra $6000 in materials. The only way to cut costs was not to get the 800sft over top of the garage and the 200 sft on the first floor 20x14 connecting segment finished. I actually think $6000 extra is not bad.

So we are getting close to the end of the project and it is NICE. However, I am left to finish the second floor and the rear of the first floor. I have no worries that I can finish the job. After all, we rented the house from a family member for 6 years before purchasing. When it got close to closing I replaced the old 1988 trane 3 ton heatpump with two more appropriate rheem 2 ton units. I did the work myself using only knowledge from the internet and the manual. I hired a local contractor to handle the filling of the systems with r410a. The guy asked me a few questions about what I did like flowing nitrogen while brazing and the lot. When he was done testing for leaks and filling he told me that if my current carrier didnt work out I could easily do HVAC work... I have also replaced stoves, done wiring, replaced carpet, conditioned my crawl space, and every other project known to man so this is just a realization of it all. I have no doubt I can finish this project myself.

Now on to my question on insulation.

Some of my ideas might seem radical. They are my own custom modifications of what is here and there about spray foam with the fraction of the cost.

I added up the needed supplies to refinish today and I am pleased. I am also pleased that using the cellulos insulation from Lowes that I priced I would be saving ALOT of money even if I had to use TWICE what the mfg estimated for my sft needs.

First off the cost for R19 in insulation is maybe $50 in blow in product over R13. The addition of 2x2 furring strips to the interior wall to make the 2x4 construction support 6 inches of material is only $150. I have a few options here.

A) Lay them directly on top the 2x4 frame nailing them in.
B) Lay them 90deg to the 2x4s and parallel with the floor. Would require less furring strips and eliminate thermal bridging. But be less sturdy then option A.

I then have three options for applying the cellulos now that I effectively have 6 inch cavities.
A) add the wallboard over the gap putting one inch holes at the very top where the molding will later cover. Blow in insulation behind the wall board.
B) do similar to a but with a 1/2 inch sheet of r3something rigid foam insulation. Once full I would then install the wallboard over. Advantage airseal and eliminate thermal bridging.
C) use the stupid expensive net to hold the cellulose at bay until I installed the wallboard/Sheetrock.

For the attic I already have a challenge. I want to use the attic as storage but there are only 2x4 trusses. I have to effectively build a platform in the attic to help support the weight independent of the trusses to avoid sag. Not a big deal but extra work. My current structure has the HVAC for the second floor in the attic. I am unfortunately stuck with this and it makes no sense to do something different with the new structure since part of the underneath garage will be finished by the contractor for code reasons and would require removal of wallboard and insulation to put the ducts in the floor of the top part of the structure.

I intend to insulate the floor to about R19-R30 of the attic with the same cellulose insulation. I will also be bringing the existing structure up to code. Now here is the tricky part. The old section has a ridge vent I retrofitted as well as a new one. I intend to keep the ridge vent. Use the underside roof deck baffles from lowes and then insulate the underside of the roof with either bat insulation of the cellulose if I can get it to stay. Next I plan on putting half inch rigid foam on the underside and air seal. This way the roof deck will not cook bc of the airflow from the ridge and soft vents. I will also stop additional heat flow downward into the space with the insulation. Some heat will naturally still flow up into the attic to keep the air handler and ductwork closer to living space temp. I will also be adding a very small supply and return vent just like the crawl to help with moisture. I will also have a dehumidifier in the space.

I am looking for thoughts good or bad on these combination of ideas... I have already ruled our spray foam due to the extreme cost. I can finish this whole thing for less than just the spray foam under the roof deck will cost.

Are you familiar with the Mooney Wall? If not, check it out below.

Mooney Wall

Another option I had considered for a modest priced moderately high R DIY wall was to build standard 2x4 @ 16" wall, then rip strips of 1/2" polyiso "Rboard" (R3) ~2" wide on my table saw using a metal cutting abrasive blade and glue or staple those strips over the 2x4 framing.  Then rip 2x4s in half and screw them over the top of the Rboard strips with a single bottom plate and single top plate for attaching sheetrock and trim.  I had then planned on having wet sprayed cellulose applied to fill the cavity (cost here for material and labor is ~$1/sq ft).  But you could do the netting and dense pack dry cellulose if you have access to a blower strong enough to dense pack.  The advantage of this approach is that the framed wall width is exactly 5.5" wide (3.5" + 0.5" +1.5") which exactly matches 2x6 wall framing and standard door jambs for 2x6 construction will work.  The Mooney wall approach is only 5.0" wide.  If door jambs aren't a concern, you could further increase the R value of my approach by using 1" Rboard instead of 1/2" Rboard.

First, before you blow anything in the walls, seal every stud bay to the exterior sheathing with either acoustic-sealant type caulk or 1-part can foam and don't forget to air seal between double top plates and between the bottom plate & subfloor. Exterior sheathing is a much more reliable long term air-barrier than anything you do on the interior (but the interior side is important too.)

The Mooney-wall /lateral furring strategy works and works well. Whether interior-side rigid foam is worth it or even advisable depends on your climate and the whole stackup from the exterior paint to the interior paint. Netting makes it easier to blow the cellulose from the interior in a readilyi inspectable fashion, but it can also be done after closing it up with air-tight gypsum, either by drilling the gypsum (use a hole saw an save the plugs for a cleaner finish repair) or from the exterior. But be sure to air-seal the blowing holes when you're done.

On the roof deck insulation you need 2" of clear vent from soffit-to-ridge on each rafter bay for that approach to be sound. Batts can work if you're meticulous about details, with no gaps or compressions. But beware the low-density fiberglass batt- unlike cellulose and rock wool they are somewhat translucent to infra-red and will underperform during the cooling season. High density "cathedral ceiling" fiberglass batts are much better, but for about the same money you can go with rock wool. Blowing cellulose into rafter bays with ventilation chutes is a bit tricker than in wall assemblies, and if it crushes the vent chute or somehow blows by it can clog the vent space putting the roof deck at a high risk of rot at some point in the future.

Note, splitting the R value between roof deck and attic floor is technically a code violation, but whether it actually creates a problem depends on the climate. Got a zip code?