Spending a few grand on going from R20ish to R40ish seems worth it to me in Minnesota. It also allows for other options for heating versus natural gas. In my case with R40 or close to it, my heat load goes from about 17000btu/hr at design load to about half that with a well sealed 2100 sq ft house. I'm looking to add about 128sq ft of solar thermal and cutting the natural gas($144 in connection fees yearly and the MN PUC has a rate case for increasing the basic service charge) and using one of the super efficient mini-split heat pumps in its place will also let me lose ductwork losses and put the heat where I want it versus the whole house.
Insulation in my case is going to allow me to avoid spending $6000-8000 on replacing my existing 8.5 SEER AC and 30 year old natural draft natural gas furnace and instead spend it on one to three mini-split heat pumps that I can install myself much easier than installing a central system(gas furnaces are illegal to DIY where I live and I'd get no warranty coverage if I did and a mini-split is easier than a central AC too).
So in my case, blanketing the house in an additional 4" of XPS before replacing the siding and windows(both need to be replaced so it is a sunk cost anyway) makes sense.
If I was building a new place, I'd aim for R40 or so cellulose done with a double stud design with advanced framing with 24" on center in Minnesota. If I lived in a more comfortable place that doesn't need high latent reduction and high heat loads, comfy places such as Seattle or Portland, I'd probably be cool with 2x4 6" framing with R21 fiberglass or a cellulose fill with inverter mini-split heat pumps. Either way, I'd be looking for a well sealed enclosure ACH ideally under 2, especially so I can fight less latent gains in the summer.
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