Renovation - Spray foam or Batt Insulation! Advice please!

Goatman Saint said
He said it’s a wall that has the sun hitting it in the afternoon. If he has a standard 2x4 wall with fiberglass it’s only giving him an R value of slightly over 6. If he has 3 inches of foam applied it’s almost an R value of 20. That’s a huge difference on a wall the sun beats down on. That part will be noticeably warmer due to that. Also, if he’s not upsizing his A/C unit, there is that to think of also and a foam insulated room costs very little in additional AC. There really is no substitute for foam, and certainly if the price is the same. It will be 3x the insulation R value per inch, and the quietness is very different in a foam house.

Where I live and work the code requires r13 minimum in the wall. R13 is standard insulation batt in a 4" stud wall. You can increase it by adding rigid foam to the outside or inside, but R6 is incorrect in a 4" space.

Oh, and this is factually incorrect:

Goatman Saint said
If he has a standard 2x4 wall with fiberglass it’s only giving him an R value of slightly over 6.

If you wanted to increase that one space you could do it with a higher R product and still not need foam or you could use the foam.

If you're worried about the radiant heat from the exterior due to sun exposure, you could also add a radiant barrier by using RB sheathing on the exterior wall.

Also, you said "if he's not upsizing his AC unit" and I don't understand what you're talking about.

You can't just add on 30% to a house and not increase or add ac. Building code won't allow it unless you're in some place where they don't have codes. Further, if you over insulate and then upsize your AC you're likely to cause all sorts of problems with excess moisture and short-cycling your ac.

Bigger AC units and more insulation are not sound building science in a vacuum.