I’ve read the “installing closed-cell spray foam between studs is a waste” article here outlining the meager difference in r-value gain by using closed foam due it not coming to the stud face.

My question is – does formula change substantially if there is an air gap? I have a log cabin with 2×4 framed walls on the interior with a 1/2 gap between the logs and wall (flat log on interior so “consistent-ish” spacing compared to log wall). I got two bids for closed cell spray foam – first guy bid 4″ to get r-28 and second guy bid 3″ and says itll be r-24. From reading it up it seems like with 2lb closed cell, we aren’t going to get 4″ average anyways, but should be more like 3.5 with the 1/2″ gap. Just wondering if I should be looking into going to the face with open cell or if closed cell makes more sense since there will be a thermal break for the studs.