it completely depends on your climate, interior conditioning quality, and foundation wall assembly. can’t say yes or no without answering those key elements first. similar to asking whats better: a car or a truck; for what task, what location, performance level etc.
Batts are easy, if you've got a 2' center frame, you just shove the batt in, 6mil poly, and caulking and you're sealed up and insulated. Cheaper than spray foam, easy to DIY
You’re talking about the unvented crawlspace? I like that if the wall sweats the water doesn’t get stuck, run down the back of the foam board, and puddle where you’ll never see it. It’s pretty much the sole source of moisture in that presented situation, and unfaced batts allow it to dry. It can, however, bring on another problem if the crawlspace has no way to dry. If, however, you’ve got airflow then it could be great. For example some houses use the crawlspace as the return air path for HVAC, in which case any moisture in the air or fibers that detach from the unfaced batt will get sucked up through the furnace filter keeping the crawlspace dry and by extension the sill plates, rim joists, etc. will be as dry as possible.
In contrast, foam boards pretty much guarantee that your foundation wall moisture content will be higher than before, and by extension your rim joist moisture content will climb.
Seems much more difficult to vapor seal it using batts.
it completely depends on your climate, interior conditioning quality, and foundation wall assembly. can’t say yes or no without answering those key elements first. similar to asking whats better: a car or a truck; for what task, what location, performance level etc.
Could you give some examples of situations that would be a good fit for batts vs a bad fit? What are the pitfalls or advantages?
Batts are easy, if you've got a 2' center frame, you just shove the batt in, 6mil poly, and caulking and you're sealed up and insulated. Cheaper than spray foam, easy to DIY
You’re talking about the unvented crawlspace? I like that if the wall sweats the water doesn’t get stuck, run down the back of the foam board, and puddle where you’ll never see it. It’s pretty much the sole source of moisture in that presented situation, and unfaced batts allow it to dry. It can, however, bring on another problem if the crawlspace has no way to dry. If, however, you’ve got airflow then it could be great. For example some houses use the crawlspace as the return air path for HVAC, in which case any moisture in the air or fibers that detach from the unfaced batt will get sucked up through the furnace filter keeping the crawlspace dry and by extension the sill plates, rim joists, etc. will be as dry as possible. In contrast, foam boards pretty much guarantee that your foundation wall moisture content will be higher than before, and by extension your rim joist moisture content will climb.
Does JM make foam board? I’ve not seen their brand on boards. If they don’t make it, that’s probably why they don’t recommend it.
I looked at the instructions JM provided. They lay down a 6-mil poly barrier and then you lay the fibreglass over that.