Definitely install insulation in the floor if your temperature differential is that high. Fiberglass batts between the joists are your best bet. Spray foam is better but much more expensive.

faced or unfaced? and thank you!

Colleen, I advise a lot of people who post here to consult an energy rater (HERS or BIP). I'm going to do it again. How to insulate is very dependent on climate conditions. A local energy rater will know how to do it where you live. Don't depend on contractors to know. They are a conservative bunch and will often follow outdated traditions. You might educate yourself by reading at https://buildingscience.com.

In a damp, hot climate you have to have the vapor barrier to the outside so the floor assembly can dry to the inside. In a primarily heating climate, the assembly needs to dry to the dry to the outside so the vapor barrier needs to face the living space. In some cases, no vapor barrier is recommended due to the mixed conditions. Depending on what you have under your feet right now, you may introduce a second vapor barrier. That is a real no-no. The materials can't dry to either side.

Yes, sealed, semi conditioned crawlspaces are more in vogue, but that is conditional too. Your soil is, apparently, damp. The thing I would be most comfortable doing there is a vapor barrier with a rat slab. Plastic with gravel will work, however., the plastic will degrade.

Insulating directly under the floor will work, but that leaves the pipe problem if they project very far under the floor at all. Insulation will not protect pipes if there is no way to get heat transfer into the pipes unless freezing temps are very transient and not very deep freezes. The other thing to think about is the nature of your floors in their present state.

If there was more room, I might spray most of the underside with foam if the pipes are recessed enough to be a lot closer to the floor than to the exterior. I'd be tempted to put rigid panels under heavily plumbed areas and tape the joints so plumbing problems don't require a lot of re-foaming. Then your crawlspace can be left open and the moisture down there does not matter. It won't pay to insulate under the floor and seal the crawl.

New Jersey soil temperatures are decent and will moderate your home temps in both summer and winter. Unfortunately, unless your crawl does not communicate with the adjacent ones, you have to close that off too unless you can convince the neighbors to follow your lead.. That is a pretty miserable prospect with 12-18" depth, although maybe you can come up with a way to easily fit some two-piece runs of panels together. (Now way i'd try to cut them individually so they all fit. If you are already isolated, I'd seal up and insulate the open sides, closely monitor the humidity down there and call it a victory if the humidity is low enough.

The energy rater will also tell you if this is the low-hanging fruit for heating fuel savings. It probably is, and keeping your feet warm in the kitchen is worth more than a couple of bucks. A rater will also know of local programs to subsidize energy efficiency.

Here in warm SC I had the same dilemma. Insulate the floor joists or insulate the basement/crawlspace walls to the outside. I had an unfinished basement with vented crawlspace with 18" to dirt under 3/4 of the house. I ended up doing the wall/brick insulation. I first encapsulated the dirt with a 12 mil vapor barrier and taped seams and to foundation. I then sealed up all the vents and sprayed diy spray foam 2" thick layer on the brick over the vapor barrier and up including the rim joist.

This made a huge difference in the temps. It now stays around 65 degrees year round. I could have added an hvac vent down there but didn't need to. The other advantage is the musty dirt smell is gone as is the very fine dirt dust.

My moisture levels have stayed around 30% so no problems there.

If I were to have insulated between the floor joists I would have used polyiso foam board insulation and spray foamed to seal the seams.

Rich, it certainly seems like you found a good solution. I wish there was a pat answer to how to treat crawlspaces. It is very difficult for many homeowners to figure out and many builders don't have the training. In the humid South, when mechanical air conditioning was introduced, building standards did not, not to mention the retrofits. What you get is a rotten mess, literally. The LSU article below is somewhat dated and there may be follow-on studies on their web site and elsewhere. Again, this is not all-purpose instructions, but can be used to illustrate how different the problems can be in different climates. The (early 1940s) home I bought most recently was an early adopter of cooling and had glass batts in the floor supported by wire mesh (hardware cloth). The flooring was a mix of hardwood, carpet over plywood, vinyl tile and Congoleum roll, It had extensive sub floor rot as a consequence. It is not a good combination in this Gulf coast climate. The impermeable flooring combined with the falling-down batts is a perfect storm.

LSU floor insulation study

Certainly NJ is not the Gulf coast (and I realize you weren't implying that).

I think it is safe (reasonably) to seal crawl spaces most of the time in the North (and most of the South). First, you want to make sure there isn't a source of water due to poor grade or leaking pipes. Then you cover and tape the plastic. Monitor humidity of course. Then move on to sealing the walls and insulating. When you do that, the crawl space will get dramatically warmer during the winter. This will keep pipes from freezing, reduce energy use, and make your feet warmer. Typically foam board on the walls is the easiest option.

Now - you need to keep an eye on humidity in the summer and run a dehumidifier if necessary.

The big question I didn't see the answer to is - are there ducts down there? I am going to guess you have radiators and not forced air. This complicates things but still makes sealing worth it. Last week your crawl was probably 20 degrees or less. Seal and insulate and it would probably be 50 in the same conditions.

Here in NC, we have forced air so it is pretty easy to condition that space - in fact it usually happens without any work just from duct leakage.

I would recommend spray foam the entire thing. It is difficult to get the vapor barrier correct on the floor joists in a crawl space. The VB needs to be on the warm side, and how can you protect the floor joists from moisture when the bottom of the joists are on the cold side?

If anything, do a butter coat on all the joists and floor, then you can put non faced batt between the joists. That would save some on cost.

"Seal and insulate and it would probably be 50 in the same conditions." That would make my toes happy.

In addition to energy raters, another source of information for home improvement with respect to energy efficiency that is specific to your local climate might be your local land grant university (Rutgers/NJ Institute of Tech.) or adjacent states' land grant universities (Penn State, Cornell, University of Delaware/Delaware State).

Thank you everyone for your incredible insight! I already have the vapor barrier so I think I'm going insulate the plumbing pipes, close the vents, spray foam the air leaks, and insulate the walls with rigid foam. Questions - Do I need to tape the vapor barrier? I have it weighed down around the edges right now. Could I just insulate the walls of the front and back since the side walls are shared with other houses? Should I just monitor humidity levels and turn on a dehumidifier as needed?

Side note - We have forced air but no ducts in the crawl space.

Thank you again!

If the adjacent homes are open to you and to the outside, you will gain very little or nothing closing off just the front and back, unfortunately. You will have to close off the sides of your space as well. If you close off the sides and don't insulate them while insulating the front and back, you will have some gain, but I can't say how much and if you do that, it might be a waste of time insulating the front and back. If you are going to close off the sides, your solution to closing it off, might just as well include some insulation (board) as long as you are messing with it.

If you enclose (and insulate) the crawl, one advantage is that you may not have to insulate the pipes and ducts. The benefit of doing so will be greatly reduced. You may not have to worry about pipes freezing if they are not within walls in the wrong side of the insulation. The soil temp in S. Jersey at 2" depth is now in the 30s. increasing with increased depth. Mean temp at 30' will be in the 50s. I can't say for sure how low the temp will go with heat above or if heat fails because I am not that familiar with that region.

Yes you must tape the vapor barrier seams and also around any columns. I ran out of my good vapor barrier tape and used tyvex tape for the last bit of mine (stuff they use on house wrap). It worked well.

I ran my vapor barrier up the walls about a foot and used double sided "foundation tape" to secure it, then spray foamed over the vapor barrier. This completely seals the vapor barrier. You want an air tight barrier between the dirt and the crawlspace. Check your vapor barrier for tears, holes etc and tape it.

If the walls to your neighbors is a solid wall I'm not sure if you would need to insulate that or not. My guess and what I would probably do would be just insulate the exterior walls (edit--what Ionized said above!). I closed my vents by cutting a piece of rigid foam and wedging it in then spray foamed over it.

When I was considering rigid foam I was looking for used "polyiso". This has a higher R value than regular rigid foam and you can sometimes find it on craigslist.

Also make sure you get closed cell spray foam (versus open cell).