I am also in the Northeast and have had to tackle this type of situation a few times. There are several ways to attack it.
In your case, the best solution, but unfortunately the most expensive would be to use an open cell foam kit to fill the joist bays. They are fairly easy to use and give you about an R-4 rating per inch. They are a two tank kit with a hose and wand.
Here is an example product: Touch N Seal 1000 Kit Open Cell Spray Foam Insulation
These type of kits start around $400, depending on the square footage and depth required.
A cheaper method would be to use 24 inch R-19 unfaced blanket insulation cut to your custom widths. You can use wire springs to hold it in place and keep it from falling. I would then cover the entire area with 4 to 6 mil plastic, stapled to all the joists. Tape the joints, if any. Moisture should not be a major problem since you have already done what sounds like a good job sealing the dirt floor. In the summer months, some ventilation in the crawl space will also help avoid excess moisture.
Rigid foam glued to the foundation walls would also help if the area is fairly air tight. If there is free air flow, insulating the brick walls would be a waste of time and money.
There are two primary uses for caulk:
- aesthetic gap filler
- functional moisture/air barrier
For the latter, you'd see that wherever you need to waterproof joints (shower panels, exterior windows, etc.)
For the former, it can really be used in any situation where you have two pieces of material meet. Typically this is done when two different materials meet and you want to cover a small gap. Example would be where door trim meets the sheetrock. Higher end finishing would have that caulked before painting.
So, as for these shelves, it's really up to you. One option is to caulk it. Another would be to attach some trim moulding.
As for how to properly caulk, the basic method is:
- cut tip to size
- apply to joint
- smooth to finish
There are tools to smooth it, but I prefer to just use a wet finger (and plenty of paper towels to wipe finger off as you go...)
Note if the gap is too wide, you may need to fill the gap first. I've used foam weather stripping in the past for this.
Best Answer
A thick tape like Wigluv would only be used where a flat surface isn't needed or any such bulge will be concealed.
Caulk would be used for small and shallow cracks or gaps (1/8th inch or less wide) and a flush or completely flat surface is needed.
Foam is for moderate to large gaps (1/4 to 3/4 inch, depending on foam type) and deep or full depth straight through filling.
Of course, all 3 can be used, or any combination of them, that addresses the need for a specific result or longevity.