pressure tank insulation

Insulate the building instead (or as well)...

Depending on the size of the tank, one could perhaps find one of the "made to fit" water heater blankets that would be simpler to fit.

What a mess and what a pita when one needed to do anything in there... :(

If there's nothing in there that generates any particular amount of heat, just stuff the entire "doghouse" with loose straw.

What an astoundingly bad idea! Straw will rot and draw infestations. The mold will draw the insects and eventually rodents. Which will end up gnawing on things like wiring...

Better to reinsulate the well house AND wrap the tank. It won't do much good to wrap just the tank if the other parts freeze up.

Well, pursuant to my question I went to Lowes to see what options I have. I think I'm going to try a roll of that foil bubble stuff with the shiny side pointing inward. You get a pretty high R for the given thickness. Thanks for the replies.

Plumbing supply will have pipe insulation that will fit. I know it comes up to 20" diameter, maybe more. Downside is you may have to buy a case of it.

But it is cheap, it works and it gives the mice somewhere to live.

Actually insulating the building is a really good idea but focusing on the tank; you can try expanding foam insulation. Probably too tedious to use the stuff in the cans for sealing doors (but it would work) but you can get it in a 2 part mix (mix a small amount and apply with paintbrush) and also larger sprayers (like what we use to foam pack boxes at work). The stuff is way more messy and expensive than batts but you can probably find a contractor to do it and make it look nice.

Look for encapsulated rolls of insulation as it is cleaner to work with. Johns mansville makes an R-13 for 2x4 wall I used in walls and inside a spa tub enclosure that wasn't too bad to work with. The plastic bag like wrap can be closed up with a few pieces of duct tape to seal the fiberglas inside completely.

They also make fiberglass insulation which is surrounded by a layer of plastic, making it much nices to handle.

Bob

According to wkearney99 :

And peeing on the tank. Which'll eventually corrode through. Mouse piss is quite hard on steel. Don't ask...

If the OP is objecting to batts because fiberglass is a pain to handle, he might want to consider using rockwool instead. Denser, and not ANYWHERE near as "picky"/"dusty". Wrap the end result in heavy plastic (leave some gaps at the bottom to let moisture escape).

Spray-on insulating foam (_not_ the canned stuff) would be quite nice/neat if done well, but I'd be a little worried about entrapping moisture from condensation with an exterior tank.