There are a couple of terms and conditions here.
Two fundamental concepts to remember when building your cooler are:
- cold sinks
- water falls
It sounds obvious, but again and again I get calls from folks with converted box trailers or structures built up on decks that did such a great job insulating the walls and roof but then put nothing or half as much in their floor!
Coolers built up on a deck, need at least as much (preferably more) insulation in the floor as in the walls. If you build on a deck instead of a slab, you need to insulate the floor to r25 or preferably to r30.
If you have an untreated wood floor under your cooler, you will need to put in a vapor barrier. This is how we did ours:
- We put three layers of carpenter plastic between the styrofoam and our top layer of plywood. (Some people happily report using single pond liners instead.)
- We stretched the plastic out into a “bathtub floor” that goes up the walls a couple inches
- There it's gathered under “Roof Edge”* that is screwed about 3 inches up from the floor all the way around the wall like metal baseboard trim.
- We caulked the entire top of the Roof Edge so the water dripping off the walls won't slip between the plastic and the wall and pool up under the floor.
*Roof Edge is cheap and available in the flashing and roofing sections of hardware stores
If you're building on a concrete slab (or dirt floor) and plan to stay:
- above 45 degrees then insulating the floor will never pay.
- down to 38F... there's a bit of an argument for insulating the floor, especially depending on your region, but typically you can still reach 38.
- below 38... you really need to insulate that floor. Even just 2 inches makes such a big difference.
Haven't poured the concrete yet? It's so easy to add below grade (and cheap) I would definitely do it.
- First layer, put down gravel
- Second, put down a plastic vapor barrier
- Then below grade rigid foam insulation. 2 inches is even fine, four is great.
- Next Rebar or mesh
- Lastly, pour! We mix in fibers to make it stronger
How To Insulate the Floor:
Whether you're building up on an existing trailer deck, on a basement floor or on a slab you do NOT need to frame out a floor.
- Place rigid foam directly on the floor
- Lay plywood painted with porch paint/ exterior paint directly on top
The plywood spreads the load out enough over the rigid foam that even after 10 years, our foam still looks fine.
Studding out a floor is not only slow, but fitting insulation between the studs generally leaves leaks and allows for thermal bridging through the framing studs. Save time, save money and do a better job -- how often does that happen!
Tilt the floor towards the door so that any water that gathers (due to condensation or dripping veggies) can naturally drain out. Don't forget to site your structure so the water has someplace to go once it leaves the front door if you plan to take a hose in your cooler.
If you're building on a trailer or a deck-type floor, this is obviously easy to do. If you're pouring a new slab, just build it above grade a bit and put your form-work a barely noticeable “off-level” towards the door.
The only people maybe needing drains are folks who do meat processing and regularly spray down the cooler. Otherwise... It's more trouble than it's worth.
There isn't enough water accumulating to necessitate a drain and putting one in would just compromise the integrity of the cooler. The only thing the drain will really be letting leak out is cold air.