A down economy might be waking the rest of the country up to the importance of savings and DIY, but for farmers working on low margins and high risk it’s business as usual. Although we still live off the income from our 200-family CSA, I developed and now sell the CoolBot, a device that makes a standard window air-conditioner run down to 33 degrees, turning it into a walk-in cooler compressor. We built the “CoolBot” because we couldn’t afford a real walk-in cooler compressor. We couldn’t afford a real walk-in cooler box either, so we built our own there, too. We’ve since helped many other people build their own walk-in boxes as well as getting tips and information back from hundreds more folks that have shared what to do (and sometimes what NOT to do) when building a room for cold storage.

If you’re reading this article, you probably fit into one of two groups. The first group has basic building skills and won’t think twice about putting up a new shed or barn. Those folks just want to know how much and with what to insulate the walls, what to do about the doors and floors and how to deal with the high humidity levels found in the cool fall night that will be your walk-in cooler’s perpetual reality.  The second group has very little building experience, but can’t afford a “real” walk-in box. They need a primer on everything from how to put down a concrete slab to basic framing and roofing questions –  and then they get to all the same questions the first group has.

There are plenty of great basic-building resources already out there (library, Internet, hardware store) on how to build small shed-type structures, so this article won’t cover basic building techniques. Instead we’ll focus on what’s specific to making an inexpensive but energy-efficient  walk-in-cooler box. Once you understand the basic concepts, you can apply them to whatever size and type of building appeals to you most.

If you don’t have a lot of building experience (or even if you do!) you might find that starting out with a pre-built shed, box trailer, shipping container, or even just the inside corner of your barn or garage might be a huge time and money saver. These structures are already structurally sound and weather-tight and just need to be cooler-customized.  

Cooler placement and the roof
You know how in the middle of summer it’s so much more comfortable in the shade rather than out in the hot sun in the middle of your field? It’s the same with your cooler, except instead of dripping sweat, a cooler sitting out in the sun drips dollars – lots of them each month of the summer you keep it in operation! Site your cooler inside an existing building, under a tree, on the north side of a barn, or build a shading overhang onto an existing structure. If none of those are possible, then at least make sure you have a ventilated shading roof on the cooler box. Because coolers are so small, it’s not difficult to build a flat roof that is still structurally sound in wind, rain and snow loads.  Because insulation is so expensive, people tend to build flat tops to save money, which makes sense, but they would save even more money if they added a ventilated shading roof overhanging the structure to shield it from the sun.

People in colder climates tend to put lots of extra insulation in the ceiling of their coolers. In a house, heat rises and that’s why we put twice as much insulation in the roof as the walls. But in here we’re building a box to keep cold in –and cold just sinks. Keep your cooler roof out of direct sunlight and you can put the same insulation in the roof and you’ll save money both in construction and long-term operating costs.

The floor
Two fundamental concepts to remember when building your cooler are that cold sinks and 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! Perched floors need at least as much and preferably more insulation in the the floor as in the walls.

If you plan to keep the temperature above 40°F, you could get away with two inches of insulation in the floor. Below that, go with the industry standard of four inches.

When you’re building up on an existing trailer deck (or above a basement) or on a slab you do NOT need to frame out a floor. Save yourself time and money by just putting pink or blue rigid polyurethane insulation down flat on the floor and covering it with plywood. The plywood spreads the load out enough over the rigid foam that even after 10 years our polyurethane 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.

When we built our first cooler we put a vinyl floor product on plywood thinking it would protect it. The vinyl has long since disappeared – it just couldn’t hold up to the mud and gravel abuse we exposed it to. The plywood underneath, however, still looks fine since no water sits on it.

Which leads to… “Water Falls”! Cold air, like a cold heart, leaves no space for love. When you open the door of your cooler and the warmer, more water-vapor-laden air comes whooshing in, within moments it sadly suffers the pangs of rejection and condenses in tears on the cold heartless walls of your cooler.  To make matters worse, most farmers spray their veggies, so they are already dripping wet as they’re coming into the cooler. Kate and I actually bring the garden hose into our cooler and regularly spritz the veggies inside during the week – and give it a full spray to clean it out after each distribution.  So… water falls, and it needs to go somewhere. If your floor is tilted towards the door even just the littlest bit it will drain out right quick in a lovely waterfall! Don’t forget to site your structure so the water has someplace to go once it leaves the front door. 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.

Because we have an untreated wood floor under our cooler, 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 where 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 all the big box home improvement stores. 

Insulation
Don’t use fiberglass insulation! I know there are people out there who’ve done it and seem happy but inside, they are not happy at all and you might want to tell them that. What’s happened with them is that (even with a very good vapor barrier) moisture-laden warm air from outside somehow found its way into their walls where it condensed back into drops of liquid on the cool inner wall of their cooler. It saturated the fiberglass batts, reducing the insulation value and ultimately growing a goth-style black mold which dragged down both the spirit and loft of the fiberglass even further until they have whole areas of their walls with gaps in the insulation. If they have a small enough cooler and big enough compressor then it will stay cold, fooling them into thinking they are happy, but they could be buying several packs of expensive clove cigarettes and black eyeliner each week with the money they’d be saving if they had sprung for the higher-cost rigid foam insulation to begin with.

The industry standard for walk-in coolers is 4 inches of rigid foam insulation. R-values vary based on different kinds of foam and the age of the foam but you should try for at least R20 in your floors, wall and ceiling. Going up to R30 will save even more money, especially if you are keeping your cooler at 40°F or below. It’s probably not financially worth it to insulate above that. With rigid foam insulation, vapor barriers aren’t necessary.

There are three types of rigid foam insulation and the cost and availability seem to vary around the country:
•Pink or blue polyurethane loses a significant percentage of its insulating value over the first 5 years. After that it stays stable. While it’s the best choice for a floor, unless it’s cheap in your area it’s probably not the best choice for the walls. Make sure to use at least 4 inches of this material in walls, floor and ceiling, and you’ll be fine. It comes in 4×8 foot sheets.
•Gray or yellow polyisocyanurate insulation also comes in 4×8 sheets, and it’s a better choice for the walls and ceiling but a bit soft for the floor. It’s usually foil backed on one side (foil goes to the hot outside) in Lowe’s or Home Depot. This product can be a bit irritating to skin so wear long sleeves when you install it. Four inches would be ideal here.
•White expanded polystyrene (EPS) is the older (it was actually discovered back in 1839!) white beaded insulation that packing peanuts, some plug trays and 1970s bean-bags are made of. It’s manufactured as a by-product of gasoline refining so it’s often the cheapest insulation you can find on a per-inch basis. It won’t ever off-gas or break down for thousands of years, but it has less of an insulating value per inch so if you decide to go with EPS, make it 5-6 inches thick instead of 4 and you’ll be in great shape.
•A practical and affordable solution in some parts of the country is spray-in-place foam insulation. This is usually professionally applied and it’s a polyurethane product that is blown against a wall or between the studs of an existing building. Here in NY, grungy male farmers who happen to be suffering from tree allergies and sneezing into their shirts will find that this stuff is cost-prohibitive at over twice the price of the rigid foam products. But if you happen to be a more attractive neighboring female farmer, the same company might spray-insulate your 12×20 cooler for around $100. Go figure. Folks living in the Midwest seem to be finding prices are neither gender specific or prohibitive, so check with your local spray-foam installer. Spray foam applications can turn leaky barns or pre-built sheds into tight, respectable well-insulated walk-ins and require no time or skill. Don’t be fooled by the high insulating values some installers quote. They aren’t accurate (they use “equivalent values” based on the leakage in fiberglass insulated structures). Insist on getting a full 4 inches of spray foam if you’ll be keeping your cooler at 40°F or below and at least 3 inches if you’ll be keeping it at higher temperatures.

One note about basement coolers. I get a lot of calls from people that insist that they are building in an underground basement with concrete walls, so they don’t need insulation. Here in New York, the average underground temperature is about 52°F. At my parents’ house in Virginia it’s closer to 62°F. The insulation value of a concrete block basement wall is about 1/20th that of standard cooler insulation. So… if you want your room to be 52-62°F, then, you’re right! You don’t need insulation. But if you want it to be 40°F, you can get there but you’ll constantly be spilling your cold through the walls into the ground, which is pretty silly. Building a cooler in the basement is a fine idea (though access can be annoying) but you still need to insulate it!

Build it tight
For folks who have access to spray foam, this is really easy. For the rest of us, it can be a bit of a challenge. Building your walk-in cooler airtight is just as important as the amount of insulation you put in! The bigger the temperature difference between the inside and outside air the FASTER your expensive cold air flies out even the tiniest cracks and seams between insulation sheets, under the door, around the air conditioner and in every corner. Buy several bottles of spray foam and caulk and apply it to every corner and seam even if you don’t see a noticeable gap. It takes less time and makes more of a difference than you might think!

Don’t cut your insulation up and fit it between the studs! Not only will you invariably leave a few gaps and holes, no matter how careful you are, polyisocyanurate shrinks a bit over time, making your kids think you were a sloppy builder when they rebuild the cooler 15 years down the road because the cooler costs too much to keep cool! EPS doesn’t shrink but it’s still better to use the full 4×8 sheets of whatever insulation product you buy and tack up on the outside of the studs. None of the big box stores sell rigid foam insulation 4 inches thick, but you can use that to your advantage. Buy two layers of 2-inch foam and overlap the seams to keep things sealed up.

The door
I’ve seen really beautiful home-made doors that do an excellent job, but unless you’ve got great building skills, I don’t recommend it. It’s cheaper and faster to buy a standard pre-hung insulated exterior door. This is because the key to the door is that it seals up tight and that’s not so easy to get right! If you leave the smallest gap in your door, much of the effort you put into insulating and sealing up the rest of your structure will be wasted. Glue another layer of 2-inch rigid foam to the inside of the door. (It’ll break down in the sun on the outside.)  You also have to remove the bottom metal piece of the pre-hung door so it doesn’t dam up the water inside your cooler (and that means you have to cut the sides of the door down so the bottom stays flush against the floor. Since you are now mounting the door flush against the floor, do make sure your door opens to the outside world or it will scrape and stop against your inner floor. Obviously, you need to make sure that your cooler is up a bit from the surrounding ground level so the door can swing outwards freely — but you’ll have done that anyway so the water drains out!

Inner and outer sheathing
Many people just leave the exposed insulation on the inside of their coolers. If you don’t have employees and you’re careful, that’s fine. We have aggressively destructive employees and our cooler is open to a public I seriously suspect to be drunk most of the time by the way they ransack the place, so our inner walls are sheathed with $7 sheets of OSB (oriented strand board). We could have used plywood; it was just more expensive. There are proper, waterproof inner cooler siding materials but they are over $1 per square foot and our neighbor built his walk-in cooler in 1985 with OSB he painted with white porch paint and it’s still completely solid 24 years later!

If your cooler is built inside an existing structure you don’t need to sheath it, but if it’s outside, it has to be protected from the sun or the insulation will start to break down. On our second cooler, we ripped cheap half-inch 4×8 sheets of CDX plywood into 1×8-foot strips to make home-grown overlapped siding! We live in a registered historic district and we are often in trouble with the village elders for not taking their “hysterical” district as seriously as we should, but our cheap plywood siding solution (properly stained a dull green) apparently looks nice enough from the road that it’s one thing no one has ever bugged us about!

Building your own cooler can be much more cost-effective than buying a ready-made cooler box and since you can add extra insulation, the extra time you put in building it can also pay off in long-term energy savings down the road.

For more, visit Ron’s website, www.storeitcold.com.