New industry standard is now r25 minimum.
Different insulation products have different insulation values per inch. It will always be printed on the insulation you buy (or you can look it up in wikipedia!) Don't think about inches of insulation, think about total r-value. You need to hit a *minimum* of r20 (that's about 3 inches of greyish cyanurate foam, and 5 inches of white EPS foam). The more the better. Coolers insulated to r20 will work, but you will spend more in electricity than you spend in insulation, and insulation is only something you have to buy ONCE.
Fiberglass insulation in coolers is a POOR CHOICE. It can get soggy, sag and mold within 1 year of full time use in a cooler. Avoid it if you can, otherwise see the UK Hybrid design that intelligently integrates it (even still we've learned to reduce the advertised r-value of fiberglass by 1/3rd in cooler design. So if it says on the package that it's r13, it's only r9 in the cooler!)
Seal everything inside AND OUTSIDE.
This includes seams in the walls, joints between walls and ceiling and critically even between the floor and walls! Regularly check the gaskets on your door. Use sprayfoam or soft-foam pool toy floaties or the soft foam pipe insulation to insulate between the air conditioner cabinet and the walls.
This is more important than insulation! Even if you can't see the gaps, there is a physics principle that will make even a pin width crack the bane of your electric bill and possibly the structure of your cooler -- and it can be solved so easily by using a couple $5 cans of spray foam or tubes of caulk *before you start your cooler for the first time*.
This is even more critical if you use any fiberglass insulation in your cooler and don't seal every crack and joint on the OUTSIDE of the cooler including between the ground and walls (the opposite of what you might think!). Fiberglass coolers without an impermeable barrier to the outside fail within 1-2 seasons. We cry with dozens of people every year who didn't read this page of our website and have to tear down their moldy, mildew-ridden no-longer-insulating cooler walls -- even in Phoenix, AZ! (This is another interesting physics principle.)
Size It Right!
And get the right brand of air conditioner.
To get the best results make sure that you have the right sized a/c for the job. Not only is it important to have the right size, but it is also important to get the right brand. For more information check out our chart
Keep your cooler out of direct sun.
Coolers in direct sun require suck down 70% more electricity --almost double!! Put your cooler under a tree, on the north side of a garage or barn (or inside), *or* just built under an openly vented roof that keeps the sun off the insulated ceiling of the cooler.
Acceptance of limitation of this system!
Check out our page on Who Should NOT Buy a CoolBot
Allow Enough Room For Your A/C to Breathe!
Air conditioners need at least 2 feet of clearance overhead on the back side to vent properly. If there is not enough room, they will overheat and die a sad premature death never fully realizing their potential.
Vapor Barrier Note
Before we get to the actual article, I want to highlight one thing, and that's VAPOR BARRIERS in home-built walk-in coolers. Although we discourage people to build with fiberglass insulation, lots of you still do, and a professor at KSU has designed a CoolBot cooler I link to above that is sort of a hybrid that mostly uses fiberglass insulation and his design is REALLY good. But you can't change or subtract from what he did (you can add, though!). The biggest issue is the VAPOR BARRIER.
As you are building your cooler, imagine that there is a constant spray of water against the OUTSIDE wall of your cooler. low pressure, but constant -- shooting straight sideways. You can't see it, but that's exactly what's going to be happening. Warm air OUTSIDE your cooler contains more moisture than the cold air INSIDE your cooler. This is even true for those of you in Phoenix, AZ! In a normal house, you put a vapor barrier on the INSIDE of the house, and so that's what lots of people do when they are building their coolers. Big mistake!
Use a continuous sheet of polyethylene (carpenters plastic from Home Depot). And when you are putting it up on (between the outside air and the fiberglass insulation) just keep thinking about that CONSTANT side-ways water-spray and protect against that. The KSU document has you put up rigid foam insulation on the outside of the carpenter's plastic and I think he even has you caulk the seams. That's GREAT! Do that! We have to stop that water from getting into the fiberglass.
Okay, that's all... Now onto the regular article :-)
WAIT! NO! ONE OTHER THING! If you build a narrower rectangular cooler, like 4 or 5 feet wide, you need to make sure you are blowing down the LONG way. So... if you build a 4x6 -- you want to blow the air down the 6 foot direction, not across the shorter 4 foot direction. If you have a 6x10 foot cooler -- you need to blow down the 10 foot direction. If you have a 6x20 cooler... you need to buy 2 air conditioners and put them on either end (and 2 CoolBots).
Okay, that's REALLY it...
Where to Put the Cooler
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.
Flat roofs on a small cooler, may still be structurally sound in wind, rain and snow loads, but they don't protect from sun exposure. Building a standard peaked roof (over the insulated cooler ceiling) will shade the cooler and provide passive ventilation that will keep air circulation over your cooler and save money on cooling costs.
In a house, heat rises and that's why we put twice as much insulation in the roof as the walls, but in a cooler, 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 as you did in the walls and you'll save money both in construction and long-term operating costs.
Now that you know where you want to put the cooler is time to consider the foundation. 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 r24.
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 all the big box grocery 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.
- 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, but 2 inches is absolutely enough.
- 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 (or above a basement) 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 your 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.
The industry standard for walk-in coolers is an R value of 26. Going up to R 30 will save even more money especially if you are keeping your cooler at 40F or below. It's probably not financially worth it to insulate above that.
R Values can vary based on the different kinds of foam and the age of the foam. No vapor barriers are required with the use of rigid foam.
3 Types of Rigid Foam Insulation:
Extruded Polystyrene (XPS):
- Pink or blue
- R Value of 7-8
- After 5-10 years: R Value of 5-6 where it stays
- Best choice for the floor
- Comes in 4x8 sheets
Use at least 4 inches in the walls, floor and ceiling and you'll be fine.
- Grey or yellow
- R Value of 6.8
- After 5-10 years: R Value of 5.5 where it stays
- Best choice for walls and ceiling
- Comes in 4x8' sheets
- Available at Lowes and Home Depot
Use at least 4 inches and if it has foil back then be sure to face that towards the outside. This product can be irritating so be sure to wear long sleeves.
Expanded Polystyrene (EPS):
- White beaded
- R Value of 5
- Retains its R Value over time
- Comes in 4x8' sheets
- Available at Lowes and Home Depot
Discovered in 1839, this is the same stuff your bean bags are stuffed with. Never breaks down but has the lowest R Value so be sure to use 5-6 inches.
Roxul Mineral Wool
Roxul is a rock-based mineral fiber insulation made from basalt rock and recycled slag, a by-product of steel. The insulation is made by melting the basalt and slag and spinning it into fibers.
Because of its moisture resistant and mildew resistant properties, its high R value and has zero off-gases, it makes a great material for insulating coolers.
Since it is not vapor resistant we do need to add some rigid foam to the cooler. You would probably be fine with 3/4 inch, but going up to 1 or 1.5 inches would be even better.
This is a professionally applied polyurethane product that can turn leaky barns or sheds into well-insulated walk-in coolers in a short amount of time. R Value and cost can vary drastically. Depending on your location this may be a practical solution for you, check with your local installer for costs. Four full inches are necessary to keep your cooler below 40F.
Note: Rarely (just a few people in 8 years) we've heard from people that are very sure the offgassing from spray-in place cyanurate based insulation is affecting the taste of fruits and vegetables. I'm not an expert but I did google about this a bit and I learned that if the spray-foam mixture is not quite right (it changes as temperature changes) one of the components may apparently be a concern because it is quite hydrophilic -- the gas quickly binds to water (or water-based vegetables/fruits or humans). It sounds like somewhat of a controversy online... people ripping out all the foam in their houses, for example... but it also sounds RARE considering how much spray-foam is used. The solution is time or blocking... months (but in some concerning cases on the internet YEARS!) can finish whatever that component is (and again we are NOT educated or experts about this and it's not common!) The other option is somehow blocking off the offgassing foam so it can't gas your vegetables. building an inner wall of FRP panels that are silicone-sealed for example.
Spray-in Place foam is so convenient and effective it's hard to want to give it up, but do your own research. No one has reported this to be a problem with the pre-made isocyanurate panels that you buy from Home Depot/Lowes (perhaps because they are completely sealed within aluminum foil!) They are also about 1/2 the price per insulated square foot, so maybe that's the safer approach).
Cellulose insulation is terrific! There is LOOSE PACK cellulose and DENSE PACK cellulose. Loose pack is MUCH more common... you have to have someone awesome to be able to do dense pack.
That's a terrific base, but it is "breathable" and in the case of a cooler... breathable means that ultimately we DO get moisture condensation INSIDE the wall (against the cold side).
It's super easy to fix, you just need to add an inch of rigid foam insulation to the INSIDE of the cooler (preferably before the plywood or FRP if that's possible, but if it can't be, that's totally fine). Not only will this eliminate the condensation inside the walls, it will also stop the thermal bridging through the studs, so you'll solve a problem AND save plenty of electricity. Make sure you caulk/spray foam all the corners as explained above.
DON'T USE IT! Regardless of whether or not you have a vapor barrier, moisture seeps into the fiberglass insulation, becoming not only a moldy nightmare but decreasing your insulation value. This results in higher electricity bills since you a/c has to work constantly to cool your cooler since the cold air is leaking out through the now uninsulated walls.
University of Kentucky Cold Room:
HOWEVER if you want to use fiberglass then use the excellent plans from the University of Kentucky. This hybrid system uses rigid foam panels to protect the walls from water in the air condensing inside them.
Modifications to the UK plans:
- Use more rigid foam. Instead of 3/4 inch, use 1 1/2 inches. This will save considerable money in the end
- If you put the rigid foam on the INSIDE of the cooler you MUST use at least 1 1/2 inches of rigid foam and be sure the vapor barrier on the outside is solid.*
- You could put 3/4 of rigid foam on the inside and 3/4 of rigid foam on the outside
* If the foam is on the inside you can skip the inner layer of plywood. If it's not a cooler that is going to be open to the public and won't be subject to bins being thrown around and puncturing the foam, this is a great way to save money... well... you'll actually break even, because you'll save the money on the plywood, but then you'll spend it on the extra foam insulation. But it's worth it!
A concrete block has a .5 R Value at best. If your average underground temperature is 52-62F and you want to be 52-62F then you don't need insulation. If you want to be lower than that though, you will need some sort of insulation, otherwise you'll be spilling out all the "cold" through the walls into the ground.
Build it Tight
Building your walk-in cooler air-tight is just as important as the amount of insulation you put it! The bigger the temperature difference between the inside and outside air the FASTER your expensive cold air flies out.
Using several bottles of spray foam and caulk be sure to close up:
- seams between insulation sheets
- under the door
- around the air conditioner
- every corner
It takes less time and makes more of a difference than you might think!
Around the air conditioner is a common place where hot air can sneak in. Instead of spray foam, use pipe insulation from Home Depot. It comes in 6 foot long strips and you can cut it to the size you need before stuffing it in the gap between the wall and the air conditioner. It is soft and smooshy, but still "closed cell" in structure, so water doesn't get into it, and air can't pass through. This way if you ever need to pull the a/c out you don't have to carve away a bunch of spray foam first.
Over Not In Between:
Tack the rigid foam up on the outside of the studs. Cutting up your insulation to go in between studs will not only leave a few gaps and holes, but no matter how careful you are, polyisocyanurate and polyurethane will shrink a bit over time.
Stagger The Seams:
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.
Electrical and Lighting
Due to the moist environment, we recommend using exterior rated light sockets and outlets.
The key is actually LESS about the fixture, and more about the perforations that wires coming into the cooler make! A vacuum effect can be caused by the warm outside air being sucked in through the back of the outlet box, by the cold cooler air. This can result in moisture gathering and could cause the circuit to trip.
- Take the cover off the outlet
- Find where the wire enters the back of the outlet box
- Using silicone caulk seal any gap where the wires enter the box so no air can move through
- Replace the cover and thats it!
Note: Besides silicone caulk, clay or feldspar can be used to seal up the gap
AC and CoolBot Placement
The A/C will preform at it's best when installed so the bottom of the a/c is at eye level. Cold sinks, so the higher up the better. This also makes maintenance a lot easier.
If you have a space that is less square and more rectangular in shape, then the AC should be mounted on the small wall so that it is blowing the longest way possible. For example, on a 5x10 room, the AC should be mounted on one of the 5 foot walls.
Side By Side:
The CoolBot should be mounted inside the cooler next to the AC. If there is a troubleshooting issue, you have to have the CoolBot and air conditioner right next to each other.
Note: If it's an issue of wanting to know the temperature from outside, there are pretty cheap ($19) wireless temperature sensors you can get from Amazon -- I've regularly seen them on sale for $12 from WalMart/Home Depot, and more expensive ones ($80) you can monitor on a smartphone or online.
Also, CoolBots can't be in direct sun, so if you still decide to mount it outside, you'd have to protect from that as well.
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.
Pre Hung Insulated Exterior Door:
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.
Heres a few tips to get the most out of your door:
- Make sure your door opens to the outside world (make sure the cooler is up a bit from the surrounding ground level so the door can swing freely)
- Glue another layer of 2" rigid foam to the inside of the door (it'll break down in the sun on the outside).
- Remove the bottom metal piece of the pre-hung doors 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). inner floor.
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.
Our inner walls are sheathed with $7/sheets of "OSB board." We could have used plywood, it was just more expensive. There are proper, water-proof inner cooler siding materials but they are over $1/square foot and our neighbor built his walk-in cooler in 1985 with OSB board he painted with white porch paint and it's still completely solid 24 years later!
Many restaurants and meat processing plants will use FRP panels which are great! They are easy to clean and durable.
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.
We did this by ripping cheap half-inch 4x8 sheets of “CDX” plywood into 1x8 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.
Common Cooler Construction Questions
For common cooler construction questions, please see our frequently asked questions.