A cold storage room in a finished basement

2016 harvest in cold storage
Fridges and freezers are great inventions and are so convenient that most people put just about any perishable foods in them. Not too long ago, maybe only 60 years ago. fridges were newfangled devices and every house had a root cellar or a basement cold room to keep produce and beer cool and ventilated.

An uninsulated corner of the basement with access to outside air can stay between 5 and 15 degrees celcius and at a high relative humidity through fall and most of winter, keeping your root vegetables nice and crisp for several months.  If you have an unfinished basement (or building your house and designing a cold room), making a cold room is not complicated; unfortunately my house was built in the 90s and has a fully insulated and finished basement.

I grow quite a bit of food in my garden and this year I had too much for my freezer. I also grew Belgian endive which must be kept dry and at cold temperatures until you are ready to force them throughout the winter, so I needed a proper cold storage. Winter temperatures here drop well below freezing (-30C),  making outside storage very impractical, so what could I do?
I guess I could rip out the insulation from a corner of my basement and drill holes in the basement wall above grade to let cold air in. That was too drastic for me, I was looking for a less invasive solution. I googled "finished basement cold storage", but found only articles and discussions about finishing an un-insulated cold storage room in a basement, which is the opposite of what I wanted to do.

Space under the stairs, with the extra door.
My house has the piping for a central vacuum system, which we have never used. A 2 inch pipe starts in the garage and has an outlet on every floor. I thought perhaps I could use a fan to push cold air from the garage into an insulated box, so I followed the central vac pipe and came upon the room under the basement stairs, which was full of Christmas junk ornaments.  I figured that if I put in a door, this would be the perfect size.

In addition to low temperature, air flow is an important feature for a cold storage room. Proper airflow prevents mold and gets rid of the ethylene gas which speeds up the ripening and rotting process. To optimize the airflow, I wanted the cold air to come in at the very back of the room, near the floor. The air exit would then be at the top (where hot air rises) near the door, ensuring an airflow through the entire space. The exhaust air can go into the furnace room next door. The furnace room already has cold outside air coming in so I figured it was a good place to exhaust the cold storage air into.

I picked up 2" foam board insulation, which would give the walls and ceiling a 7.5 R value. The floor itself is bare concrete, so that was ideal.  I also bought a few 2'x4's to frame the door and some 2'x2's to back the foam board ceiling going under the stairs. I already had a 19 Watt Fantech FR 100 inline fan that I bought 8 years ago to do some experimenting with soap bubble greenhouse insulation.  I also needed some central vacuum pipes and elbows; I learned that central vacuum uses very thin PVC pipes that are quite different from the black ABS pipes used for plumbing. I happened to have an extra door around.

The door framed by 2'x4's
Framing the door was pretty straightforward: screw 2'x4's to existing studs and frame as closely to the door as possible, to minimize air leaks. Wood shouldn't be touching bare concrete, so I put a layer of tinfoil duct tape and tuck tape on the concrete where the new studs would go.

Hanging the door in the new frame is the most complex part of this project. First, you put both hinge plates on your door and mark where the frame side plate lines up on the stud. Take the frame side plates off the door, put them on the stud where you marked their positions, and make an outline of the plate.
Then you use a wood chisel to carve out the plate outline, about a quarter inch deep. That takes some chiseling skills, which I had perfected while working on my log shed.

After screwing the frame side plates into the stud, you simply attach your door! Ha ha, it is of course never that easy. You will need a wood plane to shave the bottom and possibly top of your door so it can actually open. I was very lucky to have the door fit almost perfectly in my door frame; I attribute it to the lumber being very straight and square.

2 inch foam board insulation applied
With the door hung in its frame, I used a tape measure and box cutter to cut out foam board insulation. I started with the ceiling; after screwing four 2'x2' runners to the back of the stairs and applying foam board glue, I attached the foam board. I also hammered 3" cap nails into the foam board (the blue circles in the photo), after marking where the runners were with a chalk line.

Insulating the side walls was pretty easy. I used foam board glue to attach the foam board to the drywall and hammered the same 3" cap nails into the drywall just for good measure.

Seams all taped up, intake and outlet pipes installed
Finally, I cut out a rectangle for the ceiling and somehow squeezed that in between the wood framing and the insulation on the walls.

Unless you have perfectly straight cuts and walls, you will end up with some gaps in the foam board joints (I had some substantial gaps). You fill those up with strips of foam board you custom cut. When the biggest holes have been filled, use tuck tape to tape all the seams.

Next, I diverted the central vacuum pipe and cut a hole in the ceiling foam board to bring the pipe in. I extended the pipe to go all the way to the back bottom of my cold storage room. I cut another hole for the exhaust pipe, which I squeezed in between framing to end up in the furnace room (about 8 feet of pipe).

With all the insulation and piping in place,  I finished the door. I attached back stops to the stud closest to the swinging end of the door, for which I used pieces of 2'x2'. Then you'll use your chisel once again to cut out a hole for the latch. Make sure you put the latch hole far enough back to have your door pushed against the back stops when the latch locks.

I put some weather stripping around the door to minimize the air leaks. At that point, I had a pretty airtight cold storage room.
Fan pulling air from the attic into the central vac line.
In my insulated garage garage, I cut a hole in the ceiling to get a pipe into the attic. The attic is open to outside air through soffits and an air intake, so it should deliver some good fresh air. I didn't want to get the air from the garage, since we park in there and I don't want exhaust fumes into my cold room.

Connecting the inline fan to the pipes was tricky, I used various plumbing parts, hose clamps and foil tape (the aluminum film stuff, not duct tape) to make an airtight connection and connecting the fan's 4" outlet to the 2" pipes.

The fan is on a mechanical timer, which turns it on for several hours after sunset and during the day for 30 minute intervals.
Temperature and humidity in the cold room
I put a wireless thermometer and hygrometer in the cold room to see how it was performing and to evaluate the timer settings; as you can see from the chart, the temperature stays around 14.5 degrees Celcius this time of year. Relative humidity is around 60%, which is a lot better than the 40% inside the house.

I will keep an eye on this chart as outside temperatures drop and expect to decrease fan activity as it gets colder.  I hope it can provide temperatures between 5 and 15 celcius until April, when most of my produce will be gone.

I may have to add insulation to the door, to prevent more heat loss if the room stays too hot. When the garage air is at -15 celcius, the uninsulated central vac pipes may create condensation so I may have to wrap some insulation around them.

Stay tuned for more performance data and troubleshooting!

UPDATE  October 10 2016
Insulated door
In an effort to bring the temperature down, I insulated the door with 2" of foam board.

The first snow fell here last weekend, causing the outside temperature to drop and the intake air is now around 5 Celcius. With the insulated door and the fan left running all day, the cold storage is now between 12 and 13 degrees Celcius.

The fan uses 19 Watts of electricity. Left running non-stop (it is designed for continuous duty), it consumes 456 Wh per day or 166 kWh per year. In contrast, a modern large fridge (32 cubic feet) will use around 800 kWh per year, or four times that. Of course, my cold room will only stay cold from fall through spring whereas a fridge will keep the temperature year round, and it will keep it colder.

UPDATE October 27 2016: I calculated the effective R value of this cold room.

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