The following project has been excerpted from The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit by Stephen and Rebekah Hren
Project Time: Weekend.
Cost: Inexpensive ($50–100, but can be built with scraps, except the thermometer).
Energy Saved: High. Average refrigeration uses 8 percent of the average American energy budget.
Ease of Use: Medium. Food temperature can be uneven, making it a little less effective than a conventional refrigerator. Requires some attention to ensure proper function.
Maintenance Level: Low. A well-built box, protected from the rain, should last longer than a conventional fridge.
Skill Levels: Carpentry: Moderate.
Materials: Plywood, insulation, wood glue, screws, paint, caulking, gallon jugs, closable heating vent, thermometer.
Tools: Hammer, flat bar, drill, saw caulking gun, paintbrush.
Many existing homes have an excess of windows on the north side. Ideally, no more than 5 percent of a home’s windows will be on the north side, as windows inevitably lead to draftiness and heat loss in the wintertime, and northern windows are the biggest offenders. If you live in a cold climate and are in a home that has a north-facing window convenient to the kitchen, removing that window and converting it to an icebox could provide your refrigeration needs for over half the year. And this energy savings will come at a time when solar electric energy production is low due to short days, saving precious watts for other needs like lighting and movie watching.
Your goal in building a cold box is to take advantage of the coldest part of your home to do the work of preserving food, rather than using an energy-intensive appliance that cools down food in the heated part of your home. You will want to regulate this cold box to provide optimum unfrozen freshness while keeping the cold from entering your living space.
The basic cold box structure consists of a plywood box with an insulated door. A closable vent allows for temperature control. The vent is opened at night to allow in cold air and closed in the morning. In extremely cold places, it might be necessary to do the reverse and open the box for some part of the day in order to keep the box closer to 40 degrees F (4 degrees C) and above freezing. The shelf arrangement can vary to suit your needs but should allow for some movement of air within the box to maintain an even temperature. If you live in a climate where the average temperature is below freezing for a substantial part of the year, you can include a separate freezer compartment at the bottom of the box (remember, cold air sinks).
A great way to regulate temperature is with some kind of thermal mass that takes a while to heat up and cool down. The best readily available thermal mass is water. Plastic water jugs can be placed on the bottom shelf to maintain an even temperature, reducing overheating in warm spells and freezing in cold ones. Generally, the cold box will work whenever the nighttime temperature falls below 40 degrees F (4 degrees C), although it will still be somewhat effective at higher temperatures. Operation in colder environments may include replacing frozen water jugs with room-temperature water jugs, or increasing the insulation on the exterior of the cold box, or both. A thermometer (especially the min/ max recording type) can be a big help to determining how well your cold box is working. During the coulder seasons (spring and fall), adding ice in a bucket or tray to the top shelf can prolong the box’s usefulness. Using a cold box into the summertime becomes difficult. In addition to the warm days, there’s a good chance the outside of the box will get heated up by the morning and afternoon sun.
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Measuring the window. If working during cold weather, before removing the window you’ll want to build your cold box and have it ready to install. By removing the trim surrounding the interior of the window, you will expose the framing that locks the window unit into place without exposing the space to the outside, and you’ll be able to get an accurate measurement of the rough framing in order to construct the cold box.
To remove the interior trim, first break the paint around the window by using a utility knife to cut around the window where it hits the wall. Then use a hammer to knock a flat bar between the trim and window unit (on the inside of the window so as not to damage the wall material) and pry it off. The framing should now be exposed, making an accurate measurement possible.
To take accurate measurements for your box, you’ll need to determine if your window is a single unit or if it is double hung. Newer windows are single units and are connected to the house framing with finish nails through the exterior trim. Since the entire unit will be removed, you can measure the rough opening (the 2 °— 4s) to determine the size of your box. Doublehung windows are built in place and are typically found in older homes. Your cold box will fit into the space occupied by one or both sashes. By removing the bottom sash (after removing one of the stops), you can measure the opening and build the exterior dimensions of your box to this size (actually a little smaller to make sure you can fit it in). For both types of windows, you’ll be sliding the cold box in from the outside, so keep this in mind as you measure and build.
Constructing the cold box. Building the box is straightforward, just like building a basic cabinet. The easiest material to use is plywood. Three-quarter inch is ideal, although 5⁄8-inch or 1/2-inch should also work. Cut the back of the cold box first. Next, cut out the perimeter sides, top, and bottom. You’ll want these to be at least as deep as the water jugs that will rest on the bottom, with clearance for the insulated door. Glue the sides, top, and bottom to the back and screw together with 2-inch screws.
Building the shelves. The shelves need to leave room for air to circulate to maintain an even temperature, so if you’re building them with wood, be sure to leave an inch of clearance between them and the insulation of the door. Alternatively, these could be built out of a simple frame stapled with hardware cloth. Old fridge shelves could potentially be modified to slide in. The unit needs to be primed, caulked, and painted on the inside and outside to ensure longevity. The interior environment will be moist, so protecting the wood is important. If the back will stick outside near to or beyond the eave of the roof, the exposed portion of the box will need some kind of roofing to shed water away from the house.
Removing windows. To remove a single-unit window, you’ll need to pry the window free from the interior. When your cold box is built and ready to install, start the removal of the window unit from the inside of the house by sliding the curved end of your flat bar between the exterior casing and the outside of the rough framing and prying. Work your way around the window, making sure not to free the window completely, as it could drop to the ground. If you’re at all worried about this, have someone else outside to steady and catch the window, in which case you can continue prying until the unit is free. If you’re by yourself, loosen the window until you can see a gap between the casing and the adjacent siding on at least one side. Then go outside and, still using the flat bar, pry the window free from first one side and then the other. The moment when the window comes free can be tricky, especially if you’re up on a ladder. Employ a helper to steady the window from the inside if you’re at all uncomfortable.
If you have a double-hung window (composed of two separate sashes), the window will be easier to disassemble. The casing, stool, and apron can be left in place and the cold box can be fit inside the window casing from the outside once the sash or sashes are removed. If you remove the window stop on one side the bottom sash will come free into the inside of your home. The top sash could potentially be held in place by an additional stop, or it may be possible to easily remove it once the bottom sash is removed.
If the bottom sash is weighted, the ropes will have to be cut to remove it. Potentially, you could build a small cold box that simply replaced the bottom sash. Or, alternatively, the bottom sash could simply be opened and screwed into the top sash to steady it, and a cold box could be built underneath it. If you’re going to remove the bottom sash, you want to make sure not to lose the weights into the wall when you cut the sash free. Put a screw into the casing, hold on to the rope above where you cut it, and then tie the rope around the screw.
Installing the cold box. To install your box you’ll need to enlist some help. Once your box is built, remove the sash and slide the cold box into the opening from the outside. Have your partner properly position the box and then screw it into place while you hold it steady.
Adding the cold box door. The cold box door can also be made of plywood, either a sandwich with plywood in between or just a single sheet of plywood with some insulation board glued to the inside. Attach with hinges. The door can be held closed using a sliver of wood that turns on a nail or screw or a locking clasp.
Place your jugs of water in the bottom and once the cold weather arrives, you’re ready to go. Since installation in a traditional window is straightforward, you might consider seasonal use of the cold box, replacing it with the sashes during summertime.
Simplifying Coldbox Operation
The northern window box has the potential to be operated with much lower daily maintenance with the incorporation of a thermostatically controlled vent. While this would involve a source of electricity, the amount of power used would be minimal. Incorporated into the back of the icebox and set to open when temperatures are in a given range, from, say 32 degrees F to 45 degrees F, the automatic vent would regulate the interior temperature, keeping it from wild diversions outside of this range that would spoil or freeze food.