Project: Insulating a Worm Bin

I often joke that the worms  are my only pets, and to a degree, it’s true. I feed and care for the tiny red wigglers (Eisenia fetida) that live in bedding instead of in the ground and eat my kitchen scraps. This time of year, that means helping them stay warm on the coldest winter nights.

Here in Seattle, we often get our coldest weather in January, after which the Longest Spring on Record™ breaks bud in February. So I have to bring my garden through a month of cold. For the plants, that means an extra layer of floating row cover on the veggies that are surviving in the cold frames and cloches.

For the worms, that means insulating their box.

What to use

It’s as simple as wrapping the bin in some insulating material. You can use straw bales, bags of compost, or whatever you have sitting around that would keep out the cold.

I use half-inch expanded polystyrene (EPS) rigid insulation. It’s light, easy to work with, and provides just enough help. And it is weather-resistant, as it is rated for ground contact (used to insulate concrete) and to not retain moisture (used under the siding to wrap houses).

A 4-foot by 8-foot sheet is about $10, and can be cut to cover the sides and top of the worm bin.


As the photo shows, it is covered on one side with a metallic facing material. On the white side, it has a clear plastic facing. Both surfaces are weather-resistant. I install it with the silver facing in, hoping that the heat radiating from the worm bin will be reflected back into the box. There are other types of EPS without the silver facing.

Cut and tape

I measure and cut pieces to be roughly the size of the four walls and the lid. The four wall pieces are held to the box with duct tape, which can be cut and left on the pieces when I remove them. Thus you can reuse the material many times.

Cutting this product can be a little messy. I use a sharp utility knife, but it still creates a ragged edge, because the material is made of tiny chunks of foam pressed together. No doubt a contractor would have a special tool for this, but it’s not worth it for my limited amount of use. Cut almost all the way through the material, then flip it over and carefully cut the facing on the other side. Clean up the foam bits that flake off.

The ragged edges will tend to flake off a bit more as you work with it and store it. For a cleaner look and more longevity, wrap the cut edge in duct tape.

The top is set on the lid of the box and held down with heavy objects. This makes it easy enough to feed the worms without having to tape anything to the wooden box, as the tape would either pull the paint off the box or leave residue behind.


Two more tips

The worms have their own defensive reaction to the cold: they huddle together. In the winter I often find them in one warm mass in the center of the box. The collective heat will keep more of them alive. So to help your worms survive better in the winter, feed only rarely, and do it at the warmest point in the day. Dig a hole in the center of the bin and bury the food, covering it well.

Also, make sure the bin is completely full of bedding (leaves, straw, shredded newspaper, etc.). The bedding will also act as a great insulator.

bedded worm bin

The bedded worm bin, with a decaying pizza box to keep my plastic digging tool on top of the bedding.


If you’re just thinking of building a worm bin, check out the plans for a standard-size wood box (the one in my pictures) at the Seattle Tilth compost resources page. That section will educate you about compost, and there’s also a plan for an “off-the-shelf” worm bin that you can keep indoors.

For more details on caring for your squirmy pets, pick up a copy of Worms Eat My Garbage by Mary Appelhof.


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