Cold Room


 Last summer’s garden potatoes and garlic.

The Mill Creek Net Zero Home has a cold room built under the front porch.

The cold room, or root cellar, has long been a friend to those living in northern climates. It takes advantage of the fact that the temperature six feet underground remains a relatively constant temperature year-round.  Therefore, it keeps the room cool in summer, and warm enough not to freeze in winter.

Our cold room has been working perfectly since it was built. During the winter it is about as cold as a fridge. Our potatoes and garlic are still in perfect condition four months after harvest! Plus I only need to buy beer a couple of times a year.

The room is about 6’ x 10’. It is thermally separated from the rest of the house, and we don’t actively heat or cool it. Its concrete floor was poured directly onto the earth because we actually want the heat/cold from the earth to enter the room.


Cold room, Mill Creek Net Zero Home

We insulated the interior of the walls with two inches of polyisocyanurate board (about R7 per inch), and added R28 worth of pink insulation in the ceiling. There are also two vents to the space (see above picture), but we’ve kept them plugged with rags so far. The idea is that we could attach a fan to one of them that runs during the Fall and Spring nights to further cool the space. The concrete floor acts as a heater (in winter) or a heat sink (in summer).

Some construction details: when you are standing on the front porch, there is 5” concrete on a reinforced 8” grid, a layer of plywood, 7.25” of pink insulation, and then another finishing layer of plywood under you.


The room is thermally separated from the heated space.


A reused interior door was insulated by gluing a spare piece of foam to it.

Our cold room provides us with a free way to store food from the garden long-term. It would easily act as a fridge for milk and such during the winter if we ever needed it to.

It saves us energy in today's world, and it provides us a hedge against future disruptions to our energy supply. I suspect that we will be very grateful for this cold room at some point along the way.

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Great detail here! Thanks for the inspiration... maybe this will be this summer's project.

Love the door idea. I built mine out of 4" of polyextruded boards sandwiched between plywood. I'm no carpenter, so building custom doors and casings was the toughest part of building my cellars. Your idea's a lot easier.

I've just completed a small coldroom and am intending to insulate the door just as you have, but what kind of adhesive did you use? In the course of construction I PL300 (foam compatible glue) WAY to offensive in terms of off-gassing. What did you use ?

We used the foam-compatible glue. The sheets could be screwed on I suppose.

A question about how any rain that lands on the porch is kept out of the joint between the porch floor and the basement wall. I am considering a cold room under the porch as you have built. However, any concrete porch that I have seen has a cold joint between the house wall and the concrete slab floor of the porch that is subject to movement and opening up slightly. My question is related to any special details that were used to make sure no rain water entered that joint (and thus into the cold room) should the joint open up a bit as the house shifts over time.

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