Building a basement cold room

I'm building a house with a basement and one corner will be used as a cold room to store veggies and preserves.
The room will have a concrete floor and I'm wondering whether we should lay out Styrofoam insulation UNDER the concrete floor just like the rest of the basement before we pour concrete on the ground, which my builder tells me to.
I think that the cold room needs the coldness from the earth and the concrete on the floor and that we shouldn't have insulation under the concrete. But my builder says we should otherwise the moisture from the ground will spread to the rest of the basement.
I don't think he understands how cold rooms work. A cold room needs to be humid at 80-90 RH with good ventilation and the moisture shouldn't be a problem for the rest of the basement as it should be sealed off.

Edit: I'm going to have ventilation through the joist on the top of the cold room as my builder's suggestion. I wonder if it's okay. But the concrete walls have been already done.

What do you think? Thank you.

Hi Julia welcome to Permies;
I am no expert but I think you are correct.
Your cold room would be insulated from the rest of the basement.
No need to insulate it from the earth.
Did you plan on any ventilation? I suspect that you would need it.

I'm just starting to plan how to retrofit a cold room into my basement and I wish I had the opportunity you do. I also think you specifically don't want to insulate from the earth -- you want to make use of its cooler than home-interior average temp.

thomas rubino wrote:Hi Julia welcome to Permies;
I am no expert but I think you are correct.
Your cold room would be insulated from the rest of the basement.
No need to insulate it from the earth.
Did you plan on any ventilation? I suspect that you would need it.

Yes. I'm going to have ventilation through the joist. I edited my posting. Thank you for welcoming me.

Christopher Weeks wrote:I'm just starting to plan how to retrofit a cold room into my basement and I wish I had the opportunity you do. I also think you specifically don't want to insulate from the earth -- you want to make use of it's cooler than home-interior average temp.

Thank you. That's what I thought. My builder is also an avid gardener growing and selling greenhouse flowers in the spring and it's hard to argue with him on these things because he thinks he knows better than me in terms of gardening and building. I've been researching on building a cold room but I wasn't able to ask questions anywhere. Then this morning I realized I can here. Cool.

Hi Julia,
My only concern would be if the entire basement has a solid concrete floor and one corner was missing insulation. This could cause the moisture to come up, but instead of staying just in the cold room, it would wick out across the floor to the rest of the basement. In that case I would be tempted to insulate across, and then deal with cooling other ways. If the cold room has a separated floor, this should not be a problem.

I think "it depends"...  

Are old (uninsulated) basements in your area damp and musty and gross?  Or are they, more often, dry enough?  If they're not all gross, I'd say you don't need insulation in that corner.

Where in the world might you be?  One reason to not insulate is to harness the coldness of the earth below the basement.  But if your earth temp 6' deep is warmer than a root cellar, insulating might help you keep it colder.  Even in WI where my basement root cellar is, the soil temp below my basement floor is likely 45 degrees in winter, warmer than the root cellar itself.  My cellar gets cold because part of the basement wall is exposed above grade and not insulated so it gets very cold.  That, plus some forced ventilation on cold nights, helps my cellar get cold enough in October to start storing things.

If the floor doesn't get insulated, one concern may be the high humidity of the cellar rotting out the nearby house parts (floor joists, etc).  So you'd want to build the cellar so that humidity from it can't get out into the basement.  Vapor barrier would be one way...

If you do insulate it, you should be fine as long as you can get cold air in there in the autumn.  Regardless of the insulation detail, I'd put a fan in one of your vent pipes so you can force cold air in when the temps outside are favorable.

Did you make a decision? I'd trust the builder/pro on this. It does not seem wise to me to have the construction and temperature so variable in something as critical as a basement floor with that much thermal mass. What I would do it pull cold air in to the floor through strategic venting/piping and insulate the room like crazy. Have you seen the book Root Cellaring? They have good plans on how to do this. Your location surely matters too. What zone are you in?

On a new build, I personally would never, ever compromise the concrete foundation. It has to act as a single unit, and the thermal and moisture elements cannot have large differentials. Anything else is a recipe for disaster IMO.

The most successful root cellars I have seen have been add-ons, located outside the engineered envelope of the house. They communicate with the house thermally, and share its heat/cool, as well as the dry zone under the eaves which also adds insulating properties in my dry locale. One was a well house under a concrete slab that was insulated; the other was deliberately built with controllable ventilation. Both had insulated and well sealed doors to the basement.