I was moving some plants (in planters) in to the garage (for winter protection). The agricultural extension folks told me to put some cardboard / insulation material below the planters, before laying them on the garage concrete floor to protect them from “cold temperatures”
Question : There are two identical planters (say as big as the homedepot buckets) and filled with dirt. Both planters have a thermocouple glued to the bottom of the planter. Planter A is place directly on concrete , Planter B is placed on a thick carboard then place on concrete. The planters are never watered during the winter. The air temperature is mostly in the 60s but can fall in the 30’s.
Is there a situation where the temperature of thermocouple in A shows a lower temperature than B ? In other words, which one is more : heat transfer from ground below to concrete surface or heat transfer from air to concrete surface.
Sure.
The concrete is going to be a relatively constant (or at least very slowly changing) temperature. It’s both inside, and connected to the ground, which is an enormous heat reservoir. So, if there is a sudden warm spell, the air in the garage will get much warmer than the concrete. Conversely, if there is a sudden cold snap, the concrete will be warmer. Think of it as a huge heat buffer.
Air is a poor conductor of heat. Consider this comparison: heat your oven to 400 F, then open it and put your hand into the 400 F air for 5 seconds
Ok, now heat a frying pan to 400 F and press your hand onto it for 5 seconds. Which will transfer more heat to your hand?
Here’s a cite that says the thermal conductivity of dense concrete is roughly 50 times that of air.
Unscientific personal anecdote here. I worked for 2 years in the basement of former brewery. Concrete floors and exposed brick walls. The ambient air temperature was normal, same as outside, but after an hour I somehow found that the building had sucked all the heat out of my body. If you’d taken my temperature it would be the same, but my hands and feet were blocks of ice.
But why? Here’s my guess. There are several types of heat transfer. Direct thermal transfer is only one type. There is also radiant or infrared heat. This is why thin foil blankets provide outdoor backpackers with some insulation from the cold. And in an ordinary building, our bodies give off radiant heat and normally the walls reflect it back. So the exposed brick and concrete really were drawing heat and energy out of me.
I’ve also had experienced winter campers tell me that you never lay a sleeping bag directly on the cold ground. Always use a good insulating mat.
What this has to do with plants in the garage… I dunno. I’d listen to the agricultural extension folks, though.
Absolutely. That’s why even if you keep your house at the same temperature year around it will feel cooler in the winter because of the colder walls, floors and ceilings.
When I remodeled my house I put foil faced foam insulation on the inside of a some rooms. I was once sitting on the floor with my back to a corner but several feet out. In a short time my back was toasty warm
