I live in a newly constructed, single story, 3500 sf house, that is built on a concrete slab with an attached 3 car garage. There is an electric heater in the garage, but I never use it since it takes too long to heat up the garage to a comfortable temperature in the winter. I live in NW Montana, and it’s been in the mid-20’s for the past few weeks. There isn’t any snow on the roof at the moment, but there is insulation in the attic space above the garage and in the garage walls, which are all sheet rocked.
We keep the house at a comfortable 65 degrees during the day, and let it drop down to 60 degrees at night. There is a single ~20 foot common wall that connects the house to the garage, sheet rocked and insulated on both sides, so 3 of the 4 garage walls are exterior. There are windows in the garage and two garage doors that have some amount of insulation in them. Otherwise the garage is made of standard building materials, including cement board sliding. Two of the garage walls are exposed to passive solar radiation in the morning, but only for a few hours a day. The ground source heat pump is not anywhere near the garage and there is nothing that generates heat in there that I know of.
I have a thermometer I keep in the garage to ensure it doesn’t get much below freezing. I’ve been checking it every day and it is generally at 38 degrees, + or - a degree or 2. So what is keeping the temperature in the garage from dropping to something closer to the outside temperature? Is the heat from the house likely leaking into the garage through the common wall or attic space? Does the passive solar heat add enough energy to keep it 15+ degrees above the outside temperature? I would expect it to be much closer to the outside temperature, but having not lived in a cold climate I don’t know why I had expected that.
When you say “mid-20’s” do you mean that is the average, the high, or the low?
The insulated thermal mass of the garage + the warmth of the common wall will keep the garage slightly warmer than the average outside air temperature. If it’s been in the “mid 20’s” as a high, you must be losing a lot of heat from the house.
The Earth is itself a source of warmth (BTU’s, per se). Not much, of course, but its not ‘neutral’, for sure. There may be enough heat conducted from the foundation/flooring to make a difference, especially if its a one-piece foundation (in-common with the house, per se). Just a thought…
Sorry for being so vague about what I meant by 20 degrees. The outside temperature has been between 10 and 20 degrees at night, and between 20 and 30 degrees during the day, so I just averaged it all out. For example, right now it’s calm and 25 degrees outside and the garage temperature is 41 degrees, so again there’s that 16 degree delta between the two.
If the house was leaking heat to the garage I would expect the heater to be going on fairly frequently, and that isn’t the case. It was cloudy all day today so I don’t think there was a lot of passive solar heating going on.
It is a continuous slab, so perhaps the heater is keeping the house-side of the slab warm and heat is transferring (conducting?) over to the garage-side of the slab and that heat is radiating and therefore keeping the well insulated garage relatively warm (as Ionizer suggests).
A question like this is a challenge because there are so many inputs and outputs, and we don’t know all of them.
For example, many people have their HVAC/furnace and water heater in the garage, both of which produce a pretty sizable amount of waste heat. My garage also has exposed water pipes and the water tends to be about 50-55 degrees when it comes in from the street. So even cold water is heating my garage in the winter.
There is definitely heat entering the garage from the house, but it probably isn’t all that much. Likewise with heat coming in from the ground.
Sun on the roof and walls is probably the biggest contributor, depending on the color of the roof, whether it is covered in snow, and how much shade and cloud cover there is. The sun can easily make dark objects much warmer than the surrounding air temperature. Just look at how snow melts in a parking lot - there’s often an overhang of snow around the edge of the pile where the sun can heat up the asphalt and melt the snow faster than the air can.