A common truism that isn't is "heat rises". Actually, what rises is air that is warmer than the surrounding air. Anyone who has lived with a woodstove knows this - it's a lot hotter at the ceiling in the room with the stove than it is at the floor. But heat flows from hot to cold, so it readily goes from our houses down into whatever connection they have with the ground, because the ground is cooler than the temperature most of us like our homes to be at.
A surprising number of buildings have no insulation between the finished, occupied portion and the ground. In a concrete slab-on-grade building, such as many schools, having no perimeter or sub-slab slab insulation leads to cold floor edges, and especially if the floor is carpeted, mold.
Most houses in New England have basements (or crawl spaces). There are two locations for insulation between the finished above grade space and the ground. The first is in the first floor joist cavities, above the basement, and the second option is to insulate the basement walls and floor slab. Either can work well to slow down the flow of heat between the house and the ground.
Insulating the frame floor is usually done with fiberglass batts, those pink or yellow items that facilitate coughing and itching. This method is cheaper than bringing the whole basement into the thermal enclosure. Putting the thermal boundary at the first floor, if done well, works fine at reducing heat loss to the ground. The weak point of this strategy is that the basement gets colder. How cold depends on a number of things:
1 - Is the basement leaky to the outdoors? If so, it will get colder than if it's air tight. Even in cold climates such as northern New England, uninsulated basements won't freeze if they aren't leaky. And frozen pipes, the bane of the northener's winter, are most commonly caused by air leaks at the sill area, with the pipe located in front of the leak.
2 - How much of the basement is above grade? The more foundation that is exposed to the ambient air, the colder the basement gets.
3 - What is the ground temperature? In much of New England, the temperature at the basement footing level is somewhere near 50F. If the basement is mostly below grade, the (uninsulated) basement temperature won't drop much below the ground temperature. The entire area of the floor slab is in contact with this stable temperature, and during the summer, when the basement warms, the sub-slab areas warms also, storing some heat that comes back in the winter as the basement cools.
4 - Are there heat sources within the basement? The most common, of course, is a forced air furnace and uninsulated, leaky ducts, or a boiler and uninsulated pipes. As the outside temperature drops, the heating system fires more often to keep the house warm, and losses from the system keep the basement warmer than it would be without the heating system. In the most extreme cases, the basement is actually warmer than the house!
So one good way to get a cold basement is to insulate the frame floor above well, and also insulate below the floor slab. Here on MV, SMC did this in a lot of houses, figuring that an owner could always add wall insulation to the basement but it would be hard to add insulation to the floor slab (partially because of headroom issues). House 5 has one inch of extruded foam beneath the slab, for example. But insulating beneath the slab cuts the heat flow from the earth in the winter. In one case, some houses were built with spray foam in the frame floor, and two inches of rigid foam beneath the slab, and no basement wall insulation - basically the culmination of the typical MV strategy. These houses, because they had very good thermal envelopes above grade, were heated with a point source propane heater in the main living space, and therefore had no basement heating system. The first winter people were surprised to see the basement temperature drop below 45F! This shouldn't have been a surprise, though - very little heat came from above or from the deep ground, and so the basement temperature headed to a point somewhere between the ground surrounding the basement (which is coldest at grade) and outdoors.
In humid climates, the drawback to insulating the frame floor goes beyond energy, to an air quality and aesthetic concern. After winter, the ground has cooled down. The ground at grade warms in the spring, but the basement walls, particularly down near the footing, are cool. When summer humid air gets into the basement, condensation occurs as the air is cooled down below its dewpoint. This is typically noticeable at the bottom of the walls where they meet the floor slab. The moist surfaces, especially if they are in contact with, and therefore cause to be wet, organic materials like wood, cardboard, paper or fabrics, support biological growth and you get the classic moldy dank basement odor.
House 5 was insulated with fiberglass batts in the frame floor, and as noted, has one inch of foam beneath the basement slab. As we got into mid-late December, the temperature in the basement dropped to 58-59F and seemed to level off. Why? Because the boiler was running, heating the house and our hot water, and all the piping for both heat and hot water were left uninsulated, which makes them very effective heat emitters. On December 26th, as previously noted, we started using the heat pump instead of the oil boiler for heat. Within two weeks, the temperature had dropped into the upper 40Fs. This situation promised to make the walls even colder as we entered the summer season, potentially making the smelly basement problem worse. Considering that we were heading to removing the oil system completely, we decided that we would add insulation to the basement walls, the method of which will be the subject of another post.