Insulating a basement is a great idea if it wasn't insulated when built, as before we began insulating foundation walls and below slabs (see slab on grade), it was generally estimated that basement heat loss accounted for about 1/3 of the total heat loss of a house. It is likely that such an abysmal starting place made the addition of any insulation at all seem sufficient, so basements built to code are still often chronically under-insulated.
Current building codes vary by region and state, but what is consistent is that if you insulate to the minimum requirements of code, your basement will still account for a significant and unnecessary portion of the heat loss in your home. Somewhere between R5 and R10 under a slab tends to be what we typically see now, yet energy modeling shows that additional insulation will have a relatively quick payback period.
The three images below are thermal models showing what you could expect from a concrete floor in a typical cold climate, either in Canada or Northern US; first un-insulated, the second insulated to meet Building Code (R10) and the third insulated to R20, the minimum we would recommend in Canadian and US cold climate zones, for comfort and a good return on investment.
Modelled using a 4-inch concrete floor, you can see that in the first image of an uninsulated floor, the colour gradient from warm to cold shows that heat lost to the ground below is evident to a depth of many feet, while in the images with insulated floors you can see that much more heat is contained by the insulation. Also worth noting is the light red colour of the uninsulated floor indicates a cooler temperature that the white-coloured insulated floors, which is less comfortable for occupants.
Despite the constant temperature of the earth just below frost depth (approximately 2 or 3°C in winter), the type of ground on which you are building will have a dramatic effect on the rate of heat loss through your foundation walls, as heat moves more easily through some materials than others.
Heat will be drawn from your home much quicker through rock than clay, and quicker through clay than soil. Heat will also move quicker through wet clay than dry. We would recommend at least R20 under slab floors in cold climates (the third image above), but depending on the type of ground on which you are building, you might need even more to double that in order to maintain the same level of energy efficiency.
The rate of heat loss through foundation walls also varies between below-grade and the parts above. Over a heating season, there will be much more heat loss through above-grade walls than those below.
Therefore, with one wall and two rates of heat transfer, this leaves the dilemma of insulating more than you need below, or less than you need above. There is the option of beefing up insulation at the level above grade; however, this is a less common building technique. To be truly effective, it would need to be done on the exterior of the wall to prevent thermal bridging.
Building code requirements for foundation walls vary across the US and Canada, and range between R19 and R24.5. This is a minimum and, depending on location and climate we would recommend increasing that in the interest of saving energy, and money in the longterm.
When renovating or building a new home in the US or Canada with a Basement or a Slab on Grade, in the EcoHome best practice for building Foundations & Basements guide section you will find the basics on foundation, basement & slab design, under slab insulation, site selection & preparation, excavation, drainage & radon gas prevention and mitigation.
Technical edit and thermal images by Denis Boyer