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February 13, 2019
If you are deciding to insulate your basement, you will want to consider whether to insulate your foundation to 2' below ground level, or to insulate the full foundation wall from top to bottom. Below are some considerations to help you make this decision.
Spray foam is an expensive material to insulate with, as is appropriate rigid foam insulation. If you are trying to do the highest impact work at the least cost, or have other upgrades that you are budgeting for, you might consider insulating the foundation walls to 2' below ground level, rather than from top to bottom.
The image above demonstrates the value of insulating the top half of your foundation wall, compared to the bottom half of the wall. This image shows a foundation wall (in winter) getting warmer the farther down the wall you go. It is evident by the temperature gradient that the coldest part of the wall is above grade. That's because the outside temperature is much colder than that of the earth. The deeper below ground you go, the warmer it becomes. Your best bang for the dollar in this instance is to insulate the coldest part of the wall, again typically to 2-feet below grade.
However, there are many instances where insulating the full wall is the best option, such as if your walls are stone, or the floor is earth or crushed stone with no vapor barrier. The reason why you'd want to insulate the whole wall is not so much because of the energy savings, but because a stone wall, and dirt or crushed rock floors let in a lot of moisture.
This moisture can lead to a series of other complications, such as rot and mold. Through a combination of full-wall spray foam and an addition of a vapor barrier on the floor, many of the moisture issues will taken care of through the creation of a full-basement vapor barrier.
A couple other reasons to insulate the full wall is if the basement will be heated directly, let's say through oil or a wood stove. While the bottom half of the wall will be warmer than the top half, it will consistently be much cooler than what you'd comfortably heat the space to.
Finally, current construction codes require all new builds to be fully insulated. Which, in the end, is not a bad thing!
Phoebe C, Waterville
Bo, Thanks for your note. Nice to know you're thinking of me. I am very satisfied with everything you did. My oil consumption is down from the previous year, but I have not yet figured exactly what the numbers are. I will let you know. No water in my basement during the rain these past couple of days. Hopefully that will hold up during a heavy downpour. Stay well and enjoy these warming temperatures.