Frost-Protected Shallow Footer Slab
I have read quite a few articles here over the years about various versions of the shallow frost protected slab (see for example here: https://www.greenbuildingadvisor.com/article/high-performance-raft-slab )
One of the questions I have is about coupling the slab to the earth by removing much of the under-slab insulation in order to mediate heating & cooling loads in a home that is only occupied a few months of the year. The thought goes like this:
— In a moderate climate such as the central mid-atlantic region where deep soil temperatures remain at around 54-degrees f year round
–A shallow frost protected slab with insulation ‘wings’ extending significantly past the slab essentially keeps the soil near the surface closer to the 54-degree temp year round. During the summer, it reduces air-to-soil conduction when the air is hot and solar heating as well. During the winter, that same reduction in conduction makes the near-surface soil warmer.
— A well insulated home with an earth-coupled slab and insulation ‘wings’ extending just below ground for 4-feet or so would experience much smaller temperature swings inside than the same building WITH insulation under the slab.
–Without AC, the home would be cooler in summer with a nice cool floor
–Without Heat, the home would be warmer in winter and perhaps warm enough even without another heat source to keep pipes from freezing
Perhaps not ideal for a home lived in full-time but perhaps useful for a less-used vacation home/cabin/cottage.
Thoughts???
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Replies
With a cool floor in Summer, you will probably need to run a dehumidifier more, even when not there. If occupied in Winter, the floor will be uncomfortable.
> would experience much smaller temperature swings
Not really. Thermal mass beyond a few inches of concrete has little short term effect.
Note that once you starting adding or extracting heat, 54F is no longer the right number.
The calculation here is not one of thermal mass like in old passive homes. The calculation is based on the rate of heat transfer into (or from) what is effectively an infinite heat-sink (or heat-source).
Josh, there is a system that allows you to take advantage of the earth's thermal mass and gradually bring it to a relatively steady temperature. It's called an Umbrella House, or Passive Annual Heat Storage (PAHS) system. The key to the system is to put foam insulation a few feet away from the house so the earth can reach a somewhat steady temperature year-round. If the calculations are correct, you end up with a flywheel effect, due to how long it takes to heat the insulated earth.
But I think Jon is correct--with an infinite heat sink, you will always be fighting cool floors. Cool floors can be nice in summer, with adequate dehumidification, but not in winter--if the floor is less than room temperature it's going to feel cold.
How deep is the 54 degree year around level? What about 4 feet down, how does that change winter to summer? I am suggesting that the temperature in the ground 4 feet down may not be close to 54 degrees in July or January. Have you got data for your region?
The depth at which you find 54-degree earth depends on the amount of insulation that you place on top.
Hi Josh,
I meant before construction. What is the natural temperature in the ground; Winter, Summer, Spring, and Fall. These are the temperatures that you are insulating against, the temperatures that tell you how much insulation you need. When you know what those temperatures are then it makes it easier to decide on a below ground insulating strategy. For instance, in Tucson, a fairly mild climate, edge insulation makes the most sense. On the north slope of Alaska there is a great deal of insulation under and on the edge of the slabs.
According to this article https://www.scientificamerican.com/article/the-temperature-of-the-earths-inter/, starting at about 12 feet below grade the temperature is constant, close to the average annual air temperature.
Of course it will vary depending on the annual temperature swing at a given location. Where I am the annual temperature is about 41°F, so it won't be 54° until far below that; according to the article, somewhere below 40'. In other places it will be 54° much closer to the surface: https://www.currentresults.com/Weather/US/average-annual-state-temperatures.php.
My thought is that you may use this building in a certain way for a limited period of time, but if it's a decent building it will be in use long after you're done with it. Things that can only be done up front (eg, sub-slab insulation) should probably be done with a longer view. Especially if the benefits of doing otherwise are uncertain or minimal. My $0.02.
Andrew,
Good p0int. A fair number of posts here in the Q&A are about how to retrofit a solution to uninsulated slabs.