Insulate under a slab if house is on a hillside with a deep basement
Richard Clark| Posted inEnergy Efficiency and Durabilityon
Location: Southern B.C. Cranbrook
The deep walkout basement (8′ ceilings) is set into the hill so the north side is at least 8′ underground. (70′ long east west rectangle footprint design w/ good solar gain windows on the south)
Some say at this depth the temp. of the earth has a heat regulating effect and they advise against any sub slab insulation. The front is a walkout and so is at ground level w/ a 4′ deep foundation wall so this side it makes sense to insulate.
Any thoughts, data, experience using the earths ambient temperature.
Richard
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The heat regulating effect that people are referring to is the fact that at certain depths the soil maintains more or less a static temperature. Here in Western North Carolina 6' below grade and deeper tends to be around 50 degrees. The stable soil temperature depends on climate zone location. The colder you climate the deeper one must go in order to find soil temperatures that do not fluctuate with the air temperature. In addition the soil temperature stability would also depend, to a lesser degree, on soil composition and water retention.
I don't know what procedures and/or manuals your heating and air contractors in Canada use to calculate heat loss, but we use Manual J distributed by the Air conditioning Contractors of America.
In manual J a 'soil path' R-value is assigned to varying depths of soil; 2'-5' below grade = R-4.85, 5'-8' below grade = R-7.84 This assignment of R-value reflects the heat flow resistance of the path through the soil to ground level. In addition, an adjustment factor of the U-value (0.85) is given to account for the benefit of heat storage capability of the soil. These R-values and adjustment factors may be different in Canada, I am just not familiar enough to be certain.
In a final answer to your question, yes I would still insulate under a slab and between the foundation walls and the earth. There is a reduction of temperature changes that occurs at various depths. However, the temperature difference between the earth and your interior (70-50 = 20 F) will still cause heat loss.
Richard,
Easy answer: your soil is considerably colder than your indoor temperature, so you need insulation.
Richard, I was going to post what Martin posted. That is the simple answer too. The only way it works if it is done is that you are producing more heat than you need so it slowly is absorbed by your deep slab heating it up from ground temp of say 40-50 degrees to 68 if you are lucky and do the design right. Where the excess heat comes from would be determined by a very in depth study and plan detailed for your build.
Also if you research this further elsewhere you will find that in some cases it takes a season to several seasons and other adjustments to make things work out. One more note. Those that spend enormous amounts of time and funds doing a one off project like this often consider their "experiment" more of a success than it actually is. Human nature at work Richard. Nothing easy about making this perform better than just insulating the slab from the earth.
Good luck.
Sorry for the wordy answer. I am a little too prone to giving complete answers when I understand a topic well. I supposed I did word vomit a bit.
So the 50 degree soil is robbing heat from your 68 degree room . Insulate under the slab and you slow down that loss. Did I get it right?
What about the cooling effect of deep soil in summer. Would it have a cooling effect if left uninsulated?
Yes it would have a cooling effect. However, unless you live in a primarily cooling climate zone you will waste more energy during the heating season than you could ever recoup during the cooling season.
Very helpful. Thanks a million. I will pass on the info to those I am building with.
Just back from a group tele conference.
One of the other builders makes two points I wonder if you could comment on them.
"Laws of thermodynamics show that heat rises " So...
1. Not much heat will be lost in a downward direction (certainly not enough to get a return on investment on all the insul and labour to install it)
2. As heat rises, the ceiling is where it is 70 degrees while down at the floor it is much cooler so the temp differential (and heat loss ) is less than the straight 70-50 calculation stated earlier.
Thanks again for your feedback
Richard,
Heat doesn't rise; heat moves equally in all directions, from hot to cold.
Hot air (and hot water) rise.
There are three laws of thermodynamics, and one of them is NOT "heat rises."
First law: energy can be transferred, but it cannot be created or destroyed (the law of the conservation of energy).
Second law: you cannot move heat from from a low-temperature object to an object at a higher temperature unless work is added to the system (the law of entropy).
Third law: although you can approach absolute zero, you cannot reach it (the Nernst heat theorem). As Henry Gifford points out, "You can't reach absolute zero because you have to have something colder to absorb the last bit of heat."
So we are dealing mostly w/ #1 and we would lose heat via conductivity to the floor. Is that right?
What about the stmnt that near the floor the air is coolest so less heat is actually lost than the floor temp. vs room temperature of 68.
Richard,
Homes that experience strong temperature stratification -- with ceilings much hotter than floors -- are usually leaky homes. The tighter your home, and the better insulated, the more likely that floor temperatures will be close to ceiling temperatures.
However, you are correct that if the floor is cooler than the ceiling, it will lose heat at a slightly slower rate than the ceiling -- all other factors being equal, which they rarely are.
Richard, if you are not using the cellar, then no huge need to insulate. If you are finishing the space, then insulate. My concrete guys add the foam for no extra labor cost, The foam is not that much of an expense compared to the whole project. It seem you are trying to not use it. Then don't. Simple.
Is there a heat loss calculation to determine how much heat a slab would lose and if so who would do it?
The other builder I am working on the plans with wants to know the ROI(return on Investment) on all that insul and the labour involved in placing it.
m