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Why thermally insulate a below-grade basement?

Please sheath your literary swords, put the pitch forks back in the barn, turn off those torches, I am well aware that every source I have reviewed recommends insulating a below grade basement, and doing it well. However, thinking out of the box, given my climate zone 7, where the below ground temp is like mid 50's all year round (my assumption), wouldn't it be welcome to have a basement in the low 60's while the outside temp is either 30 or conversely 85 degrees F? My current 1950's construction uninsulated basement feels great in the summer. I would think, if you can waterproof it, keeping the moisture out of the walls, so you don't have the efflorescence and constant dampness requiring running dehumidifier 365 days a year, like I have now, the temperature differential is a sort of primitive geo-thermal concept, no? So why bury all that foam/money?

Asked by Sal Lombardo
Posted Jan 10, 2013 10:23 AM ET
Edited Jan 10, 2013 1:28 PM ET


5 Answers

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Let's discuss a typical basement with an 8-ft.-high concrete wall. Let's assume that 1 ft. is exposed to the outdoor air, and 7 ft. of the wall is below grade.

First of all, concrete is a conductor. So if you want to insulate the wall, it's a good idea to insulate all of it -- not just the top 1 ft. That's because heat will find a way around your insulation through the concrete.

Climate zone 7 includes northern Minnesota. That's colder than climate zone 6, where I live. Where I live, I can assure you that the soil isn't at 50 degrees all year round. The reason we bury our water lines 4 feet is that the ground freezes to a depth of 3 feet some winters. I know that it does, because I buried a water line 3 feet, and the water line froze one winter when the snow wasn't deep enough.

So, the top foot of the wall, on a cold winter night, is -30 degrees F. The top foot of soil might be 0 degrees F. Two feet down, the soil might be 20 degrees F. You get the idea.

Even at the bottom of your wall, where the soil might be at 45 degrees or 50 degrees, you are still continuously losing heat from your 65 degree basement.

Heat loss through basement walls is complicated to model, but it has been modeled. Trust the experts -- in your climate zone, it's worth insulating your basement wall.

Answered by Martin Holladay
Posted Jan 10, 2013 11:00 AM ET


Well stated, Martin.

And, though not intended, it points out why it was not a smart idea for the city of Hazelwood, MO to amend their local energy code to only require basement insulation if there is 4 ft. (or more) of above-grade exposure. The frost line in that region is, depending on the jurisdiction, between 18-30". But what you highlighted above shows why homeowners will get some unpleasant winter temps.

Answered by Mike Collignon
Posted Jan 10, 2013 1:09 PM ET
Edited Jan 10, 2013 1:10 PM ET.


It seems obvious after you put it that way.
Staying on the same concept, is it safe to say one's money is better spent insulating the walls more than the slab underfoot? Granted it likely would not obviate the need for heat in winter.

Thanks much, great site!

Answered by Sal Lombardo
Posted Jan 10, 2013 5:54 PM ET


SAL: i also agree with Martin.

First of all, your assumption is wrong ..
During summer in Zone 7 ( where are u at? )
temps might stay in the 12-16C region
but during winter ( goes down with 1-2 months of delay )
during late january and all of february, soil temps are under 0C for the first 1-2 ft

But even there , without insulation , even a few degree temp difference will make u loose your ideal temperatures ..

if the basement is unused and unfinished then maybe u can get off differently ,
else it requires a minimum of insulation

Code just changed here in quebec for something like R5 minimum insulation for under slab and underground basement walls.

( and a welcomed R4 minimum thermal break from any framing to interior or exterior !!! )

Do you believe that Passivehaus designs use 8-12" of insulation under the floor slab for no reason ?? :)

Answered by Jin Kazama
Posted Jan 10, 2013 6:21 PM ET


I've got CMU walls with 2 feet above grade on average, and when
I shoot the walls with the IR gun anywhere there's exposed
concrete it's pretty uniformly cold all the way down. Because
the column of all those *holes* in the cinderblock let cooled
air tumble all the way down in what's probably a big convective
loop, so the exposed part winds up chilling the whole wall.
It's definitely colder than most of the slab.

The downside of insulating all that, of course, is you lose the
ability for your 55 degree sink to turn into a source in the
event the house loses power/heating for a long time. Leaving
the basement as is or only lightly insulated also gives a certain
amount of passive freeze-down protection...


Answered by Hobbit _
Posted Jan 10, 2013 9:07 PM ET

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