Posted By want to build on 03/06/2009 12:02 PM
What are the advantages of insulating an unfinished basement vs. insulating the ceiling of the basement to isolate the living space?

[b]My friend has a house built in the 1920's. Basement is not insulated. Summer or winter it's a pretty constant 65 degrees.[/b]
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The reason it's 65F in the winter it most likely distribution loss from the heating system, and the reason  it's 65F in the summer is air leakage from the outside. This sounds more like a heat-leak DISASTER than a hallmark of efficiency!  

(I know this intimately, 'cuz I live in a 1920s house that FORMERLY had an uninsulated basement, and yes, it WAS an efficiency disaster.  ;-) )

How is it better to insulate the walls instead of isolating it from the living space above? Let me tick off some ways...

*Insulating the walls tends to have fewer errors in installation than trying to perfectly air-seal & insulate without gaps between the basement & first floor.

**It thermally earth-couples the house helping summertime cooling somewhat while dramatically reducing wintertime losses.  (The R-value of a poured concrete wall is typically about R1 or less.)

***In many homes it's also simply less material- a smaller surface area to insulate + less labor to apply = lower cost.

****It puts the heating plant (furnace/boiler, ducts/pipes) completely inside the thermal boundary of the structure, resulting in improved system efficiency (heat leaks from the distribution stay on the warm side of the insulating boundary.)

*****It puts the wood framing of the floor joists, etc on the conditioned-space side of the insulation, keeping it above the dew point, reducing condensation & mold growth hazard.

******It keeps the potable water plumbing inside of the thermal boundary, lowering it's freeze potential.

I could go on, but it's just better- trust me. (I'll stop here. :-)  )

The easiest DIY method of retrofitting is gluing XPS, EPS, or un-faced ISO to the walls, sealing the seams with mastic, then applying furring strips & the code-required 1/2" sheet rock thermal barrier over that.  The easiest (an perhaps best from a perfectionist point of view) is to spray-foam the foundation, sill & bandjoist all in one operation, and build a thin profile mini-stud wall (2" steel studs, not wood 2x4s) to mount the thermal barrier sheet rock.  Even if you use sheet goods, air-sealing & insulating the band joist & sill should be part of the project (that's typically the single largest air leak in older homes- bigger than all of the doors & windows combined!)

You have to be careful when using fiberglass or cellulose insulation in basements- it's easy to set up situations where the vapor-pressure from the soil/foundation creates condensation within the insulation for part of the year. In fiberglass that leads to mold, in cellulose it can lead to saturation that takes too long to dry. Permeable & semi-permeable plastic insulation is more reliable, best if set up as a perfect air-barrier (DO mastic-seal the joints of sheet goods!)  Use permeable paints if you finish the walls too- no vinyl or foil wallpapers, etc. That way the ever-present vapor pressure of concrete is always inward drying but doesn't allow massive air-transport of moisture from room air to condense & freeze on the now much-colder concrete wall.

There's a bunch of detailed stuff on basement insulation issues here:

http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems

Once you've air-sealed the basement, if you have any atmospheric-drafted heating equipment there (hot water heater, furnace/boiler that aren't sealed-combustion with an external air supply) it's a good idea to test for back-drafting & hood spillage, especially if you have a clothes dryer in the basement that can de-pressurize it considerably (in really tight homes, a dryer even on the first floor can create this issue, but it's hard to retrofit that tight.)  Have a pro do it, or look up on how it's done- it's not rocket science, but I don't want to waste the space here other than to flag it as an issue.

[edited to add...]For new construction, going with insulated concrete forms is almost a no-brainer.   For a crawlspace or single-story foundation wall it doesn't take a contractor with a decade of experience.  Even if a rank newbie at it screws up and experiences a blowout, it's generally pretty easy to rectify (been there, done that, on a 300square foot addition at my own home. :-) )  Rocket science it is not.