# Estimated savings by insulating a basement

OK, I'm hoping you will all take a crack at this so I can compare your numbers to mine!

I need to estimate the energy savings that would be achieved from taking an uninsulated basement wall of a home in Spencer, Iowa and insulating it to a total R value of R13 using either XPS or spray foam that would would be framed and covered with drywall to meet code. Assume NO additional insulation in the stud cavities.

Here's the details of the home:

Ranch home, 1538 square feet of basement space. Total wall square feet is 1,288 sq ft.

Basement is conditioned.

Basement walls are bare concrete block, 8".

Heating degree days in 2010 were 8,047 (using degreedays.net, 65 degree baseline)

Customer heats with an electric furnace.

If you come up with a number, can you share HOW you arrived at it? Again, I want to compare to see if the calculator I am using is at all accurate. Thanks in advance for your help!

Posted May 13, 2011 10:15 AM ET

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Curtis,

Here's some information for you:

A CMHC study (http://www.arxx.com/Assets/CMS/WYSIWYGBase/file/CMHC%20Basement%20Assess...) concluded that for 5 Canadian locations (Toronto, Ottawa, Halifax, Edmonton and Victoria), "For all types and sizes of basements assessed in this study, the lowest total life-cycle cost was associated with basements insulated internally, full-height to a nominal level of R-20."

An American study (see http://www.ornl.gov/sci/roofs+walls/insulation/fact%20sheets/basement%20...) found that annual energy savings from R-20 basement insulation ranged from $280 in Washington, DC to $390 in Buffalo, NY.

Posted May 13, 2011 11:29 AM ET

Curtis,

If you have access to an ASHRAE Fundamentals Handbook they do have a method for manual calculation of foundation heat loss. I have found this to be reasonably accurate as compared to say Energy-10 or the like.

Posted May 14, 2011 9:38 AM ET

It always strikes me as odd that so many decisions regarding insulation focus on cost justification, payback time. Maybe the promotion of energy conservation should read like what I think is a credit card commercial.

Insulating the wall $xxxx

Building stud wall $yyy

Sheetrock over studs $zzz

Comfort from all this: Priceless

R13 over the block wall equates to about 2.5" of XPS, something easily applied with either glue or powder actuated fasteners. The studs are just for fastening sheetrock, so they can go on the flat (to save floor space), sitting on ripped PT plates fired into the slab and anchored at the top by the floor above. Shallow boxes provide electrical outlets. Sheetrock goes on the usual way. But you've already got this figured out, I imagine

Posted May 14, 2011 10:43 AM ET

Hello,

You've put your finger on a blind spot in energy calculations. The textbook explicit solutions depend on the Gauss error function, so there's no simple solution. Gintas Mitalas from NRC Canada did nice work to simplify foundation heat loss in the early 80s applying factors to different insulation profiles, and his work can be found with some digging on the web. US DOE did a nice job with their Building Foundation Design Handbook in 1989, which is hard to come by.

ASHRAE Handbook Chapter 29 has a method. They draw circular arcs from the foundation (shown in section) to the outdoors, and these arcs represent the heat flow lines, or “streamlines”. Heat flow is then calculated from U A delta T. Delta T is the indoor-outdoor temperature difference. The incremental area A is a matter of computational convenience. U is derived from the insulation in the wall, the conductivity of the soil, and the length of the heat flow line. The whole works gets integrated. This method is nice in that it shows how wall sections near the top have much more heat loss than parts of the wall deeper in the soil.

Another way to calculate it is to use a two-dimensional heat transfer program like THERM from Lawrence Berkeley, and assign a lot of dirt to the outside of the assembly. You have to create your own new building material “dirt” based on soil conductivity conditions that your local ag extension people ought to know.

These results, and all results, are very rough estimates. The biggest wild card is soil conductivity, which may vary easily by a factor of 3 to 1 depending on wetness and 5 to 1 depending on makeup, though our midwest soils can have an even wider difference going from dry to wet. Then there are corner effects and also heat exchange to below—not to the surface.

I did a presentation on the different methods of below-grade heat flow calcs at the Passive House Conference from two years ago or so in Urbana IL, which you might be able to access through their site.

Good luck. I won't run the numbers, you'll have to do that.

Posted May 14, 2011 5:48 PM ET

Curtis,

You've received soem excellent suggestions. My difficulty with this calculation is that HDD really don't apply since you are in a somewhat mild climate. Presuming this is an in-ground basement (and not a walk-out), the difference in temperature between the ground and the space you are trying to heat is more applicable. It's also somewhat important to know if this is a block basement or a pured concrete foundation.

Assuming that the wall is medium density concrete and the ground remains about 55F, I calculated about $3.87 per heated day for the bare wall and about $1.07 per heated day of energy cost with 2.5 inches of XPS. I used a simple Q=U A dT calculation as is used in ASHRAE. There is a film factor for the interior wall but otherwise is a straightforward calculation. Just estimate the number of days you heat your home, and the annual cost is there. For example, if ou turn your heat on in the basement about 100 days per year, you would pay $387 for the bare wall and a bit mroe than $100 for the insulated wall.

More importantly, you need to consider moistture. The cost of heat is nothing compared to the cost of remediating mold growth in basement wall structures. You're on the right path. We recommend that you insulate the walls with R-10 or greater XPS (STYROFOAM) and that you build the stud walls on the inside surface of the XPS. This eliminates thermal bridging at the studs and keeps the studs protected from wicking, seapage, and condensation. Fiberglass directly against the basement wall runs a good chance of getting wet from condensation or leakage.

I would be happy to work with you directly to estimate your savings given some more specific information. But, given the 1288 SF of wall and the approximate savings we calculated of $287 for a 100 day heating season, your payback will be 2-3 years. Approximating the cost of XPS at $20/sheet, the insulation cost will be $800. You need to check locally for exact cost,

If you want to discuss this further, please feel free to shoot me an E-mail and I'll try to help get your specific needs met.

Scott Cummings

Dow Building Solutions

BPI Analyst and Envelope Professional - Certified

slcummings@dow.com

Posted May 16, 2011 6:37 PM ET

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