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I have an unfinished, well waterproofed basement with cinderblock walls and a concrete floor. All but the top 6 inches / 15 cm or so is underground.

Would it be worth insulating the walls down there? How would I do it? We have pretty long, cold winters and the summers are generally mild - a few 90 degree F days, but mostly high 70s to 80s.

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2  
I hope you mean 90 degree F –  Vebjorn Ljosa Jul 22 '10 at 17:44
    
@Vebjorn LOL 90C .... yikes that's a bit toasty !! :) –  Scott Vercuski Jul 22 '10 at 17:46
    
Ack! Yes, F! Hahaha. –  ceejayoz Jul 22 '10 at 17:59

6 Answers 6

up vote 24 down vote accepted

Absolutely it's worth insulating.

Concrete is a poor insulator. 8" concrete blocks have an R-value of 1.1 - about the same as 3/4" particleboard.

In my neck of the woods the frost line is about 3' down. So, assuming that a house here has a 40x40 foot square footprint and the basement is 8' high:

160 lineal feet x 6" = 80 square feet exposed to air; 160 lineal feet x 3' deep = 480 square feet exposed to ground above the frost line; 160 lineal feet x 4.5' deep = 720 square feet exposed to ground below the frost line.

Calculate the heat loss, using the formula (Area * (Tinside - Toutside) / (thermal_resistance) See here for a complete explanation

Assuming it's 0F outside and 70 inside (you said long and cold :) ), we'll say the above the frost line is 20F and below is 40F.

Above ground: 80*70/1.1 = 5091 BTU/hr Frost line: 480*50/1.1 = 21818 BTU/hr Below Frost line: 720*30/1.1 = 19636 BTU/hr Total: 46545 BTU/hr.

Now sheath and insulate with R20 insulation. Call it an effective R15 due to thermal bridging by the studs and because you don't do this every day so there are some gaps, etc. Then your total comes to 3414 BTU/hr. You save more than 16,000 BTUs/hr, or almost 400,000 BTUs/day.

In a 6 month winter that's 72 million BTUs.

Your $ savings will depend on the price of energy in your area, what you're heating with etc. $15/million BTUs is not unusual: that's higher than it was in Alberta where I'm from, but lower than California where I'm at now.

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Highly informative, excellent explanation. –  Wayne Werner Jul 23 '10 at 21:06

You would insulate it by building a false wall in front, insulating with fiberglass batt's(sp?) and putting on a vapor barrier of plastic.

For the false wall, some do 2x4's, others 2x3's. Some put the studs on 16" centers, others 2'. If you ever plan on finishing off the basement, I would recommend 2x4 & 16".

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3  
Set the vertical 2x4s out about 1 inch from the concrete wall, you don't want them touching the concrete. –  aphoria Jul 23 '10 at 0:42
    
Personally, I'd prefer 2x6s in this setting, but I also like having the extra wall space for running spare cables/pipes/thicker insulation/etc –  warren Jul 28 '10 at 19:22

I live in the midwest with the same climate and my parent's house is almost exactly as you describe yours. Well waterproofed, cinderblock (painted with UGL drylok), concrete floor (their basement is carpeted) and no insulation. As a child I spent many an hour playing down there and was never cold, even on the coldest winter days. So I'd say no, I see no need to insulate.

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Ours gets pretty cool - I'll have to measure this winter, but it feels 5-10 degrees cooler. I plugged some drafts with foaming insulation which made a difference. –  ceejayoz Jul 22 '10 at 18:00
    
@ceejayoz Ahh yeah if ya have drafts that's a whole different issue. Definitely plug those. –  Scott Vercuski Jul 22 '10 at 19:10
    
Yeah, whole house was pretty drafty when we bought it. Made a lot of small, fast fixes that made a huge difference - insulated the hot water pipes, new furnace, etc. Now I'm looking for the less obvious stuff. :-) –  ceejayoz Jul 22 '10 at 19:27
    
@ceejayoz I posted a reply in another question about hiring a company to look at your house/basement with an infrared camera. They can tell exactly where temperature differences are. Might cost you a few hundred for the service but if your bills drop enough it'll pay for itself. –  Scott Vercuski Jul 22 '10 at 19:36

Can't really answer the worth it part, but for the how I would say use polystyrene foam board. Here is a quick run down on how to do it.

It might be a good idea to insulate. You might not feel cold/hot due to the outside temperature, but you most likely are wasting money on your heating/cooling bills. Just because you don't feel cold in a room, doesn't mean your not running the heater too much to keep it that way.

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Considering that it's underground and insulated by 5 or so inches of concrete, so I would bet it's already much better insulated that the rest of your house. Also, in the winters the ground temp is usually warmer than the air temp, so that would tend do make things even better.

If it's cold down there in the winter it's probably because you don't have any heat vents in the basement (many houses don't, or they are always closed).

If you have those crappy single-pane "basement windows" though, I would recommend you replace them with glass block. That would almost certainly be worth the money.

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1  
Yeah, I want to do the glass block thing. –  ceejayoz Jul 22 '10 at 18:57
    
Basement windows are generally required if there's living space down there, as an escape route in the case of fire. –  Rod Fitzsimmons Frey Jul 22 '10 at 20:32
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@Rod - Maybe your codes are different than mine, but everywhere I know of that only applies if you have actual bedrooms down there. Regardless, a "stock" basement window would never qualify as an egress window anyways. It is too small and too high off the floor. –  Eric Petroelje Jul 22 '10 at 20:52
    
@Rod Fitzsimmons Frey Our basement windows are far too small to escape through (they'd have to be dismantled with a crowbar, too). –  ceejayoz Jul 23 '10 at 2:03
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I think you're right even for Alberta where I was thinking of - just bedrooms. –  Rod Fitzsimmons Frey Jul 23 '10 at 15:05

I just completed my basement finishing project. This is something I did quite a bit of research on. Here's what I ended up with.

I glued 1/2" rigid foam insulation directly to the blocks with a foam-friendly adhesive, and caulked in between the cracks. This approach:

  • Provides a vapor barrier
  • Keeps the studs out of direct contact with the blocks
  • Adds a little bit more to the R-value of your walls.

Then I framed the walls with 2x4s on 16" centers, and put fiberglass batts in for insulation. A few things to think about when framing the walls:

  • Your floor/ceiling are probably not perfectly parallel, so you'll either have to cut each stud to height, or do a lot of shimming
  • If you're not going to cover the insulation with drywall, make sure to use insulation that DOES NOT have a kraft-paper backing. Leaving the kraft-paper exposed can pose a fire hazard.
  • I chose not to leave the corridor between the block wall and the stud wall. I prefer to keep the extra floor space. I put in a flush-mount tile ceiling so I can run future wiring that way. Even with a corridor behind the stud wall, you'll still be cracking into the drywall somewhere...
  • If you do build the stud wall flush to the blocks, make sure that you at least leave enough room to accommodate any curves/bows in the block wall. You don't want to install drywall over a curved stud wall. :)
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