I have a home built in 1925, a "Chicago Bungalow" style, in Louisville, KY.

I have roughed in the basement, trying to take advantage of every square inch. I am planning on using it for short-term guests and also for a family room/reading area with loks of bookcases.

How much do I have to insulate the exterior basement walls? I'm leaving the ceiling open and painting it, so that isn't an issue. My family is fine with cooler temperatures and when we have guests, I could put a small space heater in the room where they'll be staying. Honestly, insulation is more expensive than I thought it would be and this thing is holding up my project. I want to do the right thing but am unsure of what that would be, within my budget.

The basement is approx. 900 sq. ft. (25' x 35'). There are 4 windows, 2' x 2.5' each. Since it is a "Chicago Bungalow" style, about 2.5 feet is above ground. The ceiling is the hardwood floor, with no subfloor and not finished, nor going to be finished (there is just too much duct work to consider working around it all). The budget is about $500 for insulation.

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    "Best" and "Cheapest" are rarely descriptors for the same product.
    – DA01
    Aug 25, 2014 at 16:23
  • 1
    If you want to use every square inch, then closed-cell spay foam is the best, but also the most expensive. You shouldn't just look at the insulation costs. Running a space heater all winter for a few years will probably cost as much as insulating with the most expensive product.
    – Steven
    Aug 25, 2014 at 16:25
  • This question is primarily opinion based.
    – user23752
    Aug 25, 2014 at 16:33
  • 2
    The windows do not appear to meet the requirements for Emergency Escape and Rescue Openings per the International Building Code Series or per NFPA 101 Life Safety Code. These are required in any room used for sleeping primarily to allow fire and rescue personnel the ability to enter with breathing apparatus and to exit while carrying a rescuee. Secondarily they further facilitate self preservation by the building's occupants. Contact the building department regarding permit requirements.
    – user23752
    Aug 25, 2014 at 18:36
  • 1
    Yes, ben makes a good point. If this is intended as a bedroom you need to make sure egress is to code.
    – DA01
    Aug 26, 2014 at 1:38

5 Answers 5


You are in the same climate zone as me. Your walls do not have to be insulated from top to bottom according to building science reports (please read their definition of basement which is totally below grade). They only suggest floor to ceiling insulation in our region given spray foam or rigid foam - in "typical" home basements.

What I have been doing the past 6-7 years with great results - including my own house - is only insulating about 1 foot below grade, using BS advice. I use rock wool. I stuff any joist areas until they meet wall level. A decent size house costs about $250. The results can be felt right away. My crews ask that this not be done until right before the drywall is put up because the basement instantly retains a ton of heat.

This is the exact reason why it is not necessary in our region to go floor to ceiling. By over insulating you are not allowing for any cool air to come in during the warmer 6-8 months out of the year. I for sure have noticed an increase in AC usage while a decrease in furnace usage in my own home.

Also by leaving your bottom half exposed you are creating a high volume area for air flow. Meaning any moisture that gets in - and there is always some - will evaporate quickly.

So we simply stagger our scrap 2x4s about 1 foot below grade (sometimes this is just the top 1-2 feet of framing), stuff rock wool against wall and then above our framing into the joists all the way to the outer wall. This is super cheap and super effective and all inspectors in our area were more than pleased with this. Again this is not for cold climates but will work in the Midwest.


You appear to live within Climate Zone 4, where basement-wall insulation of R10 to R13 is recommended.


One of the most efficient (based on thickness) insulation is polyisocyanurate foam boards. It is R-13 at 2 inches thick. These can be mounted using 2x3 studding with an air gap at the back that increases the R value a bit. You lose only 2 1/2 inches and get a huge insulation value. The stuff also handles easily.

It is not cheap at a bit more than $1 per square foot, but a lot of R in a small space.

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  • Don't get me wrong, I love polyisocyanurate foam. Two things, though - first, it's awfully porous so it wicks up considerable moisture in a hurry. Second, the foil should be on the heated side, and should have a minimum 1/2" (1" preferred) air gap between it and any other material. Only then does one get the benefit of reflective insulation. Aug 26, 2014 at 2:15
  • Furthermore, polyiso's performance falls with low temperatures (such as those experienced in a basement). I would not recommend it for a basement.
    – iLikeDirt
    Sep 25, 2014 at 14:45

To start off: when you're adding insulation to the inside of your basement wall, you need the insulation to do three things:

  1. Block moisture that wants to enter the basement through the walls that are in direct contact with damp soil.
  2. Cling tightly to the basement wall so moisture can't condense between the wall and the insulation.
  3. Be protected from ignition, if it's flammable.

#1 rules out polyiso foam (since it's water-absorbing), and EPS foam, mineral wool, and fiberglass batts (because they're moisture-permeable in the thicknesses you'd be using, and some absorb water too).

#2 rules out flat rigid foam boards if your basement wall is rough; say, made out of stones, bricks, etc. Only if your basement wall is flat--made out of concrete blocks or poured concrete--would you be able to fasten rigid boards of insulation to them.

#3 requires that if you use something foam-based, it be covered with drywall or another suitable ignition barrier to prevent it from catching fire (it's basically a sheet of petroleum with fire-retardants added).

It sounds like you have about 960 feet of wall space that needs insulation. Rounding that up to 1,000, and taking into account your $500 budget, then it seems that what you need is insulating material that 1) meets code, desired performance characteristics, and an acceptable level of safety and 2) can be obtained for less than $0.50 per square foot.

I'm not sure you're going to be able to do it for such a limited budget. First of all, if you have a rough basement wall, your only safe, practical, code-approved option is around two inches of closed-cell spray foam covered with an ignition barrier like drywall. That is certainly going to cost you four times your budget or more. If you have a smooth basement wall, you can use XPS foam, which can often be had for less than $0.5/square foot, but that would be for a single inch, which yields you R-5; better than nothing, but not a lot. As TDHofstetter points out, code minimum is R-10. R-10 worth of XPS would therefore cost you $1,000, and you still need to drywall it. Foamglas could be used in place of XPS, which would avoid the expense of drywall, but is only about R-3.3 per inch, so you would need at least three inches of it, and it's very expensive. And all of these numbers are for materials alone, assuming you do all the work.

Bottom line: you're going to need to expand your budget to adequately and safely insulate your basement.

  • Now that I think about it, the cheapest way to do this might be to spray the walls in one inch of either closed-cell spray foam or XPS (depending on your wall's smoothness), and then cover that in rigid mineral wool boards. The mineral wool acts as an ignition barrier, and is protected from moisture by the foam. You could cover the mineral wool in drywall later as funds permit.
    – iLikeDirt
    Sep 25, 2014 at 15:17
  • Actually, some of this goes against modern building science recommendations. Namely that insulation is NOT meant to block all moisture. In fact, they actually recommend EPS and XPS because it does allow for some moisture permeability.
    – DA01
    Sep 25, 2014 at 15:27
  • XPS is effectively a vapor barrier, especially at more than an inch. EPS becomes effectively a vapor barrier at a few inches, and plastic or foil-faced EPS is a vapor barrier. If moisture permeability is desired, mineral wool is probably the right product, as it transmits moisture, but does not absorb it and is not harmed by it.
    – iLikeDirt
    Sep 25, 2014 at 15:39
  • Here's what Building Science recommends: buildingscience.com/documents/bareports/…
    – DA01
    Sep 25, 2014 at 16:03
  • I should note that their opinion of permeability has changed a bit over the years. They now don't consider permeability of the insulation a requirement. However, they still recommend XPS or EPS foam boards.
    – DA01
    Sep 25, 2014 at 16:04

The short answer is "probably not".

"Best and cheapest" are contradictory terms.

The idea of insulating a basement seems a little silly at first because it already has insulation: 15 feet of dirt. That provides way, way more insulation than 6" of fiberglass. If the basement is fully underground, "insulation" would be pointless (unless you have some unusual condition, like a glacial underground stream running in direct contact with your wall or something like that).

Nevertheless insulation does make sense if part of the basement is above ground, especially if it has windows. Any kind of window in a basement, that's where your heat loss is going to occur. The heat loss through a window will be hundreds of times greater than through, say, an 8" concrete wall.

So, if you are looking to economize, the best practice is to just focus on the windows and doors , that's really the only area where you can have major heat transfer. Remember, its all about the RATE of heat transfer per unit area. So if you have 400 square feet of wall transfering heat at 1 Joule per hour, its meaningless compared to 10 square feet of windows and transfering heat at 500 Joules per hour.

The easiest thing to do is brick up the windows, 12" thick, with some insulation inbetween, but then, of course, you have no windows. One alternative is just to grout plexiglass or glass over the opening (make sure it is airtight). The more panes you add, the more insulation you get.

Cellar doors, like storm doors, are harder to insulate. The cheapo option is to get a couple bales of fiberglass insulation and just duct tape to the outside of the door (this assumes you have a outer bulkhead door). You can't use the door in the winter obviously if you do this. When spring rolls around you throw out the insulation. Repeat every year. You can add weatherstripping to a door, but in my experience it does not do too much.

If you really want to start insulating walls, you can, but truth to be told, it will not do that much if the wall is below ground level. All your heat loss goes through windows and doors, anything that has a short pathway to the outside air (or water).

Brief Note about Water

Water, by the way, is bad. It will transfer heat INSANELY faster than air, so if you have an underground cold stream in contact with your basement wall, it will wick heat INSANELY fast. You can tell because if you touch the wall it will be freezing cold compared to other parts of the wall. An IR temperature detector will do the same thing. If you have this situation, you definitely want to use heavy insulation on that part of the wall or floor, because it will just suck heat out of the house. This is a rare case though, most people don't have cold streams.

  • 1
    An 8" concrete wall has an R value under 1.5, barely any better than the cheapest windows.
    – Zhentar
    Aug 25, 2014 at 17:11
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    And you're overestimating the value of dirt as insulation a bit - it's just a huge thermal mass that really doesn't want to change temperature, and unfortunately that temperature is generally 15 to 20 degrees colder than what people generally consider comfortable.
    – Zhentar
    Aug 25, 2014 at 17:21
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    Also, sealing the emergency egress window in a basement room intended for habitation is dangerous and usually illegal
    – Zhentar
    Aug 25, 2014 at 17:23
  • @Zhentar the Emergency Escape and Rescue Opening requirement only applies to rooms used for sleeping. It does not apply to other habitable rooms [though the question does mention using it as a guest bedroom].
    – user23752
    Aug 25, 2014 at 23:41
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    Are you talking about a literal stream of water? If that's the case, there are much bigger problems than heating issues.
    – DA01
    Aug 26, 2014 at 2:46

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