If you want to a surefire way to start an argument between 10 home remodelling expert, ask them “What’s the best way to insulate a basement?” You will almost definitely get 10 different answers with 10 different reasons for why that answer is correct.

Most basement remodeling books offer many different insulation methods.

To be fair, there really is no one 100% correct way to insulate a basement. There are different types of basements, different underground situations, different building codes and a number of other factors which vary from state to state, town to town and even house to house. Additionally, there are now different insulation technologies and techniques available than there were just ten years ago, so the process of insulating a basement is constantly evolving and changing as new tools and products come to market.

The best insulation for a basement wall would actually begin on the outside, but that’s something that’s still not done as commonly as it could be. Foam board insulation properly installed against the outside of a basement wall can prevent water from entering and provide an adequate insulation against colder temperatures.

Reasons For Insulating A Basement

This seems pretty obvious at first. Insulation in any part of the home helps block the flow heat (or cold) from one space to the next. This transfer of heat is part of a whole branch of science called thermodynamics. The idea of insulation is to slow down the “thermal bridging” of heat and cold from one area to another.

So you insulate the outside walls of your home to keep the cold temperature of winters from transferring to your warm living room. Conversely, insulation helps keep the heat from your living room from immediately slipping out into the cold air of the night. The other coverings of your outside home walls (siding, stucco, shingles, plywood, drywall) also help slow the heat transfer from inside to outside, but their primary goal is keep rain and excess humidity out of your home while keeping in some comfortable moisture in your house.

Insulating your basement properly could save you some money on heating and even cooling bills, but estimates range from 3% to 30% so ultimately your specific basement and your specific insulation strategy will determine what savings you experience. Generally, adding ANY insulation to a home will have some slight savings on energy costs as long as it’s done correctly.

It should be noted, however, that insulating basement walls is not like insulating the rest of your home. Most basement walls are concrete or cinder block, which means they conduct temperature (and moisture) a bit differently than regular home walls. Basement walls that are mostly underground will remain cool all year round, which can sometimes lead to condensation problems. Basement walls are also subjected to a lot of standing water in the soil around them, and that can lead to leaks and water seeping in through concrete or block walls. Ideally, your insulation choice should help prevent excess moisture from collecting in your basement as well.

Methods of Basement Insulation

There are several accepted ways to insulate basement walls, and within those methods are a lot of variations.

Blown or Applied Insulation: This has become more popular in recent years as more stores now offer rental machines to blow in insulation and the entire process has become easier and cheaper to use. This will work in some basements where you may already have studs and/or finished areas or if you’re using a blown insulation that has an adhesive mixed in. There actually a few different ways to “blow” insulation onto the walls and these different types of insulation have advantages and disadvantages. Blown in insulation includes wet cellulose insulation, closed cell foam and open cell foam.

Fiberglass Batt Insulation Between Studs: The old tried and true method of insulating walls in other parts of the home is often used in basements as well. That’s a shame because it’s generally a pretty lousy way to insulate a cement basement wall. Why? Because of moisture. Just a little bit of moisture behind those walls can make a mess of fiberglass. Even if you put up a tight vapor barrior, a single pinhole anywhere in the plastic could allow in enough moisture from the air for it to condense and start mold growth. While this basement insulation method used to be popular for

Foam Board Insulation: There are several different foam board products that you can use to insulate your basment. These rigid polystyrene foam boards are fairly easy to attach (screw or adhesive), are moisture resistant, so a little water won’t hurt them and they can serve as a sort of insulation and vapor barrier all in one. You do need to have fairly smooth and even walls to adequately install foam board insulation in a basement.

Air Pocket Insulation: Okay, this sounds crazy, but lots of older basements have been “refinished” this way – without any sort of insulation between the studs and finished panelling or drywall other than a pocket of air. While your basement definitely won’t be as warm as it could be this way, there are some actual advantages: you don’t have to worry so much about moisture because any moisture that comes through the walls will hit the air and have a chance to dry out. If the air in your basement is often humid, then you may actually have a problem with moisture condensing on the cool walls. The idea of simply having a “pocket of air” between the finished wall and the unfinished block wall is a simple one: it works the same way a storm door might work – by keeping the coldest air off the main door and therefore keeping the main door a little warmer. For this to really work you either have to live in a climate with a lot of dry air or use a dehumidifier pretty regularly in your basement (which is a good idea anyway in most cases).

An affordable dehumidifier can save you thousands in moisture damage repairs and make your basement feel warmer.

Moisture Control Measures

Controlling moisture and water vapor in a basement is a big challenge. Just a little bit of moisture can damage finished areas, promote mold growth and make your basement feel much colder with damp air.

In the “old” days of basement remodeling finished walls were framed out with wood studs that were placed tightly against cement block or concrete walls, filled in with fiberglass insulation, covered over with a plastic sheet as a vapor barrier and then covered again with drywall. That was all well and good for temperature, but it for moisture control it was (and still is) a nightmare. A little trickle of water seeping in from the outside walls could start mold growth and go undetected for years. Likewise, moisture from the air could condense and collect behind the vapor barrier and end up soaking all your insulate (and again initiating mold growth) rather easily.

Most of the newer methods of insulating a basement try to eliminate the problem of having “sitting water” on your basement walls for any period of time. Foam board insulation can either be installed flat against your concrete and block walls with construction adhesive or they can be attached to the wall with screws using furring strips along specially designed edges. If you use adhesive, you should also tape over the seams with some sort of plastic seam tape.

Another method for dealing with moisture is to put up basement framing with a small (1 inch) gap between the framing studs (metal or wood) and the actual concrete wall of the basement. This allows air to actually circulate behind the walls, which means that small amounts of moisture that happen to form on the walls will actually have a chance to dry out.

Some experts even argue for a dual approach to basement insulation: insulating the top half of the wall for temperature control, while leaving the lower part of some basement walls exposed for drying.

Again, no matter what you do, if you have moisture and condensation problems in your basement you’ll want to invest in a quality dehumidifier. By slightly drying out the air in your basement you will prevent mold growth and make your basement feel much warmer.

Insulation and Vapor Barriers

Vapor, in the loosest sense, in moisture contained in the air around us. So a vapor barrier is really a moisture or water barrier. There are still lots of people who feel as though a vapor barrier is a necessity in a basement. When really pressed, though, they often don’t have a good reason as to why. In theory a vapor barrier is used to block any and all moisture from the air inside your basement (like you breathing out) from reaching your cooler block walls and condensing into liquid moisture. This idea only works if you have a 100% completely air proof vapor barrier.

I have never, ever seen someone put up a 100 square feet of plastic sheeting without having a single teeny, weeny air hole gap somewhere in that wall. It’s been stated by others that a single hole from a pin in a plastic vapor barrier can allow in up to a cup of moisture to condense behind a wall in a single year. Even if you staple every inch and then tape over every staple, I simply have a hard time believing that a vapor barrier can be 100% sealed in a regular home basement. And if you’re going to screw up drywall, forget it, because one slipped screw off the stud and you’ve just ripped your plastic barrier.

The more current thinking with vapor barriers and basement insulation is this: Moisture in air is going to enter and exit your basement walls. That’s almost impossible to completely avoid. So if it’s going to happen, use insulation materials that are not harmed by moisture and give them an opportunity to dry out.

Insulate and Fill The Gaps

A big part of basement insulation doesn’t actually have anything to do with the cement or block walls that make up most of a basement, but rather it concerns the wooden floors and framing of your house above your basement. That wood frame around the top of your basement that holds up your entire house is often a prime source of heat loss in a basement. Look around and you’ll see all sorts of pipes and wires going out of your house through various holes. All those holes and gaps should be insulated tight, to prevent cool or moist air from entering.

You can use foam board or other types of water resistant insulation to fill in the areas around the joists but one of the easiest (though slightly more expensive) ways to insulate that area is to use some sort of expanding foam. You just spray it in the area and the foam expands (there is a little bit of an art to it) and fills in the gaps and empty areas quite well.

Final Thoughts on Insulating A Basement

I’m just about to start my basement insulation project (I just have one unfinished room to insulate for now) so I’ll write up the details after I’m done. I’ve been looking into my various options a lot, and there’s one free basement insulation guide that I keep referring back to. It’s a free .PDF file called Basement Insulation Systems and it’s written by Nathan Yost M.D. and Joseph Lstiburek, Ph.D., P.E. It’s on the US Department of Energy’s website and it really breaks down the problem of basement insulation in a scientific but readable manner.

I think I’ve decided to go with the foam board insulation method, but I might have to get a little clever with what I’m doing. I have a fairly dry basement with very little moisture anywhere. I’ll post photos and let you know how my home insulation project goes once I get started!