Knowing how to insulate a basement is a little higher up on the skill level scale for the average handyman. Sometimes it can help to know why you need to do it. Primarily, properly insulated, your energy costs will be reduced, but the basement can be a bit problematic because it is the area of the house most prone to moisture and most difficult to seal.

Details matter

In insulating basements, you first need to take into consideration where you live. Your local climate, the kind of heating system you already have, costs of fuel and how you live will all have a bearing on your energy costs and you may just find out that your basement may be just fine without insulation. Why go to the expense if you won’t be any better off, financially?

Interior Insulation versus Exterior

Why insulate the exterior of your basement? The primary reason is to eliminate high humidity levels.

In warmer seasons, warm, moist air will enter the basement and come into contact with the cooler walls and floor. The result is condensation, just like what happens when droplets form on a cold glass of ice-water. Because of this condensation, your basement’s air will have a higher Relative Humidity than the upstairs air, and the higher the Relative Humidity, the damper the air will be.

When the outside of the basement walls are insulated, they tend to stay closer in temperature to the indoors air, rather than get cooled down by the outside soil.

How to tell if you have a condensation problem in your basement

If you’re unsure if you really need exterior basement insulation, one way to tell whether you have condensation problems or not is to do the “aluminum foil test”. Tape square of aluminum foil, each about the size of a slice of bread, to the walls and floor in areas where you suspect there to be moisture. Leave the squares there for a day or so. When you come back and look at them, if there is moisture on the foil’s surface, then you have condensation. But, if there is moisture on the walls or floor underneath the foil, it is likely some kind of groundwater seepage, and you need to look at better drainage.

How to Insulate a Basement Exterior

Unfortunatly, installation of exterior basement insulation is costly for existing houses. It may be cost-effective during a major renovation or if a perimeter drainage system is also being installed. It basically involves excavating away all the soil surrounding the basement walls, preparation of the wall surface, installation of rigid foam board insulation, installation of some type of siding over the insulation, and replacing all the soil.

Pros and Cons

These are some other advantages of insulation on exterior basement walls:

• Minimizes thermal bridging and reducing heat loss through the foundation
• Protects the damp-proof coating from damage during backfilling
• Serves as a capillary break to moisture intrusion
• Protects the foundation from the effects of the freeze-thaw cycle in extreme climates
• Reduces the potential for condensation on surfaces in the basement
• Conserves room area, relative to installing insulation on the interior.

The disadvantages of basement wall insulation include the following:

• Costs may exceed those for insulating the basement ceiling, depending on materials and approach selected
• Installation is expensive for an existing building unless a perimeter drainage system is also being installed
• Many exterior insulation materials are susceptible to insect infestation
• Some contractors are unfamiliar with proper detailing procedures that are critical to performance
• If surrounding soil contains radon gas, the house will require a mitigation system underneath the basement floor.

Adding insulation to the interior of the foundation is often more cost effective for an existing building. Interior insulation has the following advantages and disadvantages.

Advantages:

• Interior insulation is much less expensive to install than exterior insulation for existing buildings
• Almost any insulation type can be used, giving a wider selection of materials
• The threat of insect infestation is eliminated
• The space is isolated from the colder earth more effectively than when using exterior methods

Disadvantages:

• Many insulation types require a fire-rated covering since they release toxic gases when ignited
• Interior insulation reduces usable interior space by a few inches
• It doesn't protect the damp-proof coating like the exterior insulation
• If the perimeter drainage is poor, the insulation may become saturated by moisture weeping through the foundation walls
• Superior air-sealing details and vapor diffusion retarders are important for adequate performance

Insulating a basement from the interior – although it uses up a bit of space – can stand you in good stead. It’s actually less expensive than exterior insulation, particularly if yours is an existing building. You also have a bigger range of options in terms of insulation materials that you can use, plus you eliminate threats of insect infestations.

For a more in-depth discussion of basement and foundation insulation techniques, see the Building Sciences Corporation e-book here

Can you handle the job?

Unfortunately, knowing how to insulate basements is one thing, but actually doing the job is quite another. You really should seek the advice of a professional, or at least discuss your job with a tradesman or someone who works at a tradesmen supply center.

Do all your measurements, make up some drawings and list the materials already found in your basement. Give your advisor something to work with or ask them to visit to see for themselves. Insulation is an important part of the integrity of your home and if you do it poorly, you might as well have not done it at all.

When asking professionals their advice on how to insulate basements, be armed with as much information and knowledge as you can. No one has the time to stand there and discuss intricate details with someone who has no idea what they’re talking about. Do some research, understand the various methods of insulation, the materials used, the benefits and drawbacks and then go along with your questions.

Once you learn how to insulate basements and have done a job yourself, you’ll be armed with a set of skills that you’ll take with you to your next home. You might even like to take notes as you go and re-read them before you start a new job.