Insulating Basement Walls
Insulating basement walls properly is extremely important if you want to enjoy your finished basement investment. Every year thousands of basements are finished with poor insulation details that lead to mold, mildew and moisture problems that render the newly finished space un-usable.
One of the most popular topics on this site has been how to insulate a basement. Insulating basements walls isn’t that difficult once you understand a few basic concepts.
Moisture Is Present In EVERY Basement
First of all you need to come to grips with the fact that every single basement has excessive moisture. Moisture is present in ALL concrete and block regardless of it’s age. The micro-structure of concrete looks much like a sponge and it holds on to moisture throughout it’s life. The key is to always assume concrete (or block) is fully saturated with water. Even if your basement appears super dry I can guarantee there is a significant amount of water and water vapor in your foundations walls and slab.
Check out my Video On How To Insulate Basement Walls for additional information and tips.
Avoid Fiberglass Insulation In Contact With Foundation Walls
Fiberglass insulation is cheap and readily available and often used in poorly insulated basements. Fiberglass insulation is probably the single biggest cause of moldy basements today. Fiberglass insulation is nothing more than a ready and willing sponge that will soak up every ounce of water in your basement and promote the growth of mold and mildew. There are some exceptions to this rule which I will point out but it’s extremely important that you not use fiberglass insulation in direct contact with your foundation.
Expectations – Be Honest With Yourself
Not every basement is suitable to be finished. Often times I see basements that are finished off and they had zero chance of being successful finished space. If your basement has flooded in the past, or your basement is wet on a regular basis it’s not really a great idea to finish the space. You really need to be honest with yourself and only finish a basement if it has a history of being dry (no standing water, no visible puddles). Of course if you take the steps to correct past flooding issues (better site drainage, sump systems, gutters, etc) then it might be possible to successfully finish your basement. The bottom line is don’t waste your money and time if you have a questionable basement.
There have been tons of questions about whether or not to use a vapor barrier. The discussion is complicated so we wrote another article about the topic. Please check out: Vapor Barriers For Basement Insulation.
Basement Wall Insulation Details
There are several good ways to insulate basement walls. I’m going to list them in order from most expensive/most effective to least expensive/least effective. It should be noted that even the last option works quite well even though I list it as the least effective.
Spray Foam Insulation
The best approach to insulating basement walls today is the use of spray foam. Spray foam is sprayed directly onto the concrete (block, stone or masonry) wall, sill plate and rim joist. The spray foam completely encapsulates the concrete and effectively insulates the wall while locking out moisture and creating a great vapor barrier. Spray foam i s the most expensive running as much as $3 per sq foot installed.
Rigid Foam Board
Using rigid foam board (read more about Foam Board Types and R Values) is an excellent alternative to spray foam. Foam board is cheaper to install and it can easily be done by a DIY’er so this option is more attractive to home owners that want to save on labor costs. Properly installed foam board (joints sealed) can also create an effective vapor barrier while preventing the growth of mold and mildew. Depending on where you live in the Country you’ll want to install at minimum of 2 inches of foam board and be sure to tape or seal all joints. This type of insulation can be installed for just over a dollar a sq foot.
Foam Board and Fiberglass Insulation
As I said previously there is an exception about fiberglass. One of the solutions that I’ve used several times with great success is a hybrid approach that involves using both foam board and fiberglass insulation. In this approach you use a thinner (cheaper) layer of foam board (minimum of 1 inch), properly sealed, then a framed wall in front of it with fiberglass insulation in the stud cavities. This is the cheapest solution and one I’ve used in my own home. However, this should only be used if your basement is reasonably dry throughout the year.
How To Insulate Concrete Floors
If you want to insulate your concrete floor then the following article is a must read:
Basement Wall Insulation References
Building Science – Basement Insulation – This is a great resource. Building Science.com has some great publications and information on basements.
US Department of Energy – Basement Insulation – This is also some pretty good information minus the detail with batt insulation in direct contact with the concrete.