Best Methods For Insulating Basement Walls
I’ve written several articles on insulating basement walls and evolved some of my methods over time. Basements provide for a very challenging insulation problem. Basements are a challenge because of high moisture levels and cooler temperatures. In this article I’d like to summarize some of my experience with insulating basement walls.
Basement Insulation Reference
Most of my methods are based on information from Building Science.com. They have a really great publication that you can download called: “Renovating Your
Basement” Research Report – 0308, 2003 (revised 2007) by Building Science Corporation. This publication is really concise and full of great information.
Understanding Basement Insulation Problems
I can’t tell you how many finished basements I’ve seen with wood framed walls directly against the concrete and the cavities of the framing filled with fiberglass insulation in direct contact with the concrete foundation walls. This type of details almost always leads to a serious mold problem and obviously a potentially risky health concern for your family.
Most people don’t understand the microscopic composition of concrete and often overlook the significant moisture levels present in concrete. I often have people say to me “the concrete walls are very dry”. I hate to be the bearer of bad news but ALL concrete has significant levels of moisture present in it. If you were to look at concrete under a microscope you’d be surprised to see how many small voids or “pores” exist. Concrete actually acts like a sponge and those pores contain lots of water. So you MUST assume that your concrete is fully saturated and full of water in order to properly insulate your basement walls.
Insulation Materials Are The Key
The key to successfully insulating basement walls is selecting insulating materials that stop moisture movement and prevent mold growth. Basements are the perfect location for foam type insulation products. Cellulose is also an option for basements but it’s not a product that I’ll cover here. I wrote Insulating A Basement with Cellulose which uses both foam and cellulose to insulate a basement for a Energy Star Home.
Spray foam is probably the best possible insulation material for basements and damp areas. Closed cell spray foam is perfect for locking “out” the water vapor that inherintely wants to migrate from your damp basement walls to your finished basement rooms. Spray foam offers several benefits including additional structural stability, great vapor barrier, easily covers pipes, wires and other utilities and it provides exception R values. The problems with spray foam are it’s substantial price tag (although this will get better with time) and it’s a very messy task. However, if you can afford the cost and mess then this is the ultimate insulation product.
The next best basement insulating method is using foam board products. This is the method that we use most often and it’s the method that most DIY folks can handle. It’s also quite a bit cheaper than spray foam so it’s an attractive alternative. There are lots of foam board products on the market so I suggest you read Foam Board Insulation Types and R Values for a quick education on the different products available.
The key to using foam board is choosing the correct thickness and sealing it properly to create an effective vapor barrier. If you’re just going to use foam board then you’ll most likely need 2 to 4 inches in thickness depending on local energy code requirements. I recommend sealing all the joints with Tyvek (or similar) house wrap tape. You can also use “Great Stuff” spray foam in a can to seal around all your utilities and also along the bottom of the foam board. For more information on using foam board I recommend you read How To Insulate Basement Walls with Polystyrene Insulation.
Hybrid Foam & Fiberglass Insulation
The last method that I’ve used is a hybrid system of foam board and fiberglass. This method is the least expensive yet I believe it will perform well in basements that don’t have visual signs of water infiltration. This method should NOT be used if you have a history of water, even small amounts.
In this method you’ll be installing a layer of foam board, sealing it as noted above. Then you’ll frame a wall (wood or steel as I don’t believe there is much difference) in front of the foam board. Finally you’ll install fiberglass insulation in the stud cavities. Again the key here is to come up with the proper R value based on local energy codes. Most references say that you should NOT use a vapor barrier over the fiberglass insulation in this method. I’m not sure if I agree with that but it’s certainly a gray area.
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 Insulation Summary
The bottom line is not all basements are acceptable for finished space. Too often people try and “force” a finished basement when it’s not practicle due to flooding and water problems. All of these methods assume you don’t have any serious water problems in your basement. The key is using foam insulation effectively to separate the damp concrete or block from framing and insulation products that promote mold growth. I hope this article will help you decide which basement insulation method is best for your home.