Written by Todd Fratzel.
Basement Insulation – Walls
Basement insulation is far more complicated than insulating regular wood framed walls. In fact, improperly insulated basement walls are one of the biggest reasons for mold in homes. If you don’t do this correctly you’ll be exposing your family to dangerous mold exposure. In this article I’ll show you How To Insulate Basement Walls properly.
It’s All About Water Vapor
Basement Insulation is difficult to understand because of water vapor issues. While this is a topic best left to another article entirely, it’s important to realize that there are two sources of significant water vapor in a basement. Just like in the rooms above the basement, water vapor from inside the room wants to escape to the outside.
In addition, and likely more serious, is water vapor escaping from the concrete wall and trying to escape to inside the finished room. Concrete looks just like a sponge under a microscope and it holds water just like a sponge. In fact, over the entire life of concrete it holds water. So concrete is ALWAYS letting off water vapor and that water vapor is trying to either dry to the outside or into your finished space. To fully understand this please read our basement insulation vapor barrier article.
What NOT To Do When Insulating A Basement
Under no circumstances should you try either of these methods for insulating basement walls. Both of these are similar, both of these are methods I see all the time, and they ALL end up with mold problems:
- Layer of plastic poly installed over the concrete wall. Next a wall is framed and fiberglass insulation is installed in the stud bays. In some cases this is either kraft faced or another layer of poly is installed. This situation is bad because any water vapor that gets into the stud bays will end up condensing on the surface of the poly (the poly will be as cold as the concrete wall, around 50 degrees F.), the water then causes mold to start growing in the nicely tented space between studs.
- A stud wall is built such that it’s an inch or two away from the concrete. In this case only fiberglass insulation is installed between studs (no plastic on concrete). A vapor barrier is installed, then drywall. This situation is really bad, water vapor from the concrete escapes into the stud cavity and condenses on the back of the vapor barrier behind the drywall. Again, water creates a nasty mold situation in the stud bay.
Closed Cell Foam Is The Answer
Whether you spray it in place or install sheets, closed cell foam is the answer for basement insulation. When properly installed, closed cell foam (spray or board) provides two benefits: first and foremost it’s a great insulator with MUCH better R values per inch than fiberglass, and secondly when you use closed cell foam that’s at least 1-1/2 inches thick, it actually acts as a vapor barrier. So essentially you get a vapor barrier and insulation all wrapped into one easy to use product.
DIY Basement Insulation Using Foam Board
The following is a great example of proper DIY basement insulation. If you’d like to see a more detailed article on how to do use this approach you can see that HERE.
Several years ago I insulated the basement walls in our basement storage room. I installed 1-1/2″ extruded polystyrene foam insulation board by DOW. I used a special Styrofoam adhesive to adhere the foam board to the concrete (you can also use “Great Stuff Pro” foam in a can). The foam board I am used comes with a tongue and grove so installation is pretty easy. Just cut the boards to length, apply some adhesive and press it against the wall. Note, you may need to apply pressure to the foam with boards until the adhesive dries.
If you’d like to see a video of this type of approach check out our DIY Basement Insulation Video.
You’ll notice that I installed the boards horizontally. Since doing this project we’ve been installing them vertically at work and it’s much easier to do! Placing them vertically makes it much easier to keep them from falling down while the adhesive cures.
Seal Insulation Joints
After I installed all of the polystyrene foam insulation I sealed all of the joints with Tyvek tape. Tyvek tape is a really strong tape that will stay adhered to the foam board and it keeps out moisture. Concrete holds a tremendous amount of moisture so creating a moisture barrier is essential.
Frame Basement Walls
Once all the insulation was in place I started framing up 2×4 walls. I placed the walls directly in front of the foam insulation and then insulated the wall cavities with fiberglass insulation. You’ll need to nail the pressure treated plate of the wall to the concrete with a Trigger Tool Kit, 22 Caliber.
You may be wondering why I used fiberglass insulation after giving it a black eye above. We can use it in this situation because we have an insulated vapor barrier between it and the concrete. By doing the combination of insulation materials I achieved approximately an R values of 16. This was much cheaper than doing it all in foam.
There is also some really great information at the Building Science Corporation.
Insulated Concrete Floors
For this project we didn’t insulate the concrete floors because we have radiant heated slabs. However, if your home doesn’t have radiant heat and you’re looking for information on that topic then we recommend you read How To Insulate A Concrete Floor.
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