Basement Insulation

By Todd Fratzel on Basements, Framing, Insulation

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.

blue board installation Basement Insulation

It’s All About Water Vapor

Unfinished Basement Detail Basement InsulationBasement 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.

These are horrible details, no amount of water proofing, concrete sealer, etc is going to make them better. My advice is simple: DO NOT EVER USE those details.extruded polystyrene insulation Basement Insulation

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.

blue board tyvek tape Basement Insulation

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.

tyvek tape on blue board Basement Insulation

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.

basement wall framed over blue board Basement Insulation

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.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the author of Tool Box Buzz and Today's Green Construction. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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35 Comments

  1. susan says:

    My question is : do you still need foam insulation to grade on the exterior of the basement walls if you insulate as directed on the inside? Thanks so much for your helpful information.

    • Todd says:

      Not necessarily. Many people do. One reason to consider it, because it helps keep water away from the foundation wall. It’s a good first line of defense.

  2. ron says:

    Great advice. I was wondering what to put directly on the concrete walls if drain tile was installed in the interior and dimpleboard was was run about one foot up the basement wall. This would would obviously cause the foam board to be away from the wall or do I just rest it on the dimpleboard? Also, what stops the vapor from traveling up the wall and entering the ceiling joists. Thank you

  3. Dave says:

    When doing the framing for the walls, do I want to insulate the floor first. That way I can get foam board down before I put the 2×4 baseboard down, or do I want that 2×4 to be on the concrete floor.

    David

  4. Lori says:

    If the basement has already been framed, how can we seal the foam boards behind the 2×4 to create the barrier needed.

  5. James says:

    How do you keep a continuous foam insulation/vapor barrier when you have drains and stacks right against or maybe an inch from the concrete walls? Is it okay to use 2″ foam everywhere and just slip a 1″ piece the width of the pipe tightly behind the pipe and tape it to the 2″ on either side? Or should I just insulate around it and frame around it?

    • Todd says:

      James – You do the best you can! :) Obviously in a remodeling situation it’s nearly impossible to get it perfect. Your solution is spot on. Good luck.

  6. Omar says:

    Todd,
    The bottom half of my foundation wall is 12″ poured cocrete and continues with an 8″ CMU the rest of the way up, creating a 4″- 5″ ledge on the interior face. I would think using the foam board method would not work as well in this condition. Any ideas.

  7. Joel says:

    Todd- Great blog! Can I install my 2″ of XPS foam over a painted concrete wall? I am having great difficulty removing the paint (have gotten it down to 1 to 2 layers and bare in some spots) and contractors I’ve called are not interested in helping complete the paint removal task. I am concerned the paint could become a food source for mold behind the XPS foam. Is that a concern or does it not matter if its behind the vapor barrier? Should I add a skim coat of concrete to the existing wall to cover up the paint so it isn’t directly touching the foam? Thanks!!

  8. Herman says:

    Hello Todd – I’m finishing my basement and the baseement was comleted with the big silver padding insulation that is nailed to the concrete. What is your recommendation on how to finish the basement walls with this type of insulation already there?

    • Todd says:

      I’m assuming you mean that your basement has some sort of “bagged” fiberglass insulation? If it were my home I’d remove that insulation and use closed cell foam board. In my professional opinion fiberglass insulation has NO place in insulating basement walls.

      Good luck.

  9. Jeff says:

    Hi Todd,

    Thank you very much for your write up. In the mechanical room of my basement I currently have a foil backed fiberglass nailed to poured concrete wall (it is not ‘bagged’ so fiberglass is directly against concrete I guess it is something similar to http://www.specjm.com/products/faced/fsk25.asp). I was planning on installing the foam boards as you suggest. My question is, can the foam boards just be left exposed? I have no need/desire to frame in walls in this part of the basement (rest of the basement is finished). I basically just want to get rid of the exposed fiberglass insulation.

    • Todd says:

      Jeff – Technically you should not as it poses a fire hazard. Most building codes specify a minimum flame spread rating for occupied spaces. Check with your local building code official on the requirement, then check the product information on the foam you’re considering. Most do not meet the standard and therefore need some protection.

  10. Peggy says:

    Hi Todd, Love the articles here. I have a 50′s house with a “finished” basement. The foundation walls have plastered applied directly onto the concrete. I want to frame some of the exterior walls so that I can extend the electrical and drywall nicely around my new windows. Can I apply the closed cell foam directly to the plaster? Many thanks!

    • Todd says:

      Peggy – In some situations plaster can promote mold growth. So there is a slight risk that mold could develop behind the foam if the conditions will support mold growth. Having said that, I think it’s certainly a possible solution. You might want to consider some type of paint that helps prevent mold to at least seal the plaster first. Good luck.

      • Peggy says:

        That is terrific advice, I’ll do that. Living in Colorado I don’t worry about mold too much but it is possible. Thanks again!

  11. Andy says:

    Hi Todd
    I am trying to insulate my basement wall. There is a 4 foot block wall below grade then a 4 foot 2×6 framed wall on top above grade wich creates a 3 1/2″ bench. Can I put blue board on block wall , insulate 2×6 with roxul . Then if I frame a2x4wall 8 foot high and insulate it with roxul for additional r value can or do I have to use poly vapour barrier? I know it sounds confusing. My basic question is can you go block wall, blue board roxul, poly, then drywall, or no poly?

    • Todd says:

      Andy – You can install the foam board over the lower block wall. You can insulate the exterior wall with Roxul, no vapor barrier. Then insulate your new 2×4 wall in front of all that with Roxul and vapor barrier. Good luck.

      • Andy says:

        Hi Todd
        Thank you . I was worried about putting poly over the area that has blue board and having two vapour barriers. Again thank you. Great info.

  12. Ed says:

    Hi Todd,
    My basement is already 2×4 framed about 1.5″ from concrete wall. The previous owners used fiberglass insulation which I have removed. I was able to slip 1″ XPS behind the framing, but can not get them to adhere due to the concrete wall being uneven. Any advice on how to get them to stay in place? As an alternative, can I just attach the XPS to the inside framing and then drywall directly over that?

    Thanks

    • Todd Fratzel says:

      Ed – This is what I would try.Keep the 1″, then install a piece of 1-1/2″ or 2-1/2″ between the studs, foam it in place. This should hold the 1″ in place. You REALLY need to the 1″ to create a vapor barrier. This all needs to be sealed well. Good luck.

  13. Tom P says:

    Hi Todd

    Due to low ceilings, I was going to use 3/4″ XPS foam on the floor. I used 1 1/2″ on the walls – I know it’s not ideal, but is 3/4 ” better than nothing? My ceilings are about 6’9″ so I’m trying to minimize the sub-floor impact on height. Basement gets humid in summer but I have 2 dehumidifiers and a Slim Jim AC/Dehumidifier going most of the time.

    Thanks,
    Tom

  14. Tom P says:

    Thanks Todd. Your site is great. I’ve learned a ton reading through these topics. I have one more question, if there are some uneven spots on the floor, what type of shims should I use? I imagine that you would shim under the foam board?
    Thanks

    • Todd Fratzel says:

      Best approach is to use a floor leveling compound so that the transitions are as smooth as possible. Good luck.

  15. Dave says:

    Thanks for all the great info. I’m going to be refinishing about 2/3 of my basement and plan on using 1 1/2″ closed cell insulation like you recommend. Do I need to insulate the exterior walls that won’t be part of the finished area? I’m thinking this could lead to a moisture problem in the unfinished part of the basement or at lease an increased humidity level in the basement if these walls are left uninsulated.
    Thanks for all your help.

    • Todd Fratzel says:

      Dave – Not necessarily, but you’ll need to pay close attention to any walls that separate the spaces. The issue will be creating a proper vapor barrier to keep moisture from the unfinished space from getting trapped in the separation wall.

  16. Mike says:

    Todd,

    Have you checked the compatibility between the Tyvek tape and the XPS foam board? I taped the joints on my XPS boards just like you did with the common transparent red tape. About 9 months later, the joints under the tape were bubbling up, and the tape was lifted up off the XPS board. You may want to go back to a house that was completed 1yr+ ago to see if you have the same issue as I do.

    After some research, I found that XPS reacts to the plasticizers in the tape adhesive. I’m guessing the Tyvek brand tape has the same plasticizers in it as well. I’m planning on calling Dow this week to see if they can recommend a tape that will not react with XPS.

    The quest for a perfect basement finishing solution is a difficult one!

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