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.

Picture of XPS Blue Board Being Installed on the inside of a foundation wall

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.

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

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.

Picture of foam board sealed with Tyvek tape in a basement.

Seal Insulation Joints

Updated (Oct 2014)

Years ago when I first started using this method there were very few options available for tape products to seal foam board insulation panels. Back then I used Tyvek tape but that product is no longer recommended for use on foam board. Most of the foam board manufacturers now manufacture tapes made specifically for sealing these products. DOW offers the WEATHERMATE™ Construction Tape, and Owens Corning has JointSealR™ Foam Joint Tape. These products are more expensive, but they are specifically designed for this application and very important to the long term performance of the insulation barrier.

Tyvek Tape on DOW Blue Foam Board

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.

Framed Stud Wall In Front of Foam Board Basement Wall 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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  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.


  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:

    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 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?


    • 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.


  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?

    • 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:


    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!

  17. Hector D. says:

    Todd, what type and/or brands do you recommend for the Styrofoam adhesive for gluing the XPS to the walls? Thanks!

    • Todd Fratzel says:

      If you go to the hardware store you’ll typically find one or two choices of adhesives that list foam board. The glue is only to help hold up the sheets while you frame the walls, in some situations we don’t even use it.

  18. Russ says:

    Mike,your point is a good one. Tyvek themselves now recommends against using their tape on rigid foam board. From what I have read, they recommend against it’s use because the tape is made specifically for Tyvek wrap and does not expand and contract at the same rate as the foam board and over time, it will crinkle and peel off the foam board’s surface.

    Most of us that do a rigid install won’t even know if the tape job eventually fails, because as you know, once the rock goes up, it becomes an out-of-sight, out-of-mind situation.

    Owens Corning makes an XPS specific seam tape called JointsealR for use with their Foamular product line. It’s a 3.5 inch wide tape that has a waxed paper backing that has to be peeled off before applying. The tape itself is very rubbery in nature and pulls and stretches in all directions fairly easily. The adhesive on the JointsealR tape seems to be a good bit stronger than the glue on the Tyvek tape.

    I had originally used the Tyvek tape on a recent 700 square foot basement install because it had been mentioned in a few forum posts online, as well as a few blogs such as this one. After the install, not being 100% happy with the strength of the bond of the Tyvek tape in some areas, I decided to look into the matter. It was at that time I found out that Tyvek didn’t support the use of their tape as a seam tape in rigid foam install situations.

    So I personally decided to make a change…

    Two days later after I received a few rolls of the JointsealR tape, I easily removed the Tyvek tape from the entire job in about 20 minutes time. It peeled right off with no effort whatsoever. I mean it peeled right off; entire eight foot long strips -top to bottom- with one very easy, few-second pull.

    I then installed the Foamular JointsealR tape. Once on the wall, I could not peel the JointsealR off more than an inch at a time. The difference was night and day!

    The JointsealR tape costs about 3 to 4 times more than the other tapes commonly used and is hard to kind of hard find. I could only find it at one online insulation product retailer. Those might be a few reasons it is not mentioned or used more often.

    • Todd Fratzel says:

      Thanks for the update guys. I plan on updating the article accordingly. Lots of things have changed over the last few years with more manufacturers addressing issues like tapes. If you go back about 5 years ago, the only other tape available was one from DOW. Now there are several options.

  19. Hector D. says:

    Todd, when insulating rim joists, do you recommend XPS, Polyiso, any other type? Any advantage of one over the other? Thanks!

  20. Hector D. says:

    And as a follow-up question, does it have to be foil (or something else) faced? Thanks!

  21. Hector D. says:

    Todd, thanks again for your help and guidance!

    Before installing the XPS insulation, do you recommend applying hydraulic cement to the base of the wall where it meets the slab? In my case I haven’t seen any water issues but it seems it’s another way moisture or water vapor can get inside. Thanks!

  22. Dave S. says:


    Thanks so much for sharing this information. If I have relatively frequent (4 x a year) seepage issues in my basement during heavy rain, is there a step I should follow before installing the foam?


    • Todd Fratzel says:

      Dave – With that much “seepage” I’d question whether you should be finishing the basement. I’d first try to solve that problem, better drainage, possible interior drain. Otherwise the foam will likely be hiding trapped moisture that can cause other issues.

  23. Matt says:

    Can you please address when you would use 1 1/2 inch XPS versus 2 inch XPS. Both thicknesses are recommended in various articles on the site. Thanks for all of the great information.

    • Todd Fratzel says:

      Matt both are mentioned for a couple reasons. 1-1/2″ is the minimum thickness recommended to provide a vapor barrier to prevent moisture from the foundation from getting through the foam. Having said that, some publications are now recommending a minimum of 2″. 2″ is also typically more available. Like many things in life, for most people it comes down to money. I hope that helps.

  24. Terry says:

    We are insulating a basement wall as you have shown with the blue foam and then a stud wall. Just wondering- if this is airtight that means the maoisture has to escape to the outside? Would this cause the parging to crack on our foundation? Live in northern ontario. Thanks

    • Todd Fratzel says:

      Terry – Not at all, remember the foundation material (concrete) and most parging materials are cementitious materials. They need water to activate chemically and are not damaged by water.

  25. Rich says:

    Should I leave space between the foam board and the frame wall? If so, how much space? I’m trying to maximize space, but if it means taking more space to do it right, than that’s what I will do.


  26. Dave says:

    I have a garden level bathroom with shower. On top of the concrete foundation is the stud framing of the house. Only 2 1/2 inches of foil backed fiberglass was in the walls between the studs with 4+ inches of open air then the drywall. Also, the one inch between the concrete and drywall was not insulated. Can I add 4 inches of foam board behind the batt insulation(between fiberglass and interior drywall) and spray foam the gaps, then also add 1″ foamboard to the concrete foundation, butted up against the drywall?

    • Todd Fratzel says:

      Dave – I would not recommend that. I’d recommend you remove all the fiberglass, install the foam board first, then you add the fiberglass after. Good luck.

  27. Jon says:

    Hi Todd,
    I’m dealing with trying to finish 2/3 of my basement. The 1/3 I wasn’t planning on putting anything up on the concrete walls. For the other 2/3 I have framing up already and it is away from the concrete wall but foam insulation wasn’t put in first. On some of the walls i can slip the foam in behind the framing but not on all.
    I’m not sure what i should do as it seems impossible to seal up the insulation to be airtight. There’s going to be airflow between studs and from a closet that opens into the unfinished space and due to an open ceiling in another part and various other reasons. I could go crazy with spray foam but still pretty sure i couldn’t have a real vapor barrier. Should I just get the foam board up where I can, seal up what i can, and otherwise be ok with air movement? I run dehumidifiers during the summer to keep moisture levels low.
    Thanks for any help.

    • Todd Fratzel says:

      Jon – Your situation is very common. Unfortunately the best advice I can give you is to take a step back, move the walls in order to properly install foam, then slide them back in place. This may seem really difficult, but with some help it’s not all that hard. Other than that I’d not feel comfortable recommending anything else as I see too many mold/mildew problems. Good luck.

  28. Adam says:

    Hi Todd,
    Thank you for this extremely helpful thread! I am about to finish my basement using 2″ XPS polystyrene foam on the foundation walls. I have existing “bagged” pink fibreglass insulation directly on the concrete now that I will be removing. I will be building a 2×4 wall in front of the foam. I live in southern Ontario and our building code now asks for an R-20 on basement walls so I was planning to insulate the stud wall make up the extra R-value. My question is, can I use the pink fibreglass insulation that I removed (R-12 value) between the studs? This will give me the R-value I need. It is 4′ wide so I can cut it into 3 16″ strips. This will save me a bunch of money for new fibreglass insulation. I also plan on drywalling the wall without an additional vapour barrier as I have read earlier. Am I on the right track? Thanks for any advice!

  29. Fred says:


    Can I install 1-1/2″ of xps on my poured concrete wall, frame the wall with 3-1/2 metal studs, then add additional insulation in between the studs to get an “R” value of at least R10 per code. This room is a bath with a shower.

    The problem I ran into If I use 2″ xps, is my toilet drain
    is to close to the wall by 1/2″. I end up with 11-1/2″ to C/L instead of the 12″ required.

    I was thinking of adding 1/2″ if xps between the studs on this wall, or would faced or nonfaced fiberglass insulation be better?

    WOULD this be acceptable? The insulation would end up laying in the lower track.

    I also need to run an 1-1/2 ” pvc drain line running through these wall studs for my sink drain.

    What Are your thoughts.

    • Todd Fratzel says:

      Fred – That will work fine. Also, many times you can get a close rough-in toilet that would buy you another inch or so. Either approach will work fine.

  30. Mark says:

    I have a rental property that was built in 1935 with a dirt floor basement (that’s how they did it back in the day). I would like to insulate it and will be using the blue boards you mentioned. The foundation is not a concrete wall (typical old foundation type) so it is not exactly smooth and I am wondering if applying the glue to the board would be effective or if I should be taking a different approach?

    • Todd Fratzel says:

      Mark – Foam board is really hard to use on an old stone foundation. Typically spray foam is the best approach when dealing with an old foundation like that. Glue simply won’t hold it in place. Good luck.

      • Mark says:

        Aye, that’s the challenge with an old basement like that. Two questions, does foam boards have to be snug against the wall to be effective (glued or otherwise) or if there’s a gap here and there behind the foam board, does that make it ineffective?

  31. Nick says:

    Great site. I’m going to put the foam boards up but will get to the point where my breaker box is mounted to a sheet of OSB which is mounted to the concrete wall. How should I insulate that? Spray behind and around it?


    • Todd Fratzel says:

      Nick – Two options really, either just butt up to the board with your insulation or have the panel detached from the board and insulate behind it. Good luck.

  32. Fred says:


    Is there any issues screwing 1/2″ gypson drywall directly to 12″ engineered beams. Will the screw holes affect the integrity of the 12″ beams? Do I need to use adhesive?


  33. Another Nick says:

    I’m finally getting around to finishing an office in the basement. I have the XPS up. Two hopefully quick questions. Can I use foil tape on the joints? When I insulate between the framing, do I use fiberglass without the paper on one side?

    I really appreciate your site. It’s been a great source of information.

    • Todd Fratzel says:

      Nick – Foam tape most likely won’t stick long term. It’s best to use tape specifically made for that foam, if you’re using DOW then they make a tape. Similarly if you’re using OC, they likely have a recommended tape. Fiberglass without the paper, or if you use paper faced, cut holes in the paper. Good luck.

  34. Tom says:

    Hello-do I need to apply any kind of concrete sealer to the walls before putting up the insulation board? Thanks!

  35. Scotty says:

    Hi, I am finishing a basement room and have to insulate and drywall a concrete foundation wall. I only have 1 1/4″ space due to the door placement which cannot be moved. How should I do this?

    • Todd Fratzel says:

      Without knowing the specifics I’ll throw out a few things to consider: Smaller door? Transition framing so you’ve only got a small area with foam less than the minimum suggested thickness.

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