How To Insulate Basement Walls

Insulating a basement properly is rather challenging in cold climates. In this article I’d like to discuss how to insulate basement walls in cold climates.

Insulating basement walls in cold climates is a great way to keep your home warmer and drier. However, special care should be used when insulating concrete (or block masonry) basement walls. Below grade concrete foundation walls are very col and damp. If you were to insulate the walls with regular fiberglass batt insulation it is very likely that a mold problem would develop.

Recommended Basement Insulation

Therefore I recommend using a combination of products to insulate basement walls. In order to create a vapor barrier and separation between the concrete walls and wood framing I like to use extruded polystyrene insulation (blue board from DOW).

Next we frame a traditional wood stud wall in front of the polystyrene insulation. We do take one special step in framing the wall. First we install a layer of composite decking between the concrete slab and the pressure treated bottom plate of the wall. Finally we install some type of insulation in the wall cavities.

Step 1 – Install Extrude Polystyrene Insulation Boards

The first step in insulating a basement wall is to install the polystyrene insulation (for more information we recommend reading: R Values Of Foam Board Insulation). We like to use a all purpose adhesive that’s approved for Styrofoam products. After the polystyrene insulation is installed each of the joints is taped with construction tape (there are several available from Dow, Tyvek, and other house wrap manufacturers) or some other approved tape that adheres to polystyrene insulation. By taping the joints you are helping keep moisture and cold air from infiltrating into the stud wall cavity.

I recommend you seal the top and bottom of the foam board using spray foam from a can. You can buy products like Great Stuff that will seal those joints very well. Be sure that you install foam board on top of the concrete wall up tight against the wall plate and seal that well.

Want to see how this is done? What the following short video:

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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 Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

View Comments

  • One advantage of using closed cell spray foam insulation is that you can avoid having to put the polystyrene and taping it. Simply offset the studs from the wall, and the foam, when applied, will completely seal the wall and insulate it. The moisture drive in a basement is from the interior to the exterior, so the spray foam prevents any condensation or mold growth while providing the highest R-value per inch (>6).

  • One thing I'd add that if you go with the foam board approach you should either caulk or spray foam a bead along the bottom of the board and the floor to eliminate any air gaps b/t the likely uneven concrete floor and the bottom edge of the boards.

    I like the composite decking idea. I am going to use tigerfoam with offset studs as this is a much better air and vapour barrier and will prevent both condensation - from the moisture inside - and evaporation from the wicked water in the concrete walls.

  • What's the best way to frame the wall around piping? I have the drainage pipe running along side two walls. I could either bring the wall outabout 10 inches or frame around the piping creating a sort of ledge.

    Also, should I consider doing anything with the floor first or can I let it go until I decide what I want to do with it?


  • @ Hutty - There are two ways to take care of the pipe, either bump out the wall as you suggested or build a soffit around the pipe itself.

    Build your walls first then deal with flooring.

    Best of luck.

    • I've read a lot of the posts here on using ridged Styrofoam, properly adhered to the concrete/block basement walls, and made hermetically sealed. Let's say I do all that and I use 2" thick or better and instead of building and finishing a stud wall in front of the Styofoam, just plaster over it? Or, Drivet? I'm not concerned about "code" police and I agree with the guy who pointed out that below grade spaces maintain heat very well.

      • What's your question? yes those are viable options. Not sure why you call them "code police" as many building code officials that I deal with on a daily basis are extremely helpful and very knowledgeable.

        • Hi Todd,

          I'm using 2" wallmate XPS to insulate my basement. It is a recatangular basement of a townhouse with one wall shared with the neighbouring unit. For the shared wall i am using Roxul Safe n Sound just for sound. There is a wall about 8' from the end of my unit that will divide my basement into a heated and unheated space. Is there any problem with stopping my 2" XPS halfway along a wall, and then insulating the dividing wall with fibreglass batts? I'm mainly concerned with moisture issues. Is it bad to only have half a wall insulated top to bottom? Also, is it bad if some cold air DOES end up behind my studded wall and contacts the XPS?

          • Is there any problem with stopping my 2″ XPS halfway along a wall, and then insulating the dividing wall with fibreglass batts? No

            Is it bad to only have half a wall insulated top to bottom? I see no reason why you'd want to do that, it simply won't help much.

            Also, is it bad if some cold air DOES end up behind my studded wall and contacts the XPS? No

        • So just so I'm clear I can adhere the rigid insulation directly to the basement walls and then use a cementitious coating of some sort on the inside? Would a gypsum type product like easy sand work as well or what would you recommend for a coating to use directly on the insulation?

          • Nathan - I guess you probably could do something like that although I'm not sure what the details might be.

  • It is hard to tell but I believe in your case your walls are non-load bearing and really just partition walls. In the case of bearing walls would you still use a piece of composite decking under a mudsill? I don't know the compressive strength of the composite or if there would be other factors to consider in that case.

    Thank you.

  • @ Ian - You are correct. The walls shown are just partition walls. I too would not use this detail for a load bearing wall. I'm only suggesting that this detail be used adjacent to a concrete wall in case of water leakage or excess moisture. For interior partitions or load bearing walls this problem would not be a concern.

  • I like the theory of the composite decking, but doesn't it have mold issues. I have Trex on our deck and the areas that don't get sun get pretty moldy. There must be some cellulose in the composite material. I have to clean the deck with a mold cleaner at least once a year.

  • I have Trex for our back deck and have to clean the mold off at least once a year in the less sunny areas. Mold might be a concern for use of the use of the composite for this application. I like the idea, and there may be other composites that don't mold as easily now.

    • New product out called Azek, solid material versus coated like Trek, will not fade or attract mold, and will not stain with oil products. Its made of solid PVC material, pricy but you never have to do any maintenance for the remainder of your life.

  • I'm not sure your outdoor mold issue is quite the same. After all, mold even grows on vinyl siding and asphalt shingles on the shady side of homes.

    The point of the Trex is to have a product that won't wick the water up into the wood framing, drywall and insulation.

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