Categories: Insulation

Walk-Out Basement Wall Insulation

Insulating Wood Framed Walk-Out Basement Walls

Walk-Out foundations create an interesting dilemma in when considering wall insulating details. I’ve written extensively on the subject of how to properly insulate basement walls which deals with concrete (and block) walls but I’ve never addressed the occasional wood framed walk-out walls that exist in some many homes today. In this article I want to discuss the options most suitable for insulating these wood framed exterior walls that exist in homes with walk-out basements.

Typical Wood Framed Walk-Out Basement Walls

Typically we see most wood framed walk-out basement exterior walls insulated with fiberglass insulation and some type of vapor barrier. This detail is used in millions of homes in the exterior walls above grade with some success (the industry is slowly moving away from this detail, but it’s still the most widely used today). So if this detail works ok above grade, why might it not be the most suitable solution in a basement? The answer really revolves around the high humidity levels that typically exist in a basement regardless if there’s a walk-out wall or some other means of egress like a bulkhead.

Many times when we remove fiberglass insulation from a wood framed basement wall we find signs of mold and moisture. In most situations we find frost (during the winter) building up behind the fiberglass, on the surface of the exterior sheathing, around nails (from siding and framing). Moisture from the basement penetrates the wall system, hits the back side of the sheathing and exposed fasteners and condenses and forms frost on cold days. This cycle goes back and forth through the seasons and helps promote mold and mildew growth.

Improved Wood Framed Walk-Out Basement Wall Insulation Details

If you’re going to take the time and money to properly insulate the rest of the conventional foundation walls it’s definitely worth spending a bit more money and time on the framed walk-out walls as well. There are several options to consider including:

  • Spray Foam – One of the best options is to have the framed walk-out walls spray foamed with closed cell foam. The spray foam will not only provide the highest R value but it will also create the tightest wall system preventing air infiltration and moisture movement. This option is the most expensive and also requires a professional installation in most cases.
  • Flash & Batt (Spray Foam & Fiberglass Combination) – A process that’s getting lots of attention and I’m seeing more on job sites is called “Flash & Batt”. In this detail the stud bays are “flashed” with spray foam, typically about 2″ of closed cell foam which creates both a good starting layer of insulation value (higher R value per inch than fiberglass) and it seals the wall against air infiltration which drastically improves the efficiency of fiberglass insulation (fiberglass significantly loses R value when air infiltrates the wall section). This option is more cost effective than the one above but typically involves a spray foam contractor plus yourself or additional labor for the fiberglass.
  • Foam Board & Fiberglass – Another option is to use a combination of closed cell foam board (XPS Foam Board or Polyiso Foam Board) and fiberglass insulation. This detail is a great option for DIY’ers and general contractors that don’t want to involve a specialized contractor like spray foam applicators. In this approach a layer of XPS or Polyiso foam board are cut to fit in the stud cavity tight against the exterior sheathing. Then the foam is sealed to the framing using canned spray foam. This effectively seals the stud bay from air infiltration and vapor transmission. In order for this to work effectively the foam board should be a minimum of 1-1/2″ thick (2″ preferably). Finally the stud bay can be filled with unfaced fiberglass insulation. It’s VERY important not to install a vapor barrier over the fiberglass as this would create a double barrier trapping any moisture between the insulation and vapor barrier.

Example Basement Walk-Out Framed Wall Insulated with Foam Board and Fiberglass Insulation

In the picture above you can see a fairly typical walk-out basement wall. The wall has two features that are fairly common, on the left side of the photo is a portion of the concrete foundation wall that sits about 4′ above the slab due to the changing grade along the side of the house. On top of the wall is a wood framed wall which transitions to a full height completely wood framed wall at the rear of the house. For this walk-out wall there are several details to consider:

  • The exposed concrete foundation wall (above left, and lower) is covered in closed cell foam insulation board. The next step will be to frame a short wall in front of it (it will have a decorative cap on it after, instead of framing the new wall all the way to the ceiling). In this case the lower portion of the wall is insulated exactly as I’ve discussed in my Basement Insulation Article.
  • Each stud bay was insulated first with 2″ of DOW XPS foam board. The pieces were cut so they fit easily into the bay. Then a can of spray foam was used to seal the foam board to the framing. It’s best to do the spray foam last after all the foam is fit so you can use an entire can before it clogs up.

  • Lastly, the remaining stud bay is filled with unfaced fiberglass insulation. It’s extremely important that a vapor barrier is NOT installed over the fiberglass with this detail. It will trap moisture in the stud bay leading to serious problems.

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

  • Thank you Todd for all the great articles on insulating cement basements!

    Scenario:
    I am working towards finishing my basement. I have a standard split-level mid-western home (cold winters, hot summers). 4 ft cement walls in the basement and another 4 ft of wood wall above that. I would like to frame up 2x4 walls on the inside of the cement and have the "shelf" look around/above the cement rather than framing 8ft studs all the way to the ceiling. I plan on applying 2" rigid foam insulation to the cement, framing a 4ft wall, run wiring, inserting bat insulation between studs, and finally sheet rocking the entire wall. I plan on attaching the 2x4 wall to the cement floor using Tapcon screws (treated sole plates).

    The basement windows are roughed in with only 1.5 inches between the top of the cement and the bottom of the rough sill. It would be nice to have a continuous flat shelf the entire length of the wall rather than cutout around windows.

    My question is: How do I attach the top plate of the new 4ft wall to the existing wood wall above the cement with the limited space between the top of the cement and bottom of the window sill?

    Possible Solution:
    Lay treated 2x4 "studs" (about 4" long) flat on the top of the cement wall, and attach them to the top plate of the new 4ft wall and the base of the existing wood wall on top of the cement? That would fill the 1.5" space between the top of the cement and the bottom of the rough sill. Then insert 1.5" rigid foam insulation between these "studs", apply expanding foams between "studs" and rigid foam. Then cap the shelf with a 1x8 board. That would leave me flush with the rough sill, add stability to the top of the new 4ft wall, and allow me to attach sheet rock to the shelf.

    Do you see any problem with this solution? Do you have better suggestions?

    Thanks in advance!

    • Scott - Great question and one that is relevant to lots of homes. First I'd would establish the finished elevation you're looking for. In many of these details the shelf becomes integral/flush with the finished sill trim on the window. That should give you a bit more space. From that you should leave yourself 1/4" of space to shim the trim piece. Ideally you get a piece of foam on top of the concrete knee wall to stop cold from penetrating up through the trim cap. Sometimes when I've done this detail, I'll install some blocking over the foam, into the concrete wall with Tapcons, near the top of the short framed wall, then you can tie the wall into the block. This holds the wall tight to the concrete/foam. Then the cap on top is just covering up the foam and nailed down through the front of the framed wall. Make sense?

      • Is there any potential issue with puncturing the rigid foam with tapcon screws as you suggested? Any concern with moisture creeping in to the bat insulation through those holes?

        In regards to finishing the window trim, I was planning on adding nice 1x8 board painted white on top of the rough sill. That should leave me flush with the finished shelf if I use 1/2" sheet rock on top of the cap. That was my plan anyway.

        • Scott - The potential is small, but if you're concerned you can spray foam the block, and the tapcon to seal it. Good luck!

  • Todd,

    Another good article on a relevant issue. My questions relate to existing and future mold adhering to the wood framing, especially the stud section of the framing either touching the damp cement foundation wall or very close to it – say ¼ inch away. How do you prevent existing mold and moisture from reforming on the sections of 2x4 touching or close to the foundation wall?

    1. Where a small air space is present between the stud and the wall – say ¼ inch away from the wall – do you recommend spray foam in this space?

    2. What about using the new anti-mold paint primer from Rustoleum/Zinsser?

    Per their website:

    Zinsser Mold Killing Primer is a water-based fungicidal protective coating that can be used to paint over and kill all existing mold, mildew, moss, fungi, odor causing bacteria and any other fungal organisms. The Mold Killing Primer contains an EPA registered antimicrobial to prevent the growth of mold, mildew and other fungal organisms on the paint film.

    Do you or any of your readers have any experience with this product?

    Ken W.

  • Todd, it appears that you a top-notch contractor who believes in "doing it right the first time". I have learned a great deal from you by reading your posts. Thanks for taking the time to write these tips for us people who DIY. Kevin

  • Todd, I've read your articles on basement insulation which were very helpful. I live in the northeast and my basement has a framed walk-out foundation on approx. 40% of the walls, the balance being poured concrete. The basement is dry most of the time, perhaps because of the walk-out, and I don't want to create a dampness problem.

    Your opening comments say, "If you’re going to take the time and money to properly insulate the rest of the conventional foundation walls it’s definitely worth spending a bit more money and time on the framed walk-out walls as well". This sort of implies to me that the big benefit is in the concrete walls. So, I'd appreciate your thoughts on just insulating the framed walk-out portion of the foundation. Is this worthwhile or would it just be like insulating half a wall?

    Also, I'm wondering about R-values. Adding 2" insulation may produce an R-value of 9-10. What R-value should I be working toward in Zone 6?

    • Tony - Not sure I understand your question. In the article I discuss the options I'd recommend for the walkout framed walls. In my opinion, it's best to include some sort of foam in addition to just standard fiberglass. Every State has requirements on minimum insulation values typically listed in the States adopted Energy Code. In many places the R value depends on the entire system of insulation in the home, so it's not specific to the foundation walls. R10 is a good start, but certainly not as high as would typically be required. In most situations you're looking at a requirement of at least R19 or greater. Good luck.

      • Todd, thanks for the prompt reply. I'll look into the R-value aspect as you suggest. The first part of my questions is very basic. My basement is 60% concrete, 40% framed walk-out; all without insulation. I don't intend to insulate the concrete portion for several reasons. My question, though, is whether it makes sense to insulate the framed walk-out or is that a waste of time and money without insulating the concrete. For example, I wouldn't insulate a wall without closing the windows.

        • Tony - Absolutely worth insulated the walkout walls. The difference in the basement temperatures from just doing that will be significant.

  • Hi Todd,
    What about the concrete frostwall below the slab? It needs to be insulated with rigid insulation but everyone seems to disagree on which side of the wall? Also, we want to tie the floor slabs (interior & exterior) into the top of the frostwall by with rebar but how do we deal with thermal bridging?

    • Brian - Yes, lots of opinions out there and there are several ways to skin the cat so to speak. If you're starting from mew construction I'd do the following:

      - Insulate the interior of the frost wall below the slab with 2" foam board insulation.
      - Insulate below the slab with 2" of foam board insulation.
      - The wall insulation should come up flush to the top of the slab (thermal break). This means there is 2" of foam exposed at the slab surface around the perimeter, not a big deal. This also means that we never tie the interior slab to the frost wall, with a good sub-base it doesn't matter. This also means exterior slabs can be tied into the wall as the insulation is on the inside.
      - Finally insulate any above slab concrete walls with foam on the inside as well.

      Good luck.

  • Hi Todd,

    Thank you for great article! I have a related question.

    You mention that "Lastly, the remaining stud bay is filled with unfaced fiberglass insulation. It’s extremely important that a vapor barrier is NOT installed over the fiberglass with this detail. It will trap moisture in the stud bay leading to serious problems."

    However in another article
    http://www.homeconstructionimprovement.com/basement-wall-insulation-detail/

    You say somthing different as a last step: "Install a vapor barrier. Either use faced insulation in the wall cavity or install a vapor barrier (if you use Spray-In-Place Cellulose Insulation you may omit the vapor barrier)."

    Can you help to understand the right way?

    Thank you!

    • Constantin - I can understand your confusion. These two situations are nearly identical with one major exception. In the case of the foam board against the concrete, that layer is continuous. In the walk-out basement example, the studs interrupts the foam insulation. The discontinuous foam can allow moisture from the outside to get into the stud bay, and that's why I wouldn't use a vapor barrier in that case. The other situation is sealed very well from the outside. Honestly, I prefer to use the paper faced in the other example vs plastic which I never use in a basement.

      Good luck.

  • Hi Todd,

    I have a summer house on the ocean in Maine that's built on a concrete slab. The bottom floor is all garage, framed with wood, with living quarters above. We are converting 3/4 of the bottom floor to bedrooms, while leaving 1 garage door in place and keeping that area for a workshop. In all areas except the workshop I recently took your advice and put down a 6mil vapor barrier, foam insulation and Advantech sheets screwed down with Tapcons. I am totally amazed at how well this came out!

    I also removed all the old moldy insulation from the exterior walls and ceiling, and I now need to re-insulate the exterior walls and the walls that border the new workshop area. The exterior walls are framed with 2x4's, so I'm just wondering what the best approach would be to insulate. The exterior is pine boards with Tyvek paper under it on the studs, which I see from the inside. Since the Tyvek paper is already a vapor barrier, should I be using an unfaced fiberglass before installing the sheetrock?

    The other concern are the walls that border the workshop space. The garage door is old and the floor is still bare concrete, so that area I expect to still get some moisture. What do you suggest for those walls and between the ceiling joists?

    Thanks, John

    • I didn't see this post when I responded above.

      So you're insulated all wood framed walls, not against concrete. Ideally you'd insulate the full 3-1/2" cavity with foam (2" and 1-1/2" thick?). TYVEK is NOT a vapor barrier. It's an air infiltration barrier. I would install a 6 mill poly over the foam board before drywall. I would use the same detail on the wall adjacent the garage (vapor barrier on finished side). Good luck!

      • Thanks Todd - No concrete walls, just the floor and 1 row on cinder block on top of the slab. I did insulate the row of block with foam and will block it in with studs. The rest is built with 2x4's on top of the cinder block. I did find out that Tyvek is not a vapor barrier after I posted the questions above, but just so I am clear - by putting the poly under the drywall, the wall can still breather through the other side through the pine exterior? Can I use fiberglass insulation instead of the foam?

        • Yes, most homes are built with Tyvek (or similar), wall sheathing, studs bays filled with fiberglass or cellulose, then a vapor barrier followed by drywall.

  • Hey Todd,
    I have a full basement with the backside being a full 8' walk out block wall with a 4' frost footing under that. this wall is very large (about 38-40') and I loose a lot of heat through that wall every winter. Id like to insulate that wall but I want to make sure it is done right. there is also a curtain drain around the outside of the basement floor that moisture can drain into. (typical) I was wondering, do I seal the wall? do I stud right against it? if I use hard foam insulation do I attach that to the wall, the back of the studs or stud against the foam board? leave an air gap between the block wall and foam board? hang any plastic on the block wall?

    • Steve - I'd install 2" of XPS foam board in front of that wall, and seal all the seams well. Then I'd frame a wall in front of it (the framing can be tight to the foam, which helps hold the foam up). Insulate the stud wall (Roxul is a great product in the basement), do not use a vapor barrier.

      Good luck.

      • Hey Todd, thanks for the advice. Would you recommended doing this to all the walls? Or just the walkout wall? Figuring the wall opposite is completely under grade and the two side walls are 3/4 under grade? Or would it be ok to just foam board and insulate the box?

        • I recommend insulating every wall in the basement. The foundation walls need to be done with the foam board detail, the question then is for the framed walls. For those I again like using foam. No vapor barrier for either application.

      • Hey Todd,
        One last question, you said no vapor barrier, did you mean on the block or insulation? Should the batts be unfaced?
        Thanks again
        Steve

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Todd Fratzel
10 years ago

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