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Community and Q&A

Basement wall and insulation method

Evolver00 | Posted in General Questions on

I am finishing my basement finally and with the interior walls of my basement I am planning to insulate between the studs only.  I live in climate zone 5 and here is my plan.

Step 1: Build wooden stud walls 16” on center roughly 2.5” away from the poured basement wall

Step 2: Install mineral wool between the studs on all exterior walls

Step 3: Apply moisture resistant drywall directly over stud / mineral wool combo without any vapor barrier on all exterior walls then regular drywall on interior walls and ceilings

My home is 7 years old and it was built for us so we have seen for this long at least that the walls are dry.  It had a membrane brushed on the exterior concrete along with foam board applied on top the membrane then backfill.  Again interior concrete walls I do not plan on installing foam rather building off the wall to allow breathing and or drying behind drywall of any moisture in the air.  Is this method acceptable for climate zone 5?

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  1. GBA Editor
    Martin Holladay | | #1

    Whether or not the method is acceptable depends on (a) the thickness and type of exterior rigid foam, and (b) whether or not the exterior rigid foam extends all the way from the footing to the top of the wall.

    In Climate Zone 5, a conservative approach would require at least R-7.5 of exterior foam before you could safely install mineral wool insulation on the interior side of the concrete. The worry is that you'll get condensation, mold, or puddles of water on the interior side of your concrete.

    The exterior rigid foam needs to extend all the way from the footing to the top of the basement wall. If the rigid foam terminates at grade, all bets are off.

    For more information on these issues, see these two articles:

    "How to Insulate a Basement Wall"

    "Calculating the Minimum Thickness of Rigid Foam Sheathing"

    1. Evolver00 | | #2

      Thank you

      The foam R value I am not positive of but it was about 2” thick I recall.

      The foam terminates at ground level as you mentioned and then it is just poured concrete wall up to siding for the last foot or so on the fully buried sides and then more gets exposed as the land slopes to down little by little till I have a full open walkout area at the bottom with siding again.

      So what you are saying is that leaving a 2.5” space between the concrete and framed wall will not give adequate open air space to allow any condensation that may form to dry out naturally as it might be doing now unbeknownst to me since it is 100% open to the inside currently?

      Is there some sort of math equation that tells you how much open air you need before walls drip water down to the floor and mold grows because obviously at 100% open to the air it doesn’t so at what % does it turn into a pool of water and mold?

  2. GBA Editor
    Martin Holladay | | #3

    Adding interior mineral wool makes the interior surface of the concrete colder than it used to be, thereby increasing the condensation risk.

    What you need to do is insulate the interior side of your basement wall with rigid foam or closed-cell spray foam, not an air-permeable insulation like mineral wool.

    Again, useful advice can be found in this article: "How to Insulate a Basement Wall."

    1. Evolver00 | | #4

      Thanks again Martin -

      I am reading the articles but not seeing anything on method of leaving air space between the interior concrete and the framing. Most people that don’t apply foam to the walls just place frames wall right up against the concrete then insulate, so I thought leaving air space would heIp and I have adequate space to do so. I fully understand that placing a wall with insulation in it will cool concrete and I know that could potentially cause condensation but what I don’t know and can’t figure out is if air space will help this wall to dry out?

      I.E., If my post said it is 100% open now and no condensation and I plan to build an interior wall 10’ off the concrete and mineral wool that wall would the response then be “it’s fine cause there is adequate breathing room.”?

      Then if that is indeed true at what point does it become not true...? Just at 2.5” from wall like my plan or 10” or 24” etc?

      Hope that makes sense? I cannot get info anywhere where it specifically states that building a wall with a large air space between it and the concrete is a no-no so to speak. Do you have related article to that or just your word?

      1. GBA Editor
        Martin Holladay | | #5

        The reason you don't find much information about your suggested technique -- leaving an interior air space, and then installing mineral wool batts on the interior side of the air space -- is that it's not a recommended method, and is likely to lead to condensation.

        The type of air space you want to create is likely to encourage air movement and convection loops -- and this air movement and these convection loops will introduce more humid indoor air into the cavity, encouraging more condensation, not less condensation, against the cold concrete.

        1. Evolver00 | | #7

          Okay thanks

      2. Evolver00 | | #6

        And I do see where it states in the article you sent me that allowing concrete to dry to the inside isn’t a “good idea” cause you don’t want to invite moisture in the home ever but as mentioned that is how my basement is right now just plain ole concrete open to the air and it doesn’t condensate or have mold on it....I am simply asking at “what point does that fact change when the wall you build gets closer and closer to the concrete when does it suddenly say uh oh I can’t dry out anymore I want to get soaking wet and grow mold?”

        Those are the articles I am missing and the studies that I am not finding. I assume they are out there if indeed my method is wrong but I can’t find it? I could hardly try and would be able to find research on some of the craziest who cares type stuff out there that got funding to test that doesn’t mean anything but I can’t find any “tests” on my question...just peoples opinions from the field (which I respect) but opinions and nights and maybes and coulds are not true evidence and is exactly why they do actual studies to begin with. I was hoping you could point me to those articles not just ones that say this is best put foam here and foam here cause if you don’t this “might” happen.

        I appreciate your time and if there are no articles out there then that is fine.

        1. GBA Editor
          Martin Holladay | | #9

          Q. "At what point does that fact change when the wall you build gets closer and closer to the concrete when does it suddenly say uh oh I can’t dry out anymore I want to get soaking wet and grow mold?”

          A. The point occurs when you add interior insulation -- the interior insulation is what makes the interior surface of the concrete colder (thereby increasing the condensation risk).

          1. Evolver00 | | #14


            So you are saying “John Doe has basement, John Doe builds interior wall 15 feet away from concrete end to end and puts up insulation, John Doe tears down walls in X amount of years and finds mold and water everywhere due to insulation”

            By what point I meant what distance from wall and I assume “any” distance is not acccurate so again back to square 1.


      3. Jon_R | | #8

        It's a good question.

        With no interior wall, there is lots of airflow, which warms and drys the concrete. With an interior wall, there will be less airflow, less warming of the concrete and an increased chance of condensation.

        So is the airflow necessary? It's a question of R value ratio (not spacing). Say you have R10 on the outside of the concrete and only R2.5 (drywall and 3 air films) on the inside. If it's 0F outside, the interior side of the concrete could be around 56F. Not a problem, even with no airflow. Add much mineral wool and it becomes a problem. You can fix the problem with an air gap + high and low vents in the wall, but that just cancels the effect of the mineral wool (ie would be silly).

        There is also a moisture distribution effect. Say you have a small localized leak in a concrete wall. An air gap behind the wall can allow this moisture to be distributed over a large area, making it less harmful than if concentrated (eg foil facing tight against concrete would prevent most distribution).

        Dry concrete is less likely to grow mold and an interior side that is moisture permeable (or vented) will result in drier concrete.

        1. Evolver00 | | #10

          So then what you would be saying is that in theory it would make more sense and cause less problems to build off a basement concrete wall with empty stud bays than it would with stud bays with mineral wool that is also permeable and allows air to move thru it some inside to out and outside to in?

          Mineral wool will not completely stop air movement like say a plastic vapor barrier would so it wouldn’t “trap” air and moisture as much I’d think.

          Like you said open basement = plenty of air flow and concrete warms and drys etc....add a wall and that stops and now you have mold and water on walls...but at what point does that magic happen is what I am struggling to find.

          To Martins point I don’t find much info because it ain’t recommended method...I get that, but in the early 80’s not wearing a seat belt was not recommended but there was still tons of info why that wasn’t recommended prior thus proving and making our modern wasn’t just all research saying wear a seat belt, they had to “prove” why it was safer and have test dummies and statistics on deaths etc...they didn’t just say “wear seatbelts Americans or you “may” fly thru a windshield or you “may” break your face off the dash.

          Extreme example but houses with basements have been around longer than cars with belts so I’d expect more info...but maybe we are still in the infancy of this after 243 years and more info needs and be found and tested before we can definitively say one way or the other?

          1. Evolver00 | | #12

            I find tons of pics and info showing mold and water on walls when they build right up against concrete but not when they leave air’d think I’d see at least pics of this method failing somewhere on this beautiful World Wide Web.

            I am putting feelers out there to may other sites as well not just green building and all I get are a ton of “I’m right no I’m right”...ugh

            What happened to the old science of proving something....

            Make an observation.

            Ask a question.

            Form a hypothesis, or testable explanation.

            Make a prediction based on the hypothesis.

            Test the prediction.

            Get results / actual answers

  3. Jon_R | | #11

    > less problems to build off a basement concrete wall with empty stud bays

    Less moisture risk. But more heat loss.

    1. Evolver00 | | #13

      That makes sense.

  4. GBA Editor
    Martin Holladay | | #15

    First of all, if you want to test your hypothesis, be my guest. It's your house. If you like your suggested insulation method, go ahead and implement it.

    My advice is free -- worth what you paid for it.

    The farther away your stud wall is from the concrete wall, the less effective the insulation will be, and the smaller your basement becomes. At 15 feet away, you'll end up with (a) ineffective insulation, and (b) a very small basement.

    We all agree -- if you don't insulate your basement wall, your chances of condensation stay low (but your energy bills will be higher than they would be if you added insulation on the interior).

    Once you start adding mineral wool batts on the interior, your energy bills go down, but your chances of condensation, mold, and puddles go up -- because mineral wool batts are air-permeable, and because mineral wool batts lower the temperature of the interior side of the concrete wall.

    If you install rigid foam or closed-cell spray foam, you get lower energy bills without the condensation risk.

    1. Evolver00 | | #16


      I appreciate your advice, was merely looking for info on someone who had already tested it prior to giving advice on what’s wrong and what’s right.

      I know it’s hard to say if something will or won’t grow mold over time if you leave air for it to dry out, that’s why experiments and test are done, maybe I should start my own and then have a final scientific tried and tested site that proves one way is best or not rather than searching and searching thru tons of free advice that all say something else.

      Who knows in 20 years spray foam may prove to lead to cancer (already reading about the sickness caused by off gases if not mixed correctly) and rigid foam could attract some crazy foam eating parasite that kills us all...and mineral wool might turn into a monster if you feed it after midnight. Haha

      I will keep searching for definitive results on my particular question and take all of your free advice to heart.

      Appreciate your time.

      1. GBA Editor
        Martin Holladay | | #17

        It's not as if building scientists and home performance contractors haven't investigated what happens when you insulate basement walls on the interior with air-permeable batts -- with or without an air space. We know what happens.

        If the conditions in the basement are unusually dry, these walls can work well. If the conditions in the basement are damp, and the climate is cold, you get mold and puddles. These facts are well established -- they aren't conjecture.

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