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Community and Q&A

Insulating a Walkout Basement: Walls or Ceiling Trusses

kermit49 | Posted in Energy Efficiency and Durability on


New construction (zone 3 southern Utah), with unconditioned walk out basement. Half concrete walls, half framed walls. One door to backyard, no windows.

Super insulated and well air sealed main house. Basement is storage only. All energy efficiency focus has been on the house, being very comfortable and energy efficient. And it will be.

Need to decide real soon if basement walls or basement trussed ceiling gets insulated. My understanding is that code requires one or the other. No one says it can’t be both. Although I like the idea of both, it can get even more expensive.

Now, at 74 and retired, with this being our first home build, I find myself seeking advice and opinions and welcome all responses. Given this a lot of thought and am still not sure what is best approach to take with our basement’s insulation.

Thanks again.


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  1. Expert Member
    DCcontrarian | | #1

    Do the walls. Usually it's cheaper and you don't have to worry about keeping the mechanicals from freezing.

    No reason to do both.

    1. kermit49 | | #4

      DC Contrarian, yes doing the walls is cheaper since it's easier than dealing with floor trusses. Haven't heard of freezing being an issue around here. Having said that, outdoor winter temperatures can get to low 20's. Per manual J, heating 99% drybulb is 21F. Good point. Thank you.

    2. Deleted | | #18


  2. MartinHolladay | | #2

    I don't know what building code, if any, applies in your local jurisdiction. But according to the 2018 version of the International Residential Code, basement wall insulation is required in your climate zone (Zone 3). It is not optional.

    1. kermit49 | | #5

      Hi Martin, been following you and Allison for a very very long time. Appreciate the education you guys, Fine Homebuilding and too many to count forum members have offered. All this turned hobby reading into what became our energy efficient dream retirement home design.

      Will double check code but last I heard from general contractor, it's the walls OR the 18" floor trusses. You are correct, it's not optional.

      On one hand I would love a super insulated floor to complement the efficiency of the house. On the other hand, eliminating access to all the plumbing and electrical just under the house floor bothers me a lot. No ductwork there as it's all inside the house and only basement mechanical is a heat pump water heater. Attic has nothing but insulation and minimal electrical wires.

      Thanks Martin.

  3. Expert Member
    BILL WICHERS | | #3

    Another vote for insulating the walls, especially considering this is a walk out basement. Make sure that exterior door has good weatherstripping and ideally that it's an insulated door too (many are these days). Good compression weatherstripping is pretty standard on most newer doors.

    I would also insulate under the slab, which may be required by your local codes too.


    1. kermit49 | | #6

      Bill, appreciate you take, it's moved the needle more than expected.

      Yes the single exterior door will be well sealed and is insulated. The slab, well that ship sailed. I discussed this with general contractor prior to pouring slab. Had this been a single story home (house on slab), he would not have been able to convince me to drop slab insulation. Looking at Manual J, it's clear how many Btus are lost through slab perimeter. Not concerned at the time since the house was main focus. Totally regret my decision should I decide to insulate walls and not floor trusses.

      OFF TOPIC :
      Never having gone through this process, home building from design to construction, the process was an eye opener.

      My design is foreign to my local construction industry. Had to fight to get my way every step of the way. They all point to the great homes they built to code in my area. I could tell they did not take me seriously. They think we will sweat bullets in the summer and freeze in the winter. I will prove them wrong.

      Being out of state and not in tune with local building industry, nor local "culture", it became a much harder job. Did not help that my exterior house design is modern and all homes here are traditional looking housing families with lots of children (they are everywhere). Fortunately, wife and I love our neighbors and like seeing all the children playing all day outside like I did in the early 1950s. So for past 5 months wife and I live in our DIY camper van parked on site everyday standing and engaging side by side with all working subs and inspectors that visit. A learning experience to say the least.

      1. Expert Member
        DCcontrarian | | #8

        I still remember how it was described to me:
        "Two years of screw-ups and everybody doing what they want. Then in the end you have a building.

        "If you're lucky."

        1. Expert Member
          BILL WICHERS | | #15

          Ha, that's a lot like what I tell people: "construction is all F'ed up, and then at the end, you have a building, if you're lucky.".
          I also like to say that "gov't exists to make things take too long and cost too much", which certainly applies to many permit processes!

          The reality of things is that for the first maybe 2/3 of the project, it looks, to an outsider, like little is getting done besides making messes and moving boxes from one side of the lot to the other. The last 1/3 of the project seems to have lots of stuff come together very quickly. Eventually you get used to this, and don't worry so much about the beginning looking like it's not going anywhere, but for "first timers", it's scary -- especially if they're the ones putting up all the money for the project.


          1. Expert Member
            DCcontrarian | | #17

            There's also typically a long period at the end where it looks like everything is almost done but you're actually a ways from being done.

  4. Expert Member
    DCcontrarian | | #7

    I'm going to do a rare thing, and disagree with Bill. Looking at the county stats ( ) Houston* has a heating degree days to cooling degree days ration of 0.3 -- three times as much cooling as heating. That doesn't include solar gain, which only increases the ratio. An uninsulated slab will sink heat in the cooling season as well as in the heating season. That uninsulated slab will save you energy.

    But definitely insulate the basement. Pay particular attention to air sealing, and in particular make sure the rim joists are detailed properly. Read up on this site. In a humid climate like yours you really want to keep air infiltration out of the basement, it will hit the cool walls and cause condensation and all sorts of problems

    I think it will help your approach if you stop referring to the basement as "unconditioned" but rather as "unfinished and conditioned." It's part of the conditioned space of the house.

    *(I'm assuming when you say Houston you're in Houston county. Note also that your heating design temperature is 27F, not the 21F that was used for the Manual J. That may have been an attempt to put a thumb on the scale to talk you into a bigger system.)

    1. Expert Member
      BILL WICHERS | | #9

      Actually, you're probably right about that slab, or at least should be pretty close. I hadn't realized this project was so far South, and I'm usually in "frozen North mode", being in Michigan and all :-)

      For the OP: As you go further South, the BTUs lost through the slab starts to help you, since it essentially removes some BTUs from the home that would otherwise be pumped out by your air conditioner. In cooling-dominated climates, you want to optimize for cooling, since that's where most of your energy use (and money for utility bills) goes. As your go further North, you get into heating-dominated areas, where it's makes the most sense to optimize things to keep BTUs in, since you are spending the most money putting BTUs into your home with your heating system.

      We've had a string of cooler than normal days (low was only 41.7F last night, which is very unusual for this time of year here!).


  5. kermit49 | | #10

    Hi guys, a misunderstanding here. My retirement home is being constructed in southern Utah. Washington county.

    Will change my calculations using the Energy Star data Heating 99% db = 104F Cooling 1% db = 28F HDD/CDD = 0.5. I purchased my Manual J a few years ago.

    My Manual J tells me the Design Grains range from 55%RH (-36) to 45%RH (-23).

    I will see what the heat pump water heater output does to negatively affect basement temperature during the cold season. Plan B is to divert cool exhaust outside during heating season should that be a problem. Great in the summer though.

    I think I am quickly becoming convinced that I should insulate and treat the basement as part of the envelope. On air sealing, yes I personally do a lot of it. From attic to basement slab.

    Great advice, thank you.

    1. Expert Member
      DCcontrarian | | #11

      Diverting the heat pump water heater outdoors when it's cold makes it consume more energy. The air you're exhausting will have to be replaced with outdoor air, which will seep in through cracks in the building envelope. Almost certainly the outdoor air is colder than the air coming off the water heater.

      1. kermit49 | | #13

        Great point DC Contrarian. I definitely don't want to entertain make up air as I did in the kitchen (interconnected to the range hood). After going through the house in great detail with my foam guns from roof to basement slab, I don't expect much infiltration, crossing fingers.

        Crossing water heater heat pump exhaust off the list.

        1. Expert Member
          BILL WICHERS | | #16

          You'll always have some leaks, no matter how careful you are. You'll also find that over time, your home will get a little leakier, since some seals will fail and some new cracks will open up. This is all normal, which is why you want to avoid doing things intentionally that will exacerbate any air leaks. Things that make leaks worse include anything that creates a pressure differential (either pressurizing or depressurizing the interior of the home) between the inside of the house and the outdoors, since those pressure differentials will force air to flow through ALL leaks ANYWHERE in the building envelope.


    2. Tim_O | | #12

      EDIT: I should have refreshed before posting. DC beat me to it.
      Be careful ducting cool exhaust outside with the HPWH. It's cool relative to the basement, but it's potentially warm relative to the outside air, so you are losing heat from your house. If your basement gets overcooled in the winter by the HPWH, you'd be better off switching it to resistance mode. Especially depending what your heat source on the house is, it may not be a huge loss in efficiency.

      It's cool to hear your story. We are planning a build next year, probably similar things where we are slightly off the normal path for our plans. We do have a retired consulting builder who has done passive houses, so I am excited to have the help there! Sounds like you did all your own calculations and possibly HVAC/Mechanicals design as well?

      1. kermit49 | | #14

        Tim O, I now see the problem with ejecting heat pump hot water heater effluent. Not going to happen. See my reply #13 above.

        Your planned build for next year with help of a consulting builder that knows what a passive house is sounds like the way to go. I did not have that luxury. To make that shortfall up, wife and I had to "educate" our builder every step of the way. Wasn't and still isn't easy but we seem to be getting results.

        Around here, subs are not what I remember in my youth. Skillset not to my expectations. Attention to detail also not to my expectations. To make up the difference, we need to monitor and check every step taken. We find it easier to correct simpler things ourselves than get these subs involved. The home construction industry needs to get with the program. Building practices are energy wasteful. No concern with gap of all sizes left behind knowing it will be covered up. and look pretty. All trades do this. I was there and know what they do. Repaired their sloppy work (from an air sealing point of view) by filling all gaps and cracks with foam and material/foam when gaps are too big.

        Things are working out but it takes a toll on us as we are not spring chickens. Two old people living in a home-made camper van five months now isn't easy. July-August temperatures 110-115F. Did not have time to install AC but did manage to insulate it very well. We survived the heat using a swamp cooler that took much of our little floor space away. Was almost useless beyond 105F.

        Yes, after reading GBA, Fine Home and Allison for years, I was educated enough to design our retirement home 100%. Only items I did not design were the structurals. Got wet signatures from professional engineers on those.

        Designed a modern passive house. All mechanicals as well. This includes a Zehnder Q350 (all steel ducting I designed) and will be learning to program and commissioning the ERV in about six weeks. Two Mitsubishis, one ducted and non-ducted. All electric house, no natural gas service. No fireplace.

        Happy you will build a passive house. It will be an experience you will always cherish. Stay in touch.

  6. kermit49 | | #19

    EDIT: Accidentally posted up above. Moving down.

    It's been close to a month since you guys gave me great advice I took to heart.

    Zone 3, walkout basement has no windows, one exit door to yard, no HVAC system and will be used as storage only. Wall assemblies are 50% 2x6, 50% concrete walls. No insulation on basement ceiling (per advice given). Tight staircase door.

    I sealed all basement wall cracks and openings on the half of walls comprised of 2x6 wall assemblies. Also sealed all main floor utility penetrations, wiring and plumbing mostly.

    Scheduling as much insulation as possible to the unfinished basement's timber walls and concrete walls. It just dawned on me that this arrangement will, thermally speaking, turns our home into a two-story house !

    Without a walkout basement HVAC system, won't the house's main floor side lose heat/cold through floor to basement?

    Using Manual J, I took the unconditioned basement load into account, under Passive Floor loads, using R-0 on basement's ceiling when I sized the main floor's HVAC system. Should I be concerned? The numbers tell me all is well.

    Thank you DC and Bill for all the advice given. I was moving in the wrong direction. Ok now.

    1. Expert Member
      DCcontrarian | | #20

      Every house loses heat through the ground. Let's spitball the magnitude. Let's say in winter the house is at 70F, the ground below the house is at 50F. Between the heated interior and the ground you've got a concrete slab and an inch and a half of wood for the subfloor and finished floor. Concrete is about 0.2R per inch and wood is about 1.25 so you get about R2.7. There's also an air film on the basement floor and on each side of the first floor, you usually ignore them but we're talking so little insulation here they are meaningful, figure about 0.5 each so a total of R4.2. With a temperature delta of 20F that works out to a heat flow of about 4.8 BTU/hr per square foot. So if your footprint is 1000 SF that would be 4800 BTU/hr, which is pretty big. But you'll be losing the same amount of heat in the summer, and I think we already established that you're cooling dominated so the summer should more than make up for the winter.

      The other question is comfort.

      The temperature at each level is going to be directly proportional to the amount of insulation. The profile is going to look like:
      Ground: 0 insulation, 50F
      Top of slab: R0.8, 53.8F
      Air in basement: R1.3, 56.2F
      Underside of subfloor: R1.8, 58.6F
      Top of floor: R 3.7, 67.6F
      Air in room: R 4.2, 70F

      So the floor will be about 2.4F below room temperature. I don't think you'll find that noticeable.

      Note that I didn't account for heat losses through the basement walls, since you said they're going to be well-insulated I'm assuming they're small relative to the losses through the basement floor. If you wanted to model them, the equation you want to solve is to find the basement air temperature where the heat flowing through the first floor down to the basement is equal to the heat flowing into the basement slab plus the heat flowing out of the basement walls. For all three surfaces, heat flow is (temperature difference)*(area)/(r-value).

      If there is significant flow through the walls, the basement will be colder, which will reduce the flow through the basement slab. You might find that the surface temperature of the first floor becomes cold enough to be noticeable.

  7. kermit49 | | #21

    DC , my brain hurts. You are making me think again 8>)

    Appreciate your professional input. It's the catalyst I needed to take my walkout basement / main house heat transer to a higher level ... thank you.

    Have been concentrating on the main floor's construction, wife and I keeping a careful watch on the sub-contractors by living in our DIY camper van at the job site. Six months now.

    Now I need to plan how the walkout basement will be insulated. Using what material and wall assembly R value based on the information you generously forwarded me. I am a home energy efficient "civilian". Never done this before, just have a passion for it. Will post the results soon.

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