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Insulating around a concrete safe room

Jason_K | Posted in Energy Efficiency and Durability on

Seems every time I find more answers, I end up with more questions…

Building a house in middle TN and so tornadoes are a concern. We have a concrete safe room on the interior of our floorplan, on top of a sealed crawlspace.   Walls, floor, and ceiling are all poured concrete, with access from inside the house (see attached snippet of floorplan).

We have framed out walls on both the interior and exterior of the structure and the question I have is — should we put insulation in these walls?. I believe the insulation contractor our builder uses says he wouldn’t.

The room is obviously inside the conditioned envelope, but due to being concrete and ultimately connected to the ground (via the poured walls and the footer) it will be a large thermal mass that will want to seek equilibrium with the ground temp I would think?

Obviously moisture is a potential issue and the framing needs to be spaced from the conctete (it is), but I’m wondering if a couple inches of EPS against the safe room walls would be good to slow the flow of heat to the concrete.

Thoughts?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    You don't need to insulate things that are within the building envelope unless you intend to condition them differently from the rest of the space (such as is sometimes the case with something like a wine cellar). In your case, the only "exterior" side to the safe room is the ground, which is best handled with subslab insulation. I would use XPS here for slightly better underground performance over time, but EPS will work too. You're likely to have higher than normal compressive loads on that foam due to the weight of the relatively large poured concrete structure on top, so you may want to consult an engineer here prior to building this. I would, at a minimum, use one of the higher rated foams such as Foamular 250, which has a 25PSI rating instead of the more common 10 or 15 PSI.

    Bill

    1. Jason_K | | #3

      The concrete footing, walls, and ceiling are poured already, so not sure there's still an opportunity to insulate sub-slab... they have yet to pour the floor yet (need to fill above the footing up to floor height with gravel and then pour)...would that be worthwhile?

      "You don't need to insulate things that are within the building envelope unless you intend to condition them differently from the rest of the space"

      That makes sense, but is the concrete likely to be a different temp than the conditioned air around it in the wall cavity due to the direct connection to the ground? And if so, would that not be noticeable if you were to stand next to the, let's assume cooler object?

      Or is there just a temperature gradient in the concrete object, where it's "room temp" at the contact surfaces with the conditioned air, and "ground temp" where it connects to the ground?

      Just trying to wrap my head around how the concrete room may behave differently than the rest of the house, if at all.

      Thanks!

  2. Expert Member
    Michael Maines | | #2

    I wouldn't say that the concrete will seek equilibrium with the ground, but that's close--the concrete temperature will seek equilibrium between the indoor air temperature and the ground. Because the ground is naturally cooler than the indoor air, the concrete will act like a radiator, moving Btus from the house to the ground.

    The temperature differential isn't huge, probably around 15-20°F in TN, and over time the ground temperature under the house will increase a bit. You could save some energy, and improve comfort and mold resistance a bit, if you insulate the exterior of your safe room.

    More effective and less material-intensive would be to disengage the concrete from the ground with a layer of foam insulation. It would need to be dense enough to resist crushing, but 25 psi or 40 psi foam would probably be enough and both are readily available, though they may have a lead time.

    Edit to add: I see that Bill had a similar response. One difference: I believe that the materials we choose matter; XPS comes with a large carbon footprint due to the blowing agent used in manufacturing it. EPS is available in the same range of densities and once the density is above 15 psi the working characteristics of EPS vs XPS are very similar, but EPS has a much lower carbon footprint. EPS is often less expensive than XPS as well. When there are two equally-performing materials, I don't understand the desire to go with the more-polluting option, especially on a site meant for "green" building.

    1. Expert Member
      BILL WICHERS | | #5

      I only recommended XPS here because it has been shown to be somewhat less likely to take on water over time when used underground, making it's long-term insulating performance more stable. I base that on the Alaskan highway study that I think was published here on GBA some years back, although I don't remember where I read it.

      Aside from that, I agree EPS would work too. As I understand it, XPS manufacturers have been changing blowing agents, making the XPS less of a problem in terms of greeness. I am not sure how prevalent the newer variants are, but hopefully the changeover will be fairly rapid.

      Bill

  3. Expert Member
    KYLE WINSTON BENTLEY | | #4

    As someone who used their tornado shelter this morning, I'd like to congratulate you on putting one in!

    One thing to remember is that while concrete is a good thermal conductor, there's an area component to consider too. The base of the walls has a much smaller area than the entirety of the rest of the structure, which is in a heat bath of conditioned air.

    Mine is outside the house, and underground, and insulated like a basement would be - in the event we lost power (certainty if we're seriously in there), I didn't want it to lost heat too fast, and be easier to heat with less power, as we, no surprise, can get tornado's this time of year too. So if you consider a similar case, where perhaps you may be in there for a little while, you might consider insulating the outside walls with something you can glue on directly, unless you want it to look nice. And in that case you could glue it on and glue something on top of it. EPS is a good logical choice here as it's not fibrous, easy to adhere to on both sides, and performs as well as anything else you'd put there.

    1. Jason_K | | #8

      Luckily we've not had the sirens go off/a warning issued with the storms moving through. But they're common enough (and also I grew up in IL so am used to them) that I'd much rather have a fortified room than just hunker down in the closet under the stairs.

      Sounds like insulation isn't needed, but may be mildy beneficial/not harmful, I assume as long as it's vapor permeable (so EPS or rockwool - we'd just put it between the studs/framing). We'll see how many other "chits" we'll have spent with the builder when we get there and whether it's worth pushing for.

  4. DC_Contrarian | | #6

    Do you insulate basement slabs in your climate? Here (DC) we are very slightly cooling-dominant and the slab saves more cooling in the summer than it costs in the winter. So we insulate basement walls but not slabs.

    1. Jason_K | | #7

      Basements are rare around here due to the limestone..I guess maybe walkouts are somewhat common, but much of the building is on crawlspaces so I honestly don't know. If I had to guess I'd say it's unlikely they insulate slabs here.

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