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Community and Q&A

Dense packed cellulose against above grade concrete

Indiana Guy | Posted in General Questions on

Hello.

I’m building a 2-story house in climate zone 5. Plans are to pour 6″ concrete exterior walls above grade. Then use 8″ metal studs on the outside of the concrete walls for insulation and cladding. The house wrap will go on the metal studs, then a drainage gap, and then cladding (steel).

For the metal studs, we have two options – 

1. Screw in the metal studs vertically into the concrete layer. But add a narrow strip (2″ thick and 2″ wide) of EPS between the studs and the concrete to minimize thermal bridging. Then use dense packed cellulose to fill the stud cavities.

2. Build a double wall system with a 2″ cavity between the metal studs and the concrete. Then fill the 2″ cavity and the stud cavity with dense packed cellulose right up against the concrete.

I know the general advice is to put foam or mineral wool on the outside of exterior walls. However, missus would like to minimize using foam insulation in the house. And mineral wool boards are outright expensive. 

So this brings up two questions – 

1. Which option is better – #1 with metal studs on 2″ EPS on concrete, or #2 with metal studs as a separate wall and a 2″ cavity between the metal studs and the concrete?

2. Is it okay to put cellulose in contact with poured-in-place concrete? What about fiberglass? What if we put poly or house wrap between the insulation and the concrete?

Please advise!

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Replies

  1. User avatar
    Dana Dorsett | | #1

    First, metal stud are going to rob the cellulose of half it's insulation value due to the extreme thermal conductivity of steel. The edge strip of 2" EPS doesn't really fix that. You'd be better off applying the dense pack money toward thicker (and potentially continuous) EPS, if it can pass muster structurally, and installing rock wool batts designed for fireproofing & insulating commercial buildings with exterior steel stud framing. I suspect even 2" of EPS spacer (or a 2" gap) between the concrete and stud is a problem with a 2x8 steel stud.

    For a bit of bedtime reading, try this:

    http://www.steelframing.org/PDF/energy/thermal_design_guide_2015_edition.pdf

    The U-factor you need to meet IRC code minimum residential walls in US climate zone 5 is 0.060 or less. You might just squeak by that number with no EPS and a 2 x 8 dense packed cavity with 24" o.c. spacing, but I'll leave that (and any higher performance goals you may have) for you or your designers to run the math on.

    The capillary draw of concrete is quite high, so even above grade ground water could potentially wick up and into cellulose that was in direct contact with the concrete. Fiberglass & rock wool won't wick quite as much moisture as cellulose, but are still susceptible. That said, if you are meticulous about installing a capillary break between the footing and wall, drainage and water proofing details the concrete should dry primarily toward the interior conditioned space, not into the insulation layers. Putting 6 mil polyethylene between the concrete and exterior side fiber insulation would be cheap insurance.

    If there is 6" of concrete between the indoors and the foam board, why do you care? From an outgassing or fire retardent exposure point of view the EPS is "outside" the pressure boundary of the house, even though it's inside the thermal envelope of the house.

    1. Deleted | | #3

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      1. Akos | | #4

        Is there a specific reason you are looking for 8" studs? With metal studs, the thicker the stud the less effective cavity insulation is. With wide steel studs the insulation is essentially wasted.
        For example:
        3.5" ~ 60% cavity R value
        5.5" ~45%
        7.5" ~40%

        If you really want to use steel studs, stick to 2.5" to 3.5", which should be enough for most siding applications. Depending on your wall height and wind loading you would need ties back to the concrete (L shaped brackets), a structural engineer can help you with spacing. The good thing with light gauge steel studs is they are made to length. If you know your wall dimensions, you can order them to fit, saves a lot of trimming on-site.

        Taped foam over the concrete or a layer of poly as Dana suggested is cheap insurance.

        Also talk to your insulation installer. Dense packing large open spaces is not as simple as filling stud bays, you might need to split the wall up into more manageable section.

        1. Deleted | | #6

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      2. Trevor Lambert | | #5

        I think you're misunderstanding thermal bridging. No matter what you do in that gap between the inner and outer wall, the thermal bridging of the metal studs is the same. It looks to me like the concrete is the structural wall, and the metal studs are just to hold on the cladding. I can't see any good reason for that to a)be made of metal or b)be 8" deep. If it's 8" deep for the purpose of installing insulation, then it's not an efficient use of insulation or wall depth. You're only getting 4" worth of insulation equivalence in that 8". In a traditional double wall assembly, the studs are only as deep as they need to be for structural strength; if higher r-value is desired, you increase the gap between the inner and outer.

      3. User avatar
        Dana Dorsett | | #7

        "1. Is the 4" air gap (updated from 2") between the metal stud wall and the concrete wall not enough? There would be no contact between the two walls, except that they will share the same footings."
        -------------

        A continuous 4" of cellulose is about R15, which would meet IRC code minimum on it's own in zone 6 or lower on the exterior of a mass wall (such as 6" of poured concrete.)

        Dropping back to 2x6 on the exterior wall stud with 24" o.c. spacing you'd hit about R9 for that slice, so now you're up to R24 "whole wall".

        The 6" concrete wall + siding adds another R1, as does the interior & exterior air films, so it's about R26 "whole-wall".
        __________

        ----------------
        "2. Would it help to optionally add 2" EPS under the concrete wall for the metal studs (pending approval from the engineer)?"
        -----------------

        Not enough to matter.
        ____________

        ---------------
        " 3. The 4" EPS insulation will come up a minimum of 1' above ground. That is when the cellulose or the fiber glass insulation will start. Do you think wicking would still be a problem?"
        ----------------

        Install EPDM flashing tape formed as Z-flashing on top of the exterior concrete wall and EPS as a capillary break.

        Install another capillary break between the footing and everything that's on top of it (both walls, and the EPS). Waterproof the exterior side of the exterior wall up to and slightly above grade.

        If there is soil under that 6" slab (rather than a crawlspace) a sheet of 10 mil polyethylene or EPDM between the soil and foundation wall from the foundation on up to meet the sub-slab vapor barrier would reduce wicking issues to inconsequential levels.

        FWIW: That's looking like a lot of concrete, which has a significant carbon footprint.

  2. Deleted | | #2

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  3. Indiana Guy | | #8

    Thanks for the response, everyone.

    We have over 100 8" load bearing, exterior metal studs leftover from previous work. And I'm trying to find a way to use them. Any ideas on that?

    But based on the advice, we'll make sure to switch to 2x4s to build the wall for the siding.

    Thanks, again!

  4. Malcolm Taylor | | #9

    Is the decision to use a concrete structure a purely aesthetic one? All other criteria - energy efficiency, carbon footprint, cost, complexity, adaptability - lead towards other choices.

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