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More berm or more insulation?

Jeremy D | Posted in Green Building Techniques on

Currently I am finishing up a design of a house that I want to build this spring. The way the house is designed necessitates it being bermed up with earth on the north side a couple of feet more than the south side of the house. So I have the option to either add more concrete to the north side, berming it up an additional two feet ( to make a 4ft berm) or just building my stud walls and insulating with dense pack or spray on cellulose (I haven’t decided yet which). My stud walls will be made out of 2 x10’s–all reclaimed lumber, so there’s no expense there–and I plan on putting insulation on the exterior to prevent thermal bridging. So my question is which is a better option–earth or super-insulated walls? Thanks ahead of time for the response.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Before you add exterior insulation to 2 x 10 framing, bear in mind that as the cavity-R goes up, so does the necessary amount of exterior R required to keep the sheathing temperature average above the interior R dew point. The ratio of exterior-R to total-R (or cavity-R) is highly dependent upon climate, since that's what determines the average exterior air temp (and by extension, the average temperature of the structural sheathing.)

    First, if you're in the US, find your climate zone on this map:

    https://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg

    In Zone 5 you need about 27% of the total R to be on the exterior of the sheathing. So if you have 9.5" of R3.7/inch dense packed cellulose that yields a cavity-fill R of about R35, which means you would need ~ R13 of exterior R to gain that resilience, which isn't too tough- 3.5" of EPS gets you there.

    In Zone 6 it takes about 35% of exterior R, which means a minimum exterior-R of at least R19, which is a bit tougher. If you used polyiso you'd get there in 3" from a labeled-R performance, but in a zone 6 climate the mid-winter performance of exterior polyiso plummets. A safer bet would be 5" of EPS which is getting a bit awkward to build. If you used 4" of XPS you'd get there from a labeled-R performance too, but over the next 5 decades it would eventually fall below the R19 mark, since it's R5/inch performance depends on it's HFC blowing agent, most of which is lost in the first few decades.

    The alternative is to use rigid rock wool (any thickness), and rainscreened siding, which allows the sheathing to dry toward the exterior, combined with a "smart" variable- permeance vapor retarder on the inside to limit the moisture adsorption rate in winter without increasing the drying time during the warmer months. At ~30 perms rock wool is as vapor permeable as housewrap, and smart vapor retarders are under 1-perm when the proximate air is dry (under 35% relative humidity indoors, as it would be during cold winter weather) but over 10 perms when the proximate air is over 50% RH, as would normally be the case in summer. (Either Certainteed MemBrain or Intello Plus would work as the interior side vapor retarder.) With rock wool exterior sheathing you could cheap out and use polyethylene vapor barriers on the interior, but that would lower the moisture resilience of the assembly as a whole, since it cuts the drying rate by at least half.

    With less than the requisite amount of exterior R as foam, the foam is too vapor retardent for much drying toward the exterior, and using polyethylene on the interior would create a potential moisture trap. You could use smart vapor retarders with skinnier exterior foam too, but it's still not as resilient as keeping the interior side relatively vapor open and keeping the average temp at the sheathing warm enough to limit moisture adsorption to reasonable levels.

  2. GBA Editor
    Martin Holladay | | #2

    Jeremy,
    Q. "Which is a better option -- earth berming or superinsulated walls?"

    A. Superinsulated walls.

  3. Jeremy D | | #3

    Dana, thanks, this is really helpful. I am in zone 6, so it looks like the rigid rock wool with an interior vapor barrier would be the best solution.

    The question remains would it be better to berm up with earth on the north side of the home? Are those extra 2 ft of earth actually going to matter, say with rock wool on the inside to prevent heat loss from the concrete or would I just be better off sticking with the plan that you outlined and save on concrete? We do get a hard wind from the northwest side of the house, and I want to keep it as protected as much as I possibly can.

    Also, I'm already planning on a rainscreen, as I'm using board and batten siding. From what I've read rainscreens or furrings, with board and batten need to be nailed horizontally, which kind of seems like it would prevent its effectiveness as a moisture control(see for example, http://blog.buildllc.com/2008/07/board-batten-siding). Do know of any solutions to this?

    Thanks.

  4. Jeremy D | | #4

    Thanks, Martin. I'll save on the concrete. Any thoughts on a rainscreen for board and batten siding?

  5. GBA Editor
    Martin Holladay | | #5

    Jeremy,
    There are several approaches:

    1. Just install horizontal furring strips and don't worry about liquid water drainage. (There really isn't much liquid water that gets by the siding, and the few drops that do can readily evaporate.)

    2. Install diagonal furring strips.

    3. Install two layers of furring strips: first vertical strips, and then horizontal strips.

    I am attaching some images.

    .

  6. Expert Member
    Malcolm Taylor | | #6

    Martin wrote: "Just install horizontal furring strips and don't worry about liquid water drainage. (There really isn't much liquid water that gets by the siding, and the few drops that do can readily evaporate."

    I think that really depends on the climate. Board and batten is one of the worst sidings for bulk water infiltration, and horizontal battens also mean the cavity is no longer vented. We know rain screens vented at both the top and bottom perform better than those only vented at the bottom. The more air movement the better. With no vents you are taking another step down and depending on the permeability of the siding - which also acts against you allowing moisture to move into the cavity.

    I think it's a good rule of thumb to detail rain screen walls with the cavity open at the bottom as a minimum.

  7. Jeremy D | | #7

    Thanks, all, for your answers.

    Here are a few more questions:

    First, just to satisfy my own insatiable curiosity, why is an superinsulated house preferred over an earth berm? (I know those are not mutually exclusive categories.) But it seems that earth would protect the best from harsh winds, coming from the north side of the house, and also, it has a nice base temp of 50 some degrees. I know there are sometimes moisture problems with berm houses, if not designed right, but what else makes a superinsulated home a better choice in your opinion?

    Second, I plan on putting an entrance into an air locked entry from the north side of the house. This entry will be insulated on all sides from the rest of the house. There is a second story to this entrance that is also insulated from the rest of the house and is accessed from the upstairs. The reason I'm insulating this second story off from the rest of the house is that it has a lot of windows (so we can look out over our property). I didn't want to have windows on the north side sucking a lot of warm air out of the house, and so I thought, if I insulated it, we could have a supplemental heater upstairs and heat the room only when we wanted to use it. Does this idea seem sound? Am I going to run into any problems with not heating these rooms? An extra benefit to having this design, I think, and also to having a pretty open floor plan, is that I can keep these rooms and their windows open in the summer time and get a nice stack effect sucking hot air out of my dwelling.

    Last, I'm from Michigan, and I confess, this is my first home build. But I'm pretty sure that code demands a housewrap on the outside of the house, so I'm not sure how hard it will be to get the rockwool on the exterior sans wrap sold to the building inspector. If, as Dana says, they're pretty much serving the same function then perhaps I can argue that point. Also, is the interior wrap necessary? If I went without, would there be that much moisture infiltration into the home interior? I understand why you need exterior insulation to prevent condensation build up but what's the reasoning behind the smart wrap inside?

    Thanks, ahead of time.

  8. GBA Editor
    Martin Holladay | | #8

    Jeremy,
    Q. "Why is an superinsulated house preferred over an earth berm?"

    A. In theory, if your proposed below-grade wall were insulated to the same (superinsulated) R-value as your proposed above-grade wall, it would perform just as well as, or better than, the above-grade wall. However, (a) it would be more expensive (because of the need for waterproofing details, and because below-grade insulation almost always costs more than above-grade insulation), and (b) it wouldn't perform much better -- certainly not enough to justify the higher cost.

    Yes, there is wind above grade. You deal with that by including a good air barrier. That stops the wind.

    Once you (as you put it) superinsulate your wall -- say, to R-40 -- then it doesn't matter very much whether the exterior of the wall sees 0 degree air or 40 degree soil. There isn't much heat flow through the wall.

    Finally, most below-grade walls aren't insulated to R-40. Builders skimp on the insulation, because below-grade insulation is expensive. That's why your proposed above-grade wall will (usually) perform better.

  9. GBA Editor
    Martin Holladay | | #9

    Jeremy,
    Q. "I'm pretty sure that code demands a housewrap on the outside of the house, so I'm not sure how hard it will be to get the rockwool on the exterior sans wrap sold to the building inspector."

    A. Every wall needs a water-resistive barrier (WRB). This is a good idea -- not simply because it is code-required (it is), but because it makes sense.

    If you don't want to locate your WRB on the exterior side of your mineral wool insulation, you don't have to. You could locate it on the interior side of your mineral wool, if you wanted. But you have to put it somewhere, because it is necessary.

    For more information, see:

    All About Water-Resistive Barriers

    Where Does the Housewrap Go?

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