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Limited budget: Dense-packed cellulose or Zip R-3?

user-6890381 | Posted in Energy Efficiency and Durability on

We are building a custom house in central Maryland, which is in Climate Zone 4 and has adopted the 2015 IECC.

Our budget is already stretched pretty far, so it looks like we can only afford one of these upgrades to our walls:

(1) Dense-pack cellulose insulation (instead of inexpensive fiberglass batts – exact R-value unknown)

(2) Zip R-3 (instead of regular Zip)

Which of these options would the Green Building Advisor Community recommend?

We are leaning toward the dense-pack cellulose for the slight increases in R-value, airtightness, and STC rating of the walls. The Zip R-3 would be a bigger boost to the R-value, but it wouldn’t have the other benefits.

A few other details:
– The wall assembly will be quite simple: HardiePlank siding, Zip sheathing, 2×6 studs + insulation, 1/2″ drywall, latex paint. A rain screen probably will not be included due to the extra cost it would incur, but we’ll have some pretty wide overhangs, which should mitigate this somewhat.
– We’ve asked about 24″ o.c. stud spacing, but it’s unclear if our builder has bought into that idea yet, so we might end up with 16″ o.c. studs.
– Our builder’s practice is to install interior strapping when using dense-pack cellulose to avoid concerns about bowing and fussiness with drywall installation (i.e., gluing drywall over stapled InsulWeb). This increases the cost of the dense-pack upgrade, but it seems they are unlikely to agree to install it any other way.

Thank you for any insight.

Joe in Maryland

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Joe,
    I would choose the Zip R option (because I'm a big fan of continuous exterior insulation). I don't like the idea of all that thermal bridging through the studs.

    But it's your house.

  2. Expert Member
    Armando Cobo | | #2

    In most South and SW markets, for the same price you install regular Zip OSB, you can install regular taped OSB + 1" R5 rigid foam on the outside of the OSB, . You save even more compared to Zip-R3.
    I would set my foot on the 24" o.c. studs. Its your house, and you don't need sacrifice efficiency and material savings on the comfort level of the builder. I'll get another builder that is more competent before I give in.
    You need to think about rainscreens (1x4 or 3D mats) before you install your Hardie siding. They recommend it for their best practice.

  3. Stephen Sheehy | | #3

    I'd go with the Zip-R, but bump it up to R-6. Labor will be about the same. And I'd install the strapping as a rain screen instead of inside under the drywall. As cost seems to be an issue, install batts yourself, either fiberglass or Roxul, skip interior strapping. I'm with Armondo as to 24" o. c. stud spacing.

  4. Expert Member
    Dana Dorsett | | #4

    Dense packing is expensive- damp sprayed cellulose is far cheaper and still pretty good. ZIP is so easily air sealed, and with air tight sheathing in new construction the additional air-retardency of 3lbs+ density cellulose doesn't buy nearly as much performance as it does in retrofits to existing walls that aren't being gutted, and aren't very air tight.

    Spending the difference on upgrading to R6 ZIP-R would probably yield a better comfort & efficiency bang/buck. When the framing fraction is running ~R6, a mere R3 thermal break isn't much of a thermal break, whereas R6 sheathing doubles it.

  5. Expert Member
    Armando Cobo | | #5

    I happen to see it different on two points.
    1) Dense packed cellulose, installed at 4 lbs./ft³ is a lot better than wet sprayed cellulose, which tends to be installed at 3 lbs./ft³. The R-value difference is around R3.8 per inch with dense pack vs. R3.5 per inch with wet sprayed. But the biggest reason not to use wet sprayed is that it shrinks when it dries, living around 1/8”-1/4” gap around the cavity. Also, wet sprayed should not be sprayed with ≥40% humidity, try that in MD, and it can take between 48-72 hours to dry up well enough before you install drywall. In our market dense packed or wet is the same price installed.
    2) When it comes to Zip-R panels, not only is more expensive than regular OSB + outsulation, but I’m of the opinion that the foam on the Zip-R panel is on the wrong side of the sheathing, which places a much greater emphasis on quality control on the Zip tape installation. The reality on jobsites, in all states I work, is that it is very common for installers (framers) not to use the roller or to use it incorrectly. Installers miss-align, twist and miss-joint the tape constantly. Tape is installed weeks after sheathing installation. Sheathing installation was deliver in the mud or soil, not covered and weathered before installation.
    Now, exactly the same can happen with regular taped OSB, but you get a second chance to cover the panel joints with taped rigid foam on top and with staggered seams. You can choose 1” outsulation boards with R-values from 5-6.7. Check on recycled rigid foam, it’s common and cheap.

    Update: Is it just me or what... We seem to be answering the same questions with in a day or two?
    All dense pack cellulose manufacturers require to roll the wall cavities. Unfortunately, must builders don't know it, and in some cases, you have to force the installer to do it.
    If your builder does not want to frame your house at 24" o.c., get a builder that does. That tells me he doesn't know about advanced framing or he's set on old ways... either way, you loose in energy efficiency and higher costs.

  6. Jon R | | #6

    Armando, I'd like to see data supporting your R value claims in #1.

    I've seen enough damp sprayed cellulose to know that a 1/8"+ gap isn't typical.

  7. user-6902085 | | #7

    Joe - we are in similar situations.

    Is R3 zip the same price as cellulose? That surprises me because for me R3 is much cheaper.

    We are Zone 4, not on 2015 codes but that is not an issue. My builder is comfortable with 24 O.C. and I too struggle with cellulose vs exterior foam. Everyone complains about batts but is similar R rated cellulose really worth $1.50 a sqft? OCF is cheaper for me but I am not crazy about its origin.

    We might suck it up and do cellulose and R-4 exterior foam. Overkill since we are at the border of zone 3. But a quality build should last 100 years and that is the way I look at it.

  8. user-6890381 | | #8

    Thanks, all, for the helpful input.

    I did see the related question by David/user-6902085. Sorry for the similar post, but I do appreciate the comments for our particular situation.

    It seems most are in favor of Zip R-3 (or R-6) sheathing over dense-pack cellulose, so we’ll reconsider our plan. A payoff in energy savings is nice, but we’re probably most interested in “comfort”, which is harder to quantify.

    For those curious about the cost differences, for our 2400 sq ft home, the Zip R-3 upgrade would be about $2500-$3000 and the dense-pack cellulose would be about $4500 (including the extra interior strapping). So, they aren’t the same, but we can’t swing both. Maybe we can try to get Zip R-6, as several people suggested -- we’ll inquire about the added cost for that. (On the other hand, Dana’s latest comment [#7] on the <a

  9. Expert Member
    Armando Cobo | | #9

    Jon, et. all - The R-values we get are provided by the manufacturers and retailers, as those values are need to be included in the HVAC design and HERS ratings of our houses. R-values in cellulose are dependent on quality of material, wet vs. dense and installation density.
    Wet sprayed cellulose drying gaps occurrence is more prevalent in drier climate zones in the SW, but I've worked with many builders in AL, MS, LA, OK, KS, AR and now here in TX to see the same results. Everyone of the builders I design for and work with are installing dense pack now.
    Once you use a thermal imaging camera to check your houses, you'll know the difference.
    Designing and building Zero Energy Homes exclusively over many years have giving us a clear understanding of what works best in real world, and the cost differential between good products and even better products. Learning about ease of installation, quality control and end results gives us a huge advantage with our clients. Must builders we work with don't own a hammer, so all work is performed by subs, which in most cases, don't have a clue about codes, good processes, and even less about building science.

  10. Keith H | | #10

    I'm guessing you don't want to spend nights and weekends insulating but if you are doing any of the work you could. Roxul comfortbatt is very easy to do a nice install. Just a cost saving thought. Might not be relevant to you but perhaps to another reader.

  11. John Clark | | #11

    Have you looked at dense pack fiberglass?

    Below is a link to a GBA contributor who used a combination of ZIP R and dense pack fiber.

    https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon

  12. Mike6789 | | #12

    Cellulose is at 0.44 cent a pound so the material for a r48 dense packed wall (4 lbs./ft³) cost $1.76 + installation +- $2 sqft. Beware of contractors who offer you a much lower price / fluffing

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