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Community and Q&A

Better insulation option than cellulose?

htdoc | Posted in Energy Efficiency and Durability on

I’ll leave the specific history of my build aside for a minute and simply say my builder is trying to convince me to go with a different air sealing and insulating strategy for my house than what is explicitly spelled out in the current sales contract (dense pack borate only cellulose in walls, blown borate only cellulose in attic, and foaming the rim joist areas).

If the builder had built the envelope and framing the way he advertised and promised in terms of materials and steps to ensure super air tightness, I likely wouldn’t even be considering this.  But because of how leaky the house is right now and how little confidence I have the air sealing step yet to come right before they insulate will be done well enough to overcome the issues I already see, I’m considering my options in an effort to end up with something as air tight as possible and as well insulated as possible that is a viable overall wall assembly that won’t create issues down the road…

I’m in central NC and building a house with a double wall assembly, raised heel roof trusses, etc with a vented attic currently framed but amenable to sealing it up if that makes sense.  So i have 8-9 inch thick wall cavities between the 16″OC framing.  Climate zone 4 technically but only a few miles from ashrae zone 3 border on the map.

Builder is offering 4.5 inches of open cell foam in the walls, vaulted ceilings, rim joists, etc and then filling in the rest of the cavity spaces with fiberglass batts (or all blown fiberglass in the attic spaces). 

I’m not on board with this offer as is as that let’s him get away with a wildly cheaper overall solution than the upgrade charges I paid for the extra air sealing, closed cell rim joists and dense pack cellulose solution in the contract right now.  

The idea of foaming the entire wall cavities and rim joists and all other areas to a depth of foam that gets better air sealing is appealing to me, be it open cell or closed cell foam. It would ensure better air sealing results for the house overall without having to wait for the blower door test after the house is nearly finished to learn they didn’t really air seal as well as they promised they would and now I’m stuck with whatever the results show… 

In an ideal scenario, should I just plan to oversee the air sealing activities and force them to do a good job there and stick with the cellulose in the contract?

Or would doing the house entirely in open cell foam everywhere be better?  If they balk at doing 8-9 inches full depth as open cell foam, how much minimum should be done to get true air seal with icynene open cell foam options?

Or should I try to get the house done with a minimum of 2-3 inches of closed cell foam (or as much thickness as possible given its higher board foot cost and my current contract allotment for the insulation) and then finish with fiberglass batts or netted and blown fiberglass?

The builder’s primary insulation contractor doesn’t do cellulose.  So the builder has likely lied to me from day 1 and always planned to try to pull this crap no matter what he signed on for in our sales contract.  So part of me wants to force the contract to be honored and do the dense pack cellulose.  But if I can get an even  better performing wall assembly and better air sealing overall than what I expect if i force the cellulose, I’m willing to explore it and make the builder pay for it after all the things he has been pulling for the last 2 years on this house build…

Thoughts?  Thanks in advance 🙂

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  1. exeric | | #1

    I think your instincts about the honesty of your builder is spot on. It sounds like your builder is the type that gives contractors in the building industry a bad name. Doing double wall construction in most areas of the country implicitly means dense pack cellulose insulation. Dense packing cellulose is often a very regionally available service. It is also (generally) a difficult job to do well, particularly in homes with the thickness of walls that exist in double wall construction. It really requires expertise and techniques that have to be well honed.

    What I'm saying is that it is very likely, though not impossible, that you won't find "any" insulation contractor that is qualified to dense pack cellulose in your region of the country for your house. Maybe you'll get lucky and can find someone who will do it and has done it for double wall construction. I hope so and it is very worth doing your own research to find out.

    I think if I was in your position I would not want to reward your builder's dishonesty. It may be very difficult depending on how far the construction has already gone. You may be stuck with his plan depending on the insulation contractors available in your area. If you are stuck I would certainly not pay for any upcharges the contractor has negotiated.

  2. Expert Member
    Dana Dorsett | | #2

    Closed cell foam thermally bridged by framing is a waste of money, and very un-green. At 2" it uses the same amount of polymer as 8" of open cell foam, and the more affordable closed cell foam is blown with HFC245fa, a powerful greenhouse gas soon to be banned in this application under the Kigali amendment to the Montreal Protocol.

    Given the shaggy dog background story, a full fill of 8-9" of open cell foam is probably the best "Plan-B" option here. To be done safely & right it needs to be installed in two passes of no more than 6" per pass, which is probably why they're only offering 4.5" (one pass.)

    Stuffing batts up against the uneven surface of open cell foam will result in many voids and potential bypass channels. Blown fiberglass in netting will work with though, but probably more expensive than just doing the second pass of open cell foam after the requisite cooling/curing period. Some installers would install 9" all in one go despite the manufacturer's explicit instructions, but that is a fire hazard during the curing period, and results in more shrinkage & potential separation from the framing etc.

  3. GBA Editor
    Martin Holladay | | #3

    A typical flash-and-batt job is done with closed-cell spray foam, not open-cell spray foam. I've never heard of a contractor suggesting the use of open-cell spray foam for a flash-and-batt job.

  4. Expert Member
    RICHARD EVANS | | #4


    I believe open cell foam becomes an air barrier at around 5.5". (GBA has a great article on this so I'll see if I can track it down)

    If it were me, I would research to see if there are other insulation contractors in the area that know how to dense pack (dense pack fiberglass is great too if they don't do cellulose). Your builder shouldn't be offended as he wasn't honest with you. He can also tell his insulation contractor that you've decided to go with another outside group so he doesn't damage his relationship.

    The other option would be to open cell foam the whole thing. It's nasty stuff but you'll get some air barrier benefits and a similar r value as cellulose. It will be way better than batt insulation which tends to leave voids around pipes/wires etc. Like Dana said- make sure they don't spray it all in at once. Houses have burned down because of this.

    1. GBA Editor
      Martin Holladay | | #5

      Here is a link to the article you're thinking about: "Air Leakage Through Spray Polyurethane Foam."

      1. Expert Member
        RICHARD EVANS | | #6

        Thank you Martin- That's the one!

        1. Expert Member
          Dana Dorsett | | #7

          >"I believe open cell foam becomes an air barrier at around 5.5"."

          Rick: DO read comments #1 and #2 at the end of that article.

  5. 300TTto545 | | #8

    I'm in Raleigh. The Cellulose quotes were crazy high. I have 2x6 with 24 O.C. My initial plan was cellulose.

    My rough numbers and memory, it was $20k for cellulose and $10k for netted fiberglass. Really rough numbers. Open cell foam was basically the same as cellulose.

    I don't know why the pricing locally is the way it is but I wasn't about to change it. I did the netted fiberglass. I did a decent amount of spot foaming - like any narrow areas that were not amenable to netted fiberglass and rim joists etc.

    Always consider the opportunity cost of solar. I got a 12kw system. Duke Progress paying $6k of the cost. So my net is in the $15k range. Not much different than the upcharge for cellulose. Enough to make my house net zero energy.

  6. charlie_sullivan | | #9

    I used to think that one of the advantages of dense pack cellulose was that it was easier to install than fiberglass batts--that it would inherently fill in any odd-shaped spaces without the work that takes with fiberglass.

    Then I had a bunch of utterly incompetent insulators try to dense pack my double-stud walls. It turns out it takes skill and training to get it right. In my case, I was lucky enough to be get Bill Hulstrunk, perhaps the world's top expert in cellulose insulation, show up and give them a lesson, and it turned out OK in the end (maybe).

    My takeway: it's better to have your crew do what they know how to do than to insist that they learn a new skill on your project, unless you arrange in advance for Bill to be there to guide them. Half open-cell foam and half blown in fiberglass sounds like a fine solution to me.

    1. Expert Member
      MALCOLM TAYLOR | | #10


      I agree. I had a wet blown cellulose job go horribly wrong when the regular installer was too hung-over to work and his eager partners took up the cause. All types of insulation take skill. I'd hate to be the job they learned on.

  7. lance_p | | #11


    I'm not a builder, but I've done an obscene amount of research into building science over the last year or so in preparation for my own build this spring.

    A recurring theme when planning a high performance build is to clearly define the layers of your envelope. In a heating dominated climate from outside-in they should be as follows:

    1. WRB (Water Resistant Barrier)
    2. Air Barrier
    3. Insulation
    4. Vapor Retarder

    The Air Barrier is usually the first to be built, the main component being the exterior sheathing. Sometimes this includes the WRB if using a 2-in-1 product like Zip. With the sheathing up, the attic drywall in place and the windows in, the envelope is "dried in". IT IS AT THIS STAGE THAT AIR SEALING SHOULD BE DONE. Blower door testing will reveal the building's air tightness, and at this stage any issues can easily be fixed as the whole envelope is naked and accessible. Once you start insulating, finding and fixing air-tightness issues becomes exponentially harder.

    Relying on spray foam for air-tightness has left many builders disappointed with the results. This likely stems from the fact that the sheathing details are not done properly prior to insulating, hoping that the foam will seal everything up. And when it doesn't work as planned, finding and fixing the leaks is a tough job.

    In my opinion, your house is in the stage right now where air sealing and blower door testing should be done. Yes, the house should get tighter as insulation, vapor retarder and drywall are added, but those layers should not be asked to do double-duty. Air sealing should be done primarily at the exterior sheathing. This also has the benefit of limiting pest and insect access to the inside of your walls.

    As far as your contractor not honoring his contract, that's plain shady. I would stick to your guns as much as possible here. As you suspect, he likely took on this project fully intending to pull this trick from the beginning. If your exterior sheathing is properly detailed and airtight, there's no need for spray foam in the walls.

    Spray foam can also be a heath risk. Many people have suffered from foam installations that were not done properly. You should research this independently of what your contractor says.

    1. lance_p | | #12

      Check out this video for some practical advice:

    2. lance_p | | #13

      And here's Matt talking about his newer approach to air sealing. He also mentions getting away from using spray foam to air seal and how he went from over 4 ACH50 to much less using these newer better methods, many of which he learned from Passive House builders:

  8. htdoc | | #14

    Thank you to everyone for replying. I really appreciate the insights and thoughts.

    Based upon a bit more reading (thanks for the links) and thinking through your great comments and insights, It looks like doing full depth open cell foam everywhere may be the most viable option if I dont force the issue and make the builder do the contractually mandated cellulose and watching over the caulking/air sealing step to make sure everything is as air tight as possible before the insulation.

    I agree that doing the blower door before insulation is the better place to do it. But a) builder won't do the prep steps in order to allow the blower door to happen now instead of later even if I was willing to pay for for an extra blower door myself now and b) the only place the builder will pay for the blower door is at the end as part of the rater doing the energy star certification testing...

    I could go on and on and on about the challenges of this build and why or why not various things won't happen (and did write pages and pages if various reply drafts before cutting and trying to be more succinct and to the point). The reality is that I'm limited in what I can get done and can't always do what is the most ideal solution or the consensus "best" solution given my challenges with this build, this builder and the settlement agreement in place given builder is more than 18 months late delivering the house.

    Thanks again for all the insight... I think I have a plan of attack now.

    1. lance_p | | #15

      Sounds like quite the ordeal. Good luck, and let us know how it all works out.

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