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Community and Q&A

Smart Membranes and Cellulose Insulation

gstan | Posted in Green Building Techniques on

Having read a variety of questions and answers on this site over the last
several years certain themes and memes have appeared with regularity.
They aren’t always wrong or right, but they are popular! Here are a few of
them along with a few questions.

1. Variable Permeance membranes are more desirable and superior to impermeable membranes and should be utilized at every opportunity.
Q. Are any of these ideas actually true? I suspect that in many cases
they are not superior (except for the expense) and in many
instances are you would be just as well off with cheap polyethylene.

2. Cellulose is more desirable and superior to most other insulation
material and should be utilized at every opportunity.
Q. What if anything happens to the R-value of cellulose when it absorbs
moisture (apparently it is capable of absorbing about 135% of its own
weight) – what is the R-value at 10%, 20%, etc.? How much settling are
you likely to get in your walls over 8 or 10 years if you have a lot of high humidity and the cellulose you had carefully installed at 3.5 lbs. per cubic
foot now weighs 4.5 or 6 or whatever lbs. per cubic foot? These are the type
of questions that should be asked of all fluffy blown in insulations by the
way, not just cellulose – and don’t get up on your moral high horse and tell
me that they don’t settle if you follow the manufactures installation instructions – there are too many anecdotes out there about people opening
up cellulose filled walls and finding settling. Cellulose in ceilings is not
much of a problem because you can get up there to see it and fix it – but
in walls it can really be a problem!

Enough for today!

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  1. brendanalbano | | #1

    If you restate the advice as:

    1. Using a class II vapor retarder instead of a class I vapor retarder, when it is allowed by code, has a greater drying potential and is often a more conservative choice.

    2. Cellulose has lower embodied carbon than other commonly available insulations, so using it when it is appropriate to do so is often the right choice if you are concerned about global warming.

    There's a lot less to argue about ;)

    That said, regarding using polyethylene, you might enjoy these articles from BSC that discuss some instances where you probably can just use cheap polyethylene:



    1. gstan | | #4

      Having less embodied carbon doesn't cut any ice for me. If I really worried about it I
      could stop wearing ski jackets and modern shoes in the winter - after all deer skin
      moccasins and a bear skin coat have way less embodied carbon - Ea-what! Same thing
      with insulation, mineral wool, polyiso, polystyrene and many others have higher R-ratings
      per inch and are nowhere near as vulnerable to problems with moisture - use them
      and Variable Permeance membranes become largely irrelevant - no?

      1. brendanalbano | | #6

        If you don't care about embodied carbon, then a good chunk of the reasons why people on GBA recommend things don't matter. That's fine, but you'll just have to account for that when reading advice. If you're not worried about embodied carbon, you can happily use XPS and closed-cell spray foam and all that good stuff! Things are easier if you're not worried about embodied carbon!

        You can make interior poly work in a lot of scenarios, although it may be unwise in climates that have significant air conditioning in the summer. From the article on double vapor barriers linked above: "For the record I do not have heartburn with interior vapor barriers in cold climates. Nor should all of you. But I do have heartburn with interior vapor barriers in hot climates and mixed climates and pretty much any place with lots of air conditioning."

        Can you link some of these anecdotes about cellulose settling in walls that you're worried about? What density was the cellulose packed at? How thick were the walls?

        My perception is that the recommendations on GBA typically recommend a wide variety of insulation types, including, but definitely not limited to cellulose. ¯\_(ツ)_/¯

        1. gstan | | #8

          brendanalbano: Actually, it's not that I don't care, it's that I don't consider it
          to be the final authority - given a choice between two equal products or
          techniques I would pick the low carbon alternative every time. But at todays
          level of technology the choices are never very equal and "low carbon" is not
          a final authoritative and trumping argument. You will find a Youtube link in the
          reply to Bill below - there are several other written articles about this on the
          web but I'm basically too lazy to look them up for you. I think that poly (or
          any other vapor retarding membrane) needs to be on the warm side of the
          wall (no exceptions) - having air conditioning in a cold climate usually makes
          this difficult to achieve.

          1. brendanalbano | | #9

            The settling in Thorsten Chlupp's home is definitely concerning! Do you know if this is an issue that has been observed in 2x6 walls at 3.5+ PCF? It would make me think twice about dense-packing a double stud wall!

            Regarding poly--it seems like everyone (in this thread at least) is pretty much on the same page regarding when it is or isn't appropriate to use it.

          2. exeric | | #10

            I wouldn't use Thorsten Chlupp's cellulose installation as a reliable indicator for settling for a 2x6 wall. He was pushing the limit for when dense packing is appropriate. In fact, I doubt you'd find a more demanding installation involving cellulose dense packing anywhere on the internet.

  2. Expert Member
    BILL WICHERS | | #2

    The so-called "smart" vapor retarders allow for some drying while still limiting the amount of moisture that can get into the wall. This is a plus, since it makes for a more robust wall assembly. In some cases, the vapor retarder is not needed (such as when using a high ratio of exterior rigid foam to interior insulation), but in those cases the vapor retarder can still provide some extra insurance. Poly doesn't allow for any drying, ever, which is why it's less desireable in comparison.

    Manufacturers typically specify an "installed" and "settled" depth for given R values of cellulose, along with a weight per cubic foot as a target to hit for a good installation. The issue in walls is when you don't "dense" pack the wall densely enough. That "not dense enough dense pack" problem is a known issue, and is warned about in these forums periodically when people are attempting to do DIY dense pack where it's more likely to be a problem. No one considers it to be a secret. Regarding moisture absorption, I'm not aware of any studies of R value vs moisture content, but remember that moisture can be both absorbed AND released by cellulose (which is part of how it can do moisture buffering), so it CAN dry out in a typical vented attic assembly, which is probably where it's most likely to be exposed to widely varying moisture levels. Even in a wall assembly, you should still have some drying ability -- unless you used poly -- which will help to keep moisture content from getting too excessive.

    Remember also that ALL building materials have their pros and cons, nothing is perfect. Part of good design is knowing the limitations of the materials you're working with and making sure that you use those materials appropriately to avoid potential problems down the road.


    1. gstan | | #7

      Bill: Sorry but I don't buy the argument that "The issue in walls is when you don't "dense"
      pack the wall densely enough". Take a look at Thorsten Chlupps house built in Anchorage
      some years ago - He went a ridiculous amount of trouble trying to prevent settling, blew it in at 4 lbs. per cubic foot (into a double wall) left the top of the wall open and piled up feet of extra cellulose over the top of the wall so that if any settling occurred the extra could fall into the wall and refill the voids. A year or two later a thermal imaging camera was used to judge
      the effectiveness of the walls. You can guess what was found - voids - it did not solve the
      problem. Here is a link to a Youtube video showing the design and discussing problems and mistakes (the voids problem is discussed about an hour and 16 or 18 minutes in) there are a few written articles on the web also which go into more detail on this house.
      I am really skeptical about the use of cellulose in walls - as near as I can determine there
      is no density which will prevent settling over the years and no information available on
      the effect of moisture contributing to settling. Buffering? Does this mean spreading the
      moisture around sort of evenly which shouldn't have gotten in there to begin with? Sorry,
      I couldn't resist a little sarcasm there. There might be a good reason that manufactures
      don't have or publish anything showing R-values at varying moisture levels - perhaps I'm
      just cynical?

      1. Expert Member
        BILL WICHERS | | #11

        No "buffering" means the cellulose is capable of absorbing and releasing small amounts of moisture without issue. That's a plus over some other kinds of insulation in some applications. As I mentioned earlier, all materials have their pros and cons.

        I'm not a huge fan of cellulose in walls myself, but there are many who have used it that wasy successfully. I prefer mineral wool in walls myself. Cellulose is probably the best option out there for vented attics though -- very good performance and very cheap. loose fill fiberglass is probably next best, and has the advantage of being lighter weight per unit R value, but it is more prone to convection air current issues than cellulose. Loose fill mineral wool is a lot more expensive, so I don't really consider it for attics, even though I much prefer mineral wool batts for walls over most other types of "fluffy" insulation.

        I don't think anyone on here is trying to push cellulose as THE insulating material. It's often recommended for attic floors where it's an excellent option, and sometimes dense packed in walls, but that's usually only double stud assemblies which are less common for most projects compared to "regular" stud walls.


  3. Expert Member
    Michael Maines | | #3

    1. In many cases, using a variable permeance membrane makes a more durable and resilient assembly than you would have without the membrane. But situations vary. Some wall or roof systems can be very safe without the membrane. Few assemblies are made more durable and resilient with the use of polyethylene sheeting, though they can work just fine if you're lucky.

    2. In areas where cellulose is exposed to very high humidity levels, to the point of saturation, it can settle. There are ways to minimize how much moisture it absorbs, such as using in in cavities with continuous exterior insulation, by using a variable permeance membrane at the interior, and by controlling indoor humidity levels. It's the air spaces between cellulose flakes that do the actual insulating, so thermal performance depends on how well they are maintained. The more moisture there is in the wall, the lower the R-value.

  4. Expert Member


    1) Poly, installed as a combined air/vapour barrier, is still used in the overwhelming majority of houses in Canada.

    The main two situations where it can make assemblies risky are:
    - In climates with a long cooling season where a wrong-side vapour barrier can become a condensing surface near the interior of the assembly.
    - Where the assemblies are not designed to dry to the outside.

    In the absence of these conditions, poly generally works fine.

    There are some advantages of combining the functions of air and vapour barrier together on the interior:
    - All the trades here are familiar with poly, and how maintain continuity and to detail penetrations.
    - It is visible, meaning both that it is obvious which material is preforming these functions, and it can be inspected before being covered.

    2) I am agnostic on the benefits of cellulose over other options for walls. It's hard to beat in attics.

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