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Community and Q&A

Ammonium sulfate offgassing from cellulose

user-426670 | Posted in Green Products and Materials on

I’m trying to decide how to best insulate a 1950s cape cod style attic being remodeled.  A local cellulose contractor proposes to put up scrim and apply stabilized (wet-spray) cellulose, but will not apply an all-borate product because, he says, it requires too much moisture to activate the binder.  He insists on using a product with ammonium sulfate instead, and says that once the initial install has dried it cannot offgas further, even if it gets wet.

I am trying to avoid dry cellulose due to the concerns about the load it would put on joists that are already flexing enough to cause hairline cracks in the ceilings below, though the cracking may also be due to the cement board used in the ceiling — a product not nearly as flexible as modern drywall.

There are a few online articles warning against using cellulose with ammonium sulfate due to corrosion and offgassing, but there seems to be a dearth of hard science online about the consequences of using ammonium sulfate in cellulose insulation.

Can anyone point me to research on this and other topics related to wet-sprayed cellulose?  Rather than trying to research the risk, would I be better off avoiding it altogether by looking for another contractor — one who is willing to install an all-borate stabilized cellulose?  The first guy took a while to find, so I’m not enthusiastic about restarting the search, being way behind schedule already, but I’m even less enthusiastic about making the wrong choice at this point in the process.

Thanks for considering my plight!


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  1. Expert Member
    Dana Dorsett | | #1

    >... would I be better off avoiding it altogether by looking for another contractor — one who is willing to install an all-borate stabilized cellulose?

    Probably. But dry blown is also still something to consider for attics.

    Is this insulation only going to be an the attic floor? Is any of it going to be at the roof deck level?

    Venting the mini- attic behind kneewalls in Cape houses is prone to leakage/infiltration problems, and fiber insulation at the roof deck has it's issues too.

  2. walta100 | | #2

    My guess is the cracks in the ceiling are from the plaster separating from the wood lath that supports the plaster. Not that it is impossible that the ceiling joists are under sized by today’s code.
    If you push up on the plaster near the cracks and it moves you need to repair the plaster before you insulate regardless of your insulation choice. Generally the best option is to cover the plaster ceiling with drywall.
    Once you are sure the plaster is not going to fall there will be no need to wet spray.
    Also make sure you do not have any knob and tube wiring before you insulate.


  3. GBA Editor
    Martin Holladay | | #3

    Are you talking about installing cellulose on the horizontal floor of an attic that is vented?

    Or are you talking about installing cellulose on the underside of the roof sheathing, creating an insulated sloped ceiling?

    If it is the latter, you need to install ventilation baffles extending from soffit vents to a ridge vent.

  4. user-426670 | | #4

    Thanks for the responses, folks!

    The thermal barrier will run from the eaves, across the attic floor to the kneewall and gable walls, up the kneewall to the rafters, and along the rafters to the ridge. The portion of each gable wall enclosing the conditioned space will also be insulated. Plywood baffles have been installed at the eaves and the part of the roof to be insulated. Blocking has been installed between the joists below the kneewall bottom plates and between the rafters above the kneewall top plates. The seams and joints at the baffles and blocking have been sealed with canned foam, but Dana's point about the likelihood of leaks and infiltration is well taken and is part of my motivation for keeping ammonium sulfate out of the attic or, at the very least, verifying that it will not cause a problem later on.

    As for the ceilings, instead of lath and plaster, the builder used a product that has a cross-section like today's drywall, but with a cementitious grout-like layer instead of gypsum between the paper facings. Each piece is 16"x48", and was installed by nailing directly to framing (not lath), then skim coating with a layer of cement or plaster. Apparently it was a transitional technology between plaster and drywall that, thankfully, did not last.

    So the question remains whether ammonium sulfate-based cellulose in an old, dry-for-now attic is or is not a risky endeavor. I found an anonymous, undated 3-page writeup on the Wayback Machine that talks about thermal decomposition of ammonium sulfate into ammonia and acid-forming sulfur and nitrogen compounds above 536 degF, but I hate to give this analysis much weight without knowing more about its provenance. It also says nothing about the less hazardous, but still possibly noxious off-gassing of ammonia from the stuff at typical attic temps, or the occasional leak or high-humidity situation.

    I'm starting to wonder if there is any research to support the long term safety of this chemical in cellulose insulation?

  5. Expert Member
    Dana Dorsett | | #5

    If it never gets wet, sulfated fire retardents aren't a problem.

    If it DOES get wet it's fairly corrosive- it'll eat into copper/brass very quickly, and it'll corrode nails (any plaster and lath ceilings below?) too, though not as quickly as cuprous metals.
    But that's only if it get WET. Humidity has to be in the nearly saturated range for it go be corrosive when dry.

    Unlike borate-only cellulose fire retardent it'll gives off a friendly cat-pee kind of odor to let you know there's been a leak, and that it's time to scoop it out. :-)

    That said, I have a few wall cavities in my own house that I've put at risk with this stuff, (most are blown with borate-only goods) and I still sleep just fine (maybe because I don't own a cat... :-) ) YMMV.

  6. Bogdan_Livinti | | #6

    It seems you haven't received an answer to your question, just opinions and friendly cat pee comments...This report might provide the answer you were looking for >

    1. Expert Member
      MALCOLM TAYLOR | | #7


      There is a pretty good chance that the OP has resolved the issue one way or another by now as the discussion is over a year old.

  7. dub12Nation | | #8

    To the OP, what did you end up doing? We’re trying to understand this as well!

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