GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Cellulose Insulation in Unvented vs. Vented Attic

keaton85 | Posted in General Questions on

This is a very air-tight house my wife and I have built.

We installed greenfiber cellulose insulation this past winter and now find that the attic is extremely toxic with off-gassing now that its hot outside. After reading, it seems that if there is any moisture on the cellulose, the heat creates the off-gassing of the borate.

the technical, its a 38X8 12/12 space in the attic which is currently unvented. I am hesitant to vent due to how tight the house is already. If i did vent the attic, the smell would just come straight down into the house every time the bathroom fan is on. Even now, if the windows are closed and the bathroom fan goes, it struggles and the smell from the attic comes into the house.

this is only a 20X38 house with a 4ft knee wall full upstairs, no dormers.

Any advice would help, this is not good for living in this space!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    NICK KEENAN | | #1

    I'm struggling to understand your post. Normally there is an air barrier on the interior side of insulation which should be keeping air from entering the living space through the insulation. When cellulose is used to insulate an attic it is usually between the ceiling of the room below and the floor of the attic. The attic itself is open to the outside to allow moisture and heat to dissipate. When people talk about a "vented" attic this is what they mean. I'm not sure that's what you mean when you talk about venting the attic.

    In a super tight house you're probably going to have to supply air when you're running a vent fan. Building codes now require that well-sealed houses have a source of ventilation air.

  2. walta100 | | #2

    You say very tight house but everyone likes to think of their house as tight.

    Have you had the house tested with a blower door?

    Does your home have a ventilation system?

    Why would your cellulose have moisture in it?

    If wet cellulose off gassing is a big problem, why is damp sprayed cellulose such a common install method?

    Generally a vented attic is the safest and lowest cost assembly.


  3. Expert Member
    Michael Maines | | #3

    Are you sure you didn't choose cellulose with ammonium sulfate instead of borates? I have not heard of borates offgassing under normal conditions.

  4. keaton85 | | #4

    The installed product was dry GreenFiber from HD. Installed by myself in the attic and father feeding it, so no dampness other then from the moisture in the air.

    The ceiling between the 2nd floor and the attic is shiplap with 10mil plastic under it. Not really the best fit on the edges as I thought I was going to keep the roof unvented. It’s definitely ammonium sulfate that I’m smelling.

    I looked around and felt for moisture anywhere and there was none. The only think I can think of is we had some condensation on the fabled ends while finishing the house in the winter before the attic was insulated. At some point it could have condensated on the gables after? It’s bone dry now.

    I’m thinking I have to pull it out and rethink what to do with insulation. NOT fun!

    1. Expert Member
      NICK KEENAN | | #7

      Are you running air conditioning? In the summer vapor tends to flow from outside to in because it's hotter and more humid outside. That 10 mil plastic will keep the vapor on the outside, but if the inside temperature is cooler than the dew point in the attic it forms a surface that condensation can form on because the inner face of it is going to be at room temperature.

  5. Deleted | | #5


  6. morganparis | | #6

    The words '4' kneewall', 'unvented' and 'cellulose' do not go together well. You need to pull out the insulation you have for sure but just changing to borate is unlikely to be the right long-term solution. It sounds like you have a partial cathedral ceiling, read Martin's recent posts about how to insulate them safely.

  7. keaton85 | | #8

    Here is a photo of the current setup.

    If cellulose is not the correct option, what would one do? pull it all out and use closed-cell for the whole thing? Wow that would be a job to undertake, it was intense just getting it up there, let alone getting it back out.

    1. GBA Editor
      Kiley Jacques | | #9

      This article by Martin Holladay, Ruler of All Things Roof Related, should be helpful: Five Cathedral Ceilings That Work .

    2. Expert Member
      Akos | | #11

      You built a roof that should have been half unvented and half vented. Cellulose by itself won't work in an unvented assembly.

      If you are in warm enough climate, a vapor diffusion vent might be the simplest solution:

      If this won't work, probably the next simple fix is to properly separate the unvented and vented sections. This means pull back the cellulose where the SPF ends, seal up the end of the sloped ceiling between the rafters with spray foam, put the cellulose back in and add in the required gable vents for the mini attic.

      P.S. Even if you are in colder climate, I would try the diffusion vent first as it is a pretty simple retrofit, it might just be enough to keep the moisture at bay.

    3. user-5946022 | | #12

      What is "RW"?

      Also please refer to Michael Maines reply - if you smell ammonium sulfate, it seems you have cellulose with that in it. If you google these terms, you will find discussions where people claim the Green Fiber product sold at the big box stores has ammonium sulfate in it as a fire retardant.

      Instead of removing it, it seems the better solution is to ID and remediate the source of the moisture that is allowing the cellulose to smell
      1. Is the plastic a good idea in your climate or is it condensing?
      2. Has your house had a blower door test?
      3. What would be involved in sealing between your house and attic and letting the loose fill cellulose be in an unconditioned attic?

      1. morganparis | | #13

        RW is an index of sound insulation. I don't understand its relevance here.

        1. keaton85 | | #14

          Sorry, Rockwool. I will replay later tonight, more work on the house.

  8. maine_tyler | | #10

    "I am hesitant to vent due to how tight the house is already. If i did vent the attic, the smell would just come straight down into the house every time the bathroom fan is on."

    These sentences are a bit confusing and speak to some misunderstanding or strange situation. 'Attic ventilation' and 'house ventilation' are not the same thing—they're completely independent (ideally/theoretically).

    Is the issue the one mentioned above: that you lack make-up air for the bath fan? Do you have blower door test numbers at hand?

    Perhaps your bathroom fan exhausts into your attic? This would be a problem.

    What sort of air barrier do you have to separate the conditioned living space from the attic?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |