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

Safest insulation material?

yumipatrick | Posted in Energy Efficiency and Durability on

I would like to know what the safest material is for an attic. We have a converted flat roof – originally it was a flat roof and then a pitched roof was added on – so we have some space as an “attic” that we use for storage and the HVAC system. There aren’t any rafters and our ceiling is wood planks (tongue and groove?). My husband put poly iso foam insulation but now I’m wondering if it’s safe. I have a small child and I’m worried about the air quality, etc. and worried about the fire retardant in it…

Thank you so much in advance!

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. GBA Editor
    Martin Holladay | | #1

    It's unclear from your description whether your husband installed the polyiso on the attic floor or on the interior side of the sloped roof.

    Also, it's hard to visualize what you describe. I'm not sure what you mean when you say that "a pitched roof was added on" but "there aren't any rafters."

  2. yumipatrick | | #2

    I'm sorry that I wasn't clear. My husband added the polyiso on the attic floor. There are no rafters on the floor of the attic.

    Thank you!

  3. GBA Editor
    Martin Holladay | | #3

    The framing members that support an attic floor are called joists, not rafters. (Rafters are the framing members that are used to support a roof. Joists are usually level, while rafters usually have a slope.)

    Even though you can't see any joists on your attic floor, I suspect that they are there. It is probable that the "wood planks" you see are floor boards installed on top of the joists.

    There are two ways to insulate the attic you describe. One way is to insulate the attic floor; the usual way to do this would be with cellulose insulation. If there is no insulation between your joists at this point, cellulose could be installed in the empty joist bays by drilling a few holes in your floor boards, and filling the bays with cellulose (using a hose connected to an insulation blower).

    The other way is to insulate along the sloping roof assembly; this method creates an unvented conditioned attic. This method would be preferable in your case, because you have HVAC equipment in your attic. Ideally, you want to bring the HVAC equipment inside the home's conditioned envelope. For more information on this type of attic, see Creating a Conditioned Attic.

    Finally, now that the background information is out of the way, I can address your questions about your husband's technique for insulating an attic:

    1. Polyiso foam has an R-value of about R-6 per inch. It's hard to get enough R-value to meet minimum code requirements with polyiso, unless you add several layers and make the insulation quite thick. In many areas of the country, the building code calls for a minimum of R-38 of ceiling insulation; that would require at least 6 1/2 inches of polyiso.

    2. Many building codes require that polyiso be covered by a protective barrier to limit the chance of ignition during a fire; the usual barrier is 1/2-inch-thick gypsum drywall. It's usually not a good idea to leave polyiso exposed. There are exceptions, however, so you should consult your local building inspector to find out the regulations in your community. (One brand of polyiso, Thermax, has been tested to meet certain fire tests, and it may be permissable to leave the polyiso exposed if your husband bought Thermax. Again, check with your local building department.)

  4. yumipatrick | | #4

    Hello Martin,

    Thank you so much for your detailed response! I really really appreciate it! I think we have some fiberglass on the roof assembly from the previous owner. My husband put in the poly iso down on the floor instead of fiberglass because he was worried about the fiberglass fiber being in our house. Is the cellulose pretty safe? Does it out gas bad chemicals? I really worry that the polyiso is not safe inside our house (even though it's in our attic). We have a small toddler and I really worry about the air quality inside the house.

    Thank you!!!!

  5. wjrobinson | | #5

    Yumi, If the foam came in rigid sheets, very little to be concerned as to chemicals. That said a very very few of any of us can have problems with whatever peanuts, foam, dust... There should be no harm for a healthy normal non allergic person not living in the attic.

    The foam sheets to be effective insulators need to be taped at the seams, fitted perfectly to each other and to the perimeter of where they are. Any air that can pass past the sheets will render them less able to insulate to even useless at insulating.

    You could add cellulose over the foam, but where you do so you will not be able to walk around or store items and someone should check out your furnace install and the attic to see if there are other concerns. It is not easy to figure this all out for you over the internet. Find a friendly reputable home remodeler to take a look at your situation for a cup of coffee or tea maybe.

  6. yumipatrick | | #6

    Hello AJ,

    Thank you for your response! I was a little bit worried since even though we don't live in the attic, our ceiling is a wood ceiling (I think it's called tongue and groove) and also has a few holes since it's wood... We just figured out that when I run the fan over the stove, I think our house is so well sealed that it tries to pull the air in from anywhere it can - including the attic... But it seems like the material is relatively safe (if it's not ignited) and I am hoping that a lot of the outgassing is done since it's been in there since last year. Please let me know if this is not the case, we can always take it out... (it came in sheets)

    Is the cellulose always sprayed? I also worry about spraying - I'm not very familiar with that either...

    It is strange because we just had our furnace replaced not too long ago - and no one said anything about the insulation around it. I will maybe have my husband call the furnace installation company.

    Thank you so much! I really appreciate your help - I worry constantly about my son....

  7. wjrobinson | | #7

    Yumi, crack a window if you want when you run the fan over your stove. Then you will know where the make up air is coming from. Sounds like you know your stove fan vents to outside your home. Many do not they just vent back to the kitchen using a charcoal filter.

    As to sheet foam it is not a risk for most people. At this site we here of rare problems with spray applied foam not sheet foam.

    Cellulose added to your attic would not add something either that bothers your air quality for most people. May be a bit dusty while being put in. The only reason to add the cellulose would be to add even more insulation though I didn't read where you live or if you want more insulation or what your energy costs are.

    I am not at your home, but it sounds like you are being cautious about air quality which is OK but if you at this point leave all as is not being there my guess is all is fine and just leave all as is and go on with life.

    I think if any competent remodeling contractor looked at your concerns that would be all to do next.

  8. yumipatrick | | #8

    Hello AJ -

    Thank you so much for your quick response. Our fan over the stove does go outside our home - I can hear the vent open when I turn the fan on. You are right about cracking the window when I use the fan - I wish I had done that long time ago!

    We live in California - so the weather is relatively mild here. Since it sounds like the foam sheet insulation is relatively safe, maybe we will keep it as is - but I think we need something to protect the insulation from the furnace. My husband said he'll look into a barrier.

    We had a very reputable contractor remodel our house a year ago - but they didn't know exactly what would be the best solution to our flat roof converted to a pitched roof... I wish we had done more research back then. But I like your idea of having someone look at it, I will look for an insulation expert/company to see if they can come out to have a look at our house.

    Again, thank you so much for the reassurance, I really really appreciate it.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |