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Considering replacing current blown cellulose with open-cell spray foam on attic floor. Is this crazy?

tutorcottage | Posted in General Questions on

My wood frame and stucco home with cedar shingle roof is almost 100 years old. We live near Chicago (Zone 5). We have blown cellulose on the attic floor, ~r45. A furnace with duct work is in the attic (there was no other place to add a needed zoned furnace), in a small room with some rigid foam board. The duct work was sealed as carefully as possible and is covered in some cellulose.

About 5 years ago, there was too much moisture in the attic. After cleaning, we applied mildew resistant paint, and we added several vents with a fan on a timer and on a temperature switch. No issues since then.

Our house gets dusty (either that or my cleanliness standards are too high). I suspect some comes from the blown cellulose in the attic or the dense pack in the walls. Most probably comes from the outdoors.

I am considering having the r45 blown cellulose on the attic floor removed and replaced with r30 open cell spray foam (ocSF). Is this crazy? A recommended technician says that we will be better off with the foam due to its air sealing properties. Plus he can better insulate the ducts and small furnace room with the ocSF.

Would replacing the blown cellulose with ocSF reduce dust in the house?
Would it make moisture problems more or less likely in the future?
Would it reduce or increase our energy costs?
Are there other concerns I am not considering?

Thank you so much!

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  1. GBA Editor
    Martin Holladay | | #1

    Your symptoms point to one cause: Whoever installed cellulose on your attic floor forgot to perform air sealing work before insulating. The leaky ceiling is the cause of the moisture problem.

    For more information on the required work, see Air Sealing an Attic.

    Your contractor's suggestion would require cellulose to be removed. If you are going to that much trouble, you'll find that it's easier to work in sections. Here's what you do: remove the insulation from a section of your attic, and pile the insulation on the other side. Perform your air sealing work. Replace the insulation where it belongs, and do another section.

    This approach will cost less, and be easier, than removing the cellulose insulation from your attic and installing R-45 of spray foam. (You don't want to replace R-45 insulation with R-30 insulation -- if so, you are taking a step backwards. R-30 insulation doesn't even meet minimum code requirements.)

    By the way, the installation of the powered attic ventilator is potentially counter-productive. It's probably adding to your energy bills (by sucking away interior air at a fast rate through your ceiling cracks), and it may even be endangering your family's health (by causing your water heater to backdraft). For more information on these problems, see Fans in the Attic: Do They Help or Do They Hurt?

    One final point: If you decide to install spray foam, at least install it in the right place. In your case, the right place for spray foam is on the underside of the roof sheathing, not on the floor. That way, you will convert your vented unconditioned attic into an unvented conditioned attic, which is what you want to do when you have a furnace with ductwork in the attic. If you take this advice, use closed-cell spray foam, not open-cell spray foam. For more information, see Creating a Conditioned Attic.

  2. tutorcottage | | #2

    Thanks so much for your response. A few thoughts I'd like to bounce off of you:

    1) I've had the bid upped from r30 to r40. That way I'm keeping almost the same r, while adding
    a) air sealing properties
    b) the foam would be sprayed to cover the duct work and the little board room the furnace is in, bringing the furnace and all duct work into the envelope of the house for the first time.

    2) Both the GC I've worked with and the insulation guy recommend against spraying the underside of the roof, even with closed-cell foam, due to the fact that it is shingle. They both say that it will make the roof rot faster.

    Based on all this: do you think this is crazy or counterproductive?

    My main concern is that, by creating an air barrier to the attic, I'll be preventing moisture from leaving the house and therefore increase the chance of mildew or mold inside the house. Is that realistic at all, or no?

    Thanks so much!

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