EDIT WITH NEW COMMENTS/QUESTION: Begin edit/ Ok...a few thoughts based on your replies:

1) I totally agree that the insulation guy is selling something. Roger that. At the same time, the GC I've worked with in the past agrees with the insulation guy, and he's not in on this project at all.

2) Yes, this isn't the main source of dust in the house, but I CAN see dust coming down around some of the can lights. I also see more dust in rooms near where the registers blow out air, so it's likely some comes from there, no?

3) Re: hiring a maid. Indeed, that's an option. However, prices here are around $100 for a visit (up to $150). At once every other week, that's $2600+ a year. I would break even there in two years (maybe and a couple of months). Then I'd be spending more that way. Plus - maybe someone comes over sometimes already :)

4) 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.

5) Both the GC I've worked with and the insulation guy recommend against spraying the underside of the roof, 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!

/end edit

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?

  • Be aware that the motivation of the spray foam technician is to sell you a product so they are going to say all the good things about it. Beware.
    – Michael Karas
    Jul 7, 2017 at 10:08
  • Also note that any space that has air in it is going to have dust carried in with any air circulation. Just the simple act of coming and going through your entry/exit door invites in loads of outside air with the dust it carries. The main reason that the dust shows up on the surfaces inside is that the rooms tend to make the air static for long periods of time allowing the dust to settle out of the air.
    – Michael Karas
    Jul 7, 2017 at 10:13
  • Keep in mind that spray foam can be applied to the underside of the roof to insulate that in lieu of the attic floor, which might be A Good Thing considering your ducts-in-attic situation... Jul 7, 2017 at 11:34

1 Answer 1


Not crazy but quite unnecessary and counterproductive.

Unless you are visiting your attic often there is no way for the cellulose to get into the living area of the house. Much less the cellulose in the walls since it is sealed behind the walls.

I suspect most of your dust comes from other sources.

You said adding ventilation has eliminated your moisture problem. Don't worry about it then.

So, I think you are contemplating spending a whole lot of money for much less insulation R-rating. Then you will also pay more for heating and cooling from that day forward.

It would be much cheaper to hire a maid service to come clean your house once a week.

Good luck.

  • 1
    @tutorcottage - This is the same answer I was going to write. I want to add that spray in foam is so hard to work around once it is in there. I would never encase my attic in stuff like that.
    – Michael Karas
    Jul 7, 2017 at 10:03

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