Spray Foam insulation in the attic

My old house needs some insulating, and I've been looking at the companies that do spray foam. I'm in the Midwest, in Chicago, where winters can get very cold. I have an unfinished attic space over one half of the house, and then a room with knee walls and a much steeper pitched roof on the other side.
One company proposed removing the existing blown cellulose in the unfinished attic, and then spraying the roof joists to insulate that space. I'm not keen on the idea of the roof not being able to "breathe", and also, I have roof vents, so the roof will be insulated, but there are still big holes in the roof! This seems dumb to me. Am I missing something here? I do have some of the HVAC duct work in that attic, so my impression is that it might be most helpful to find a way to insulate the floor of that attic AND create some sort of insulated capsule over the main metal trunk up there. The ducts to the ceiling vents are flexible foil-like ones.
Another company proposed simply blowing loose FIBERGLASS over the cellulose. I suppose that would be okay, but what a mess.
opinions? ideas?
(I'll probably post separately for the other room, and I also have a basement that needs insulation!)
thanks,
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Do you mean you have insulation on the attic floor now and a guy wants to remove it and spray the underside of the roof with foam, and your attic is open, vented for fresh air? If so it wont do anything and you will be colder. How thick is what you have now, I think you can figure 3.75R per inch of cellulose for what you have now. R 35 is minimum code but I would go to R 60 or so. With an open attic as it gets below 20 fiberglass looses R value, maybe up to 25% as it goes below 0. I dont know if cellulose does this also but its the cheapest way to go, but add an extra maybe 10-15% foor settling. I think you need some more bids from real pros and do your research first. I dont think you can foam over cellulose, it will make a vapor barrier and mold and rot the cellulose and maybe more, especialy if you humidify
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Fiberglass over cellulose, really dumb. If you added spray foam to the roof, condensation is not a problem if its done right, and you would seal ALL holes. A good thermal break will not create any condensation problems. You could add more cellulose, and add a reflective coating or cover for the rafters that keep reflective heat in and summer heat out. A cover being perforated alumina sheeting.
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Spray foam on the roof is about as dumb an idea as has come along.
His attic space is vented so you block that off. Great, now you hae made the entire attic part of the heated portion of the house before the insulation takes effect.
When insulatomg you put it where it will do the most good. That is on the floor of the attic and needs to cover a lot less square footage of area.
Curious. Why is fiberglass over cellulose a bad idea? I'm still trying tofigure a way to do the old half of this house, only access is thru a 2x2' hatch and I don't want the blown in due to the mess.
Harry K
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I dont see any reason no to put fiberglass over celulose, they both have nearly the same R value, both breath. Fiberglass may loose more R value at -20f in Chicago than celulose so that is a reason to reseach. I put fiberglass over cellulose years ago, everyone does it rather than work in a dusty mess trying to remove it, there just is no point.
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Exactly.
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Cellulose is know for its superiority over fiberglass in that it stops air movement better. Fiberglass loses its R value with extreme temp variation. I would also rather brush into paper vs glass.
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I think I figured out why zek is 'reading this differently from the rest of us.
The OP might have worded it better-- or maybe the majority of us are mis-reading.
The OP-
-snip-

I read it as; 'I already have some cellulose & the contractor wants to add to it with some fiberglass.' I think zek was reading 'The contractors prefers fiberglass to cellulose."
I don't think the OP mentioned any insulation earlier- so maybe zek is right. I think we all agree that cellulose is a better insulator.
Jim
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There are air vents in the attic. Either add R50 to the vents or INSULATE.
I already explained the benefits of paper insulation. Its blocks air flow better than fiberglass
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-snip-

-snip-
Why? That's how I'd do it. I might use batts rather than blown in-- but can't imagine what the problem would be.

But it wouldn't do any good in a ventilated attic. And why would he want to heat the attic?

Maybe below the Mason/Dixon-- but the op is in Chicago. I think he'd be better off putting more 'r's on top of his living space.
Jim
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Glad it wasn't Aizona. You've got a tough job there so I hope your company is a bit more helpful than mine [to be fair, they were in NY, but just seemed to second guess more than suggest].
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Your HVAC will benefit from having the rafter bays sprayed with foam. It will help in winter and summer, particularly if those ducts do double duty and heat as well as cool. Blocking up the vent holes is not a big deal, and the vents can be removed easily enough. As far as the roof breathing, the main reason it has to breathe is because of condensation in the insulation. With spray foam insulation sufficient thickness of closed cell foam has to be applied. If the contractor is recommending the cheaper, more porous open cell foam, I wouldn't trust them as they don't know what they're doing.
Closed cell is around twice the price of open cell, but it is superior. A sprayed foam attic has advantages that make it tough to determine a payback time, but it's almost a sure thing that choosing closed cell sprayed foam over blown in cellulose or fiberglass rarely makes sense from a strict return on investment basis. They're apples and oranges.
R
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I forget the name, Dow used to make an itchless fiberglass that was superior to regular grade fiberglass. Great for speaker cabinet damping, and it was extremely fine spun that blocked air flow better. And it was natural white like cotton
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wrote:

maybe this
http://saveenergy.owenscorning.com/2008/03/question_has_miraflex_insulati.html
which is discontinued.
I find various products which claim to produce less itch by partially encapsulating the fiberglass.
Edward
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Closed cell may have a slight edge on thermal efficiency, but it's not always the best solution.
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It is pretty much the only solution for spraying rafter bays in an attic. It's more about permeability than R-value.
R
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Well that was my thinking. The roof deck should be water tight already, and if you spray the underside of a roof deck that isn't water tight, you don't want to be trapping water there. Open cell at least allows the water to pass through and not rot out the deck.
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Granted, a leaking roof is always bad, and it is worse with a spray foam undercoat - of either type. You'll just find out about the leak a bit sooner with open cell. The trade off is that open cell is much more of a problem with moisture from humidity condensing inside the insulation in cold weather climates. As far as I know any code that allows an unvented attic with spray foam, requires the foam to be closed cell.
R
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Makes sense - thank you.
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