Styrofoam insulation panels for attic floor?

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Old 02-14-15, 04:58 AM
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Unhappy Styrofoam insulation panels for attic floor?

So I found a source for 4'x8' 4" Styrofoam insulation panels for next to nothing and thought what if I doubled them up to install on the attic floor on top of the ceiling joists, since my existing fiberglass insulation is almost to the top.
At R 4 per inch that would give me R 32 on top of what I have which would be good for my area. I'm not sure that I favor the fire rating of Styrofoam but I don't see too many houses having a fire in the attic. Perhaps once in my life I've seen that, so that's not going to prevent me from doing this. The nice thing is that I would be able to still store some things up there that would not pierce the insulation, I have a problem with fiberglass, blown is is messy, this Styrofoam is dirt cheap..... what are your thoughts? and thank you in advance.
 
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Old 02-14-15, 05:30 AM
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How wide are these joist and what's the span?
Reason I ask is it's far more common to see attics that where never designed for storage in the first place.
I'm on a volunteer fire dept. and we go to attic fires, or fires where the fires broken through to the attic and roof all the time.
Went to a chimney fire just last night that had broken through the wall down stairs and burned into the attic.
Heat rises, when foam melts or gets hot it gives off toxic fumes.
Try to burn a piece of foam, it melts when the flames removed it goes out, it's the fumes that are going to get you.
 
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Old 02-14-15, 05:40 AM
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Interesting about the fumes and doing a 'test-burn'- thanks. Perhaps I should have expanded a bit more. The span is 30 feet and the spacing is 16"OC. The house was built in 1955 and has a 1" thick plaster ceiling. There's 1500SF up there of space. Fireplaces are past the outside double withe walls, but we seldom use them anyway.
 
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Old 02-14-15, 05:53 AM
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OK, so what's the width of the joist?
2 X 4, 2 x 6 ECT.
 
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Old 02-14-15, 05:55 AM
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2x6- sorry for forgetting that one
 
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Old 02-14-15, 06:51 AM
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In addition to Joe's concerns, placing a vapor open rigid foam material above an insulated ceiling is confusing. You identify the product as Styrofoam, which has a high permeability, but 8" is a lot, thus I'm getting into undetermined territory. I know of one deep energy retrofit that applied 6" over the outside in Montana and he is a top energy professional. You can say he wrote the book, because he did, John Krigger. Not exactly the article I read, but it may provide some details. Superinsulation Retrofit in the Wild West - Saturn Energy Blog Saturn Energy Blog

On an attic floor in cold country it would initially seem like placing a vapor control layer on the wrong side. But, at that thickness the downward side will never reach a condensing temperature. But, with the vapor open nature of that produce, I wonder if moisture from below will be diffusing (slowly) through to where it does pass the dew point, leaving the moisture it is carrying stuck in the middle of that 8". Whether that would happen or whether that creates a concern, I can't say. Always a problem when we encounter un-tested methods.

The key as always is to 100% air seal from below, since air flow is the primary mode by which moisture moves. If you installed a double layer, staggered seams, and taped all seams, plus gave great attention to any leakage around the perimeter, in your climate I don't see a problem. Additional air sealing before the new layers is advised.

Bud
 
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Old 02-14-15, 09:53 AM
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How much are you paying per sheet?

Bud, what if he put the vapor barrier under the styrofoam?

The concern with any insulating foam and fire is the speed at which the fire spreads. I assume you are putting a plywood deck on top of the foam anyway. I think that will go a long way in slowing the spread of fire in the foam. There is so much toxic material in the rest of the house, I'm not sure that the burning foam is going to make things any worse. You have very little time to get out of a burning house no matter where the fire is. I just don't see that as a primary concern.
 
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Old 02-14-15, 10:16 AM
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droo, I get what you are saying, it is just that the painted ceiling, perhaps kraft faced fiberglass, and now the 8" of styrofoam starts to sound like too many vapor elements. If air flow is blocked, diffusion is very slow so I would avoid another VB.

alekos, another step might be to confirm you local codes will accept this installation. It would be a shame to have to remove it somewhere down the road. It might also require some level of fire rated covering.
 
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Old 02-14-15, 10:31 AM
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Styrofoam or EPS (expanded polystyrene)?

Here is a link to a building supply company's page that talks of insulating flat roofs and other structures, but lists all the different materials available as well as widths and R-values. So what you plan to do is very common, as a stand alone item. EPS Foam Panels | Log Home Products | Products

Disclaimer - I have no affiliation nor have I ever purchased anything with the linked to company.
 
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Old 02-14-15, 01:17 PM
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Bud, you got me researching more into Superinsulated Retrofits and expanding my knowledge further. Wow, great stuff- thank you. At the moment, there is no VB and the existing fiberglass is loose fill. I wasn't planning on using a VB anywhere.
Droo, I found it at $1.00 per 4'x8' sheet and that was for 4". So I figured placing 2 perpendicular to each other for greater R value.
What is really ideal about this idea is that in this approach, I would still have access to wiring and stuff in the future if necessary much easier than blown cellulose, and be able to utilize the flat surface. It is expanded polystyrene (EPS) since Styrofoam is just a 'brand' - my bad.
Thanks again guys
 
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Old 02-14-15, 05:02 PM
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When you say styrofoam, I picture insulation like a coffee cup. Is that what you are talking about?
 
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Old 02-15-15, 12:37 AM
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That is correct.
Wikipedia: Styrofoam is a trademarked brand of closed-cell extruded polystyrene foam currently made for thermal insulation and craft applications. It is owned and manufactured by The Dow Chemical Company.
So basically 4" thick sheets of 'coffee cup' stuff. BTW, after reading some of these articles, I'm thinking that double layered (8") could be a bit of an overkill.
Thanks
 
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Old 02-15-15, 05:42 AM
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alekos <after reading some of these articles, I'm thinking that double layered (8") could be a bit of an overkill.> A single layer requires more attention to air sealing as opposed to two staggered layers, but you are probably correct. I'll add a link on overall air sealing from the folks in Vermont. But be sure to check under the existing insulation to seal what you can find.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
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