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Discussion Starter · #1 ·
I plan on building a 45' diameter geodesic dome in Tarpon Springs, Florida. The shell is constructed with 2x6 lumber with plywood sheething on it. What I'd like to do, is spray urethane foam insulation between the studs to obtain the maximum R-value possible...before I put on the enterior wall board. If I spray the foam so it slightly fills past the studs, can it be trimmed off flush with a hot wire? Or, can this only be done with EPS foam?
 

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What I am reading says to use a handsaw, if I was facing a lot of it I would figure out something like a bowsaw using a section of bandsaw blade.
The foams that they are using seem to be open cell, a hotwire would seal the surface, possibly trapping moisture.
Any idea what the density is? Up to 4# cuts almost like it isn't even there.
 

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Discussion Starter · #3 · (Edited)
Teetorbilt said:
What I am reading says to use a handsaw, if I was facing a lot of it I would figure out something like a bowsaw using a section of bandsaw blade.
The foams that they are using seem to be open cell, a hotwire would seal the surface, possibly trapping moisture.
Any idea what the density is? Up to 4# cuts almost like it isn't even there.
Teetor, here is a site that has info on it. I think it is closed cell.
http://www.dow.com/buildingproducts/frothpak/index.htm

It's the same as Great Stuff like you can buy at Home Depot, etc.

And, yes, I would be facing a lot of it. A saw blade would take forever. It's not going to be easy, either, while working almost upside down on the uppermost triangular panels of the dome shell. Studs are 24" on center. I wonder if there is another way to achieve a flush amount of the insulation, without sawing it flush, if this the only way to do it.
 

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From what I read, this is not what you want to use. I have posted quite a few sites and you should be able to find them in the roofing area. This is new tech and I suggest that you do all of your homework before proceeding. All foams are not the same and using the wrong one or process could come back to bite you in short order.
 

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Discussion Starter · #5 ·
Teetorbilt said:
From what I read, this is not what you want to use. I have posted quite a few sites and you should be able to find them in the roofing area. This is new tech and I suggest that you do all of your homework before proceeding. All foams are not the same and using the wrong one or process could come back to bite you in short order.
Thanks, Teet. I will do plenty of research before I get to that point. Check out this link: http://www.monolithic.com/plan_design/polyurethane/
 

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Read it. It,too, is inconclusive as to the type of foam to be used other than being very light weight. If you have a problem locating those sites let me know and I'll see if I can dredge them up.
 

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professional insulation contractors who specialize in the spray foam insulation use like a big electric knife to cut it. I don't no the correct name meaning what they call that device. But that is exactly what it looks like and works like a big electric knife. Its my understanding that insulation is very expensive to go with meaning 2or3 times the price of other insulations. But I'm not a expert like teeter on pricing that is what a contractor told me. :Thumbs:
 

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Discussion Starter · #8 ·
Thanks, 747, I will check into that. I'm thinking that I will go with a pro to do this, instead of doing it myself. That electric knife sounds like the way to go. That way I can get the insulation all the way flush with my studs, which will give me the maximum R-value. The extra cost of 2 or 3 times regular fiberglass insulation will be worth it in the long run, I believe. It will give me a tighter shell; and the R-value of foam is much greater than fiberglass per inch.
 

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Spray foam for insualtion also seals, glues and contains boric acid for bugs.
 

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Discussion Starter · #10 ·
I spoke to a guy today, who owns a company that sprays on polyurethane foam insulation. I explained to him that I intend to build a geodesic dome of 2x6 construction with plywood sheathing and metal shingles, and that I wanted to spray on polyurethane insulation between the studs. He told me that I would want to use the soft open cell type, instead of the hard closed cell type, to prevent mold and mildew from forming in the walls/roof. He said that if I used the hard urethane foam, it would act as a vapor barrier, thus causing an environment for mold and mildew to form.

Does this make sense to you? I told him that I originally planned to use the hard foam, because it has a greater R-value than the soft. He still recommended that I not use it, especially here in Florida. I hope he knows what he’s doing. Obviously, he knows more than I.

Any opinions?

David
 

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Go back to post #2. My research is telling me the same thing.
Common sense tells me that the same medium that lets moisture out also allows moisture in. I would be tempted to install plywood ceilings and having it all blown with the unicellular 4 or 6 lb. A good guy with a wand should be able to eliminate any air gaps/voids and you would wind up with one he11 of a torsion panel structure. I'll come visit next Jeanne.
 

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Discussion Starter · #12 · (Edited)
Teet, I don't understand what you meant by:
"I would be tempted to install plywood ceilings and having it all blown with the unicellular 4 or 6 lb."
Do you mean installing plywood to the interior of the studs, and filling with dense closed cell foam? This is what the foam guy said not to do.

However, I don't understand how having a moisture barrier on the inside of the decking, in the form of the foam, would creat a problem...especially if there is free air space between the shingles and the decking.

David
 

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It's what they do in boats for flotation, how much more moisture can you have? The higher density foams are also used for srutuctural components. In general, the concept is called 'stressed skin' construction. It is the way that planes are built. Two surfaces with high tensile/compression loads seperated by space, the space is oftened filled to impart more ridgity to the skin and increase it's loading ability.
Your foam guy probably only knows what the company that sold the equipment told him. Just for fun, quiz him about densities and open cell vs. closed cell. Tell him that you are considering structural panel construction and watch his reaction closely.
 

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Come to think about it I have only seen the hard kind like your talking about used. But I live in a cold weather climate Northern Illinois. Down in Flordia it might be a different ballgame because of the heat and humidity. Teeter from Flordia I sure he knows what those boys use in that state. :)
 

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2lb foam

The reason your contractor is specifying .5lb foam is that he can get a greater yield per set than the 2lb. (15,000 bd ft VS. 3600) I don't think moisture has anything to do with it. 2lb foam is much stronger, doesn't absorb moisture and, as in the case of a geodesic dome, will make it a strong, monolithic structure. Tell your contractor that you'd like to spray 1" of 2lb foam, the spray the rest of the cavity with .5lb. That will give you tremendous strength, higher R value and eliminate the moisture argument.

Thanks,

Pete
 

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Discussion Starter · #16 ·
Teetorbilt said:
Go back to post #2. My research is telling me the same thing.
Common sense tells me that the same medium that lets moisture out also allows moisture in. I would be tempted to install plywood ceilings and having it all blown with the unicellular 4 or 6 lb. A good guy with a wand should be able to eliminate any air gaps/voids and you would wind up with one he11 of a torsion panel structure. I'll come visit next Jeanne.
Teet, I like your idea. The plywood ceiling would definitely make for a much stronger structure. How would the foam guys spray the foam inside the chamber... by drilling holes? Should there be air escape holes, as well? Would this tend to bulge out the plywood, when it expands?

David
 

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Plywood panels inside and out change the structure to torsion panel, a much stronger configuration. The foam is shot through 1" holes in this config. and turns the torsion panels into cored panels which are stronger still.
Panel distortion depends on a number of factors. Span, escape, foam density and operator competancy are all factors in the equation.
Ever get a heavy part delivered in 2# foam and plastic? Everything made it from Taiwan, right? The cardboard box arrived as a cube with a few dented corners, right? Same idea.
 

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Discussion Starter · #18 ·
Teetorbilt said:
Plywood panels inside and out change the structure to torsion panel, a much stronger configuration. The foam is shot through 1" holes in this config. and turns the torsion panels into cored panels which are stronger still.
Panel distortion depends on a number of factors. Span, escape, foam density and operator competancy are all factors in the equation.
Ever get a heavy part delivered in 2# foam and plastic? Everything made it from Taiwan, right? The cardboard box arrived as a cube with a few dented corners, right? Same idea.
This sounds like the way for me to go. Thanks for your advice.
 
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