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Old 04-17-2012, 04:24 PM   #1
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Two Part Spray foam in addition floor


My gf had an addition built on to her mobile home (trailer) The floor is made up of 2x10" floor joists with 3/4" tung and grove plywood then hardwood flooring. The floor is insulated with FG batts no vapor barrier. The floor is friggin cold. The skirting of the trailer is vented and not insulated so it allows air movement. In the winter time we get down to -15 C on rare occasions.

Here is my plan. Please comment if you see any issues.

Step 1
Use diy two part spray foam and spray one inch of foam on the bottom side of the floor to create some R value and a sealed vapor barrier. I know I could spray more but there are cost issues.

Step 2
Put the batt FG insulation back into the cavity.

Step 3
Put an external vapor barrier on top of the FG bats.

Question:
I understand how condensation works. However would the 1" of spray foam be enough to keep the warm air away from the poly vapor barrier so we don't get the condensation? I want to try to reuse the batt FG (dont want waste the money already spent.

thanks

Brad

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Old 04-17-2012, 10:38 PM   #2
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Two Part Spray foam in addition floor


I would apply a layer of rigid foam to the bottom of the floor joists, taping and gooping the edges to air seal them. That will help the fg batts work better, as you will be deadening the air around them. They are pretty useless w/ air moving past them. Then, close in the skirting and put a vb on the ground (condition the crawl space). Buildingscience.com has info on this, as well as many threads here.

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Old 04-18-2012, 01:14 AM   #3
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Two Part Spray foam in addition floor


http://www.energystar.gov/index.cfm?...sulation_table
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Old 04-18-2012, 01:35 AM   #4
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Two Part Spray foam in addition floor


btw: If I understand what you said about "external vb", that will likely be a disaster. The vb will be cold and condense. VB's, if used, are always on the warm side.
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Old 04-23-2012, 09:04 AM   #5
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Two Part Spray foam in addition floor


Just a note even though this is a little late. If you want to use the foam as a vapor barrier first you must use close cell foam. Next you need to check with the manufacture as to how thick they recommend to use as a vapor barrier. It seems to vary from 1 1/2 inch to 2 inch but nobody seems to say why there is a variation.

As a side note spray foam is not a fun thing to install it sticks to everything you have to wear all kinds of protection, crawling under a mobile home and spraying in a tight space would only add to the discomfort level.

Last edited by bill01; 04-23-2012 at 09:12 AM.
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Old 04-23-2012, 12:11 PM   #6
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Two Part Spray foam in addition floor


Quote:
Originally Posted by bill01 View Post
Just a note even though this is a little late. If you want to use the foam as a vapor barrier first you must use close cell foam. Next you need to check with the manufacture as to how thick they recommend to use as a vapor barrier. It seems to vary from 1 1/2 inch to 2 inch but nobody seems to say why there is a variation.

As a side note spray foam is not a fun thing to install it sticks to everything you have to wear all kinds of protection, crawling under a mobile home and spraying in a tight space would only add to the discomfort level.
I thought most of the expanding foams like Great Stuff are open cell. But Great Stuff claims otherwise:

http://building.dow.com/na/en/produc...windowdoor.htm

http://building.dow.com/na/en/produc...ggapfiller.htm

Latex foams are typically "open cell" and, as a result, can take on water. In fact, the same properties that allow you to wash latex foam off your hands with water also mean that the cured foam can absorb water. This can cause wood rot or deterioration in areas where wet latex foam is next to wood, such as a window frame. In contrast, GREAT STUFF™ is a closed-cell foam. It forms a water-resistant outer skin when cured.

A similar product, Pur Fill, also claims to be closed cell
http://www.todol.com/pdfs/purfillnf12_tech.pdf

Both of these would be a pain to try and uniformly cover a large area though. Not really designed for it.
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Old 04-23-2012, 03:06 PM   #7
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Two Part Spray foam in addition floor


Quote:
Originally Posted by M3 Pete View Post
I thought most of the expanding foams like Great Stuff are open cell. But Great Stuff claims otherwise:

http://building.dow.com/na/en/produc...windowdoor.htm

http://building.dow.com/na/en/produc...ggapfiller.htm

Latex foams are typically "open cell" and, as a result, can take on water. In fact, the same properties that allow you to wash latex foam off your hands with water also mean that the cured foam can absorb water. This can cause wood rot or deterioration in areas where wet latex foam is next to wood, such as a window frame. In contrast, GREAT STUFF™ is a closed-cell foam. It forms a water-resistant outer skin when cured.

A similar product, Pur Fill, also claims to be closed cell
http://www.todol.com/pdfs/purfillnf12_tech.pdf

Both of these would be a pain to try and uniformly cover a large area though. Not really designed for it.
Most can foams are closed cell and some of the large gap and crack filler ones are open.

Window and door foams are closed cell.

I agree with the other recommendations to use rigid foam and go that route.
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Old 04-23-2012, 11:46 PM   #8
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Two Part Spray foam in addition floor


Here is the link others have described: NO vinyl if using foil-faced f.b.; http://www.buildingscience.com/docum...n-crawlspaces/

Thickness depends on your location to prevent the dew-point from happening inside the cavity....

Gary

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