Thermal barrier required by foam board...


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Old 09-25-15, 06:00 PM
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Question Thermal barrier required by foam board...

Would appreciate your help.

Have a basement with block walls, and plan to use 1.5 to 2 inch foam board to moisture barrier the block wall, and then place a stud frame wall directly in front of the foam board. About an inch of space will exist between the foam board and the stud wall.

The foam board is flammable (Owens Corning Foamular 150 0r 250) and requires a thermal barrier.

What is best to use as a thermal barrier?

Thanks for your assistance.
 
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Old 09-25-15, 06:04 PM
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I would think that your sheetrocked wall, in front of the foam, would be the fire rated barrier needed.

We'll have to see what the pros say.
 
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Old 09-25-15, 06:07 PM
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Use 2" foam.
Once the walls framed add R13 insulation, vapor barrier then sheet rock.
No "Thermal" barrier needed.
As long as the foam is behind the sheet rock your good to go.
Just can not leave it exposed.
It's not flamable, it gives off toxic fumes when heated.
 
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Old 09-26-15, 09:43 AM
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Thanks for the help, PJmax and joecaption.

As far as the sheetrock goes, would UltraLight Firecode 30 5/8 in. x 4 ft. x 8 ft. Tapered Edge Gypsum Board be OK? I am neither big or strong to carry one of the regular sheets around.

On the vapor barrier, will the wall assembly be able to dry, since this is in the basement? Concerned about mold on the R13 insulation and studs.
 
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Old 09-26-15, 10:13 AM
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Moisture will move through the foam board you have suggested. Foil or plastic faced would not. Once past the foam board it is important to allow that moisture to dry to the inside, thus not using a vapor barrier on the inside. The amount of moisture is very small, but a layer of plastic would allow it to accumulate until everything between the vb and the dirt outside is all equally wet.

Using foam board as a moisture barrier should not include an attempt to solve a water problem. You have to recognize that most basements were not designed to be dry and well suited to be converted into prime living space. You have probably already read all of this, but start on the outside and then manage what gets through on the inside.

Bud
 
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Old 09-26-15, 04:25 PM
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Thanks for the info, Bud9051.

There is no flooding or small accumulation of water in the basement.
Also, unfortunately, I cannot afford a higher level of foam board. XPS seems to be a popular choice:
http://www.homeconstructionimproveme...lation-values/

On your comment:
Once past the foam board it is important to allow that moisture to dry to the inside, thus not using a vapor barrier on the inside. The amount of moisture is very small, but a layer of plastic would allow it to accumulate until everything between the vb and the dirt outside is all equally wet.
If I understand you correctly, a vapor barrier in this situation is not a good thing. So, frame the walls, add the insulation, and then use sheetrock?
 
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Old 09-26-15, 05:00 PM
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In an above grade wall you get a choice, dry to the inside or dry to the outside depending upon how you build it. Below grade there is no choice because those walls cannot dry to the outside. In recent articles they have been approving the rigid foam that includes a foil facing on the inside of the basement walls because the correct thickness of foil will act as the thermal barrier. More expensive, yes, but in some cases it eliminates the studded wall and the drywall. Not as pretty, but if local codes will accept it it becomes an option.

But my point is, behind that foil faced rigid foam board it will b3ecome as wet as the soil outside. The concrete doesn't care and as long as the foam board can tolerate it, it works.

For your proposed wall, "If I understand you correctly, a vapor barrier in this situation is not a good thing. So, frame the walls, add the insulation, and then use sheetrock?" should be fine. My only difference would be, I would eliminate the space and I would use Roxul. More expensive, but better suited for basements.

In addition, be sure your bottom plate is pressure treated wood and use a top plate. Then all fasteners that attach to the pt wood will need to be rated for that purpose. I also like to enclose the back of the top of the wall to be sure it doesn't become a nesting site for the occasional critters that eventually find their way in.

Bud
 
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Old 09-26-15, 06:14 PM
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Thank you again, Bud9051.

Roxul is out of my league! Also, the area involved is only 9x8 feet, where I plan to keep my 3 parrots, since I am allergic to them. They talk, so, I want to be able to hear them.

One last item, if I can have your opinion...will you post your opinion here:
http://www.doityourself.com/forum/ba...all-front.html

Thanks again for your guidance.
 
 

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