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Old 12-22-2011, 12:22 AM   #16
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Insulate floor on above grade addition


Be carefull following GBR's advice...he has given you a system with a double barrier. 1-1/2" foil faced foam is a class I vapor retarder. Couple that with his recommendation for using the kraft-faced batts and you've got a double barrier system. Being that you are in zone 4, the code may allow that a continuous layer of foam at an R-10 value will allow you to negate the vapor retarder at the floor level. I know that is the case for CT and zone 5.

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Old 12-22-2011, 07:46 AM   #17
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Insulate floor on above grade addition


Kraft faced batts are not a vapor barrier (1.0 perm and Class II or III vapor retarder).

An exterior foam layer of sufficient thickness would eliminate the necessity of any vapor control layer at the warm side beyond normal materials (i.e. sub floor etc).

Double vapor barriers would be in question in situation such as vinyl tile/floor coverings as mentioned by GBR previously.
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Old 12-22-2011, 12:55 PM   #18
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Insulate floor on above grade addition


Well put, WW. That is why I said paper facing- not plastic, which could trap condensation. Where plastic is a vapor barrier and asphalt-coated paper facing is not (it is more vapor-open (hygroscopic) the wetter it gets as is plywood). http://www.buildingscience.com/docum...vapor-barriers

AGW, with un-faced foamboard, as I said in the numbered steps, you could use paper-faced insulation.

(This space means a different thought than the previous)
( So does the "Or"- means a different approach)


OR, the foil-faced insulation without faced batts as the article said.

Please don’t mix the two thoughts/articles, AGW. Please read it again, notice the thoughts are separated by a space……I even added another entirely different thought about Zones between the two so readers would not get confused.


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Old 12-22-2011, 02:04 PM   #19
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Insulate floor on above grade addition


I did read "faced batts" in your first sentence and "facing vapor retarder" in item #4 of your post. "faced batts" are faced with kraft-paper facings and are considered to be a class II vapor retarder. This cannot be placed to the exterior of insulation envelope in zone 4. It is a code violation. If you are providing a continuous rigid insulation board, then you can provide a thickness that will negate the requirement for a vapor barrier on the warm side. The magic number for the foam thickness in zone 5 is R-10, not sure about zone 4.

Here is cut for kraft-faced fiberglass batts: http://www.specjm.com/files/pdf/bid0016.pdf

Notice the kraft 1.0 perm rating as determined by ASTM E96 at the bottom of the first page. That is considered a class II retarder. This is what code officials go by, not buildingscience.com.

To be clear, I wasn't trying to create confusion, but rather clear up the one you created...
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Old 12-22-2011, 06:16 PM   #20
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Insulate floor on above grade addition


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Originally Posted by AGWhitehouse View Post
If you are providing a continuous rigid insulation board, then you can provide a thickness that will negate the requirement for a vapor barrier on the warm side. The magic number for the foam thickness in zone 5 is R-10, not sure about zone 4.
Is a vapor barrier required in zone 4 or 5?
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Old 12-22-2011, 09:50 PM   #21
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Insulate floor on above grade addition


fothrof4, here is some information to help you understand the issue. “Figure A: The traditional “vapor barrier” on a fiberglass batt is actually “asphalt-coated kraft paper” that varies in vapor permeance as a function of relative humidity. It pretty much is a vapor barrier only in the absence of vapor. It really functions much like a valve that opens and closes depending on available moisture. In the winter in most older homes the interior relative humidity is in the 20 percent range whereas in the summer the interior relative humidity is in the 50 percent to 60 percent range. So in the winter a kraft-faced batt has a vapor resistance of approximately 1 perm—an interior vapor retarder on the correct side of the thermal control layer—reducing outward vapor flow. And in the summer the same kraft faced batt has a vapor resistance of approximately 10 perms—allowing the same assembly to dry inwards.” http://www.buildingscience.com/docum...024-vocabulary

The foam on the exterior does replace the interior vapor retarder requirements if you add in the latex paint and drywall (Class III), which you don’t have in your floor system. Plywood sub-floor (1 perm), XPS@1”+ (1.25 perm) and ” plywood (1 perm) plus the f.g. insulation with paper facing (1 perm) will permit drying either way. You don’t require the paper facing per code, but are not disobeying it by having it. They dock you when not meeting minimum code, not surpassing it. If worried about it, omit the paper facing (most box stores are not carrying it), it’s not that big a deal, just ask your local inspector.



Seattle has 4900 Heating Degree Days and 39” annual rain--- Crownsville has 4700HDD and 41” rain yearly, quite similar with both on the coast of a bay. Read this, especially the beginning and Fig. 3b; http://www.buildingscience.com/docum...nd-wall-design

Any other concerns or questions, feel free to ask, we are all here to help.

Gary
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Old 12-22-2011, 10:53 PM   #22
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Insulate floor on above grade addition


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Originally Posted by Windows on Wash View Post
Is a vapor barrier required in zone 4 or 5?
Not sure about zone 4, but zone 5 requires a minimum class II retarder at the warm side in winter. This can be negated if there is continuous minimum R-10 of foam insulation provided on the exterior of the assembly. The reason being that within zone 5, an R-10 of foam will cause the dew point to fall within the foam layer and thereby eliminating the chances of condensation within the wall cavity.

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