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Old 04-22-2010, 12:05 PM   #1
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Super TUFF-R under sheetrock


I am in the process of remodeling a bathroom and came up with an idea on how to deal with the thickness of the old plaster wall (keeping the window moulding the same) and increase my insulation R value at the same time.

My idea - fiberglass insulation between the studs, then 1/2" Super TUFF-R over entire exterior (interior side) wall with the seams taped, and then 3/8" sheetrock. My idea would also eliminate the need for a vapor barrier. My only concern was strength when dealing with the thinner sheetrock over the foam board. So off to the net I went to figure out if this was possible.

The answers that I found while searching are confusing at best. It seems that everybody agrees that the foil side (interior side) of the board would create a condensation problem within the wall cavity. I really don't understand how this would be any different that just putting a 6 mil vapor barrier behind the sheetrock, which I believe is not only standard, but required by code. Can someone explain why a 1/2" of foam behind a vapor barrier will change anything other than the R value?

Thanks, Mike

EDIT: I have noticed that DOW requires a minimum of 1/2" sheetrock over TUFF-R insulation (probably as a fire barrier). That would leave me with 3/8" of TUFF-R, which still provides an R value of 2.0


Last edited by medic; 04-22-2010 at 01:19 PM.
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Old 07-28-2010, 09:57 AM   #2
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Super TUFF-R under sheetrock


i am doing a similar retrofit and am disappointed to see that you haven't gotten any replies.

my house was built in 1949 and i am working my way through each room, one at a time, and redoing everything down to the studs to include the interior drywall and exterior sheathing and siding.

right now my house has that old 1/2 thick cardboard type sheathing with tar paper and pine beveled siding. i'm currently working on the living room, which has tounge and grove pine run vertically across the walls and ceiling. it also has the original wiring which is really showing its age. i will be gutting the room down to the studs, replacing the wiring and both interior and exterior wall coverings.

for the outside, i was thinking about layering 1/2 OSB, 3/8 super tuff-r, furring strips, and then cedar shingles. for the inside i was just going to do fiberglass batts, plastic vapor barrier, and sheetrock.

a big reason for this project is to make sure everything is proper, and i don't want to create a moisture problem. does the above method work or is there a better way?

also, i was thinking it may be easier to put the super tuff-r first and then the OSB so that i would have a nailing base for the shingles and be able to ditch the furring strips. does that work? would i need diagonal corner bracing if the foam board goes down first?

thanks for any help and/or critiques.

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Old 07-28-2010, 11:30 AM   #3
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I could see a potential moisture problem in your case. I used the Super Tuff - R as the vapor barrier, sealing all of the seams and the ends with tape and plastic.

In your case, with a vapor barrier, as per code - on the warm side of the insulation, the Tuff - R insulation may create a second vapor barrier on the cool side of the insulation, and therefore create a void that cannot breath. I am no expert, but from what I have read, the scenario I described could very well be true.
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Old 07-28-2010, 11:47 PM   #4
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Super TUFF-R under sheetrock


Where are you located?

http://www.ecohomemagazine.com/energ...gy-claims.aspx

http://www.energysavers.gov/your_hom.../mytopic=11810

http://www.buildingscience.com/docum...ecommendations

Be safe, Gary
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Old 07-29-2010, 12:05 AM   #5
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Super TUFF-R under sheetrock


Quote:
Originally Posted by GBR in WA View Post
Where are you located?

I'm in New Hampshire
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Old 07-29-2010, 09:16 PM   #6
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And what did you learn from the sites sited? Is a vapor BARRIER like foil faced foam or poly plastic required?

Be safe, Gary
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Old 08-03-2010, 09:38 AM   #7
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the sites you provided seemed to say that a vapor barrier is required.
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Old 08-03-2010, 11:59 PM   #8
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If you are in Zone 5 or 6, a poly v.b. inside is not recommended. Scroll down 1/4 and read your --- under 5000 degree days- mixed climate for the foam board and lack of interior v.r. http://www.buildingscience.com/docum..._view=1#Ref_11

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Old 08-06-2010, 08:35 AM   #9
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I am right on the border of zone 5 and zone 6.

"A Class I or Class II vapor control layer is required by the IRC on the interior side of framed walls in Zones 4c, 5, 6, 7, and 8, with the exceptions of basement walls, below grade portion of any wall, and wall construction that is not sensitive to moisture or freezing (e.g. concrete block wall). However, BSC recommends avoiding Class I vapor control layers in general in wall assemblies, except in special use occupancies in cold climates such as indoor pools and spas."

am i missing something? the first sentence says a vp is required but the bold says to avoid using one...

what does seem clear is not to use a vp on both the inside and outside - "Avoidance of the installation of vapor barriers on both sides of assemblies—i.e. “double vapor barriers” in order to facilitate assembly drying in at least one direction."

then its says this... "Vapor diffusion from the interior can be a concern in cold climates and is typically a concern in very cold climates. Vapor diffusion retarders, when specified in cold climates and very cold climates, are located towards the interior of the thermal insulation. When vapor retarders are used, walls and other building assemblies are designed and built to dry to the exterior, should they get wet or start out wet."
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Old 08-06-2010, 08:41 AM   #10
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Super TUFF-R under sheetrock


i'm not a contractor and didn't mean to get into a scientific debate over this. i'm just a homeowner who lives in NH and is wondering what to use for a vapor barrier.

i am thinking about using rigid foam insulation between osb sheathing and white cedar shingles and am concerned that the foam will create a double vapor barrier if i also use one on the interior wall.

is it not a problem? should i forget the rigid foam? should i use the foam and omit the interior vapor barrier?

while i'm sure there is a debate about this, there must be a currently accepted building practice that the pros use today.
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Old 08-07-2010, 12:06 AM   #11
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Be safe, Gary

Zone 5 (e.g. Boston)
  • Vented cladding over OSB, plywood, fiberboard or exterior gypsum
  • Insulated sheathing with an R value ≥ 5 (e.g. 1 XPS) on a 2x4 framed wall
  • Insulated sheathing with an R-value ≥ 7.5 (e.g. 1.5 XPS) on a 2x6 framed wall.
Insulated sheathing can be installed in the form of Expanded Polystyrene (EPS) with approximately R-4 per inch, Extruded Polystyrene (XPS) with R-5 per inch or Polyisocyanurate (PIC) with approximately R-6 per inch. The amount of exterior insulation required to reduce the amount of interior vapor control depends on the climate and framing width (2x4 or 2x6 framing).

A Class III vapor retarder can be used instead of a Class I or Class II in zones 4c, 5, 6, 7, or 8 where one of the criteria for the specific zone from the list below is met. These criteria may depend on whether the cladding is vented or whether insulating sheathing is used.
Class III. 1.0 - 10 perms e.g. latex or enamel paints.


I underlined it for you. The interior is painted, so the wall dries to the inside. The wall thickness determines the minimum foam thickness. I would recommend a drainage plane for the shingles or shakes: http://www.coastalcontractor.net/article/135.html
http://www.nchh.org/Portals/0/Contents/Article0440.pdf
If you tape the seams of the foam board, you can omit the house wrap. Be certain
ALL edges are taped to repel wind-driven rainwater: http://www.buildingscience.com/docum...rainage-planes


Watch the indoor humidity levels as per this article: http://www.buildingscience.com/docum...gs?full_view=1

Be safe, Gary
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Old 08-07-2010, 12:47 PM   #12
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Fastening drywall over foam board might be asking for problems with nail/screw "pops" and /or joints cracking. The drywall should be fastened tightly to the framing members or another layer of solid backing. The foam seems like it would have a lot of "movement" as you get your usual lumber shrinkage/expansion through seasonal climate (heat/AC) changes. Just a thought...
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Old 08-07-2010, 02:44 PM   #13
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"i am thinking about using rigid foam insulation between osb sheathing and white cedar shingles and am concerned that the foam will create a double vapor barrier if i also use one on the interior wall.

is it not a problem? should i forget the rigid foam? should i use the foam and omit the interior vapor barrier?"


"Bold" is mine.....


Be safe, Gary
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Old 08-07-2010, 04:14 PM   #14
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That makes more sense.
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Old 08-08-2010, 08:32 PM   #15
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Quote:
Originally Posted by mklang View Post
i'm not a contractor and didn't mean to get into a scientific debate over this. i'm just a homeowner who lives in NH and is wondering what to use for a vapor barrier.

i am thinking about using rigid foam insulation between osb sheathing and white cedar shingles and am concerned that the foam will create a double vapor barrier if i also use one on the interior wall.

is it not a problem? should i forget the rigid foam? should i use the foam and omit the interior vapor barrier?

while i'm sure there is a debate about this, there must be a currently accepted building practice that the pros use today.
Rigid foam is NOT a vapor barrier unless more than 2" thick. So if you are using thin XPS foam board (<1") it will still have enough permeance to let things dry out.

I don't remember the "building science" link I saw this info at... Pretty sure that <2" you are OK.

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