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Old 01-09-2012, 09:25 AM   #1
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knee wall insulation help


Hi i need help on a insultion issue with my knee wall . they are complete and the drywall is hung. My question is the walls are insulated with vapor barrior with insulation on warm side like it should but my problem is I put rigid foam board on the cold side. I have acces in the wall. I read online before that it was ok to put it on cold side which i thought was incorrect. But doing some more research im getting conflicting answers. If it is wrong is it possible to leave up and drill holes in the foam in each cavity to help. I dont want to have to take it down if i dont have to. Thanks Dave


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Old 01-09-2012, 12:23 PM   #2
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knee wall insulation help


Hi i am remodeling my attic and have a question about my knee wall. I have put batt insulation on and i have also put foam rigid board on but on the cold side . I got conflicting answers about if that is correct. It is already installed and im concerned about moisture being trapped . I would like to know if its possible to leave up and drill holes in each cavity in the foam board. I have access back there but would be tough to get out now . Thanks for any thoughts . Dave


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Old 01-09-2012, 12:48 PM   #3
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Hi i am remodeling my attic and have a question about my knee wall. I have put batt insulation on and i have also put foam rigid board on but on the cold side . I got conflicting answers about if that is correct. It is already installed and im concerned about moisture being trapped . I would like to know if its possible to leave up and drill holes in each cavity in the foam board. I have access back there but would be tough to get out now . Thanks for any thoughts . Dave
What type of foam board is it?

Is it rated to be exposed, or does it need to be sandwiched between two non-combustable surfaces?

These are some of the Q's I came up with when considering the same thing you did. I decided again the foam board... I installed batt insulation and then put house wrap on top of it--the house wrap blocks any wind in the space (since it's a vented attic space) which helps to retain the insulating properties of the batt (and also helps to make sure the batts stay in place).

I think drilling holes in the foam board would defeat the purpose of having the foam board at all...
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Old 01-09-2012, 03:49 PM   #4
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knee wall insulation help


Will need some additional information here.

1) what is your concern exactly?

2) Where are you? Climate Zone? This will help address the vapor barrier issue

Some other issues.... Knee walls are rarely insulated correctly. The space under the Knee wall (bays in the floor) need to be air sealed completely. Without this the cold air will simply run through the floor joists cooling the room above and below this area.

Do not drill holes in the insulation..period! Air sealing is the most important thing. Use spray foam to seal all the edges of the foam board along the perimeter and tape the seams.

look at this web page to see what I mean with sealing under the knee walls. http://www.homedoctorofamerica.com/knee-walls
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Old 01-09-2012, 03:59 PM   #5
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Best to read this web page. Http://www.homedoctorofamerica.com/knee-walls

Batt insulation is only used when you need to be cheap. It is by far the least satisfactory installation.

Stopping the air leakage is the most important thing. do not drill holes
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Old 01-10-2012, 03:24 PM   #6
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thanks for your response . i live in pa and the knee wall is insulated properly.ohter than having the rigid foam board on the cold side of the wall my concern is that in the summer months. will i be causing myself issues with having two vapor barriers with moisture .its sealed well and on both sides and it will got hot back behind knee wall. I have r 13 and i wanted a little more so thats why i put the foam board up; its about another r 5 i think . i insultated way under the knee wall in each bay so there is no chance for air to get up under the knee wall . thats why i was thinking of puttin some holes between each stud in the foam board . it wouldnt affect my r 13 insultation at all.

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Old 01-10-2012, 04:47 PM   #7
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no holes.... it will effect the R-Value.

In your case you will have no issues with the double vapor barrier. If the foam is sealed completely as it should it will be fine. If you also used batt insulation simply do not use a vapor barrier with it.
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Old 01-10-2012, 05:14 PM   #8
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I do have my vapor barrier on my batt insulation . so i have drywall on, then r 13 batt with a vapor barrier in my 2 by 4 studs . vapor barrier on warm side and then i have my foam board over everything on cold side of the knee wall. thats why i was concerned about the moisture being possibly trapped between the two barriers. its already up . thanks again for any help Bob

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Old 01-10-2012, 05:20 PM   #9
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I do have my vapor barrier on my batt insulation . so i have drywall on, then r 13 batt with a vapor barrier in my 2 by 4 studs . vapor barrier on warm side and then i have my foam board over everything on cold side of the knee wall. thats why i was concerned about the moisture being possibly trapped between the two barriers. its already up . thanks again for any help Bob
Out of curiosity--how are your joists sealed at the knee wall? Is there wood blocking w/ spray foam? Dense packed cellulose? Foam board?
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Old 01-10-2012, 05:44 PM   #10
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2" foam board and the edges get spray foamed in place
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Old 01-11-2012, 08:14 AM   #11
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my joists are 2by 8 . i ran my batt r 19 insulation from the edge of house making sure not to block my soffits. but with a good seal and took it about three feet beyond my knee wall and then stopped it with blocking . I also on the cold side put another layer of inslutation on my joists right up against my rigid foam insualtion on my knee wall to make sure no way of air leaking thru. being an old house i doubled my joists for the attic. i used calking or spray foam between each joists . hope this helps . i dont even have the heat on in attic and its at least in the 60s up there and its goin down to about the mid twenties at night . it seems to be the trick . i also used foam board on the exterior. walls . which gets them to about r 18. it is a true pain in the ass putting drywall up over it being that your using long screws and into hard wood but its warm inside.
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Old 01-11-2012, 08:26 AM   #12
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any thoughts with the moisture issue bob or bubbler

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Old 01-11-2012, 11:23 AM   #13
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Vapor barriers and vapor retarders are two different beasts: http://www.energysavers.gov/your_hom.../mytopic=11810

If asphalt-coated paper (vapor retarder) on the insulation, it is vapor variable permeable; http://www.buildingscience.com/docum...vapor-barriers

Your Zone requires a Class 1, or 2: R601.3 Vapor retarders. Class I or II vapor retarders are required on the interior side of frame walls in Zones 5, 6, 7, 8 and Marine 4.
It is not a vapor barrier- Class I: R601.3.2 Material vapor retarder class. The vapor retarder class shall be based on the manufacturer's certified testing or a tested assembly.

The following shall be deemed to meet the class specified:

Class I: Sheet polyethylene, unperforated aluminum foil.

Class II: Kraft-faced fiberglass batts.

Class III: Latex or enamel paint.
From: http://publicecodes.citation.com/ico...001_par005.htm

So any vapor from the inside will not condense due to the foam board (1") on the attic side. You don't even have one vapor barrier to "trap water". The foam board would need to be 9"or so thick to stop all moisture movement.

You want to stop air/moisture from getting to the cavity from the conditioned space (inside room):http://www.buildingscience.com/docum...wall-approach/

Sounds as if it already done, at least air seal the drywall bottom edge to the floor- as per Code minimum, with caulking or canned foam; number 6: http://publicecodes.citation.com/ico...002_par021.htm

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Old 01-11-2012, 11:35 AM   #14
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GBR's got you covered on the vapor retarders...but...unless your code is different from the majority of others you cannot leave the foam exposed within the attic space. It is a fire hazard. It must be covered by either plywood, gwb, or sheet metal of adequate thickness to meet the code. This need for an additional layer is what likely drove bubbler to use tyvek instead.
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Old 01-11-2012, 11:55 AM   #15
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GBR's got you covered on the vapor retarders...but...unless your code is different from the majority of others you cannot leave the foam exposed within the attic space. It is a fire hazard. It must be covered by either plywood, gwb, or sheet metal of adequate thickness to meet the code. This need for an additional layer is what likely drove bubbler to use tyvek instead.
gwb? gypsum wall board? Is that the same as drywall, or different?

BTW, I had a professional insulation company with very good reputation recommend leaving the foam board exposed in my attic... when I asked about flammability the guy looked at me cross eyed and said "yeah it's flammable, but so is wood"... I didn't hire them. Another company, the ones I used, also was prepared to leave the foam exposed--in fact they did use it for my access doors and left it exposed--this stuff has a tin-foil thick layer of reflective metal-like foil, it's printed right on the stuff that it is flammable and should not be left exposed, I think it says "to living space", but not sure... point is, pro companies are leaving this stuff exposed... I guess it's a lack of education in the industry, or maybe lack of inspection for work like this.

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