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Apparently we are the only ones enforcing this part of the energy code

jar546

jar546

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Builder put paper faced, fiberglass batt insulation in the knee walls of a finished space. We failed him for not having an air barrier. I just spent an hour on the phone explaining the code section and difference between an air barrier and vapor barrier. 2009 IRC applies but apparently none of the other inspectors surrounding us ever read N1102.4.1

Its all good now. Just venting.
 
Good catch. I know that kraft faced is rated as a vapor retarder,but it alwayys seems to be ripped or not quite tight on the top or bottom. what do you consider acceptable for proper fit when you inspect it?

Joe
 
Knee wall inside a ventilated attic similar to cape cod style. I don't see the relevance unless it was an unvented attic setup.

Kraft paper is only a vapor barrier and not an air barrier. I even went as far as to contact Certainteed to confirm that paper facing does not qualify as an air barrier.
 
jar546 said:
I don't see the relevance unless it was an unvented attic setup.
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That would make it relevant. You didn't specify in the OP, so I have to make assumptions.

There are situations which may not require an air barrier on the kneewall.
 
mjesse said:
That would make it relevant. You didn't specify in the OP, so I have to make assumptions.There are situations which may not require an air barrier on the kneewall.
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Other than a knee wall already inside the thermal envelope, I am not aware of any others. Can you please share?
 
Is the vapor barrier exposed? If combustible this would be prohibited per R302.10.1. Otherwise gypsum board is an air barrier and penetrations "shall be caulked, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material" (gypsum).

The IRC is silent on air wash of the insulation on the attic side.

Francis
 
Francis Vineyard said:
Is the vapor barrier exposed? If combustible this would be prohibited per R302.10.1. Otherwise gypsum board is an air barrier and penetrations "shall be caulked, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material" (gypsum).The IRC is silent on air wash of the insulation on the attic side.

Francis
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The vapor barrier is facing the gypsum so all is well in this cold climate area.

The code does not call for an air barrier for the attic area, only vapor barrier so that the moisture in the house can make it out and be carried away by the vented attic.
 
mn joe is correct that kraft paper facing is a vapor retarder. A material that qualifies as a vapor retarder/barrier can also qualify as an air barrier. However, just the facing itself is not a sufficient air barrier--laps, tears, joints, and penetrations need to be sealed.

Seams and tears can be taped. Joints and penetrations can be sealed with polyurethane foam. There are other products that are acceptable. See the Air Barrier Association of America (ABAA) for more acceptable products.
 
Certainteed for example was very explicit to let us know that their paper is definitely not an air barrier by design and by the way it is installed. Too many loose areas, rips, joints, etc.
 
What is the specific product? According to CertainTeed's specification for the SpeedyR tabless kraft-faced fiberglass batts, the insulation meets ASTM C 665, Type II, Class C, Category 1 insulation. A Category 1 classification means that the facing is a vapor retarder.

They probably mean that the tabless feature does not allow overlapping at the stud, thereby not providing a continuous vapor retarder.

Another thing to point out is that gypsum board is considered an air barrier. If the walls are finished with gypsum board and the joints are taped and finished, all that needs addressing are the floor to wall joints (polyurethane foam is good for this) and other penetrating items, such as outlet, light, and switch boxes, and recessed light fixtures.
 
Here is the language for everyone.

N1102.4.1 Building thermal envelope. The building thermalenvelope shall be durably sealed to limit infiltration.The sealing methods between dissimilar materials shall

allow for differential expansion and contraction. The following

shall be caulked, gasketed, weatherstripped or otherwise

sealed with an air barrier material, suitable film or solid

material.

1. All joints, seams and penetrations.

2. Site-built windows, doors and skylights.

3. Openings between window and door assemblies and

their respective jambs and framing.

4. Utility penetrations.

5. Dropped ceilings or chases adjacent to the thermal

envelope.

6. Knee walls.

7. Walls and ceilings separating the garage from conditioned

spaces.

8. Behind tubs and showers on exterior walls.

9. Common walls between dwelling units.

10. Attic access openings.

11. Rim joists junction.

12. Other sources of infiltration
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Fiberglass batt insulation is the issue here. If it was another type of insulation, this may not be a problem.

Fiberglass insulation is rated based on it being inside a wall cavity, between coverings such as an exterior wall where there is no airflow. A knee-wall is open on one side, allowing air to easily flow through the insulation, therefore reducing its R value and efficiency. We did a lot of research on this one.
 
You are not the only one. I don't even allow the paper to be left exposed. Says on the facing "flammable- must be in contact with drywall". As far as an air barrier, no way.

Bonus room attic kneewalls, ends of vaulted ceilings also require an air barrier. "Air permeable insulation must be inside of an air barrier."
 
jar546 said:
Other than a knee wall already inside the thermal envelope, I am not aware of any others. Can you please share?
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I'm sure there a several, my immediate thought was was the wall inside the envelope as you mentioned. (that was the basis of my first post)

jar546 said:
The code does not call for an air barrier for the attic area, only vapor barrier so that the moisture in the house can make it out and be carried away by the vented attic.
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So, you require it for the kneewall at the attic or not?

RLGA said:
A material that qualifies as a vapor retarder/barrier can also qualify as an air barrier. Seams and tears can be taped. Joints and penetrations can be sealed with polyurethane foam. There are other products that are acceptable. See the Air Barrier Association of America (ABAA) for more acceptable products.
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Ditto

RLGA said:
Another thing to point out is that gypsum board is considered an air barrier. If the walls are finished with gypsum board and the joints are taped and finished, all that needs addressing are the floor to wall joints (polyurethane foam is good for this) and other penetrating items, such as outlet, light, and switch boxes, and recessed light fixtures.
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...and ditto.

My point is, it's not so black and white to just fail a kneewall at insulation inspection for lack of an air barrier. I'm glad you researched it and came to a resolution.

But, like so many other things I am sure I know the answer to, once I read the various forum comments I learn a whole lot more.

This place is the BEST!
 
Jeff, you just lost me.

An air barrier prevents the movement of air from interior conditioned space to the exterior (exfiltration) and from the exterior into interior conditioned space (infiltration). One side of the knee wall being open does not affect the movement of air in either of these directions.

Yes, air can move laterally through the concealed space that is created, but if that air is prevented from entering conditioned space, then the air barrier criteria has been met. The conceled space behind the knee wall will likely need ventilation (like an attic), since it is outside of the conditioned envelope.
 
What about the (green sheathing) Advantec Zip wall system? If properly taped is it considered an air barrier?

Just today saw it for the first time and can't access the ESR report.
 
ICC ESR-1474 lists the Adventec product as an acceptable air barrier when the seams are taped with Zip Systems seam tape
 
To Jar I agree with RGLA who would not; I would look at infiltration either way as an interior finish material, GWB to keep outside air out. or as Plywood or exterior sheathing with a wrap to keep outside air out, a vapor barrier of class 1,2,3 could even be a good coat of latex paint (cringe) How are you looking at the air passage throught the material . In my opinion and experience if you keep the water out you are very close to an air barrier, and that barrier can be any where within the wall constructo from zero inside surfave to the water resistant menbrane at the exterior. If your kneewal is to a space that is nonconditioned types as mentioned in the attic / cape wall to roof type connect it can be the interior finisih that provides that barrier.?
 
Exterior insulated walls have sheathing on the outside and the inside has gypsum in many instances. The fiberglass batt insulation is in dead air space where there are no drafts and a vapor barrier is required on the outside such as tyvek house-wrap, etc.

The knee wall is nothing more than an insulated exterior wall with nothing on the other side and it is left exposed to drafts and airflow through the vented attic. It is obvious that the exterior walls require a vapor barrier on the outside and the knee wall is no different. Drywall on the inside does not constitute an air barrier to stop airflow from encroaching inside the batt insulation and removing it effectiveness. The code specifies that knee walls have an air barrier for this very reason. Fiberglass batt insulation loses its effectiveness if left exposed. The attic space only needs a vapor barrier and not an air barrier so that moisture can escape into the attic above.

Knee walls that are part of the thermal envelope will be covered with drywall anyway so why else would they have the separate requirement for air barrier?
 
If you are not enforcing the knee wall then what are you having them do behind shower and tub walls on exterior walls where an air barrier is required? What's the difference? Everyone seems to be in compliance with that without question in all areas that use the 2009 IR.
 
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