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



## jar546 (Aug 22, 2013)

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.


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## mjesse (Aug 22, 2013)

Do the kneewalls face the exterior, or are they under a roof? (Cape Cod style)


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## mn joe (Aug 22, 2013)

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


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## jar546 (Aug 22, 2013)

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.


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## mjesse (Aug 22, 2013)

jar546 said:
			
		

> I don't see the relevance unless it was an unvented attic setup.


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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## jar546 (Aug 22, 2013)

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*.


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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## Francis Vineyard (Aug 22, 2013)

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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## jar546 (Aug 22, 2013)

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


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.


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## RLGA (Aug 22, 2013)

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.


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## jar546 (Aug 22, 2013)

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.


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## RLGA (Aug 22, 2013)

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.


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## jar546 (Aug 22, 2013)

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
> 
> ...


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## jar546 (Aug 22, 2013)

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.


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## kyhowey (Aug 22, 2013)

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."_


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## mjesse (Aug 22, 2013)

jar546 said:
			
		

> Other than a knee wall already inside the thermal envelope, I am not aware of any others.  Can you please share?


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.


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.


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.


...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!


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## RLGA (Aug 22, 2013)

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.


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## mjesse (Aug 22, 2013)

kyhowey said:
			
		

> "Air permeable insulation must be _inside__ of an air barrier."_


Please list the Code section this is from.

Thanks


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## RJJ (Aug 22, 2013)

I too enforce the air barrier / infiltration requirement. However, I am with RGLA the knee wall explanation.


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## Mac (Aug 22, 2013)

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.


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## mjesse (Aug 22, 2013)

ICC ESR-1474 lists the Adventec product as an acceptable air barrier when the seams are taped with Zip Systems seam tape


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## RJJ (Aug 22, 2013)

Yes I would agree, but in this case there was nothing there but insulation.


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## Architect1281 (Aug 22, 2013)

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.?


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## jar546 (Aug 23, 2013)

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?


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## jar546 (Aug 23, 2013)

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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## jar546 (Aug 23, 2013)

Even the PHRC and PSU along with the PCCA teach this.  This is common knowledge.

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## jar546 (Aug 23, 2013)

And there is more that is interpreted the same way we have after discussion with the ICC, a visit to BuildingScience.com and the US Energy Department.

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## jar546 (Aug 23, 2013)

From the Energy Dept for air barriers such as gypsum.

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## mtlogcabin (Aug 23, 2013)

Jar Your second photo is what we have been taught here in MT. There needs to be an air barrier on the attic side of a knee wall and behind a tub/shower enclosure. Common practice here is batts in the stud cavities then spray foam the attic side of the knee wall. I will see if I can take some photos tomorrow. BTW the instructors have come from CO and WA and it was part of the training requirements because the state took the money to adopt the 2009 IECC from the Feds that has been mentioned in another post.


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## jar546 (Aug 23, 2013)

mtlogcabin said:
			
		

> Jar Your second photo is what we have been taught here in MT. There needs to be an air barrier on the attic side of a knee wall and behind a tub/shower enclosure. Common practice here is batts in the stud cavities then spray foam the attic side of the knee wall. I will see if I can take some photos tomorrow. BTW the instructors have come from CO and WA and it was part of the training requirements because the state took the money to adopt the 2009 IECC from the Feds that has been mentioned in another post.


                                                                                                         ......................................................................................................:agree.....................................................................................

Apparently the memo for the 2009 IRC Chapter 11 was not passed to southeastern Pennsylvania, Arizona or Rhode Island........


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## jwilly3879 (Aug 23, 2013)

As a CEO and a contractor I see this issue from both sides. When I build a kneewall that will be insulated with fiberglass we install housewrap on the cold side which limits any windwashing effect on the insulation. The tabless kraft faced product does satisfy the vapor retarder requirement. There has been much debate in my area about the requirement to face staple the insulation to provide a continuous vapor retarder but nowhere in the NY 2010 Residential Code is the word continuous used with vapor retarder. The stapling tabs on CertainTeed insulation are plain kraft paper,  just like a paper bag and have no retarder effect. The insulation with stapling tabs, according to the manufacturer's installation instructions may be installed by stapling to the sides of the studs or to the face, which can cause drywall problems down the road and if it is stapled to the top and bottom plates the drywall cannot be sealed(caulked). I prefer unfaced batts with CertainTeed's vapor retarder, I believe the unfaced batts are easier to install without the voids commonly seen with the kraft faced products.


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## Francis Vineyard (Aug 23, 2013)

jar546 said:
			
		

> 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?


I understand your logic about knee walls however my interpretation that it is not an exterior wall but it is an element of the "Building Thermal Envelope".

*N1101.9 (R202) Defined terms.*

*EXTERIOR WALL.* Walls including both above-grade walls and basement walls.

*ABOVE-GRADE WALL.* A wall more than 50 percent above grade and enclosing _conditioned space._ This includes between-floor spandrels, peripheral edges of floors, roof and basement knee walls, dormer walls, gable end walls, walls enclosing a mansard roof and skylight shafts.

*BUILDING THERMAL ENVELOPE.* The basement walls, exterior walls, floor, roof, and any other building elements that enclose _conditioned space _or provides a boundary between _conditioned space _and exempt or unconditioned space.

Could also interpret the criteria in TABLE N1102.4.1.1 (R402.4.1.1) for "Exterior thermal envelope contains a continuous air barrier."

Is "exterior thermal envelope" an exterior wall or does it include knee walls as "other building elements" within building thermal envelope?

R104.1; here's another interpretation; see page 11: DOE 2012 Air Leakage Guide



Francis


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## jar546 (Aug 23, 2013)

Francis Vineyard said:
			
		

> I understand your logic about knee walls however my interpretation that it is not an exterior wall but it is an element of the "Building Thermal Envelope".*N1101.9 (R202) Defined terms.*
> 
> *EXTERIOR WALL.* Walls including both above-grade walls and basement walls.
> 
> ...


Understoon.  The air leakage guide even specifies rigid foam on the attic side of the framing to ensure a proper air barrier.  I just don't see how someone can interpret drywall on the inside as the only requirement when an air barrier is specified for this exact location.  It is only an issue with fiberglass batt insulation which is dead air space dependent.


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## mjesse (Aug 23, 2013)

jar546 said:
			
		

> Understoon.  The air leakage guide even specifies rigid foam on the attic side of the framing to ensure a proper air barrier.  I just don't see how someone can interpret drywall on the inside as the only requirement when an air barrier is specified for this exact location.  It is only an issue with fiberglass batt insulation which is dead air space dependent.


Although not stated in the text on page 11 of DOE 2012 Air Leakage Guide, the illustration very clearly shows the air barrier on the inside of the insulation.

If you are requiring the air barrier on the attic side of the wall pictured in figure 5, shouldn't you also require it above the ceiling insulation?? If not, then why are you requiring it on the wall??


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## RJJ (Aug 23, 2013)

No southeast got the memo and understands it. Did not understand your earlier explanation. the photos are clear on your intent.


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## jar546 (Aug 23, 2013)

mjesse said:
			
		

> Although not stated in the text on page 11 of DOE 2012 Air Leakage Guide, the illustration very clearly shows the air barrier on the inside of the insulation. If you are requiring the air barrier on the attic side of the wall pictured in figure 5, shouldn't you also require it above the ceiling insulation?? If not, then why are you requiring it on the wall??


The ceiling does not require an air barrier, just the knee walls.  The ceiling has a vapor barrier and must allow some of the moisture buildup in the house to escape to the vented attic.


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## RLGA (Aug 23, 2013)

A little information on vapor retarders/barriers (VRB, to simplify things).  They do not always go toward the outside.  The location of the VRB depends on the environment and the vapor drive.

In a cold climate situation, the vapor drive is from the interior to the exterior.  As moisture vapor travels through the exterior wall materials toward the exterior, the temperature drops and eventually it will reach the dew point and the moisture vapor condenses creating liquid water that can damage insulation and other materials an cause mold growth.  The dew point is typically reached somewhere within the insulation where the greatest drop in temperature is achieved.  The VRB should be located towards the inside in this situation to prevent moisture vapor from reaching the dew-point location within the exterior.

In warm humid climates, the reverse exists:  warm moisture vapor will be driven into cooler conditioned spaces of buildings.  Again, the temperature will drop as the moisture vapor travels through the exterior wall toward the interior space and will reach the dew point within the exterior wall assembly.  This is why so many buildings years ago had mold problems because they used vinyl wallcovering--a vapor retarder--on the interior surfaces of exterior walls.  The moisture was stopped at the wallcovering, condensed due to the cooler interior temperature, and mold spread between the wallcovering and and gypsum board.  In this situation, the VRB should be towards the exterior to stop the moisture there before it reaches the interior.

In my area (southern Arizona) a vapor retarder is generally unnecessary, since our winters are mild (no vapor drive from interior to exterior) and the air is dry (no vapor drive from exterior to interior).  However, I specify VRBs for slabs-on-grade, because the soil, believe or not, has a high moisture content and the vapor drive is significant in many locations, which causes floor finish problems.

As I mentioned in an earlier post, an air barrier is to prevent the lost of conditioned air to the exterior and the addition of unconditioned air into conditioned space.  Therefore, all exterior walls and roof/ceiling assemblies must be designed with air barriers if the requirement for an air barrier exists.  Providing an air barrier over a portion of the building does little to prevent the transfer of air.  Although VRBs are good, a good air barrier system is even better, since moist air carries more water molecules than moisture vapor.  Therefore, if you have a good VRB, but the building leaks air like a sieve, then moisture is still getting into areas where it shouldn't.  The only thing that may prevent the damage of materials and the growth of mold is that eventually dry air may pass through and dry out the wet innards of an exterior assembly.  This only delays the eventual deterioration.

If an air barrier is provided in the walls, it should tie into the roof membrane (which is also an air barrier--if it isn't, you'll have bigger problems).  If you don't have a roof membrane (e.g. sloped roof with tile), then an added air barrier is necessary, generally at the ceiling level between the conditioned space and the unconditioned attic, which is why attic access openings are mentioned in IRC N1102.4--the gypsum board ceiling is the air barrier, but the attic opening has a joint that needs to be sealed.


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## tmurray (Aug 23, 2013)

The Problem is that wind washing typically does not diminish the R value of an assembly when the air flow is not directed at the assembly.

This:







doesn't happen because there are no forces changing the direction of the air entering the attic.

However in Jeff's situation, this is a concern. The airflow will presumably be directed more or less at the assembly. Since the air is being directed at the assembly AND since fiberglass batts are wind washing sensitive (the r value is developed by layers upon layers of fiberglass and air, so if the wind comes and pushes that nice warm air out of the fiberglass more heat is going to need to cross the envelope to heat the new layers of air up) it is a concern in this scenario.

BSI-064: Bobby Darin and Thermal Performance — Building Science Information


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## mtlogcabin (Aug 23, 2013)

Typical behind a tub/shower





Backside of a knee wall is foamed over the batt insulation





Same system on a coffer style ceiling


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## RLGA (Aug 23, 2013)

tmurray:

I agree that windwashing would reduce the R-value of insulation.  Fiberglass blanket insulation gets its insulating value by the air spaces it traps between the fibers.  If the air is still, the insulation is at its most efficient performance.  If air can circulate through the insulation, then the insulating efficiency is decreased.  However, that has little to do with air barriers in this particular situation--a properly installed air barrier would not prevent the reduction of performance of the insulation.  Another material (preferably a vapor permeable material), should be added to the concealed side of the kneewall to prevent air circulation through the insulation--this material is not part of an air barrier system by industry definition, or by IRC definition, for that matter.  All the material would do is keep the air within the fiberglass batts still.  The gypsum board on the interior face would be part of the air barrier system.


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## jar546 (Aug 23, 2013)

The original intent of this post by me was to illustrate that this is part of the code that is not often enforced, yet all of the training we receive from multiple sources in multiple states along with consultation directly with the ICC clearly shows us the intent is to place an air barrier on the other side of knee wall WHEN fiberglass batt insulation is used.  If another method such as spray foam or foam board is used, then this is not an issue as they themselves act as an air barrier.

I hope that everyone else that enforces the 2009 IRC including the energy chapter (without amendments) is holding everyone to this established intent and standard.


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## tmurray (Aug 23, 2013)

RLGA said:
			
		

> tmurray:  I agree that windwashing would reduce the R-value of insulation.  Fiberglass blanket insulation gets its insulating value by the air spaces it traps between the fibers.  If the air is still, the insulation is at its most efficient performance.  If air can circulate through the insulation, then the insulating efficiency is decreased.  However, that has little to do with air barriers in this particular situation--a properly installed air barrier would not prevent the reduction of performance of the insulation.  Another material (preferably a vapor permeable material), should be added to the concealed side of the kneewall to prevent air circulation through the insulation--this material is not part of an air barrier system by industry definition, or by IRC definition, for that matter.  All the material would do is keep the air within the fiberglass batts still.  The gypsum board on the interior face would be part of the air barrier system.


Sorry Ron, I didn't mean to imply that the air barrier system MUST prevent the wind washing, just that there should be an air barrier (either part of the air barrier system or a independent  system that is not part of the building air barrier system).


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## Glenn (Aug 23, 2013)

There should be an air barrier on both sides of the building thermal envelope (as Jar546 has stated).  Air transports far more moisture than vapor diffusion (passing through a solid object).  Pressure differentials from both mechanical equipment and outside wind speed push and pull air on both sides of the thermal envelope throughout the year.  You do not want your air leaking either direction into the envelope for both thermal and moisture reasons.

Vapor retarders should go on one side only, depending on the climate.

This is how we interpret the code to what we believe is the intent.  Many of our new homes use the performance method for energy compliance, and this is exactly what the HERS raters are designing and requiring as well.  We do not have push back from the building community.

Never leave the kraft paper exposed, no matter what you call it (air, vapor, wallpaper, etc).

Good thread.  Thanks.


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## Rick18071 (Aug 24, 2013)

I too enforce the air barrior in knee walls. But I am not sure if house wrap can be used. Can house wrap be exposed in a crawl space?


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## jwilly3879 (Aug 25, 2013)

6 mil poly can.

The back of the kneewall of a vented attic is what I would consider an exterior wall even though it is covered by a roof.


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## peach (Aug 25, 2013)

mjesse said:
			
		

> ICC ESR-1474 lists the Adventec product as an acceptable air barrier when the seams are taped with Zip Systems seam tape


And you'll start seeing ALOT of this stuff... (we already are).


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## RJJ (Aug 25, 2013)

Rick some approve it so I am told. I say no.


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## jwilly3879 (Aug 25, 2013)

I have been seeing Advantech products more and more over the past five years or so. It started with the floor sheathing, which holds up quite well to the weather for long periods, then the red roof sheathing with Zip Tape and recently green wall sheathing with Zip tape.

On the contractor side, we did an addition with crawlspace and the architect specified housewrap on the floor joists over the insulation and was approved by the AHJ with no problem. There was also a 12 mil reinforced poly barrier over the dirt floor, up the walls and fastened to the sill plate.


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## MASSDRIVER (Aug 26, 2013)

jwilly3879 said:
			
		

> 6 mil poly can.The back of the kneewall of a vented attic is what I would consider an exterior wall even though it is covered by a roof.


That stuff turns to shredded crap in 5 years. need something that will hold up to ozone and heat.

Brent.


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## MASSDRIVER (Aug 26, 2013)

General question here: We go to great lengths seal up walls, yet go to greater lengths to vent attics and crawlspaces. If wind wash is such a terrible thing, why are we not enveloping our insulation fully to prevent it? If we create sealed cells, could we therefore use a lower r-value of insulation? Why treat a kneewall in an attic differently than the roof?

Brent.


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## jar546 (Aug 26, 2013)

MASSDRIVER said:
			
		

> General question here: We go to great lengths seal up walls, yet go to greater lengths to vent attics and crawlspaces. If wind wash is such a terrible thing, why are we not enveloping our insulation fully to prevent it? If we create sealed cells, could we therefore use a lower r-value of insulation? Why treat a kneewall in an attic differently than the roof?Brent.


Slowly and carefully read the first 48 posts in front of yours and you will see the answer.

I for one am a fan of unvented attics.  My new house has one.  That will eliminate the knee wall issue.


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## MASSDRIVER (Aug 26, 2013)

jar546 said:
			
		

> Slowly and carefully read the first 48 posts in front of yours and you will see the answer.I for one am a fan of unvented attics.  My new house has one.  That will eliminate the knee wall issue.


I will. I didn't.

Brent.


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## jar546 (Aug 26, 2013)

I could be wrong you know.  I've been known to be wrong every now and then.  I have a special fork for eating humble pie


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## RJJ (Aug 26, 2013)

We all have that fork at times.


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## kyhowey (Aug 26, 2013)

mjesse said:
			
		

> Please list the Code section this is from.Thanks


Table 402.4.2  Air Barrier and insulation inspection component criteria

The first component "air barrier and thermal barrier", last sentence in the criteria "Air-permeable insulation is inside of an air barrier."  I slightly misquoted it from memory adding the "must be" instead of "is".


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## Francis Vineyard (Aug 26, 2013)

Rick18071 said:
			
		

> I too enforce the air barrior in knee walls. But I am not sure if house wrap can be used. Can house wrap be exposed in a crawl space?


Not familiar with this type of configuration but this section R302.10.1 would apply (major brands are class A). I've contacted manufactures in the past and though they recognized it as a air barrier they only provide installation for exterior applications.

Note as brent posted #48 it can tear when not attached to a solid wall membrane in accordance to the manufacturer's guidelines.

R302.10.1 Insulation.

Insulation materials, including facings, such as vapor retarders and vapor-permeable membranes installed within floor/ceiling assemblies, roof/ceiling assemblies, wall assemblies, crawl spaces and _attics _shall have a flame spread index not to exceed 25 with an accompanying smoke-developed index not to exceed 450 when tested in accordance with ASTM E 84 or UL 723.

*Exceptions: *1. When such materials are installed in concealed spaces, the flame spread index and smoke-developed index limitations do not apply to the facings, provided that the facing is installed in substantial contact with the unexposed surface of the ceiling, floor or wall finish.2. Cellulose loose-fill insulation, which is not spray applied, complying with the requirements of :Next('./icod_irc_2012_3_par069.htm')'>Section R302.10.3, shall only be required to meet the smoke-developed index of not more than 450.3. Foam plastic insulation shall comply with :Next('./icod_irc_2012_3_par208.htm')'>Section R316.


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## Francis Vineyard (Aug 26, 2013)

Over the weekend I was searching EarthCraft guidelines for a different issue (they don't allow a return pathway in unvented crawl spaces) and noted in their earlier version specifically did not require the attic knee wall to be encapsulated but now the newer versions do!

Keeping up with this stuff is like herding cats being chase by wild dogs.

Francis


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## markw (Aug 28, 2013)

Ive been soooooo waiting for this topic to come up. After much reading, I think the air barrier, when defined as a "system" not just an individual component, can be installed on the interior or the exterior as long as it is approved and installed correctly. The ceiling under a vented attic must have the air barrier also-so if I have a t+g ceiling under this vented attic is it an approved air barrier?? Big neg dude- its gotta be gyp board,or you need to add an approved air barrier to the bottom of those trusses before running your t+g.  Thats what I tell big Jim contractor on a new job and he looks at me like "are you F-in nuts". Right or wrong-Its what you learn after you know everything that matters....  The code only requires the air barrier in the exterior thermal envelope-Right? and it must be in alignment with the insulation-Right? So soffits/knee walls that are exterior facing will require it.Ok clear as mud.


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## bptp32 (Oct 23, 2013)

I totally agree with JAR on this one. I require an air barrier on the "exterior" side of a knee wall that is exposed to a vented attic area.  Table 1102.4.2 in the Residential code mandates "air permeable insulation is inside of an air barrier".  Inside an air barrier means totally encased in an air barrier.  Fiberglass R values can be reduced by almiost 30% if air is allowed to wash across the stud cavity due to the venting of the attic area.  The only exception that I would allow would be if the rafter bays are insulated and venting  in the individual bays is in place (air-permeable insulation not air-imperable insulation).  Great topic.  Thanks  Bob


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