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

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

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

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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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.
 
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.
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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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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??
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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.
 
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.
 
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:

bsi064_figure_10_web.jpg


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
 
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.
 
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.
 
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.
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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).
 
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.
 
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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mjesse said:
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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And you'll start seeing ALOT of this stuff... (we already are).
 
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.
 
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.
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That stuff turns to shredded crap in 5 years. need something that will hold up to ozone and heat.

Brent.
 
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