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Air barrier

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bnymbill

Member
Joined
Jan 30, 2013
Messages
41
Location
Baltimore
If there's a post already out there please link, I searched but came up empty.

In IRC Table N1102.4.1.1, General Requirements, it reads:

"A continuous air barrier shall be installed in the building envelope"

I believe this is usually accomplished with drywall with specifications to seal at joints, corners, etc.

"The exterior thermal envelope contains a continuous air barrier"

I'm having a tough time wrapping my head around this requirement but have read options such as OSB sheathing as long as the joints are taped (Zip system being an example) and WRB (house wrap) as long as the joints are taped.

Could the fiberglass batt insulation itself provide a continuous air barrier if it is kraft faced? Kraft facing is considered a vapor retarder (not the same as an air barrier I believe) and there are so many unsealed joints at each stud, top plates, etc. I don't see how insulation could fulfill the requirement for the "exterior thermal envelope contains a continuous air barrier."

So, I'm assuming the cheapest/easiest way (even if it's not the best way) to meet the minimum requirement is to ensure the WRB is taped at all joints. Is it really that simple or am I missing something?
 
bnymbill said:
So, I'm assuming the cheapest/easiest way (even if it's not the best way) to meet the minimum requirement is to ensure the WRB is taped at all joints. Is it really that simple or am I missing something?
Click to expand...
You’re right to question kraft-faced batt, they’re not air barriers, and they don’t meet the requirement in Table N1102.4.1.1. The kraft facing is a vapor retarder, not an air control layer. Air can pass right through the batts and the unsealed gaps around them.

The code only requires one continuous air barrier, and it can be on the interior, exterior, or somewhere in between, as long as it’s continuous and sealed. In a typical build, that’s often achieved on the interior using the drywall, with sealant or caulk at the top and bottom plates, around electrical boxes, and any penetrations. The insulation does nothing to stop air flow; it’s the sealing at those framing connections that satisfies the code.

As for the WRB (house wrap), that’s a moisture barrier, not an air barrier—unless it’s a tested product that’s detailed correctly. If it’s not sealed at all seams and penetrations, and not listed as an air barrier per ASTM E2178 or equivalent, it doesn’t count. Some WRBs, like taped Zip, can qualify, but not all do.

So no, insulation doesn’t qualify, and neither does unsealed WRB. Code requires a verified air barrier layer. Where you place it is flexible, but its performance is not. Does this make more sense? Also, don't forget about the additional requirement behind tubs and showers on exterior walls.
 
jar546 said:
You’re right to question kraft-faced batt, they’re not air barriers, and they don’t meet the requirement in Table N1102.4.1.1. The kraft facing is a vapor retarder, not an air control layer. Air can pass right through the batts and the unsealed gaps around them.

The code only requires one continuous air barrier, and it can be on the interior, exterior, or somewhere in between, as long as it’s continuous and sealed. In a typical build, that’s often achieved on the interior using the drywall, with sealant or caulk at the top and bottom plates, around electrical boxes, and any penetrations. The insulation does nothing to stop air flow; it’s the sealing at those framing connections that satisfies the code.

As for the WRB (house wrap), that’s a moisture barrier, not an air barrier—unless it’s a tested product that’s detailed correctly. If it’s not sealed at all seams and penetrations, and not listed as an air barrier per ASTM E2178 or equivalent, it doesn’t count. Some WRBs, like taped Zip, can qualify, but not all do.

So no, insulation doesn’t qualify, and neither does unsealed WRB. Code requires a verified air barrier layer. Where you place it is flexible, but its performance is not. Does this make more sense? Also, don't forget about the additional requirement behind tubs and showers on exterior walls.
Click to expand...
Great writeup, I appreciate the details! I bet most drywall installations would fail since sealing all the junction boxes is time consuming/easy for builders to skip.
 
bnymbill said:
Could the fiberglass batt insulation itself provide a continuous air barrier if it is kraft faced? Kraft facing is considered a vapor retarder (not the same as an air barrier I believe) and there are so many unsealed joints at each stud, top plates, etc. I don't see how insulation could fulfill the requirement for the "exterior thermal envelope contains a continuous air barrier."
Click to expand...

No, absolutely not. There would be no way to create an air barrier using paper in that manner. It will 100% fail a blower test.

So, I'm assuming the cheapest/easiest way (even if it's not the best way) to meet the minimum requirement is to ensure the WRB is taped at all joints. Is it really that simple or am I missing something?
Click to expand...

Yes. Generally, your air barrier will be your exterior sheathing. The weather barrier will be applied to the exterior of that - whether a Tyvek type product, fluid applied or layers of asphaltic felt building wrap. I recommend you pick up a standard construction detailing book which talks about common construction methods which will generally be code compliant.
 
bnymbill said:
If there's a post already out there please link, I searched but came up empty.

In IRC Table N1102.4.1.1, General Requirements, it reads:

"A continuous air barrier shall be installed in the building envelope"

I believe this is usually accomplished with drywall with specifications to seal at joints, corners, etc.
Click to expand...
In our codes, the air barrier has to meet tests/standards; the air-barrier requirements generally lend to the contractor relying on the vapour barrier. Drywall won't cut it.
 
So, Im seeing things differently than some.

The Air barrier should be in line with the insulation, which could be on the outside or the inside.

See figure 3 at https://buildingscience.com/documen...ights-newsletters/bsi-120-understanding-walls
Not saying its not found in different spots depending on the details, but generally it should be on the outside.

Filter glass or fiberglass cannot be an air barrier, air moves though it too easily.
2 staggered layers of exterior foam (cont. insulation) could be an air barrier.

I believe ASTM E2178 is the reference standard and it says at 75 pascals, the product cant leak more than .004 CFM per square foot.
 
Yankee Chronicler said:
Tyvek is not a weather barrier.
Click to expand...
Dupont thinks it is, this page has two instances where they describe Tyvek as a “weather barrier”:

Three Common Weather Barrier Myths

Don't let these three common weather barrier myths get in the way of properly protecting your next build.
www.dupont.com www.dupont.com

Honestly, I’ve never confirmed the “official” definition, I thought a “weather barrier” was something that stopped air and water and may or may not affect movement of water vapor.
 
bnymbill said:
If there's a post already out there please link, I searched but came up empty.

In IRC Table N1102.4.1.1, General Requirements, it reads:

"A continuous air barrier shall be installed in the building envelope"

I believe this is usually accomplished with drywall with specifications to seal at joints, corners, etc.

"The exterior thermal envelope contains a continuous air barrier"

I'm having a tough time wrapping my head around this requirement but have read options such as OSB sheathing as long as the joints are taped (Zip system being an example) and WRB (house wrap) as long as the joints are taped.

Could the fiberglass batt insulation itself provide a continuous air barrier if it is kraft faced? Kraft facing is considered a vapor retarder (not the same as an air barrier I believe) and there are so many unsealed joints at each stud, top plates, etc. I don't see how insulation could fulfill the requirement for the "exterior thermal envelope contains a continuous air barrier."

So, I'm assuming the cheapest/easiest way (even if it's not the best way) to meet the minimum requirement is to ensure the WRB is taped at all joints. Is it really that simple or am I missing something?
Click to expand...
The insulation won't work because it's inside the space to be provided with the barrier - not the wrap around it. Still air is part of the insulation value of cavities. The air barrier also creates a still space close to the exterior where wind-driven mists can decelerate or condense and make their way out of the wall system. In some areas determined by the energy code, you are allowed to consider exterior paint, in good condition, on the outside of a concrete block wall, as the membrane/wrap - when you are also applying an exterior finish over top of that existing paint. Otherwise, a wrap or membrane is actually a wrap or a membrane - it goes between the wall structure and the exterior finish, not inside the cavity. Your exterior finish is basically a wind-break and sheds a lot of water - but not all the water and not all the air, either. So the wrap is there to capture and help shed the inevitable moisture that gets in - and also to handle condensation and help create/defend the still air at the surface of the exterior finish and inside the structure. But you can check the energy code itself for more info on the zones and where they apply to - zone 4 is split, for example.
 
steveray said:
A little light reading...

R

Thread 'Tyvek'

Have you ever seen Tyvek used on the inside of exterior walls?
Click to expand...
I have seen people put in a "moisture barrier" on the inside of masonry walls where there is no exterior finish to speak of and also in basements, but in those cases the moisture being collected has to go into flashing and back outside via weeps or it needs a perimeter trench and sump. If someone wants the look of a solid masonry wall without the condensation and wind-blown water, they need to make an internal air space for insulation, moisture capture and drainage to the outside. With the greater requirements for insulation values, it's easier to put in the wall as necessary and then add non-bearing masonry on the inside.
 
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