Air barrier

[bnymbill]

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?

 

[jar546]

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?

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.

 

[bnymbill]

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.

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.

 

[ICE]

The people that birthed the concept had the Mason jar as the holy grail.