Insulation Question

[XR250]

Not sure if this is the right forum for this question. I am a PE who usually hangs out in the Structural Engineers Forum.
Anyhow, I'm building a house and the insulation contractor says i need to cover the back side of the insulation of my attic kneewalls with an air barrier (the attic is unconditioned). I believe he is referring this section of the IRC...

My question is that if this is the case, why would you not have to cover the top of the ceiling insulation batts? Seems like there is a hell of a lot more surface area there for air to penetrate.
Why is the wall different than the ceiling? If I have painted sheetrock on the interior surface of the kneewalls, how would air penetrate anyway?
Maybe it is to keep the insulation from pushing out into the space?


Thanks for your help.

 

[MintJulep]

Start here.

https://www.buildingscience.com/documents/reports/rr-0403-air-barriers/view

Then search for other papers on the topic on the same site.

 

[XR250]

Ok, that does not change my opinion that the code is inconsistent on this issue.

 

[XR250]

This article confirms my thought about the sagging insulation....

https://energyresearch.ucf.edu/research/buildings-research/roof-assembly/attic-knee-wall/

 

[EnergyProfessional]

I would recommend thoroughly studying the BuildingScience resources linked above. They are gold. Understand what you try to accomplish, especially regarding water in all 3 states.

You need rain, vapor, air and thermal control layers. You need to account for your climate. We have no idea if you are in Florida, or in Alaska and if you are looking at the applicable code for your area. A method that is usable in Brazil can be useless in Norway.

 

[XR250]

You need rain, vapor, air and thermal control layers. You need to account for your climate. We have no idea if you are in Florida, or in Alaska and if you are looking at the applicable code for your area. A method that is usable in Brazil can be useless in Norway.

I'm in Central NC. No rain as it is in an attic. Looks like the reason ceiling joists do not need covering is that there is simply more insulation.
If I have painted sheetrock on the interior, is moisture transfer an issue? I'll probably end up using 2" of rigid foam for the backside. More curiosity than anything.


Thanks for your help.

 

[EnergyProfessional]

The roof is the rain control layer. the roof, the attic system and the ceiling are one system to be considered together. Is the attic ventilated?

What foam? Open cell or closed cell? Big difference for perm rating and air barrier properties.

You want to prevent air movement. you want to prevent vapor going from inside out, or from inside in. But some will transfer, and you want that to be able to dry out without damaging the structure. Also some rainwater will make it in for one reason or another, that needs to be able to dry out.

Look at the Building Science website. Your type of construction is common and I'm sure they have multiple articles just for that and discuss what the perfect solution would be. He also often writes articles in the ASHRAE magazin, if you have access to that.