Just how much does adding air barrier to a knee wall insulation increase it's performance?

I know we have a lot of scientifically minded people here and I bet someone has access to a reliable test data showing the answer to the question that's been on my mind for years:

When we add Vapor-permeable Air Barrier to those knee walls just how much exactly that increases the performance of that insulation?

I've had hard time finding results of actual test showing how effective this is. 

I am not doubting it's effectiveness, I would just like to quantify it. 

Please share if you know it

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Honest Building Science question on this topic: If the drywall of the knee wall is well sealed (meaning outlets, top and bottom plates, etc) as well as the floor joist below, if there are fiberglass batts in the knee wall, is is really that important that whatever covers the fiberglass (or other insulation material like net and blow cellulose) is air tight? 

That's a really long way to ask if one air barrier is good enough. Thoughts?

Covering the kneewall back serves 3 purposes.  1) it prevents the batts from falling off later on 2) it forces the batt to contact the drywall 3) it prevents wind washing.

This is why it is very hard to measure the effect of covering the kneewall -if the batts are very well fastened then low potential for benefit #1,  if the batts are making good contact already, then no benefit #2, and  if the area is back in a corner where there is no wind washing, no benefit #3.  So the effect depends on where on the wall you are looking.

Our viewpoint is that as long as you are putting something up, why not give it some R-value so you provide 1, 2 and 3, and also 4) reduced thermal bridging.  That is why we use 1/2" styrofoam if the customer will pay for it, but always 1/4" styrofoam at least.

Ed Minch

"When we add Vapor-permeable Air Barrier to those knee walls just how much exactly that increases the performance of that insulation?"

The original post by Jack is an excellent question, asking if anyone knows of actual testing to quantify the energy savings resulting from adding a "vapor-permeable Air Barrier" to those knee walls. The only post that came close to answering the question is the graph of 'Convective Wind Washing" posted by Alexander, from a presentation by Steve Easley. All of the other posts were conjecture or statements of common facts. The posts about "The Code" were not applicable because it does not specify that you air seal the exterior of the wall. 

I am interested in the answer to the original question, if anyone has found any actual test results. I am also interested in the effectiveness of insulating the roof, rather than the knee wall and floor, if there has been any scientific testing of that. I am interested in knowing the effect of roof slope on rate of heat loss by radiation. At what point does a roof become a wall? I have encountered this question when dealing with the knee walls of an A-frame house.

I am looking for scientific studies on these questions, to help quantify energy savings and to help guide us in deciding the cost effectiveness of our "prescriptions".

I am not interested in conjectures.

thanks

Brad, my belief is that "it depends" is so much a factor here that a study would be meaningless.  The extremes would be if the batt has fallen at one end, and would be insulation properly contacting the interior drywall and no wind washing at the other.  And you will see these extremes in almost every attic.

I believe this could be there reason there are no studies. 

Wow, guys! I am quite surprised to see how much this topic has stirred. 

That's good, I hope this is the case where out of a group conversation and a common search some clarity comes up. So far it hasn't cleared that much, I agree with Brad that a lot of debate moved away from the original subject. 

My understanding of the scientific basis of my initial question is this: 

The main benefit of an air barrier on the back of the insulation is to prevent the movement of air inside of the insulation. Most of it in my understanding is caused by difference in density in the air inside that insulation. It's pretty much the same convective air movement that would have occurred there if there were no insulation, just in slow-motion. 

So the question is just how much does adding an air barrier helps for this. I did not mean to discuss support for insulation, sealing up the openings under the knee walls and tightness of the wall itself,  that's a given.

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