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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Hi Jack,

Just a quick comment.  Adding just a vapor barrier, ie house wrap, doesn't provide as much improvement as installing a rigid barrier.  A flexible covering will billow in and out with pressure changes which correspondingly pumps air in and out of the insulation on the house side. 

Quantifying the resulting insulation value is difficult and would require specifics as to the wall thickness and the insulation material.  I have taken temperature readings on 2x4 knee walls with fiberglass insulation with no covering and a well vented attic space and the results were poor.  Define the worst case you think "poor" with no covering might be and then the best case with a rigid covering and your number will be somewhere inbetween.  My guess would be somewhere between R-4 and R-8. 

Bud

You cannot quantify this because it depends on how leaky the assembly would have been without the barrier. That would vary with construction quality. However there should be no reason to compare because the energy codes require the air barrier. A comparison would be between the minimum legal requirement and something that does not meet code.

Further: The training I have received indicates that fiberglass insulation cannot perform as rated unless the barrier is there. So, without the barrier the insulation would not be performing up to its rating. How badly, would depend on how leaky the construction is.

I used to work for Guardian Building Products, they sell a perforated reflective insulation called Solar Guard. Here is a copy of a tech bulletin they did. The houses aren't truly apples to apples, but R-13 with Solar Guard over it performed much better. It's on page 2. I scanned it page by page back in the day, sorry for the annoying 4 pager.

Page 1 https://docs.google.com/file/d/0B2hVKdnHQRj_NnlYMkxUWVJydU0/edit?us...

Page 2 https://docs.google.com/file/d/0B2hVKdnHQRj_OFk0bC1KZlpWaFE/edit?us...

Page 3 https://docs.google.com/file/d/0B2hVKdnHQRj_UjdfZWhHTUo2aE0/edit?us...

Page 4 https://docs.google.com/file/d/0B2hVKdnHQRj_bnVmazZCbmo0ejQ/edit?us...

I've used this for a while on Cape Cod homes and have had really good feedback on it - if not measurements. I did it in my own Cape early into this business, along with dense packing fiberglass in the floor joists and I had to turn my thermostat down 6 degrees to have the second floor be the same temperature it was before the upgrades. My knee wall method is here:

http://www.energysmartohio.com/blog/how-to-insulate-and-ventilate-k...

As has been said - very hard to quantify because the starting point varies so much.  Bud - house wrap is an anti-vapor barrier but I get what yo are saying and we use rigid foam to combat this,  

An unquantifiable component is the  benefit that the insulation will make contact with the drywall once there is a "sheathing" on  the wall, and in the future, no batts will fall off the wall.  Using 1/2" foam board gives about an R-3 increase to the wall and reduces thermal bridging markedly, but how do you make a comparison on the varying amounts of air movement in the attic and the varying quality of installation of the batts and their current condition?

I have no quantitative data, but many scans done before and after showing improvement.

"dense packing fiberglass"?  You are kidding right?

James, you could use a better tone there...

Like I said, that was early on. I used fiberglass because the cavities were chock full of knob and tube that I wasn't going to change. I didn't know to block the joists yet. We are in a practice - every year we need to do better, and not worry too much about last year.

Knauf has done some testing on dense packed fiberglass, though, and the air leakage reductions are on par with cellulose. Don't knock it 'til you try it. It's harder to do well, as a recent article in Home Energy pointed out, but it can be done. 

What's so humorous about dense-packed fiberglass? 

perforated reflective insulation? Seems like an odd combination of words in a post about improving energy efficiency.

The purpose of it is more to seal the fiberglass enough to reduce convective losses. It's perforated to prevent a double vapor barrier. It's much easier to handle, drag into, and install in small knee walls than foam board. It is an air barrier according to ASTM (whatever) standards. To me, it was an elegant solution, but there's lots of ways to do it, what would you suggest that's better?

Code calls for a sealed air barrier on the back of knee walls.   "perforated" implies having holes to allow air passage.  " It is an air barrier according to ASTM (whatever) standards"??  What standard?  If it is perforated, how is it an air barrier?  How does this satisfy Code? .

"Convective" usually is used in relation to convective loops in enclosed spaces such as non insulated wall cavities with the sun shinning on one side causing warmed air to rise on the heated side and fall on the shaded side creating the convective loop.  The faster the air travels, the more heat it can transfer.  Installing fiber insulation slows the air movement helping reduce heat transfer across the closed space.  Spray foamed cavities work even better - no air movement.  In this case having perforations would allow any heated rising air to exit the wall through the perforations sort of like the stack effect in a house - out flow at the top, inflow at the bottom. Yes, I know any moving air in the attic will move through the exposed batt insulation to contact the sheetrock and transfer heat into the room on the other side, but that is the reason for an air tight barrier on the attic side.

Side note:  I am surprised no one has mentioned the requirement to seal the floor joist cavities beneath the knee wall.

"Reflective" has nothing to do with insulation, it has to do with radiant barriers. To operate properly radiant barriers need to have a open air space toward the area they are protecting which is not present in this application you describe. When ever I see "reflective insulation"  I cannot get pass the "NASA uses it" advertising and the rebuttal "Oh, shinny stuff".  Radiant barriers have a use in cooling dominated climates but this is not it.

I recommend any solid material you can easily get into the attic.  Some of the contractors in my area use TrermoPly or the fan folded, aluminized cardboard material HVAC contractors use. Either one gets the seams sealed with weather proof acrylic glue type tapes, canned foam or caulk.  Foam only if there is no exposure to sunlight.

You caught me being lazy, I don't have time to go look up the ASTM air barrier test it passed at the moment. The material can be left exposed, it's inexpensive, it provides an air barrier, it's easy to install - I like it as a solution.

I don't want to devolve into a radiant barrier argument - it at the very least restores the R-value of the fiberglass in the knee wall.

And you're right, we didn't mention blocking below the joists. It's critical. I wrote about it last year, I'm open to if you think I missed anything.

http://www.energysmartohio.com/blog/how-to-insulate-and-ventilate-k...

In the meantime, enough throwing stones at me, what do you do? I actually do this for a living, do you? =)

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