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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Yes, I do, from the third party consultant/auditor/consumer advocate side, not the supplier/installer side.

Anthony 

Can you cite the code section that requires a back on a kneewall?

thanks


Ed Minch

Try the Air Leakage heading  N1102.4.1.6

N1102.4.1 Building thermal envelope. The building thermal envelope shall be durably sealed to limit infiltration. The sealing methods between dissimilar materials shall allow for differential expansion and contraction. The following shall be caulked, gasketed, weatherstripped or otherwise sealed with an air barrier material, suitable film or solid material.

Here in PA the Pa Housing Research Council even publishes a booklet, Alternate Residential Energy Provisions 2009, (our code lags behind thanks to the state Home Builders lobbying effort) which includes diagrams of how to seal to meet the Code.

6. Knee walls.

Anthony

I am not getting that you need to cover the kneewalls according to this provision. You can have an airtight kneewall with no back on it.  I think what they are talking about is "below" the kneewall, between the joists.

By the way, remember Act 222??

Ed Minch

Just adding my 2 cents here regarding knee walls and insulation. First off if there is a lot of air infiltration between conditioned space and the attic knee wall (and the attic behind it) it will affect the R-value of the insulation because of the added air movement. If the home envelope is tight the insulation will retain it's R-value. The problem is attic knee wall areas are such an extra load in the summer. Insulation is like a sponge and once that sponge fills up, the heat is coming through even though the insulation is still functioning at R- whatever it's rated at. That is where a solid, sealed air barrier is a great idea - it can help stop the air infiltration and add R-value, or at the very least add R-value.

As for dense packed fiberglass, yes it can work and possibly there was a misconception that someone was suggesting dense packed BATT fiberglass, which would be kinda silly. The dense pack blown fiberglass can be substituted for cellulose in walls but you have to be careful due to the material properties and I'd be willing to bet the fiberglass is a little pricier.

Ok, so forgive me for being skeptical. Per manufacturers specifications, compressing fiberglass batts lowers the R-value. How is it that compressing blown fiberglass (ie dense packing fiberglass) yields a different result?

I realize there are a few scenario's where cellulose should not be used and putting as much blown fiberglass in as possible could be the best solution. But I find it hard to believe that, as a practice, this is a good method. In my experience, at least in the Midwest Climate Zone 5, 99 time out of 100, dense packing cellulose is the highest performing, most cost effective option.  

Sort of. 3.5" fiberglass batts can be R-11, R-13, or R-15. An R-15 batt weighs a ton in comparison to R-11. You have to get in the 2-2.5 pounds per cubic foot range to get over R-4/inch with fiberglass. If you squash an R-30 batt, you lose R-value, but it's supposed to be 9-10" thick. If you pack that batt in a 3.5" wall, I'd think you'd be close to that R-15. If you squashed it just a little, you might get in the R-20 range. Does that make sense? 

I don't have any particular loyalty to materials, I use what works depending on circumstances.

You can compress fiberglass up to a turning point where further compression starts to reduce the r per inch.  Here is a chart that will give you some of the standard thicknesses installed in different depth cavities.  Compare the r per inch and you will see the total r may go down while the r per inch actually goes up.

http://www.ibsadvisorsllc.com/_library/Fiberglass_Insulation.pdf

Bud

Here is a chart from a presentation by Steve Easley.  I am sure there is a study with more data behind this graphic, which I would love to see, but this gives some insight to your question.

Thanks for that! 

Is it possible that the perforations in the backing of the batt insulation are intended to allow for water vapor diffusion and not serve as an air barrier?  Generally moisture will move from hot to cool, and if it gets trapped in an assembly could condense, depending upon dew point.  The perforations could allow the moisture to escape.

As to the original question: I'm as confounded as everyone else.  It would seem that you'd have to perform a miniature air leakage test on the wall in question in order to measure the exfiltration rate.  As I write, one method that comes to mind is to perform a blower door test just before installing an air barrier, then another test immediately after.  From the delta you can compute the Btus saved.  Keep us posted!  (I will, too).

In the past contractors installed a non perforated material similar to this as a radiant barrier in attics, often spread flat over the insulation in the attic floor. Moisture condensed into liquid water on the bottom side and created problems.  Now I see similar perforated material being sold and installed as an energy saving radiant barrier in the same manner, often spread on the floor of the attic. As noted in an earlier post - to work radiant barriers should have an air gap on the side they are protecting and they do not work well when covered in dust. 

 

Moisture moves from high humidity levels to low humidity levels, heat moves to cold.

 

Mostly theoretical but in a worst case situation, cold attic with warm moist air moving out through the insulation, you could possibly condense some liquid on the inside of the perforated film where the moist air contacts the cold film.  Most of the moisture will ride the warm air through the perforations out into the colder attic with a lower humidity level.

 

In any case, I want an air tight sealed backing on knee walls. 

 

Also, comment on blower door testing -, some retro fit contractors (and some new home builders building very energy efficient homes) do work with a blower door to improve their air sealing results.

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