# Insulating knee wall



## msaeger (Mar 1, 2011)

Still deciding on what to put in my 1.5 story knee wall. Searching google found a thread on diychatroom talking about Prodex

I want to put something on the back of the studs then some R15 fiberglass in the stud bays. 

I am thinking I need about 600 square feet of something to put on the back of the studs. 700 square feet of Prodex is 220.00 and it claims to be R15. Polystyrene 2" thick is R10 and would cost about 360.00. I would think the Prodex would be easier to put up too. 

Is this stuff really R15 and would it work for what I need to do?


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## bubbler (Oct 18, 2010)

msaeger said:


> Still deciding on what to put in my 1.5 story knee wall.


I'm in the same position. My walls are just 1960s wood paneling, maybe 1/4" thick and 2x3 framing. 

My thought was put up R13 with the paper facing the inside space, then I was considering 1" thick foam board nailed up over the framing and seams taped. I know it would compress the R13, but I figured it's only an inch, so better than nothing. I was also thinking this could act as an air block keep air out of the stud bays and would let me insulate over the studs as well.

The alternative I'm considering is building out the 2x3 studs with 3" of hard foam, then fill with R21 2x6 batts and cover over with tyvek building wrap with taped seams.

Don't forget to install air blocks under your knee wall if they aren't already present.


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## Windows on Wash (Aug 30, 2011)

msaeger said:


> Still deciding on what to put in my 1.5 story knee wall. Searching google found a thread on diychatroom talking about Prodex
> 
> I want to put something on the back of the studs then some R15 fiberglass in the stud bays.
> 
> ...


It is not an R15.

Fiberglass with a radiant barrier exterior will do fine. Tape/seal the seams and foam the joist cavities as mentioned.


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## Gary in WA (Mar 11, 2009)

The radiant barrier has an R-1+. 5mm. = .1969 inches x R-6 (closed cell) = R-1.0014 Only 3/16" thick....

The 2” foam board at R-10. The radiant would have little effect on your total cooling bill, even negating your heating bill from solar warmth in the winter helping to heat the upstairs (a good thing). http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem

Fiberglass batt and a non-perforated radiant would act as a vapor barrier (if on the wrong side- attic) condensing any moisture trying to leave through the attic (diffusion). You would have wet fiberglass insulation reduced by 50-75% or R-6.5 or R-3+. This also covers multiple layers of radiant: http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/90-controlling-heat.html
I’d rather use f.g. and a housewrap or rock wool, cellulose, or any other insulation; http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/
The 1” compression would leave you with R-10 minus any whole wall deductions for the studs, etc. 


Gary


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## msaeger (Mar 1, 2011)

R13 is 3.5 inches thick and the wall is 2x4 so I have 3.5 inches of wall depth why would I be compressing it an inch?

Why the heck does no place sell and 3.5 inch thick insulation R11, R13, or R15 23 inches wide unfaced? The only 23 inch I can find at the home improvement stores is kraft faced.


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## Windows on Wash (Aug 30, 2011)

+1

I should have been more detailed in my explanation of material choices. You do not want to put an impermeable vapor barrier on the wrong side of the wall depending on the climate zone. The radiant barrier we use is pre-punched. 

Radiant/housewrap is much more user friendly because of the fact that you are normally passing these materials through smaller access openings.

Most of the kneewalls that we are repairing are with shared garage attics at the end of duct runs. The dominant comfort issue is cooling and not heating. Using a radiant barrier in lieu of housewrap helps interrupt the radiant heat gain from the roof deck and mitigate the summer temperature extremes.

It is absolutely correct that the radiant barrier will also slow down passive solar gain, however, most of these locations are in upper floors and are benefiting from the natural rise of warm air as compared to the inability to get cool air to the end of the duct run.

Rigid foam is a nice high R-value, lower cost, alternative to spray foam, however it is difficult to maneuver a bunch of rigid foam boards in a kneewall area or even get them through the access hole.

Rigid foam is very often recommended on this site and I think it is a great option. What I do not see discussed is anything pertaining to the thermal or ignition barrier requirements. I think it is worth noting that most customers will need to at least check that they are compliant with code. There is quite a bit or variability in the posters' interpretation of what would legally be defined as living space vs. inaccessible.

I also think it is worth noting that there are several environmental impacts associated with foams (polyiso being the least toxic of the group).


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## bubbler (Oct 18, 2010)

msaeger said:


> R13 is 3.5 inches thick and the wall is 2x4 so I have 3.5 inches of wall depth why would I be compressing it an inch?


Sorry--I think that was in response to what I had said, I didn't mean to thread jack...



msaeger said:


> Why the heck does no place sell and 3.5 inch thick insulation R11, R13, or R15 23 inches wide unfaced? The only 23 inch I can find at the home improvement stores is kraft faced.


Neither Lowes or Home Depot sell unfaced R13 or R15 around me either...


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## msaeger (Mar 1, 2011)

bubbler said:


> Sorry--I think that was in response to what I had said, I didn't mean to thread jack...
> 
> 
> 
> Neither Lowes or Home Depot sell unfaced R13 or R15 around me either...


Sorry I missed in your post that you have 2x3 walls that makes sense now


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## msaeger (Mar 1, 2011)

What would be wrong with attaching fiberglass to the back of the knee wall studs like in this PDF LINK

I could seal below the knee wall with some rigid and put fiberglass on the rest.


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## Gary in WA (Mar 11, 2009)

Ventilate your attic, as per Code. Install fiberglass batt insulation exposed directly to the attic air (outside air temperature yet wind-blocked somewhat). F.g. is very air permeable (air travels right through it- a bad thing) though it is a good filter, just look for dirty f.g. to find your air leaks through the framing/ceiling below; http://www.finehomebuilding.com/PDF/Free/021105092.pdf

With a wind washing the f.g. in the attic expect to lose up to 40% of the R-value; http://www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf

Cellulose is much better at stopping air movement (attics and walls), it is also air-permeable, just not on the same scale as f.g.: http://findarticles.com/p/articles/mi_m0KWZ/is_3_6/ai_n8582994/
Hence the reason f.g. is cheaper than cellulose (in more ways than price alone).
The R-value for f.g. is rated with all six sides protected from air in the lab……..

Gary


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