# Insulating knee walls



## joecaption (Nov 30, 2011)

Got some pictures.
In most cases you add onto the rafters to give you room for the proper amount of insulation and the baffles without compressing the insulation.
If there 2 X 4 knee walls you can add 2 X 2's and use R19 faced batts with the paper toward the room.
Just staple it in place.
Go over the back side of the knee wall with Tyvek to stop air washing.
On the room side of the knee wall go over the wall with 1/2" blue foam board then the sheetrock.


----------



## Ocelaris (Oct 9, 2012)

The only question I didn't hear about is if you have a roof to vent as well. It's better to make the area as close to conditioned as possible. For me, my knee wall was like a triangle space at the end of the room, so I made sure that the roof had ventilation by putting prefab baffles inside the roof rafters, then put 2" of polyiso under that. For the knee wall it's self I did what you suggested, batts in the 2x4 stud walls and then 2" polyiso on top of that. Now I'm not 100% sure if I should have drywalled, as the foam can release toxic fumes in a fire, but it would have been a real pita to drywall as it's only 4' tall at it's highest and drops down to a foot at the roof. 

Anyways, the plan sounds good, and for the ceiling, I am going to do blown in cellulose because of the such high reccomendations from everyone here; and like you the rest of the attic doesn't have much insulation so I'm going to do it at the same time. Insulation is great, but I've found that air sealing does 90% of the job. From my reading blowing in insulation is easy comparatively, you just have to get the tube up into the attic somehow and have a helper on a 2-way radio to keep filling it up. i.e. people say the whole process takes only a few hours, but you want to get all your wiring and sealing done before that happens. The hardest part is transporting the bales of cellulose and renting the tool (free with rental usually).


----------



## Ocelaris (Oct 9, 2012)

Also a pic of the attic with cat walk so I can navigate the attic after I blow in. You can see a picture of the baffles I used in the knee wall in the first pic here in the top right corner. I didn't have a picture with them installed in the knee wall, but you get the idea, making sure you get ventilation from the soffits up through the roof so you're not over heating the shingles which can reduce lifespan.


----------



## kristok (Apr 2, 2014)

Well, sadly I don't have soffit vents (and no real easy way to add them), but I have a dormer vent in the top attic, and some ohagen vents on the lower sections.

It looks like you used some expanding foam between the foam panels on the vertical wall section - what does that do for you? If you have faced batts in the wall with the facing towards the room and then you use the foam panels sealed up with more expanding foam, doesn't that create a double vapor barrier?

What did you do in the slanted sections?

Yes, before I do any of the ceiling areas I need to air seal the rest of the house. Possibly more painful than that will be replacing some of the damaged knob-and-tube wiring.


----------



## Ocelaris (Oct 9, 2012)

I used foam to seal between the vertical panels or where I had holes for low voltage wiring etc... But now I discovered they have tyvek tape, which is just like regular packaging tape but a bit stronger and much stickier. If I had to do it again I would seal all the joints with tyvek tape instead of foam, as the foam is difficult to get into small cracks even with a foam gun. 

I don't have a picture of it, but inside the slant ceiling there are foam baffles which allow air from the soffit up to the top of the attic where there is a ridge vent. Under the baffles I put R13 insulation and it might have been faced up towards the shingles, I probably should have placed them facing down, but I can't recall if it was faced or not... either way good point. 

This knee wall opens up to the garage and to the attic, so just getting it sealed off has made a huge difference, originally behind the knee wall was a radiator which opened up to the attic letting cold air right into the room. 

eww, knob and tube, yeah, you have to be very careful about doing any insulation with knob and tube. I took the opportunity with the attic open to rewire the entire house with NM #12. I would make sure you get any wiring done before any insulation, because you won't be able to do it after (easily anyways).


----------



## MikeMKE (Mar 13, 2014)

Faced batts on the knee wall sounds best...you should be able to staple the paper that sticks out of the sides of the batts to the inner sides of the studs. Then 2" rigid foam over the entire knee wall.
Faced batts with baffles on the sloped section would be best too...I would think blown-in would settle toward the low end over time.

I have a very similar situation as you, except I have existing batt insulation on all sides...however it's old and thin and looks like it needs replacing. You're right, getting batt insulation into the sloped areas seems almost impossible especially with nails coming down through the roof deck. I may be tearing down the walls at some point to completely re-do the insulation properly. For now I will probably just be adding 2" rigid foam to the knee wall for some added R-value.

Good luck!


----------



## Gary in WA (Mar 11, 2009)

Your CA location doesn't require faced batts (vapor retarder- not barrier), if you use them, *don't side staple *for reduction in R-value loss; pp.45-47; http://www.buildingscienceconsultin...010-03-10_When_R-Value_Doesn t_Measure_Up.pdf

Add the foamboard on the *room side* of ceiling/walls with your cooling climate and use unfaced, the paint does about the same; Fig.41, pp.83; http://books.google.com/books?id=Eq...esnum=1&ved=0CBYQ6AEwADhQ#v=onepage&q&f=false

http://www.buildingscience.com/documents/insights/bsi-073-macbeth-does-vapor-barriers

If you want to install the baffles full-length- go for it, though not really needed- mainly at the soffit openings. Interesting insulation choices for your attic roof/ceiling area; http://www.buildingscience.com/documents/reports/rr-1110-hygrothermal-analysis-california-attics

The tape on fb will fail over time, ADA the drywall, either way; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Gary
PS. Welcome to the forums!


----------



## kristok (Apr 2, 2014)

Thanks all for your input.

If I don't side staple the batts to the knee walls, how do I get them to stay in place? I was going to use the foam on the attic side to hold the batts in place. If there is concern of a double vapor barrier could I put little pinpricks through the foam? The only "vapor barrier" that would exist now is the latex paint on the walls.

We got the baffles in place today. I only installed them in the sloped sections, with enough clearance at the top/bottom that any extra insulation won't cover them up. Got the batts in a few of the sloped sections too. What a pain. Had to stuff them in from the top as far as I could reach, then go to the bottom section and again reach up as far as I could. If they were 1/2 a foot longer I'd need longer arms :laughing:. Good thing our roof is steeply pitched so I don't have to cram myself into a really tight corner.


----------



## Gary in WA (Mar 11, 2009)

If you read the second link I left above, you would see the foam needs to be on the room side of the studs/cavity insulation because your AC will cool the drywall. The FB buffers/insulates the temp/moisture of drywall/cavity insulation to prevent condensation there. Read the link...
Paint is a Class 3 vapor retarder, no worries. 
Be sure to *protect the ends* (air barrier- housewrap) of the fibrous insulation from wind-washing over the knee wall.

The batts are usually 15" wide and stay upright by themselves in a cavity IF the on-center stud spacing is 16". Add some 3-1/2" wide strips of foamboard on the stud face (one-side only-on side face) to reduce the cavity width, if necessary OR it is possible to staple the paper on the stud edges of the attic side as asphalt-cover paper is variable perms- it could still dry to the attic side- especially in your location; http://www.greenbuildingadvisor.com...facing-batt-insulation-face-inside-or-outside

Gary


----------



## kristok (Apr 2, 2014)

Yeah, I wish the studs were 16" on center. They are all over the place, with the spacing ranging from 28" to 8" or so. Most of the studs are set to one side or another of a roof rafter (which are 2' on center), and there are a few random ones in between. Nor are they 2x4's (not even true 2x4). They're more like 2.5"x3". House was built in 1926, and it seems that this room has been there from the start.

I put up some of the batts on the knee wall today, and I put the facing towards the room. For the stud bays that weren't a perfect fit for the 15" batt (which was pretty much all of them), I carefully cut another piece lengthwise to fill the cavity. At the top of the stud bay, I stapled the facing to the back of the drywall (which has some sort of backing board behind it) to hold them in place. Once I had both pieces in it was a great fit. As you recommended, I didn't use the tabs on the facing. I didn't staple all the way thorough the insulation - I carefully peeled the fiberglass off the facing in the corner, and tucked it back into place after I stapled it.

The chapter you referred to in the Moisture Control handbook for "cooling climates" seems more geared towards humid climates - they say this at the beginning of Chapter 6. The map in figure 6-1 shows "cooling climates" as being pretty much the entire south/southeast. But they also say it "will perform satisfactorily in warm, dry climates". 

Thanks again for everyone's input. I realize I'm probably not doing this in the optimal manner, but I'm learning alot about how to do it "more right" the next time I do this. I think my (mostly) hot and dry climate is more forgiving than most.


----------



## Gary in WA (Mar 11, 2009)

Great, sounds like you are on the way! The insulation at the box stores is not the same as pros use.... they stock what sells. Worked your way around it, don't you feel warmer already? Dry, as in TX; http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies

Glad we could help.

Gary


----------

