Vertical second layer of insulation

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  #1  
Old 01-02-15, 07:03 PM
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Vertical second layer of insulation

Been on an efficiency kick ever since the local coop offered a 500 rebate and I am now addressing the knee walls in my 1970 cape cod in SW MO. I cut an access hole today thinking (based on the auditors notes) that there wouldn't be any insulation on the walls themselves but found r11 batts. I want to up it to at least r19 and then lay some r30 over the existing r19 that's on the 'floor' of the kneewall. The knee wall is about 4.5 feet at the peak and about 6.5 feet deep.
The r11 fits easily in the cavity with the studs accessible. So rather than rip out the r11 I was thinking of adding r19 to it. It wont compress the r11 and gets me to r30 without wasting the r11 or paying double for the r30. I know I can't have two kraft facings like that but I also don't know of anyway to hold unfaced to exposed fiberglass. So my plan is to cut/pull the vapor barrier off but leave a two inch section for stapling every 18inches. I can't imagine this causing any moisture issues but wanted to see if anyone had any thoughts.
 
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  #2  
Old 01-02-15, 07:29 PM
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Old advice was to slice the vapor barrier to defeat its function, but the slices aren't really enough. Just install it from the kneewall side and then cut out diamonds or circles however decoratively you want to be, leaving the edges in place to hold the insulation. Leaving 80% in place will serve as a partial air barrier. They sometimes cover the cold side with tyvek as the air barrier.

As for the final r-value, it isn't always the total of the two. It is the r-value per inch times the final depth. If those are 2x6's then 5.5 x 3.5/inch is a rough guess or r-20 total. Air seal around any electrical boxes or penetrations and create a solid air block directly below the kneewall to prevent air from circulating through that fiberglass to the cavity below the floor. Below is an article on kneewalls.

Also, be sure those soffits are clear of insulation for proper air flow and install a wind block near the soffit to protect the end of the insulation from "wind washing".

Bud
http://www.finehomebuilding.com/pdf/021230088.pdf
 
  #3  
Old 01-02-15, 08:45 PM
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Thanks Bud.
1. This house for better or likely worse doesn't have soffits or any baffles. The insulation in the roof that goes between the knee wall and attic is flush against the roof sheathing. Since I've known this for some time I did a thorough inspection of the roof sheathing after opening the knee wall. It all looks/smells good and no moisture present on the nails. So I would say it relies on the fiberglass air permeability to draft but without soffits I guess that's not the case. Knowing this, is it still good to use rigid foam and block the ceiling rafter above the kneewall? The PDF says yes but I like opinions

2 The walls are 2x4. If I understand the document correctly it says to install a layer that would be the 'exterior' wall. So rather than installing more fiberglass, should I leave the r11 and add 2 inches of Dow blue foam since it would compress anything more (besides r13)? Wouldn't this create two moisture barriers?
 
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Old 01-02-15, 08:50 PM
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3. I briefly looked earlier and it looks like there's a wooden air block about two feet under the upstairs floor in from the kneewall. Since I plan to work myself out of the space would I be better served to cut a slit in the floor insulation right at the kneewall , insert a foam block, and spray foam it? Essentially creating an earlier block. Or is that a waste of time/ money?
 
  #5  
Old 01-03-15, 03:08 AM
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I believe the block you are referring to is only blocking the face of that fiberglass. It doesn't go all the way to the roof sheathing so it maintains the air path above the insulation.

However, having fiberglass insulation directly against the bottom of the roof deck is not good and although you inspected part of it any air flow that comes from the kneewall side (house air) will deposit its moisture in there somewhere. Now, this may not be apparent now, but add more humidity to the house or some future owner decides to cut in a bookcase and it becomes a disaster, thus codes in most areas do not allow.

You can work around the lack of soffits by installing low gable vents or edge vents under the bottom shingle, but then you should also establish that air path above the insulation all the way to an upper attic and or upper venting.

As an unfortunate note, I run into many homes that were built wrong and seem to be doing just fine. But I always have to be careful that any work I do doesn't upset whatever is working. In your case, that opening you created can result in lots of warm moist air depositing lots of moisture. Keep it sealed now during construction and well air sealed when finally done. Not being there and not seeing how deep your pockets are it is difficult for me to advise anything other than what new methods would follow. BTW, if you ever replace the shingles it is possible to extend those rafters to provide soffit venting.

That block farther in (if I understand) is called solid blocking to stabilize the rafters, but was not air sealed. Air sealing directly below the kneewall would be preferred.

A question I would ask if I were there to do some work would be, how long do you plan to live there. Not that anyone should do lower quality work, but that you should bump it up if this is your last home.

Bud
 
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Old 01-05-15, 08:24 AM
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Great info. I spent yesterday creating an insulated air tight door. Ive got a roof leak on the back side if the house so I am going to have the roofers take a look at the knee walls when they come. In the meantime I would like to do some insulating since the room below the knee wall is where the pellet stove is. At this point I suspect small gable vents will be the option but regardless of the roof outcome, is it safe to say that adding 1in foam blocking in the floor directly beneath the kneewall (spray foaming the edges) and then unfaced r30 on the floor over the existing r19 will be a good move that shouldnt have any adverse effects? I am going to redo the upstairs bedroom in the near future and can address the r11 walls at that time. I am also thinking of removing the drywall on the slant, adding baffles and then maybe spray foam at that time.
 
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Old 01-05-15, 01:59 PM
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Also, pending anything unforeseen (such as the unvented roof disintegrating around us ), we will be in this house forever. So while I am on a budget I either want to do it right or prioritize it so I can do it right later.
I don't want to get off topic from the knee wall but the crawlspace was a wet mess when we moved in. I installed a couple sumps for bandaids/emergency and have a french drain install scheduled for the spring. After that, I'll roll out the 10mil vapor barrier. I am hoping that will alleviate some humidity and condensation on the windows.
 
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Old 01-05-15, 02:16 PM
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Photo 5 would be an end result you would love.
BSI-009: New Light In Crawlspaces — Building Science Information

Bud
 
  #9  
Old 01-05-15, 02:40 PM
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That's actually what I am hoping to achieve albeit a diy route. Read that website many times also . Finishing it is still waiting on the french drain but I've already sealed (besides vents) the rim joist with 2" of foam and I've got 2" of foam pressure fitted against the foundation walls. Once the drain and vapor barrier are in place I'll seal up the vents and attach/seal the foundatuon wall foam. I got a spray foam quote but they recommended doing the floor only. I have ductwork, furnace, and plumbing down there which is why I didn't go that route.

Thoughts on any reasons not to do the foam plugs and r30 on the floor of the knee wall? OK to do that regardless of the current lack of venting?
 
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Old 01-05-15, 02:54 PM
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"OK to do that regardless of the current lack of venting?"
As long as you don't introduce more air/moisture nothing should change. Although adding that near the top of your list is important to ultimately get the venting and insulation working together.

I've always left my basements as open basements that allowed full access to the floor above and it makes fixing things a lot easier. Conditioning your space as you have described will be nice.

Bud
 
  #11  
Old 01-05-15, 03:17 PM
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Thanks a lot! I really do appreciate all of your help. And the venting definitely wont be forgotten. Stuff like that gnaws at me.

Since you've been so helpful, I've got one final unrelated question that I am hoping is a quick one. I was using my digital thermometer just now and noticed there's about 2 feet of wall upstairs in the hallway that's 75 vs 68 elsewhere. The chimney is a few inches behind that wall and the pelletstove has been running so it makes sense. Would it be OK to place a register low on that wall to draw some of the heat into the upstairs? Or against code/fire rules?
 

Last edited by kramttocs; 01-05-15 at 05:45 PM.
  #12  
Old 01-16-15, 01:10 PM
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Hey Bud.
Started thinking about this project again and ran across a url you provided someone a few years back:

http://www.efficiencyvermont.com/ste...ide_062507.pdf

On page 35 it shows the kneewall consisting of Room > Kraft (I assume) + Fiberglass > Rigid foam board.

This brings back a previous question I had --


-------
2 The walls are 2x4. If I understand the document correctly it says to install a layer that would be the 'exterior' wall. So rather than installing more fiberglass, should I leave the r11 and add 2 inches of Dow blue foam since it would compress anything more (besides r13)? Wouldn't this create two moisture barriers?
------

The two moisture barriers is still my main question.
If that isn't an issue, then my followup is how does this sound:

Room > Existing Kraft r-11 > Kraft (with 'diamonds' cut out to negate it) R-13 > Add stand off 2x4s on every other wall stud to prevent compression > sealed layer of 1inch dow foam. So my wall would essentially be 2x8

Theoretically this would get me to 11+13+5=29 and the benefit of the foam air sealing it. Again, since I plan on being there for the indefinite future I don't mind spending more money to do it right.
 
  #13  
Old 01-16-15, 02:59 PM
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When rigid foam is used on the outside of a wall assembly in cold country care must be taken to ensure that the house side of the rigid never drops below the dew point. To accomplish this there is a ratio of fiberglass insulation on the inside and rigid on the outside. Here's a link:
Calculating the Minimum Thickness of Rigid Foam Sheathing | GreenBuildingAdvisor.com

Zone 5 for example would require r-7.5 on the exterior of a 2x6 wall. Your 2x8 wall (I'm extrapolating) would need about R-10. Check the climate map at the bottom of the article to see what zone you are in.

Bud
 
  #14  
Old 01-16-15, 03:23 PM
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Hmm that brings up more questions
I am in zone 4 so sounds like 1inch would be good but -

"Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior. That means you dont want any materials with a very low permeance on the interior of a foam-sheathed wall or between the studs. If you are building this type of wall, you should not include interior polyethylene or vinyl wallpaper, nor should you install any closed-cell spray foam between the studs."

Since the existing r11 has the kraft facing it sounds like I shouldn't do the foam as then the moisture could go exterior or interior. Correct?
In which case how about r11 +r19+tyvek stretched over the standoffs?
 
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Old 01-16-15, 03:31 PM
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<In which case how about r11 +r19+tyvek stretched over the standoffs?>

The Tyvek or house wrap is a common solution, just air seal as beast you can.

For the 6" layer have you looked at Roxul? It is very dense, details with a long bread knife, and very vapor open.

Bud
 
  #16  
Old 01-17-15, 10:39 AM
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I did but for this vertical application I am finding that the faced (cut up) R19 would be best. I wasn't familiar with roxul until now but I've got a couple of places I need to add some horizontal insulation that it would work for.
 
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