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Attic knee wall insulation & floor joist insulation

Attic knee wall insulation & floor joist insulation


  #1  
Old 11-03-15, 07:42 PM
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Question Attic knee wall insulation & floor joist insulation

Hello,

I have a cape cod style house in Richmond Va. with essentially 1/2 sqft. on the 2nd floor than that of the first floor. The north side 8ft. knee wall/ attic space is accessible but the south is not. I am getting a new heat pump & air handler installed soon. While investigating the space I found that the insulation against the walls of the master bedroom and my daughters room to be less than needed to be efficient.

The walls adjacent to these rooms have R13 to R19(at most) faced batts installed. My thought is to add 1/4 in. perforated foam board (fanfold foamular) directly over the studs to primarily provide an air barrier and then install perforated attic foil over this as a radiant barrier. I am concerned about creating a double vapor barrier if the product doesn't have some breathing.

I have been going back and forth on whether the cost is worth the reward for this or just using a radiant barrier from attic foil.

Is this a good plan?

I found this cost of this to be the same as buying foil faced polyiso but haven't found polyiso to be perforated.

The summers are very hot upstairs and cold in the winter no matter what, hopefully the new Heat pump will help as well. We currently have central AC but use electric baseboards in the winter.

The 2nd issue:

I search this forum and others often and found that insulating and having an air barrier in the knee wall floor joists to be very important. I discovered that mine do not have an air barrier but does have some insulation.

So since I only have access to the north side knee wall would it be beneficial to install an air barrier in knee wall floor joists? Would only doing one side be beneficial?

I have attached a picture showing the wall described.

I appreciate your feedback and any other suggestions or advice you can provide.Name:  knee wall.jpg
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Size:  33.8 KB
 
  #2  
Old 11-04-15, 10:39 AM
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Hi Daniel and welcome to the forum.
Super busy right now trying to button things up before it gets really cold, but wanted to touch base with you as I live in a cape and appreciate the challenges from first hand experience. I'll add a link below that discusses some of the cape issues.

IMO, you will want to access that other side to determine what is there for insulation. you can see what they installed where everyone can see it so just imagine what they did where it is hidden.

Blocking the air flow directly below the kneewall on just one side would certainly be beneficial. Obviously both sides would be better.

The side attic pictured has a plywood floor, but we don't know what the other side attic has and how that space communicates with the ventilation.

An inch or two of rigid insulation covered with drywall on the back side of that kneewall (shoulder wall) would work great. No need for a true vapor barrier, but air sealing is very important. Air leaks transport far more moisture than what passes through a wall assembly.

I'll be back to night, if I can stand.

Bud

Two Ways to Insulate Attic Kneewalls - Fine Homebuilding Article
 
  #3  
Old 11-04-15, 12:28 PM
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Thank you for the response.

Is your suggestion to install the rigid foam over then studs then cover the foam with drywall?

What about a radiant barrier attached to the foam facing the rafters?

I don't currently have any access to the other side of the house. So there would have to be some demolition. I am relieved that blocking the air on one side of the floor joist would be beneficial rather than cause additional problems.
 
  #4  
Old 11-04-15, 12:59 PM
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When you try to put the value of a radiant barrier in terms of r-value as we use for insulation, it isn't a lot. The problem is, once the heat is inside that attic space it is difficult to get it out. The best protection from a hot attic is more insulation.

Here's another problem. There is only one foil faced polyiso insulation that is rated to be left exposed, Dow Thermax. But it is hard to find and expensive when you do. Its primary application is for commercial work, IMO. Everything else we install for rigid foam must have a thermal barrier covering it. You could ask your local authority if a lesser tested product like Tuff-R would be acceptable, but do you really want to do that? Adding a thermal barrier is for your protection.

If you want the benefits of a Radiant barrier, they say to use 3/4" strapping before installing the drywall, that adds the r-2.8 per Dow. Ironically, 3/4" of insulation would actually be more beneficial.

Moving on a bit, is your ventilation good, soffit venting, that path above the slope with insulation, and a ridge or gable vent?

Bud
 
  #5  
Old 11-04-15, 01:39 PM
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Thanks. I have been researching a product called AtticFoil? If this product was covering the rigid foam would that be acceptable instead of drywall? Is my current situation with batts rated to be exposed?

The more time I spend in the space the more things I find that could be so much better. I noticed that at the top of the shoulder wall the fiberglass batts laying on the rooms ceiling were blocking the air flow between the rafters (not every rafter had blockage but most). To my knowledge this would not allow free passage from the soffit vents to exit out of the gable style vents on each side. Is this correct? I hope I explained this so it makes sense. I can provide a picture if need.

The soffit vents are continuous for the length of the house.
 
  #6  
Old 11-04-15, 06:16 PM
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I have attached a picture showing the area that was obstructed, as explained in my last post.

The picture looking up to the top of the shoulder wall. The black border shows the area where fiberglass insulation was, blocking air flow. I moved it thinking that was not correct. I noticed between some of the rafters in the area that the plywood under the shingles has a black tint.

I don't think it matters much but right at the top of the wall the roof slope changes from about 60-70 degrees to about 20-30. The roof section in the outlined area is the 20-30 degree area.

Did I do the right thing in pressing down the insulation between the rafters to ensure proper airflow? Name:  knee wall rafter cavity.jpg
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  #7  
Old 11-04-15, 09:42 PM
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Ok, I'm catching up. BTW, I followed Gary's and Window's comments and see we are basically on the same page.
Sounds like you have either a gambrel roof or a vaulted ceiling in that cape. In either case, the depth of the rafters limits your available space for insulation with an air channel above. Typically, when you create an air path above the insulation you want a baffle to hold back the insulation and protect that insulation from the air flow. How deep are the rafter channels on the 20°/30° slope? And what is in there for insulation?

Also, how big are those gable vents, Net Free Area if you know it.

In many cases that exposed fiberglass insulation is covered with house wrap to protect it from air flow while keeping it breathable. There are also smart membranes and I just read a site on ThermaWrap by DuPont that sounded interesting. Cost and availability are unknown.
http://www.dupont.com/content/dam/as...FIN%201.16.pdf

By the way, somewhere in the back of my mind, Tyvek has some level of reflectivity. Other brands I do not know.

I did find a web page on "AtticFoil" and although it looks heavy duty, I didn't find, and didn't expect to find, where it was rated as a thermal barrier. Difficult for them to say their product will meet any particular rating without it having been tested with that specific product. Thus, no, IMO the AtticFoil probably would not suffice over a rigid foam.

Since you are researching, search "foil faced duct board". Duct board is a rigid fiberglass insulation board (R-4 per inch) that avoids the thermal barrier issue. I researched it for use inside a home with the white facing. Project did not go forward, but the product was interesting, and it is available with a foil facing. Again, cost it unknown, but eliminating the rigid foam thermal issues is a worthwhile consideration.

I'll stop here for now, but you have a big space to work and it should come out nice. Air sealing is a top priority so while everything is open check for plumbing and electrical penetrations. I'll add a link on air sealing related to the whole house.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
  #8  
Old 11-09-15, 10:44 AM
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The rafter channels on both slopes are 6-8 inches. I know there are no baffles installed between the rafters. Since I pushed down on the insulation (as seen in the picture) do I need baffles if there is clear air flow?

The gable vents are probably 2 ft x 1.5 ft. I however don't know the net free area.

I really can't decide if I should use foam on the wall to cover the studs FG installation and if the $ or effort will pay back over time. At this point (for the shoulder walls) I am probably going to just install perforated Attic Foil. I got a sample of the product and it is very tough I could install it on the wall and over the rafters in this space and still have extra for about $70.
 
  #9  
Old 11-12-15, 03:45 AM
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Space for airflow is required. Baffles aren't required, they just improve the performance of the insulation. Since that looks like something you could do later, you could wait to see if needed.

50% is the default guess for louvers with a bug screen. So NFA is 1.5 ft² each. Two of those should be fine.

The attic foil should do fine. It will close off the back of the insulation and give you a firsthand chance to judge the performance of the radiant barrier. RBs have been so poisoned by fraudulent promotions that the energy profession shuns them and the research people won't waste their time with standards or performance work. Only in the south do they get any attention. Be sure to post back with your thoughts on how they perform.

Bud
 
 

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