Vapor barrier for a true vaulted ceiling

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  #1  
Old 05-02-10, 09:42 AM
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Vapor barrier for a true vaulted ceiling

We are currently builing a new home. The entire roof system is true vault useing 16" I's with several dormers. We are putting a R52 in the ceiling with no venting. My insulator says that a vapor barrier is not needed on the ceiling, but with no venting I feel it should have a vapor barrier.
Iis a vapor barrier need under these circumstances?
Thank you for any help!
 
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  #2  
Old 05-02-10, 11:45 AM
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Hi Boo and welcome to the forum. There are actually two functions normally associated with a VB. Stopping or slowing down the migration of moisture and as an air barrier. In all applications, the two functions need to occur at the same boundry, ie in contact with each other.

To comment properly, we should know where you are and what you are planning on using for insulation. What other air sealing efforts are being used and are there any recessed lights.

Bud
 
  #3  
Old 05-03-10, 07:55 AM
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The house is being built in south centeral Idaho, We are using 1 layer of R13 and 1 layer of R38 fiberglass batting. The bottom end of the rafters (16" I's) are baffeled and foamed for a good seal, the top end either ties into the side of a 2ply 16" LvL or they lay on top of a ridge beam.

Thanks for the help Bud!
 
  #4  
Old 05-03-10, 09:49 AM
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One of the problems with I's is that the distance between them is greater than the typical 14.5" cavity, or 22.5" for 24"oc, plus the step created by the top and bottom cords of the "I" causes a poor fit. The consensus, when we discussed this problem, was to use a commercial batt that is a full 16" wide, but that still leaves the step around the top and bottom. Or fill in the space on each side with 1/2" rigid foam. Spray in cellulose was a good option as well where possible.

The unfortunate part is that you are going the extra mile with the 16" cavity, only to loose 20% to a poor fit. If it were mine, I would install 1/2" rigid foam on each side of each "I" so they would look like full 1.5" wide, thus a normal cavity for the batts to fit into. Talk to your insulation guy, but be aware, he may prefer to gloss over the issue to avoid the extra work. The new home rating systems are preparing a standard for evaluating installed insulation with Infrared Cameras and they will pick up a loose or poor fit in a heart beat. And that will not only mean heating dollars, but resale dollars/equity.

Bud
 
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Old 05-05-10, 09:09 PM
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We are using full batts so the insulation is not my concern. The issue lies with the vapor barrier. the roofer, isulater and sheet rocker say not to use a vapor barrier because with no venting and no possible moister of any kind can lead to dry rot. So there lies my concern, possible moisture and mold without a vapor barrier or possible dry rot with a vapor barrier. Im not sure which way to go.
 
  #6  
Old 05-06-10, 04:40 PM
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I will direct you to buildingscience.com. They have several papers there about unvented roof assemblies and vapor barriers. you should read them all to help you make this decision.

Personally based on what I've read on that site, I would forgo the fiberglass batts. If you were to use fiberglass batts without a vapor retarder or barrier, I would expect you to have condensation issues. The best insulation for this would be closed cell spray foam. It has great air sealing properties and acts as a vapor retarder.

To help reduce the cost of using spray foam, you can use a batt insulation along with it. That's commonly referred to as flash and batt. The one manufacturer I spoke to recommended applying at least 50% of the total R value in foam to avoid any condensation problems.

You should also consider dense pack cellulose as well. Cellulose is denser than fiberglass which allows it to be more resistant to air flow and moisture flow.
 
  #7  
Old 05-06-10, 05:45 PM
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What about gluing 2 inches of foam board up then putting your batt in? would this be similar to a flash and batt with closed cell (only less expensive) in terms of being a vapor retarder and for air sealing?
 
  #8  
Old 05-06-10, 06:49 PM
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I really don't know. Has the roof been built yet? If not, putting two layers of 2" rigid foam over the rafters is a great way to go as well.
 
  #9  
Old 05-07-10, 06:58 AM
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The first thing to consider about vapor barriers is to qualify as one it must have a "Perm Rating" of 1 or less. More importantly all building materials have a "Perm Rating". What this explicitly implies is that moisture will permeate all known building materials. The Perm Rating indicates the time it takes for the moisture to permeate that material. The lower the rating the longer it takes for the moisture to traverse that material. So vapor barriers slow down the flow of moisture because of its low Perm Rating. I support the installation of a vapor barrier in your instance because of Pressure Induced Moisture Flow, Relative Humidity and the "LOW" Perm Rating of ROOFING MATERIALS. I could write a book on this subject. To avoid writing a very long post if you need explanation I will respond.

As for the Building Science Organizations (BSO's), I happen to know most of them. I openly state that I respect and admire their efforts. However, I do not agree with their positions on "Air Tight Construction" methods.
 
  #10  
Old 05-07-10, 07:26 AM
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Originally Posted by resercon View Post
However, I do not agree with their positions on "Air Tight Construction" methods.
What do you mean by this and what is your perspective on it?
 
  #11  
Old 05-07-10, 03:26 PM
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This is quite a broad subject and a difficult one. First "Air tight construction" has been around for decades. It is used primarily with "High-Rise Structures". However, all High-rise structures are mechanically ventilated. On the other hand Residential structures are designed to be "Water Shedding". Both processes, mechanically ventilation and water shedding purpose is to extract moisture from the structures that is generated inside the structure. If you apply Air tight construction methods to residences it will impede the water shedding properties of that structure. This will result in an "Unhealthy Indoor Environment".

The "Air-Sealing" industry, which I am a part of, uses ASHRAE "Minimum Ventilation Guidelines" as an action threshold. If those guidelines are violated, mechanical ventilation is mandated. This is not mentioned in these BSO papers. Furthermore High-rise structures have maintenance people and regular inspections specifically regarding indoor air quality. You will not find that in residences.

My position is to encourage energy conservation but DO NO HARM! Or energy conservation is important but not as important as the health and well-being of the occupants of the dwelling.
 
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