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  1. #11
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    Default Re: no existing attic insualtion vapor barrier

    I understand that, but where am I going to find 6" thick batts?

    R19s are 6.25" thick, R13s are 3.5" thick, and R15s aren't available unfaced according to the Corning website.

  2. #12
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    Default Re: no existing attic insualtion vapor barrier

    Quote Originally Posted by cjwittmeyer View Post
    Thanks Canuk.

    The only question I have is the one I started with. Wont the warm air condense when it comes into contact with the insulation? I was under the impression that that was the whole idea behind a vapor barrier. Has doing what you suggested removed the need for a vapor barrier?
    There is plenty of confusion and misconceptions as to the role of the vapour barrier.

    Moisture gets into walls and attics by two paths ...... air infiltration through cracks or gaps and vapour diffusion through materials themselves.


    Vapour from air infiltration can be 100 to 1 times greater than from diffusion.

    Water vapour manages to filter its way through most building materials ....... which is referred to as diffusion. Although all building materials slow diffusion to a certain extent, only those that almost totally block it are officially classified as vapour barriers and deemed acceptable for new construction standards.


    Permeability is the technical measure of how easily vapour will get through a material and only materials rated as less than 1 perm are considered vapour barriers. Aluminum foil, polyethylene films, aluminum paints, latex vapour barrier paint, vinyl wallpaper, and several coats of oil paint all qualify ....with varying degrees of effectiveness.

    A similar technical measure is given to the permeability of materials as being an air barrier.


    The polyethylene films are the preferred materials for vapour barriers because they come in large sheets, have few joints where air can leak by, and can be efficiently sealed around openings such as windows, doors and electrical boxes.

    If we completely seal a polyethylene sheet on the warm side of the insulation, we call it a combined air/vapour barrier.
    This is important …… in order for an effective air and moisture barrier it needs to be a monolithic sealed envelope.

    So .... they create an air barrier while also meeting the code requirements for a vapour barrier.

    Keep in mind this is referring to the vapour that exists from the warm moisture laden air from the living space …. in this case.

    The problem with paper faced insulation is the paper facing has a higher perm value than 1 …. So is considered a vapour retarder. What this means vapour diffusion will be slowed but will eventually pass vapour once it reaches saturation.

    Another down side to faced insulation is properly sealing preventing air infiltration. Simply installing the faced batts and tacking the tabs doesn’t provide a continious sealed envelope.

    Here in Canada the use of faced batts has been discontinued for the last 30 years.
    The main reason was the paper face increases the risk of fire. The secondary reason was research done here showed poor performance as being a air/vapour barrier.



    Having said all that ……….

    Chances are if you live in an older home it’s likely several layers of oil and latex paints have been applied to the walls and ceilings over the course of the years. This has created an effective layer to prevent vapour diffusion …… a vapour barrier on the warm in winter side of the insulation.

    As mentioned earlier …… vapour from air infiltration is 100 greater than diffusion.

    This is a major step many people overlook when they decide to increase the insulation performance.

    Taking the effort of sealing any air infiltration from penetrations between the attic and living space with things like electrical , plumbing , HVAC , etc. before stepping up the insulation will greatly reduce issues of condensation and heat loss.

    By the way ….. this also is beneficial for the hot weather conditions.


    Hopefully this makes sense and helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #13
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    Default Re: no existing attic insualtion vapor barrier

    Quote Originally Posted by jackbird View Post
    I'm confused as to how perpendicular joists would work - an R-30 batt is 10 inches thick according to the spec sheet, and due to the mineral wool, there will be less tan 6 inches of clearance.

    This government-produced flyer says:



    and the illustration seems to show the joists nailed edge-to-edge. I imagine one would want the seams between boards staggered rather than all in a row, however.
    Unfortunately the link you provided didn't work for me.

    However .......
    Sure you can do it the way you're suggesting .... personally it doesn't make sense to do it that way.
    There are advantages to laying the additional joists perpendicular.................

    - easier to attach to the existing joists. You will have more meat to toe nail or screw in at an angle and less likely the joists will roll over.

    - this would distribute the extra weight over several joists







    Quote Originally Posted by jackbird View Post
    I understand that, but where am I going to find 6" thick batts?

    R19s are 6.25" thick, R13s are 3.5" thick, and R15s aren't available unfaced according to the Corning website.
    Use the R19 batt ..... the 1/4 inch extra can either be pealed off or just leave it .....
    This small amount (1/4 inch ) will crompress slighty anyway when the top layer of batts are placed on top ..... isn't critical.

    The idea of filling the existing joist bay and running subsequent layers perpendicular is to cover the tops of the existing joists.

    If the tops of the joists are left exposed this creates a thermol bridge. In other words .... the wood joists will transfer heat if left exposed. This can create what is known as ghosting ..... seeing dark shadow lines on the ceiling where each joist is.
    Even though you have plenty of insulation in the spaces between the joists ..... it's the thermal bridging of the joists that's the weak link.

    This is why you want to cover the exposed portion of the joists.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #14
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    Default Re: no existing attic insualtion vapor barrier

    OK, that makes sense.

    Here's the fixed link, by the way.

    Given that the R19 batt is pushing the limits of the 6", do I vacuum out the mineral wool to ensure it has room?

  5. #15
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    Default Re: no existing attic insualtion vapor barrier

    I have a similar question on attic insulation.

    We have an attached garage we want to convert to a 'man cave' for the King of the house. It was at one time an open garage with walls that were enclosed and the ceiling was finished off. After I get my wiring updated, I would like to insulate the ceiling. (Finished walls I will blow in.) There is 0 insulation in the ceiling now which makes the wiring updates a whole lot easier!!!

    I will install the foam soffit vents next to the roof decking down low with a little pillow of insulation at that point. My venting is good, I have a solar fan that does a great job.

    My question is: Since the ceiling is already on, what do I do about a vapor barrier?

    Do I use kraft paper insulation and lay the kraft side facing the heated side? (downward) Then follow up with more insulation perpendicular to that?

    Also, I want to install recessed lighting. I was looking at the insulation contact cans at the store, but they seems like they’d leak heat upward. Suggestions for my application?

    We're in Alabama, so we don't have the strong heat concerns a northern house would, but we do have heat and humidity to be concerned with.

  6. #16
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    Default Re: no existing attic insualtion vapor barrier

    Quote Originally Posted by jackbird View Post



    Given that the R19 batt is pushing the limits of the 6", do I vacuum out the mineral wool to ensure it has room?
    If you want to make extra work for yourself ...... sure ...... but it's really not necessary.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  7. #17
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    Default Re: no existing attic insualtion vapor barrier

    Quote Originally Posted by southern fix View Post
    I have a similar question on attic insulation.

    We have an attached garage we want to convert to a 'man cave' for the King of the house. It was at one time an open garage with walls that were enclosed and the ceiling was finished off. After I get my wiring updated, I would like to insulate the ceiling. (Finished walls I will blow in.) There is 0 insulation in the ceiling now which makes the wiring updates a whole lot easier!!!

    I will install the foam soffit vents next to the roof decking down low with a little pillow of insulation at that point. My venting is good, I have a solar fan that does a great job.

    My question is: Since the ceiling is already on, what do I do about a vapor barrier?

    Do I use kraft paper insulation and lay the kraft side facing the heated side? (downward) Then follow up with more insulation perpendicular to that?

    Also, I want to install recessed lighting. I was looking at the insulation contact cans at the store, but they seems like they’d leak heat upward. Suggestions for my application?

    We're in Alabama, so we don't have the strong heat concerns a northern house would, but we do have heat and humidity to be concerned with.
    Personally I wouldn't worry about vapour barriers unless you are making this space part of the home's conditioned space .
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #18
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    Oct 2009
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    Default Re: no existing attic insualtion vapor barrier

    In the process of remodeling the kitchen, I am having ducts opened to heat/cool on demand. So think that it may be of concern, but I'm not sure???

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