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Attic and Vapor Barrier

I am preparing to add insulation into my attic that has approximately 6” of blown in insulation.  The current insulation does not have a “vapor barrier” underneath the current insulation. Also, I would like to obtain at least a R30 value in my attic.


Question 1: Do I need to install a vapor barrier under the current insulation?


Question 2: Can I combine a fiberglass roll insulation with the existing blown in insulation or should I stick with all blown in insulation?



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Posted 2013-10-19T15:33:07+0000  by donathan donathan

Hello donathan.  Welcome to the community.


Attic insulation and vapor barrier


It is not necessary to have a vapor barrier for either GreenFiber Natural blow in or Owens Propink  blow-in insulation except if your local building codes require it.

If your current insulation is a cellulose product similar to the Greenfiber natural insulation your existing 6” is equal to about a R-19.  To achieve an R-30, you would have to add another 3.5” of Greenfiber.  You can also combine either a fiberglass roll of R-13 unfaced insulation over your existing insulation, rolling perpendicular to your floor joist or add the comparable amount of the Propink blow-in insulation to achieve your desired R-rating. 

For more information about insulation and installation click on this link for Owencorning.


If you have more questions please ask.




I am a Home Depot associate trained and authorized to people on the internet

Posted 2013-10-22T15:25:53+0000  by Char_HD_CHI

We do a lot of home energy efficiency testing and insulation retrofit work, and we know from our experience and having thousands of case studies to back this up, and reducing enrgy consumption for over a thousand homes; air sealing is a vital part of adequate thermal protection throughout the home and especially in the attic.


There is a good chance you can miss out on major long term savings of you just blow in insulation without knowing how much air is leaking in from you interior through the attic by "Stack Effect"


I have no particular bias on products other than to say closed cell foam is the best of them all at this time, but honestly cellulose out performs fibergass (Blown in or in batts) 


In an open ventilated attic there is a certain amount of air turbulance and it is more severe depending on atmospheric conditions, but thae bottom line is fiberglass does not stop air flow. Thats why it is chosen as a filtering material for HVAC systems. 


If you have ever demoed an interior wall that had fiberglass installed when the home was built, it is probable that you will find a high percentage of the battng to be thinner and very dirty. Thats because air is leaking into those particular wall cavitites and it slowly removes the fiberglass (probably letting some of it flow through the home) and it ends up being thinner. You can find some wall cavities where it looks reletively new and that is because the framing assemblt was tight enough to stop air infiltration.


If you listen to the fiberglasss industry they will knowck cellulose saying it is dirty and dusty and it settles, but they are typically talking about the installation process and on that note, fiberglass particles are being introduced into the air which just like cellulose dust does so proper care needs to be taken with all materials to avoid contact or drifting into the interior of the home.


The industry is changing and consumers need to know there are more effective measures to ensure higher home energy efficicences are being met since utility bills  will only increase and energy resources. Big box stores and all vendors who just want to sell insulation are screwing a lot of people telling them this is a DIY project because without verification through energy loss testing, "you don't know what the structure really needs"

Posted 2013-11-24T02:24:42+0000  by weatherman11150
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