Blown-in on top of fiberglass OK?


Old 12-12-02, 10:30 AM
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Blown-in on top of fiberglass OK?

I have an attic with blown-in insulation, with no vapor barrier, and I want to install fiberglass batts with a vapor barrier. Is it OK to just move the blown-in stuff aside, put down the fiberglass, and then push the blown-in stuff on top of the newly installed fiberglass?
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Old 12-12-02, 05:03 PM
Join Date: Aug 2000
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Adding attic insulation

Typically, when adding insulation, the old insulation is not removed. While a vapor barrier is important, even more so is having adequate attic ventilation and sealing the gaps between the house and attic. Air leaks into the attic and increases heating and cooling costs and moisture problems. The issue of moisture in the attic is of greater concern in winter to those who live in cold climates. When adding new insulation over old buy the unfaced batts.

Go to on this website to calculate how much more insulation you need to add to meet your ZIP code requirements and to learn more about insulation and its installation.

You can also use the little blue Search button in the top right hand corner of the computer screen to search this forum for threads about insulation and about ventilation.
Old 12-19-02, 06:47 PM
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Installing a vapor barrier in the ceil'g can cause moisture problems as the plastic will hold the water. Adding more FG to the existing will probably not pay for itself as 6" is about maximun for thermal value vs costs. It most likely will not reduce your a/c costs and could increase them.

See about installing a radiant barrier over the top of your insulation. It could reduce your a/c costs up to 30% and heating 12%.

You might also see about up grading the thermal value of your exterior walls.

If you want more info let me know.

Thank you for considering my opinion.
Old 12-20-02, 04:59 AM
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Thanks for you replies.

A fair amount of my current blown-in insulation has been trampled down due to several wiring projects in the attic. At the very least, I need to have some more insulation blown in.
I'll investigate installing a radiant barrier, but it seems to me that the act of installing it will cause more of the blown in insulation to be trampled down.

It looks like I need to do more research.
Old 12-21-02, 07:49 AM
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Good points.

When a RB is installed over the FG the RB becomes the PRIMARY insulator. Adding more FG will not help. You might want to place 3.5" FG in low areas to level out.

If you want to persue this I will send you drawings showing you how to do this. The method I developed abiut 20 years ago is a two layer material that you make (easy to do) that you lay over the existing FG. It is better than installing on the rafters.

The method is a panel that is about 6' (MAX) and 25 1/2" wide. You work from the center of the attic. I usually lay a board down the center.

If you want more detailed info contact me at e-m and I'll give you my phone number.

Thank you for considering my opinion.

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