no vapor barrier under the insulation


Old 12-07-02, 08:17 PM
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Question no vapor barrier under the insulation

I'm buying this house which has 6" thick unfaced fiberglass batts in the attic between the joists. There does not appear to be any vapor barriers between the fiberglass and the ceiling drywall. I want to increase the insulation in the attic substantially as I live in upstate New York and DOE calls for R-48. Do I attempt to pull up the batts and add a vapor barrier or just lay on the new insulation over the top of everything or ????

As I look towards the eaves from the center of the attic, I note that the insulation fills the space where the rafters meets the attic floor joists and that there seems to be precious little allowance for any ventilation through the attic to the eves, which do have soffit covers. I'm guessing I need to do something about that too. There are small gable vents on each end of the building, with one end over the unheated garage. At the other end of the building, I also note that there is some frozen condensation on the roofing nails near that gable end. This is occuring on both sides (north and south facing roof). Either the roof is leaking along the ridge and down both sides or water vapor from the house is having a hard time getting out and condenses and freezes on the nails!!!

Any thoughts or comments?

Last edited by oncor1; 12-07-02 at 08:42 PM.
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Old 12-10-02, 12:02 PM
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hi oncor1,

you definitely want a vapor barrier.
I would get rid of all that old insulation and start new (without a V.B., the insulation is probably in sad shape anyway).

Once you've installed your vapor barrier and/or faced insulation between your joists, run subsequent layers of unfaced perpendicular to the direction in which the joists run.

And you're rignt, stay clear of anything that would block ventilation and watch recessed lights too.
It might be a good idea to install one of those polystyrene ventilation baffles between each rafter (run it from the soffit up a foot or two) just to keep ventilation from getting blocked.

As far as the frozen nails, I'm not sure. But you would probably have a layer of ice all over everything up there if you were getting that much condensation. Maybe just some small leaks.
If it is condensation, adding the vapor barrier and getting the insulation away from the soffit vents should help.
Old 12-10-02, 08:08 PM
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Location: USA
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Adding attic ventilation

Typically, if old insulation is in good shape and has not gotten wet, then new insulation is added over old. Should you decide to replace existing insulation, purchase insulation that has the vapor barrier attached and place the faced side down between the joists. Additional, unfaced (no vapor barrier) insulation is rolled on top of the bottom layer.

Condensation is caused when warm moist air encounters the cold air. The additional insulation will help hold the heat into the areas where it is needed.

Take a look at your ventilation. Codes vary depending where you live, but one square foot of free vent area for every 150 square feet of attic is the general rule of thumb.

To learn more about insulation and ventilation, go to where you can calculate how much insulation you need for your ZIP. Go to for more on ventilation.

You can also do a search on this site by clicking the Search button at the top of the page and typing in Attic Insulation in the key word box. Also Attic Ventilation for some very informative threads on these topics.
Old 12-19-02, 07:12 PM
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Contrary to what DOE says, 6" is about maximum for the attic. There is no gaurantte that adding more will reduce your bills. The added mass could increase your a/c bills. In fact you will probably not save enough energy to pay for the insulation in 50 years. Remember FG is only about 10% effiecent. The reason the builder didn't put a vapor barrier in the ceiling is because when you have condensation the moisture can pool.

You can install a radiant barrier over the top of the existing insulation and get better results. You also want to check and see about improving the thermal value of the walls.

Thank you for considering my opinion.
Old 12-21-02, 05:21 PM
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You can paint a vapor barrier on the inside of the ceiling using a low permeance primer like Zinsser B-I-N (0.4 perm). Then rent a machine at your local Home Depot and blow in 6" - 8" of cellulose loose fill on top of the old fiberglass. If some of the old fiberglass is wet, replace it with an equivalent thickness of loose fill. It's very cost effective. Depending on your fuel source and the severity of your winters, you're probably looking a 3 to 5 year pay back. Do install guards to make sure your eave vents are not blocked. There may also be some fire hazards sticking through the ceiling of an old house - pot lights, for example, should be replaced with new units specifically designed for installation in an insulated ceiling.

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