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Question DetailsAsked on 12/13/2015

I have 8-10inches fiberglass insulation in my attic on 1983 colonial home. Don't I need a vapor barrier?

I have a colonial home built in 1983 which is in northern Ohio. I recently was in the attic and found that there is 8 10 inches of loose fiberglass installation. There does not appear to be any vapor barrier. I would like to add more installation whether that is more loose fiberglass or celluose. My question is do I remove the old insulation and install batting with a vapor barrier? Also, what is the best way to add more insulation? add cellulose on top of the fiberglass?

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2 Answers


Are you a DYIer? If so take plastic rake and move fiberglass around attic thereby exposing the air leaks from the rooms below. When area is clean take accoustical caulk, stainless steel duct tape, & triple expanding foam to seal electrical lines, tops of sliding doors, around chimneys (beware of fire hazard if not masonry), & avoid can lights. For can lites make a box or purchase pre made covers and then seal them to attic floor. Level out insulation and with neighbor tearing the bales of cellulose, crawl around attic and blow insulation mounding it over old fiberglass. It will settle LCD, so install 50% more than the R's you will need. Establish R value at local utility company's help line. Big box like Menards will loan you blower in return for selling you the insulation.

Or if not DYIer call in contractor and have him blow a cap of cellulose to needed R value.

For vapor barrier paint ceilings of rooms below with vapor barrier paint.

Best idea, make sure attic is properly vented to allow the moisture from inside the envelope to escape and not build up in your attic. Ideas how? Install continious ridge vent when reroofing,install flat 505 vents on backside of home, install needed soffit vents to feed airflow thru the attic without pulling conditioned air from the home.

You have just air sealed your home and lowered the number of air changes an hour to save you energy. Cellulose will restrict air flow. ie hot air rises etc,

Jim Casper Old energy conservation guru

ps for ideas on gutters and covers see my website

Source: www.heartlandmastershield,com

Answered 4 years ago by jccasper


Vapor barrier should be under the insulation, BTW - not on top. You never want an impervious layer on top of the attic insulation unless your house has inverted vapor barrier - vapor barrier under the siding and over attic insulation rather than under the interior drywall because you live in a constantly wet environment. Fairly rare but is done in tropical and very wet maritime areas like the Maine and Washington coastal and island areas, and in some deep south very humid areas where A/C runs year-around. Usually installed on the underside of the attic floor joists or trusses before the ceiling drywall below goes up. Can be retrofitted between joists though not as effective because you can't go under the framing with it, so you have to use foam or urethane caulk to adhere and seal to the joists.

Faced insulation is pretty useless in that environment with respect to preventing moisture transmission unless it is the type with overwidth facing that can be fastened to the framing at each side and to the next piece at ends, though it does help stop the air migration through the fiberglass which reduces its energy efficiency.

And make sure any insulating you do does not inhibit the necessary ventilation through the attic from eaves/soffits to ridge vents or ventilators, depending on your roof type. Unless you have a "conditioned space" attic of course.

Here are some previous similar questions with answers FYI - more links to other related questions below each of them -

As to the fiberglass versus cellulose issue - you do NOT want any sort of a vapor barrier or insulation facing within the attic insulation pack - can trap moisture, so if adding insulation batts over the top of existing use unfaced. Personally I do not like cellulose because it packs down badly and loses a lot of its insulating value over time, and absorbs moisture and even when treated supports mold/mildew growth, but as I think Jim Casper said in one of the linked responses, it is substantially more resistant to airflow through it than fiberglass (especially blown-in fiberglass) so an inch or two over the top of bown-in fiberglass can act to limit airflow. You do need to be sure penetrations from the living space have been blocked if doing that, because fiberglass only will pass a lot of moisture in most cases, but cap it with cellulose and you can get moisture retention issues in winter in cold areas, and in humid periods when house is colder due to air conditioning running. A fiberglass batt layer over blown-in can help limit the airflow through the insulation but with less moisture trapping and also less energy efficiency than with cellulose capping.

There are moisture permeable "wraps" that can be used over blown-in fiberglass insualtion to stop the airflow, but their performances are across the board depending on brand, and except with totally open-bay attics with no cross-bracing or many utilities coming above the insulation layer, are hard to put in place without a lot of penetrations.

Answered 4 years ago by LCD

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