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Question DetailsAsked on 8/5/2012

Cellulose insulation or fiberglass insulation for an attic

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3 Answers

Voted Best Answer

A couple of comments about what Jim said:

1) Regarding type of insulation, in cold winter environments: Cellulose and fiberglass are actually about comparable in R value when installed - blown in cellulose runs from 3.2-3.8 R value, fiberglass batt 2.9-4.3 R value depending on manufacturer and whether hig-density or low density, high-efficiency or standard, according to official Department of Energy publications. Measured values in attic test cases, in areas with a true winter, after 10 years showed a decrease from 3.4 (in the test case) down to 2.1 for cellulose, and 3.5 to 3.3 for fiberglass batt, due to packing or matting. In an attic environment, there WILL be condensation or frost on the insulation at some point during the year (assuming an area with true winters) and in highly insulated houses commonly for a substantial time period each winter. Fiberglass packs down slightly from that weight but mostly rebounds, cellulose packs down and mats and does not substantially recover, so over the years cellulose loses 1/3 to close to 1/2 its insulation value, fiberglass about 10%.

2) a note on radiation barriers attached to the bottom of the rafters - there are a lot of installers and homeowners making two major mistakes with this product that can cause major trouble: First, be sure to terminate it short of the eave openings. I have seen cases where it was carried all the way out to the fascia board, thereby blocking all airflow on the underside of the roof. Even carrying it all the way to the eaves along the bottom of the rafters will block off ventilation to the main attic area. You have to leave the air space between the rafters open to full airflow from the soffit/eave area ot the ridge vent. Second, do NOT run it continuous from eave to eave across the full width of the attic - leave a gap about a foot wide under the ridge vents so warm and moist air in the attic can vent through the ridge vent. Closing the ridge vent area off with the radiant barrier effectively puts a vapor barrier around the main attic area, causing retention of the moisture which WILL accumulate there, promoting mold.

Answered 7 years ago by LCD


Hands down cellulose is the winner. It has a higher R value and is a denser material. You make furnace filters out of fiberglass, it is a barrier to conductive heat transfer ie a hotpad. For convective heat loss, moving air is pretty much worthless. By blowing a layer of cellulose, (paper treated with boron for insects) over an existing insulation( usually fiberglass) you help to slow down the stack effect of heat loss. It is highly reccommended that if you are physically able, take a plastic rake and move the insulation out of an area over a recessed light or a pocket door or around a chimney going thru the attic and use an expanding foam to seal the air leaks before the insulation process. More important the older a home is.

Fyi insulating a ceiling in a garage with a heated room above it will substanially raise the room temperature above and alongside the ? garage.

Finally a radiant barrier applied ONLY to the roof rafters will significantly reduce the radiant heat gain (my attic temperature dropped to outside temp) and help some on heating bill. Do not block the ventalation in the attic when installing radiant barrier. I experienced a 50% reduction ( in my own attic) on A/C bill in 2011 after installing a radiant barrier. Material cost 10cents a foot.

Jim Casper

Old Energy Conservation Auditor

Source: http://www.heartlandmastershield,com

Answered 8 years ago by jccasper



Answered 5 years ago by Guest_92612071


Was looking at this one as a reference for another similar question and noticed one thing - in Jim's comments he talks about foam sealing around penetrations into the attic. Fine around air vents (bathroom, kitchen) and wiring and pipe penetrations (only use a short length o coverage around wires to prevent overheating). However -

DO NOT use foam to seal around recessed light cans or around combustion ducts. Brick or fake metal chimneys OK to seal around that outer surface with foam, but do NOT seal around the actual flues with foam - risk of it getting hot enough ot ignite, and most spray foams (most all types of foam, for that matter) burn like crazy once ignited.

For potentially hot surfaces like furnace and hot water heater exhaust flues and recessed light cans (even if they don't feel hot to you now) use rock wool (aka mineral wool, stone wool) stuffed into the gaps - looks like fiberglass, but is totally fireproof. Before insulating around light cans be sure they are rated for insulation contact - many types, especially older ones, require from 4-12 inches of airspace around them to dissipate the heat from them. Rock Wool is available in bundles like insulation and cost about 50-100% more, or in smaller packages at some box home improvement stores in either the insulation or the wood stove area, or you can probably buy just a batt or two at a wood stove and insert stove dealer/installer.

Answered 5 years ago by LCD

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