I have a section of my attic that no longer has insulation due to recent ceiling repairs.  The area is about 10' X 20'.   Which is better,  blown in cellulose or fiberglass batts.  Seems kind of silly to rent the machine for blown in for such a small area.

Any suggestions appreciated.

Blown is always better- no gaps, it fills in around all voids. Cellulose doesn't lose R at high temperature differences due to convection currents the way fiberglass batts do either.

Most blue or orange box stores throw in a free day of blower rental with more than $100 of goods. For 200 square feet @ R38 you're looking at ~250lbs, which runs you about ~$110.

If the rest of the attic is insulated with batts, consider doing a ~3" overblow on that as well which will halt the R-value loss of the fiberglass at the temperature extremes, improving overall performance far more than just the additional R10 would deliver. A 3" overblow is ~1/3 lb of material per square foot of area.

When blowing, set up several cardboard depth markers around that are 15-20% higher than your settled-R goals. eg. R38 is about 10-10.5" settled, so set your gauges to 12", blow up to your marks and gently rake it smooth. It'll lose a bit of loft over the first 5-10 years, but it'll be pretty stable thereafter. Coverage as marked on the bags will also be overstated for amateur installation w/rental blower, so over-buy by at least 10% (if you have any bags left over they'll take them back.)

thanks dana, that makes sense.

I was thinking that while I'm up there i would add an attic fan. The house is suare, with a hip roof, so there is no place to put a ridge vent. It's FL so the attic gets insanely hot. Are those power attic fans worth it? I ask because we get frequent afternnon showers that cool the air temp down considerably but because of the poor ventilation in the attic it stays hot all evening, even if it rains. If I could manually turn the fan on after it rains I think it would drop the temp up there considerably - although they seem to use a lot of watts just to run.

Attic fans are almost always a net-loser from an energy use point of view. By creating a negative pressure in the attic they suck conditioned air up in to the attic, causing humid outdoor air to be drawn into the conditioned space. For the money you'll get a lot more bang per buck out of radiant barrier applied to the rafters, which will at least be guaranteed to reduce the cooling load, not add to it. With low-flow solar-powered versions you can get single-digit reductions in cooling power. See:

http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

and

http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf


If your attic is the code-minimum R30 or less the effect of radiant barrier will be higher, but it may be cheaper/better to raise the R value to R40+ instead. This is particularly true if you're insulated with fiberglass batts, which absorb radiant heat about 2" into the top depth under a hot roof deck, so that the point 2" in is hotter than the attic air(!). A 3" overblow of cellulose blocks that problem too, and the hottest point in the insulation is now the top surface. R19 batts behave at ~ R11-R13 under a 125F roof deck , but more like R17with a radiant barrier & 115-120F air. With 3" of cellulose on top of R19 (no radiant barrier) it'll pretty much deliver ~R28-R30 against the attic air temp, and a radiant barrier would knock 10F or so off the air temp (by keeping the roof deck and shingles a bit hotter, radiating the heat back to the sky and convecting it to the outdoor air.)

Is there any reason I can't break up the bundles and place the material by hand? Got all the cellulose insulation, but the blower won't be avail. for almost a week.

I have such a small area to do, I don't mind doing it by hand.

Posted By bmancanfly on 11 Dec 2010 04:14 PM
Is there any reason I can't break up the bundles and place the material by hand? Got all the cellulose insulation, but the blower won't be avail. for almost a week.

I have such a small area to do, I don't mind doing it by hand.

You'll end up with a fairly hign and inconsistent density if you do, so you'd probably end up short on material and it may under-perform slightly.  Blowers are designed to give a very non-lumpy density that is adjusted by the air mix, something that isn't easily replicated with manure forks/rakes/hands.  Packed in bundles it's density it's typically 7-8x what you're looking for in an attic open-blow.

You're right Dana. I tried a small section - it didn't work. I'll wait for the blower.

Both systems will deliver excellent results. However, if you intend to do the job with batts, you had better be a detail oriented, patient individual. The thermal performance of fiberglass insulation is directly related to its ability to trap and maintain very small pockets of air. Blown-in fiberglass creates a seamless blanket in your attic. You might spend many hours kneeling in your attic to achieve this same result with fiberglass batts.

In attic floor high delta-T apps standard density batts & blown fiberglass are at a distinct performance disadvantage as compared to cellulose (or even rock wool), due to loss of R from higher convection losses during the heating season, and higher penetration of radiated heat off of roof decks in cooling dominated climates. (The fiberglass 1.5-2" below the top will be significantly higher than the attic air temp under a 120F+ roof deck in a ventilated attic, throwing away at least R5, and insulating agianst a higher temp than when the hottest part of the fiber is in the top 1/4" as it is with the higher density fiber.) A 3" overblow of cellulose is enough to rectify both of those issues. Higher density "cathedral ceiling" batts are significantly better than standard density goods, as are higher-density superfine " blown-in-blanket" fiberglass blowing wools installed at 1.8lbs density, such as Certainteed Optima or JM Spider.

It does indeed take a LOT of detailing to get the performance out of batts, but even that isn't enough in attics with standard density batts.