Adding new insulation

momfromthenorthSeptember 11, 2010

We have about R19 of blown in fiberglass insulation in our attic currently (which is 24 yrs old). We're in the midsouth area - zone 3 on the energy star map. Very steep roof so plenty of room in the attic to add more and no heating/cooling equipment in the attic.

We're looking to add another R19 or possibly even R29. However, both contractors we have received bids from said they would use blown-in cellulose insulation. I've read that you should NOT use blow-in cellulose over blow-in fiberglass because cellulose is heavier and will compress the fiberglass. So what do we do? Find a company that will suck out the old fiberglass insulation and put in all new?


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I don't like cellulose at all. Regardless of what it does to fiberglas, it always packs and settles over the years.

In general, in fact, I don't care for blown in insulation of any kind. In your place, I'd probably add unfaced fiberglas batts on top of the existing insulation. But I'm not an expert, and others here may have more suggestions.

    Bookmark   September 12, 2010 at 1:57AM
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"both contractors we have received bids from said they would use blown-in cellulose insulation"

If you want a quote for fiberglass, just tell that to the next contractor you call. :) No need to work with people who don't listen to what you want.

    Bookmark   September 15, 2010 at 12:42PM
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do you have 6" ceiling joists?
if so the R-19 you have should fill them completely.
adding another layer of UNFACED R-19 would make
attic R-38.
if the joist bays are completely filled lay the
batts across joist bays to stop thermal bridging.
but if the joist bays are not completely filled
put the batts in the joist bays.
if there is an air gap between existing insulation
and new insulation it will reduce R-value of insulation.

oh and do any duct sealing and air sealing before adding
if you lay batts to cover joists it will be difficult
to see where to step as joists will be covered.
usually roof rafters and ceiling joists are close
so that can be a guide. but be careful where you step
either way.

best of luck

    Bookmark   September 23, 2010 at 2:11PM
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I would blow the cellulose over the FG.
If you are worried about some compaction of the FG, just pay for a little more depth of cellulose.

The reasons I think that cellulose is better:
- loose fill (or even bat) FG is very open to air circulation through the insulation. Cellulose is tight enough to be much better at stopping circulation.

- In cold conditions FG loses half of its R value due to circulation currents. So, just under the conditions you would like to work best it performs worst. The NREL report that shows the testing on FG and cellulose for performance under cold conditions:

I pasted in a small excerpt below.

In addition, cellulose has a greater R value per inch, less embodied energy, and is easier on your body to install.


R-Value Divided by Two

Tests of both specimens revealed that as the temperature differential increased, apparent conductive resistance (R-value) of the insulation decreased (see Figure 3). Thermal resistance at the greatest temperature differential was as much as half the estimated nominal thermal resistance of the insulation. For specimen 1, thermal resistance dropped to R-9.2, and for specimen 2, measured thermal resistance was as low as R-11.1 when the attic temperature was at -8 degrees F.

The investigations confirmed that natural convection occurred within the insulation itself. The reason is that air densities changed with the temperature differences. As warmer air from the heated space below reached the top of the insulation, the air cooled, became more dense and fell back into the insulation. As part of these observations, the researchers made infrared scans of the upper surface of the insulation. The heat patterns resembled the traditional hexagonal Benard cell pattern of natural convection occurring in fluids heated from below (see Figure 4).

Cellulose Holds its Own

Loose-fill cellulose, initial testing indicates, allows no such convective patterns to develop. Using a similar test procedure on the Attic Test Module, Wilkes and Childs recently completed the first phase of testing for the Cellulose Insulation Standards Enforcement Program, using a product, Forest Wool brand, selected by the standards group as an average or typical cellulose product. R-values increased as the temperature difference across the cellulose increased-the opposite effect that the fiber glass exhibited. The researchers concluded that the cellulose tested did not allow convective losses as the fiber glass had. Oak Ridge plans to test more samples and types of insulation in the future.

Here is a link that might be useful: NREL report - effective R values of FG and cellulose

    Bookmark   September 25, 2010 at 10:00AM
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