Cellulose on top of Rockwool

Kfbeal · Nov 9, 2009

We built our home 7 years ago, and it appears the insulation has settle some below our metal purlins or some of the cross-braces. I'd like to add to this, but the Rockwool guy wanted quite a bit to top it off.

Would it be bad to add Cellulose on top of the existing Rockwool so I can do it myself to cut costs?

Our exterior walls are a true 6" with rockwool but don't appear to be sagging but I'd like to cover up all the exposed metal to keep from transferring the heat through that.

Any ideas or suggestions? Anyone use the cellulose? Or should I just pay the insulation guy?

Luremaker · Nov 9, 2009

I did just that a few weeks ago in my shop. My son and I added 10 extra inches of cellulose (over 100 bags) right on top of the rockwool (Roxul) insulation using a rented blower. It took us around 4 hours to complete the 2400 sq ft. It was much easier than I thought it would be. If you have an afternoon with a helper I'd say do it yourself.

heehaw · Nov 10, 2009

cellulose has lots of benefits over most other insulation materials: putting it over rockwool or fiberglass is a good ideal: all blown in insulation will settle in the attic: except foam of course: but a couple years ago we priced foam compared to cellulose an it was way more than double...blow it in yourself if you can, you will like the results.
heehaw

Kfbeal · Nov 10, 2009

I'm looking at this chart:

http://sutherlands.com/downloads/greenfiber_coverage_chart.pdf

If the bag is 40 sq. ft. and I only add 6 inches I should get 80 sq. ft out of one bag.

If I cover 1800 sq. ft. of our home, that should calculate to 22.5 bags correct?

The chart, if I'm reading correctly, is showing 12" thick instead of 6" at 25 bags per 1000 sq. ft. if 40 sq. ft. per bag.

Luremaker · Nov 10, 2009

Here is the chart I used and found pretty accurate. Page 2 lists the number of 25lb bags required to attain the R-value in sq ft. The chart said I required 106 25lb bags for R32 for 2400 sq ft. I used 110 bags.
http://www.can-cell.com/insulation_products/weathershield/weathershield_coverage_chart.pdf

Here is a must read article on sealing your attic before blowing your insulation along with good tips for doing the job.
http://insulation.owenscorning.com/docs/Attic%20Insulation%20Upgrade_FHB200.pdf

I think you would need more like 50 25lb bags for 6" (R-20) for 1800 sq ft.

Kfbeal · Jan 25, 2010

I finally got around to blowing in the insulation. In all, I have a total of roughly 12" throughout. Most places the rockwool was 8-10", not sure what that R-Value was but the company said R38 would be blown in. I added between 2"-4" in most places and some places 6". I only had to cover 1300 sq. ft. or so because we have quite a bit of our attic decked.

I ended up blowing in 20 bags of the Green Fiber Cellulose. It took roughly 2 hours with my wife and father-in-law handling the unit and me in the attic.

Anyway of calculating what my R-Value would be with say 8" of blown Rockwool and 4" of blown Cellulose? I estimated an R38.