This is what my attic insulation looks like: Rock wool on top of pink batts =)

A closer view of another section, notice the gap between the insulation and the rafter vent, Do I need to fill the gap?

First I extended the existing rafter vent with a black vent bought from Home Depot.

Then I layed a piece of Roxul on top of the rock wool insulation, and pushed it against the rafter vent. Then I noticed the gaps between the Roxul and adjacent trusses, which would remain if I lay more Roxul to the left and right of it, so would this render this insulation ineffective?

Overall, Is this way of adding insulation effective? Do I need to flatten the wool insulation a bit more before placing Roxul on top? I only purchased one bag of Roxul to experiment, please advice.

how are you dealing with the gaps in the Roxul under/around the trusses?

Posted By Bob I on 10 Nov 2013 01:31 PM
how are you dealing with the gaps in the Roxul under/around the trusses?

Great question, I thought about the same thing. It's like wearing a thick winter coat but with holes on the side right?

I would just blow in more cellulose.

I see. Well, the reason I like to use Roxul is because it's very easy to install. For cellulose you need a machine and the process is like working in a dust storm. If Roxul is no good then I will probably ask a pro to do cellulose or foam.

I am working on a similar project and have to install extensions on my existing air chutes like you did... I was going to blow in cellulose but its not available locally so I will use AttiCat ... it looks to be way easier to install than plopping those batts in place and will fill every nook and cranny...I think you could leave the batts in place and fill in around them with the AttiCat (see videos on youtube I found a bunch)

When the batts don't fit perfectly, the peformance is undercut pretty severely. This is a case where blown cellulose is the clear cost-benefit winner. (It's cheaper than Roxul as a DIY, probaby comparable if installed by a competent pro.)

Some of the blown fiberglass alterneatives can work OK, but have a disfavorable price point, as well as the friable airborne glass partcle issues.

Specify "borate only, sulfate free" on the cellulose product. It can be either damp-sprayed, or dry blown in this application.