Here is my dilemma. I have an addition to my house. It is insulated, but it seems to be some fibrous type of material (I'm thinking rock wool). The problem is I'd like to upgrade to a higher R-Value, but not sure how to do this. I guess one option is to rip out the existing wall board and hang new batts. option 2 would be to blow in cellulose. I like option 2 (no real drywall repair), but how do I do this when the cavity is already filled? I am thinking that the blown in insulation won't be heavy enough to compress the existing insulation in the cavity.
First off blow in rock wool has a very high R rating. It is about 3.5 per inch. So a 3 1/2" wall would have a rating of 13. This is comparable to fiberglass in 3 1/2 inch wall of R11,13 or 15 (much more expensive for R15 which is really mineral wool).
Second is that insulation needs to be fluffed up. Compressing it will decrease the insulating properties.
A good example is a down sleeping bad. I used to do a lot of mountain climbing and had a 3 1/2# down sleeping bag. When compressed I would get cold at 40 degrees but when fluffed up I could be warm at -40.
I see people who have figured that they would get extra insulation by shoving in 2 bats of R11 thinking they would get R22. Wrong, they probably got R4 if they were lucky.
What you propose is long on work and expense and short on results.
The only issues is that blow in insulation does settle over time and adding to the top might not be a bad idea if you are sure that it is settled. This is why the standard is the fiberglass that will not compress when attached to the batting.
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