Rock wool (roxul) over Fibreglass Insulation?

[AldeanFan]

I need to add another layer of insulation in part of my attic.
There's r12 fibreglass batts there now, I'm going to add r24 on top running in the opposite direction.
Cost difference is about $0.50 sqft and I'm doing an area 300sqft, so $150 for roxul over fibreglass.

I much prefer working with roxul, not itchy and I like that it's water resistant.

Any reason not to put roxul over the existing fibreglass?


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[Angelfire]

Only that the weight of roxul will compress the fiberglass which will lower it's R-value (haven't a clue how much this will really matter though). I did Roxul and Fiberglass in my ceiling but had the Roxul under the fiberglass.
Cheers.

 

[Richard Cranium]

rock wool under fiberglass is ok, but not on top of it, it is too heavy.

 

[stonesfan68]

Send a note to the folks that make Roxul and ask them. I'm sure that they've been asked before.

 

[Abeo]

I did Roxul in my walls, but blew in fibreglass in the attic. It wasn't much work at all, and zero itching (the new stuff is nothing like the fibreglass of old). It was also very light when installed, but the bales were compressed so I didn't have to haul as many. If I were you I'd consider that.

 

[Boomer343]

Rent a machine and blow in the fiberglass pink insulation. The new stuff is a lot better than previous to install and superior at insulating compared to old.

 

[forAK]

Rent a machine and blow in the fiberglass pink insulation. The new stuff is a lot better than previous to install and superior at insulating compared to old.

plus it'll get into every nook and cranny to fill the whole area. it's cheap too - free rental if you purchase 10 bags at HD.

 

[6768rogues]

If the fiberglass insulation is between ceiling joists so that laying rock wool perpendicular will not compress the fiberglass, do it.

 

[AldeanFan]

Thanks for the advice
I had not thought about the extra weight on top of the fibreglass being a problem.

I've done blown in on top of fibreglass before and it worked great, but for only 300sqft it's not cost effective to rent the machine, I could use blown in but just open the bags and rake it out.

Or I could strap the top of the joists with 1x2 to support the roxul.


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[DarrenF]

I could use blown in but just open the bags and rake it out.




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i have done this for small areas, works fine

 

[zktk01]

Make sure if they blow in insulation there is baffles so it doesn't block airflow from your eaves.