My atttice was never properly insulated. Maybe an R13 at best. The craft paper barrior is installed correctly, towards the heated space.

Would it be in-advisable to blow in on top of the fiberglass? Or does it need to be removed? Seems to me that it would be okay to leave in, just not sure.

The house is in the Pacific Northwest. We want to insulate to keep the heat in winter, we do not have need for air conditioning, as most here seem to.

Thanks folks

Mark

So long as there is no mold in the fiberglass, and so long as whatever will be holding it up is sturdy enough, this is fine, and is actually done quite frequently by insulating contractors. While the fiberglass will lose R as you compress it, so long as you understand that, and add enough cellulose it's fine, you aren't going to hurt anything doing just that. And the compressed fiberglass still keeps some of it's R value, if you totally compressed it down into a flat hard board it would probably still be about R3. You will be doing well to add in the cellulose over it, you will reduce the chance of air gaps and I'm sure your heating bill will reflect the better performance, so long as you don't have any other issues that is, haha. Good luck!

Check out a site called building science, I'm sure they have detailed analysis of this practice there, that site is awesome.

Blowing over the fiberglass is standard practice, and even in relatively thin 3" layers can improve the R-value of low density fiberglass itself by reducing convection loops within the fiberglass in winter, and stopping radiated heat at the upper surface in summer of the insulation (fiberglass passes some radiated heat absorbing some internally- the hottest temp within the insulation is an inch or so into it, not the surface!)

But before you blow any insulation, get serious about air-sealing every plumbing & electrical penetration into the attic with expanding foam. If you have any recessed can lights sticking up there, either replace the fixtures with air-tight insulation-contact versions or build an airtight box over them with a few inches of clearance. Insulating over recessed lighting cans can be a fire hazard, and non-air-tight fixtures are a heat & moisture leak which can lead to a moldy attic. Read this:

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/files/bsd-102_understanding-attic-ventilation.pdf/attachment_download/attachedFile

In many cases you can get better overall performance if you seal the attic with half-pound foam in addition to adding cellulose on the attic floor, since it's easier to get a perfect air-seal using foam, which blocks the whole-house stack-effect convection from drawing warm humid conditioned space air through the fiber insulation into the cool attic in the first place, limiting the condensation factor that the attic ventilation was designed to take care of.

If staying with a ventilated attic, preserve soffit ventilation using purpose-built chutes to keep the insulation from blocking them. (Warm air leaking up through the exteiror wall cavities can condense and soak the insulation at that point if you don't.)

The R-value of the fiberglass will still contribute to the insulation, but it will compress somewhat. As fiberglass is compressed it's R-value PER INCH goes up very slightly, but being compressed, it has fewer inches: An R13 batt compressed from 3.5" to 2.0" will still perform to ~R8 (Unless you're adding 2 feet of cellulose, you'll retain at least R8 out of the fiberglass.) Similarly, as open-blow cellulose settles over time it's per-inch numbers remain fairly constant (rising slightly), but with fewer inches it'll be lower overall R. So when specifying the job (or doing it yourself), think pounds per square foot, not just inches of depth. Manufacturers ratings per bag typically give square feet of coverage at a specified R-value, as the SETTLED value, but the initial R-value (and depth) will be somewhat higher, 5-10% higher than it will be in a coupla decades. Know the square footage, and count the bags.

Overall performance also goes up dramatically if you can get at least 2-3" over the tops of the joists as a thermal-break. The insulation value of the joist is only about R1 per inch, whereas the insulation value of cellulose is between R3.5-R3.8 per inch. If you have 2x8 joists, 2" of cellulose above the joist doubles the R-value at that thermal short-circuit.

Blown insulations tend to outperform batts in real-world-typical situations by ~15% due to the fact that they fill in all the gaps, large & small. If this is a DIY project, be sure you rake it out smooth on the top to make sure you don't have any cratered thin spots, etc. Open blow cellulose isn't rocket-science. If you have a full attic floor, you can also "dense pack" the cellulose in the floor cavities if the ceiling has sufficient structural capacity (half-inch sheet rock on 24" center joists doesn't qualify). With dense-packing air infiltration through the insulation drops by over 95%, becoming your de-facto air seal in some climates. This isn't normally a DIY project, but even beginners can get it right with a bit of reading-up on it. A guy named Rick Karg has volumes of web-published tips of the cellulose installation trade see:

http://www.karg.com/papers.htm

http://www.affordablecomfort.org/images/Events/16/Courses/295/W3-BEST_InsulationInstallation_Karg.pdf

http://www.affordablecomfort.org/images/Events/20/Courses/494/Karg_DONE4.pdf

(there's more, much more google it)

Adding ten inches to foot of cellulose over thin batts is usually advisable- more is probably overkill in the PNW. More than a foot starts running into substantial weight, something of concern about if it's only sheet-rock holding it up.