INSULATING ROOF OF A TWO-STORY STUCCO HOME

QUESTION FROM PETE of Minnetonka, MN:

My attic is not insulated very well. i will be using fiberglass blown in to add to blanket of insulation. at the four corners of the attic, the roof line goes diagonally downward into the 2nd floor. all 4 corners of the 2nd floors have angled walls from the pitch of the roof above.

Do you recommend blown insulation down between the joist spaces with or without a vent tray? There is not much room to work with and the nails from the shingle roof poke though into this cavity. not sure i can get a vent tray into the pitched roof joist cavity AND get blown in insulation on the underside. please comment.

Additional Question:
Thank you. this is helpful. I also want to blow insulation down the vertical joist spaces along the exterior walls. Are there anything I should know about this….or can I simply blow insulation down into all accessible areas, filling in the void? Again, I plan on using fiberglass blown-in for this.

Back to the slotted rafter spaces: if I keep those spaces as is but blow in the horizontal surfaces of the attic space, I should then install vent trays where attic floor meets angled roof so as to allow air movement from the slotted rafter areas into the open attic space, correct?

Question #3:
Should I be concerned that the cellulose in the exterior walls will trap moisture, or will it naturally allow moisture to travel through it without traping and causing rot in the woof sidding, behind the stucco?

Question #4:
What is your opinion on Fiberglass versus cellulose for blowing in attic space? Relating to: Fire retardent properties R-values Cost Settling Also how many inches of Fiberglass are needed to reach an R60 in attic space?How many inches of cellulose are needed to reach R60 blown into same attic space? I plan on using Fiberglass for attic and cellulose for walls.

ANSWER FROM THE INSULATION DOCTOR:
 
Thanks for your question.  It is imperative that you maintain an air space in this sloped rafter area.  If you cannot install an air baffle (vent tray) you should not install any blown-in insulation.  Failure to maintain an air space will eventually rot the roof boards and will cut the life span of your shingles in half.   

Your option may be to buy 3-1/2" R-ll fiberglass rolled insulation and try to slide it down on top of the plaster or drywall.  This can be accomplished by taking a hockey stick, wrapping about a foot of the insulation over the end of it and pushing it downward until it stops.  Once it stops bring the hockey stick back about a foot and pull the insulation toward you then push the hockey stick back down pushing the portion you overlapped the hockey stick with flat against the ceiling.  Be careful not to block off the lower end of the rafter space.  

If you cannot install insulation in this way, I would recommend you just leave it alone.  The result will be you will lose some heat, you will also have some icycles, but will continue to have a roof over your head!
 
Hope this helps.
 
Continuing Answer:
Hello Again,
 
You should install baffles (vent trays) prior to installing the blown-in fiberglass insulation in the roof rafters.  
 
Regarding blowing in the outside walls, I am assuming that this house, due to its age, is what is referred to as balloon construction.  This means that the wall cavities are open into the attic area.  I would recommend that you use blown-in cellulose in these cavities for the simple reason that it is a finer-sized product and will compact better at the original installation.  Fiberglass tends to hang up on nails, wiring, etc. (speaking of wiring - what type of wiring does the house now have?  It needs to be romax).  
 
Things to look out for when blowing in the walls:

1.  You will need to double-check that if it is a forced-air heating system, that heat runs and cold air returns are headered off above the openings.  If not, you will either fill the room or fill the furnace (Not good!).  

2. Be aware that any small opening will allow the insulation to blow out into the room or outside.  You are blowing insulation into a cavity with air so be sure to allow the air to escape.

3.  To allow the air to escape, the best method is to take a piece of blanket insulation, hold it over 90% of the balance of the opening.  So what you would have is the hose going down into the opening, a piece of blanket insulation acting as a shield with about a 1" opening at one end and this will allow the air to escape and help prevent blow-back when the insulation reaches the top.

Have a good day.  BTW, wear a mask while doing this type of work!
 
Answer #3: 
There should be no issue with moisture under normal conditions.  In other words, as long as there is no leak in the wall with concentrated water.
 
Answer #4:
The best comparison for fiberglass vs. cellulose would be the analogy of a Ford vs. a Chevrolet.  They are both good products and they both will get you where you need to go.  My personal opinion is that cellulose does a better job in a closed cavity situation such as yours and I would always prefer blown-in fiberglas in an open attic for the simple reason that even if it settles it will retain its original r-value.  Both products have the same fire resistance.  Cellulose is about 10% less expensive because it is not a virgin man-made product but is a reprocessed product.
 
An R-60 cellulose would require 16.72" and an R-60 fiberglass would need 24" of product. 
 
Good luck on your project!
 
Bruce Jones
The Insulation Doctor