Owens Corning R-30C

[Clutchcargo]

Is there any reason that I cannot use Owens Corning R30C insulation in the floor of my uninsulated attic? The Owens Corning website (http://www.owenscorning.com/around/insulation/project/cathedralceiling.asp) only lists Cathedral ceilings as the application. The floor joists are only 2x6s but I figure I can raise them a couple inches with 2x3 sleepers. Otherwise I can use R21 without the sleepers.
TIA
Cliff

[Wgoodrich]

I am curious, why are you insulating R30 between a ceiling and a flooring above. Is the upstairs storage only or living area?

You may use the R30 insulation as you discribe but only with any vapor paper backing against the ceiling if that is an unheated attic. Vapor barrier is required on the warm side being toward the ceiling.

Then I am curious if wiring is above this ceiling joist. Then I am concerned if this ceiling joist is rated only for a 10 pound live load as a ceiling or rated as required for a floor at 30 pound live load as a floor.

You raised some cencerns for me before you go and do this. Have you done your research. Right now you are leaving a lot unsaid in your post.

Curious

Wg

[scuba_dave]

If this is unheated space, and there won't be a floor installed to walk on; then I would consider R38 insulation. Check the price between the 2

In NE they recommend a total R value of R-49

The "C" stands for Cathedral. It is made to fit in a small space & still allow for venting.

[Clutchcargo]

Thanks, why does Owens Corning market this product for just cathedral ceilings. Since the high density fiberglass has a higher r-value than the standard density fiberglass, why not use the high density stuff everywhere?

The attic is uninsulated/unheated storage area only, not living area. The ceiling only allows for about 6' of headroom in the center of my hip roofed house.

When I pulled up a floorboard, I was surprised to find only about 3" of rock wool type insulation with the vapor barrier on the wrong side. If I want to keep the floor as is I can only squeek in r19. My first thought was to try keep the storage area by building up the joists a few inches and then fill between them but now I'm thinking of just filling the area with insulation and make the attic unusable. I suppose I can put 2 or 3 sheets of plywood on stilts for storage in the area of the access door.

[scuba_dave]

In a Cathedral ceiling a 2x10 is 9 1/4"
You need to allow 1" for an air gap for proper ventilation
So the R30C is 8 1/4" thick, normal is 9 1/2"
The R38C is 10 1/4" for 2x12's Cathedral, normal R38 is 12" thick
With a normal flat ceiling you do not need the 1" air gap

I'm using the R30c on my Cathedral ceilings
Then switching to R38 where the ceiling transitions to a flat ceiling

When you think of it, R38 is almost 30% more then the R30
Oil prices aren't going down

I'm also insulating the walls between rooms for some sound proofing
The floor between the 1st & 2nd floor is already insulated with R25

[Clutchcargo]

I guess my rhetorical question is why doesn't Owens make all of there sizes with this more dense fiberglass? You get a higher r value for the same amount of space. There's got to be something else to it.

[suemarkp]

I would expect it to cost more. Does it?

[scuba_dave]

I bought mine over a year ago, I think it was more $$
I think they do make more sizes & R values using the same method
There is an R-15 that fits in a 2x4 wall cavity
But most building projects will use what code calls for
The more that sells, the lower the cost
I do 2x6 construction now & R-19 for walls
I like the added insulation, wide windows sills, and extra room between the 3" electric box & outside wall

But how many builders do that unless a custom house?