insulation question

[dug]

Looking at insulation for the ceiling now and have a question regarding R30 vs. R30c fiberglass batts. All of the places I have called so far stock R30 but have to order R30c, at a substantially higher price (.75 cents vs. $1 sq. ft.).

I know the R30c is thinner to accommodate the venting space above. How much R value would I lose compressing the R30 slightly? 

Also would appreciate any other economical options to the batts that anyone has experience with. I knew insulation was expensive but $1200 for my ceiling was a little more than I was hoping for.

[John Raabe]

If you can provide for a vent space with a baffle or other supporting structure, then a bit of compression on the standard batt will reduce the R-value only slightly. More important is to do as good a job as possible to eliminate void spaces at top, bottom and sides. Too tight (and slightly compressed) is far better than too loose. Voids provide a nearly straight shot to the outside and a couple square inches of that will offset many square yards of R-28 vs R-30.

[MountainDon]

I believe I copied this chart from Owens-Corning.  It shows what R-value results when fiberglass batt insulation is compressed.



The R30C used with a 2x10 leaves a one inch space




Note what happens with the very commonly used R-19 batts when installed in a standard 2x6 wall.

[John Raabe]

Good chart Don, thanks.

[rocking23nf]

Is r30 enough? up in canada they use r40 and r50 for attic insulation. I used a double r20 in my cabin.

[dug]

Insert Quote
If you can provide for a vent space with a baffle or other supporting structure, then a bit of compression on the standard batt will reduce the R-value only slightly.

I was planning on using the baffles, but was wondering if you used R30c could you omit the baffles?

Note what happens with the very commonly used R-19 batts when installed in a standard 2x6 wall.

Everyone around here stocks R19, R21 is special order- more $$. I'm curious as to why the R21 is thinner, yet has higher R value than the R19?     Maybe I am misinformed on the thicknesses.

Is r30 enough? up in canada they use r40 and r50 for attic insulation. I used a double r20 in my cabin.

Plenty for here in SW NM. 

[John Raabe]

Don's chart is helpful here.

Say you are using 2x10 rafters. Then R-30C batts will give you R-30 in an 8-1/4" thick batt leaving 1" of vent space so you don't need a baffle. A standard R-30 batt in the same cavity will give the same R-value but will fill the cavity and be compressed 1/4". You have no vent channel and installing a baffle will compress the insulation more.

The high density batts have a greater R-value per inch, and like foam have more isolated air spaces per inch of thickness. That's what makes heat work harder getting out of your house. In fiberglass this takes more material and that is what you are paying for.