A while back I asked about adding my 1.5" thick pieces of mineral wool insulation batts ontop of my 12" thick fiberglass I am installing in my attic. As I have began doing this, I am seeing that the fiberglass is so fluffy that throwing on the 1.5" thick batts weighs the fiberglass down an inch or more. There is no way this insulation will not settle if it squishes that easily. Am I doing more harm than good with the extra layer? This 12" insulation is 10" at best.

  • I'm guessing there ceiling joists. How much does the fiberglass project over the ceiling joists? Or how much will the fiberglass compress until the mineral wool is supported by the ceiling joists? – ojait Nov 24 '15 at 16:25
  • @ojait - it sticks out roughly 5 to 8 inches. Some trusses are 2x4 and some 2x6. I'm not leaning towards no mineral wool except possibly stuffing some in the very corners (hip roof). – Nic Nov 25 '15 at 2:20
  • Ohh, Trusses! They're a pain to insulate around. So there is 5-8 inches that the mineral wool can compress before being stopped by the truss member? The 'corners' are the roof/wall location? – ojait Nov 25 '15 at 3:00
  • This is new construction so much easier than crawling in an attic. The fiberglass can compress that much if I had enough weight. The mineral wool only compresses fiberglass 2-3". But still a backwards attempt. The corners are tight kneewall areas where a bridge truss runs with jack truss members connecting to it to form the hip style roof. – Nic Nov 25 '15 at 12:52

Even though the fiberglass is getting slightly compacted by the mineral wool on top; the minor R-value that may be lost to compaction is easily outweighed by the increased R-value of the extra mineral wool. The only adjustment that could be made (and it is a laborious one) is to increase the height of the joists so they are above the fiberglass and support the mineral wool. Just to be sure the mineral wool is being installed at right angle to the fiberglass?

  • I can't find a huge difference in r-value between fiberglass and mineral wool per-inch. I'm seeing, at most, a range of 2.5 (fiberglass) to 3.0 (mineral wool). I don't think the math is actually coming out showing that the mineral wool is a net benefit after compaction of the fiberglass. – DA01 Nov 24 '15 at 15:22

You are correct, the R value of any material depends on many things, but with fiber type insulation, the internal dead airspaces are critical. If you have a 12" insulation that has been compressed to 10", you have lost 2/12 of your R value, or about 17%. The coefficient of thermal conductivity hasn't changed for the material (glass, stone, etc), but the CTC of the trapped air has because there is less of the air in the insulation. If you started with an R30 and you lose 17%, you are down to an R25. This will decrease over time as well as the insulation compresses with age. As long as the mineral wool insulation is higher than R5, you are winning. However, for a 1.5" batt of mineral wool insulation like Roxul or something similar, you probably are less than that (but it does depend on the specific thermal insulative characteristics of your actual material. You would be much further ahead to add a dense Styrofoam like SM. It is light and you could put the individual sheets on little risers (lumber, strips of foam, etc) above the ceiling joists to ensure a seamless foam barrier and prevent future settling and thus compression of the fiberglass batts below.

  • I have roughly 150 pieces of mineral wool shavings that are16" x 1.5" x 47". The fiberglass comes extremely compressed. I fluff each piece in the rafters and maybe get the full 12". But I watch it compress itself even after I fluff it since it is not dense. Mineral wool is better r value and much more dense. I don't know where to use these scraps otherwise. And I am not sure if the fiberglass benefits from having the mineral wool atop if it squishes an inch or two down from weights but gains density. – Nic Nov 24 '15 at 13:04

As Personal says, the value of batt insulation is primarily in the air. So compressing definitely affects the R-value.

Fiberglass batt insulation has an R-value of 3.14 per inch.

12" will give you about 38 for an R-value.

10" will give you about 32.

Mineral wool gives you about the same R-value.

So, yes, you are likely doing more harm than good or...at best, no harm at all but just adding time and money...by going from 12" of insulation to 11.5".

  • Added a comment to personals answer. Still thinking its a time waste and to just leave it alone? – Nic Nov 24 '15 at 13:07
  • @nic I don't think Mineral Wool is all that more efficient than fiberglass. Most show it having the same--if just slightly more--Rvalue vs. fiberglass per inch. As such, yes, it's a waste of time and money if you are ultimately ending up with fewer inches of total insulation. – DA01 Nov 24 '15 at 15:20

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