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Definition of Compressed Insulation

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Cinci-hal

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Oct 15, 2020
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Cincinnati
What exactly is the definition of uncompressed insulation? I am referring to:

1102.2.1 Ceilings with attic spaces:
Where Section 1102.1.2 requires R-38
insulation in the ceiling, installing R-30 insulation over 100 percent of the
ceiling area requiring insulation shall satisfy the requirement for R-38
insulation wherever the full height of uncompressed R-30 insulation extends
over the wall top plate at the eaves.

I have been talking with an architect who is also a plans examiner and he was saying that all of the insulation below the insulation baffle that was below the full height of the blown-in insulation was considered compressed whether or not it was literally compressed. In other words, If we have 12" of blown-in insulation that has an R-value of 38 and we have an insulation baffle at the eave of the roof with enough space above the top plate to allow for 9" (R-30) insulation without being literally compressed, but not enough space for 12" of insulation, that the 12" (R-38) insulation is still considered compressed because of the existence of the insulation baffle.

I would prefer not to limit the insulation to 9" across the entire ceiling area, but would prefer to indicate on the section detail that there is at least 9" of R-30 insulation below the insulation baffle that is not literally compressed.

Does this make sense and would or should it be acceptable?
 
What the code appears to be referencing here is that if raised heel trusses are used and the full value of R30 can be achieved above the exterior walls, R30 is sufficient. But if the full R30 value of insulation cannot be achieved here, R38 must be provided to make up for the missing insulation around the exterior walls.

The use of the word "uncompressed" by the code may be misleading in this instance.
 
steveray said:
Take a look at this if you can....

Click to expand...
I am aware of the effects of compressing insulation and that is actually the point of my question. As my statement said, the Architect / Plans Examiner that I talked to is under the impression that other plans examiners are going to assume that any blown-in insulation below the insulation baffle that is below the full height of the blown-in 12" (R-38) insulation is going to fall under the catagory of compressed insulation even though it is not literally compressed. I want to have a note that states that the blown-in insulation that is below the insulation baffle is not compressed and has an R-Value of 30 which should comply in my opinion. The question is, will a plans examiner accept that note?
 
I'm not sure how blown-in insulation would be compressed, unless you physically tamped it down after it was blown-in.

If at any spot you have less thickness than required for R-30, then you have to provide adequate thickness for R-38, except at the eaves where there isn't adequate framing depth for R-38
 
Paul Sweet said:
I'm not sure how blown-in insulation would be compressed, unless you physically tamped it down after it was blown-in.
Click to expand...
The only way coming to mind where this could be the case would be if it was a cathedral ceiling with poly vapour barrier on the inside of the building, you can see blown in insulation "slide" down away from the attic slightly, but compression would be relatively minor.
 
Cinci-hal said:
I am aware of the effects of compressing insulation and that is actually the point of my question. As my statement said, the Architect / Plans Examiner that I talked to is under the impression that other plans examiners are going to assume that any blown-in insulation below the insulation baffle that is below the full height of the blown-in 12" (R-38) insulation is going to fall under the catagory of compressed insulation even though it is not literally compressed. I want to have a note that states that the blown-in insulation that is below the insulation baffle is not compressed and has an R-Value of 30 which should comply in my opinion. The question is, will a plans examiner accept that note?
Click to expand...
I doubt the plans examiner will call it out, but if they do, the note should suffice. In the end, it is the inspector's job to verify that it is done properly, so the plans examiner should defer to the inspector to catch any issues in the field.

If I were the plans examiner and I was really concerned about it, I would add a note somewhere so that the inspectors don't forget to check it in the field.
 
The way I have enforced this requirement; if you can get the full R-30 outside of exterior wall plate to outside of the opposite wall plate you can use R-30 for the the entire attic floor. In a pinch we have suggester to use closed cell foam to get the required R-30 at the roof edge, providing that there is 5 or more inches of space with cellulose for the ballance.
 
Tmurray hit the nail on the head...
Compressed insulation is sorely misunderstood. Compressed is where the cavity to be filled now has nothing for any debth more than the staple flange of kraft facing and where the side of the framing is exposed. Whats fooling most is when a bat Compressed to fully fill a cavity.. most say the R value drops, however it actually goes up (to a certain point). Insulation work by trapping air. A high density bat traps many smaller pockets. Putting R30 into a 2×8 ceiling cavity creates a high density bat.
The key is r-something always beats r-nothing and more of something is (up to a point) better than less..

Im on my mobile, but google compressed fiberglass r value chart, the big manufacturers have tables showing the tested R value can be more than the lable in certain applications.
 
bill1952 said:
Per inch, not for the batt overall. So say an 8" batt compressed to 6" will have greater R than 75% of the batt at 8", but less than the batt at 8".
Click to expand...
Correct, as mentioned there has to be something taking the place of the compression (as in more fiberglass) for the overall cavity r value to be above what you might expect...

If I attached the correct document (again on a mobil) ...

R49 at full fuff (Uncompressed) needs 14"
Compressed down to 9.25" it behaves as R37.
An R30 batt at full fluff needs the 9.25" and is -7R less than the 49.

That R37 is better and compression isn't always bad...
 

Attachments

  • 10017857-Building-Insul-Compressed-R-Value-Chart-Tech-Bulletin (2).pdf
    91.5 KB · Views: 5
Which says denser is greater R value. Just like dense packed cellulose - 3 pcf - is greater R value than lesser density.
 
Yes, again Bill nails it with less lol...

For those who skim posts or are afflicted with the building science bug....

Generally more is better, but the entire cavity needs to have something.
Popular in my area is flash and batt install, where 2" of closed cell (R6.2 per inch unless the manufacturer spec sheet states otherwise) is applied to interior of the sheathing. That is R12.4 cavity. From the chart previously uploaded and for ease say a 2x4 cavity had 2.5" left. If R13 was compressed into that cavity (sheetrockers would have a fit!) Or R11 the cavity would have R12.4+10 or R12.4+8.9.
While the R10 is less than 13 or 8.9 < 11 the Rvalue per inch goes up.
Say you get a set of plans with 2x4 walls showing an R value of 22.4 or 21.3, its a simple matter to now check the sectionals to see if the data is reasonably accurate..
 
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