Compressed Insulation R values

Does anyone have information/data for quantifying compressed insulation R values?



Inspect-R® Density Gauge

Not sure if this is what you are looking for.

Insulation compression Chart


Thanks Gents. I wasn’t clear… If you have say 12" of blown fibreglass you’d have R40 approx. After sparky goes into the attic and sits on the insulation while he installs can lights and leaves it at 3" what’s the R Value? This was a point of contention with the selling agent this afternoon. We all know it loses R as it’s compressed, but I have no chart/gauge/notes on this subject. I was hoping there might be an article or something on this.



No, 12" of blown fiberglass in an attic is about an R-25

I do not know the R-value when it is compressed.

Sorry, yes. This was rock wool.

We would all agree that settled or “squeezed” in this case would reduce the R value though, yes?

When you compress insulation the r-value per inch can go up but the rated r-value goes down. So if it’s compressed down to 3.5" then typically it is rated at the standard r-value at 3.5". But since the whole attic is usually not compressed in every area you have to use a weighted area value calculation to get the best idea of it’s effective r-value.

BS, No it is not!

The rated value is based on the manufactured consistency of the insulation.

3" of R-11 is not the same as 3" of R-19…

Review “conduction”.

David is correct.
“All insulation materials… shall be installed in accordance with manufacturer’s instructions” is found in every published edition of the Model Energy Code (MEC) and the International Energy Conservation Code (IECC).

A read of this Article here, may help.

Insulation Inspections for Home Energy Ratings

R-Values Per Inch of Some Typical Insulating MaterialsMineral Wool Batts3.5Fiberglass Batts3.1 - 3.5Rock Wool (Loose Fill)3.0 - 3.3Cellulose (Loose Fill)3.2 - 3.8Fiberglass (Loose)2.2 - 2.7Perlite (Loose)2.3 - 2.7Polystyrene Boards (Rigid)3.6 - 5.3Polyurethane Boards (Rigid)5.6 - 7.7


Thanks guys. I just thought there may have been a study done on what “X” inches of “type” insulation is rated at when it’s compressed to “Y”.

I had a selling agent challenge my suggestion of topping up compressed areas. As I had no academic literature I though I’d see what I could learn on the subject.



BPI states R-values of installed insulation shall be determined based on an actual
measurement of the insulation depth and the R-value per inch for that product.
In this case if there is loose fill unknown manufacture. *Default Values for Insulation
When manufacturer’s rated R-values for insulation are not available, use the chart below
to estimate the R-value per inch for the installed product. *

The chart has no value for compression. It only addresses fluffing. The fact is the compression changes the r-per inch only slightly.

Thanks Mr. Adair.
Bloody nice link.
I have been using that wrench-less chuck for 15 years but you must keep it clean. Mine were for my concrete blades on 4, 4.5, 7 grinders. Sears craftsman had them many moons ago.

I developed the first table which is the estimated R-Values of blown cellulose that has been compressed. The following assumptions have been made:

  • The manufacturers of cellulose list the R-Value of their product “after settlement” which is about 10 to 20 percent.
  • The density after settlement is approximately 3.62 lbs/ft3
  • The ASHRAE handbook was used to determine the change in the Thermal Conductivity versus the change in Density. Basically as the cellulose compresses the density goes up. As the density goes up the thermal conductivity goes up. The R-Value is the reciprocal of thermal conductivity or (1 divided by the thermal conductivity), which means the R-value goes down.
  • The compressed R-Values were rounded to the nearest 0.5 so that’s why you see several numbers the same. For example R-22, R-24, & R-26 all show R-18 when compressed into a 2x6. However the true R-Values were 18.2, 18.0, & 17.8

You could use this as a guide for sections of an attic where foot traffic has compressed the insulation or plywood was placed over the insulation for storage.

The second chart is for compressed fiberglass batts produced by Owens Corning.

These charts are very generic so don’t stick you neck out too far when using them… :wink:

Its a good night to visit the MB. Thanks Mr. mayo.
Very nice work on your behalf.

Cellulose has a very nice property…it maintains its R value over a range of densities from about 2.2 lbs/cu ft to 4.0 lbs/cu ft. In that range, the old style material R value was 3.6/inch.

The newer material manufactured by the “Fiberizer” process has an R value of 3.8/inch with a density of only 1.8 lbs/cu ft…nice- higher R using less material!!


The R-factor of cellulose insulation is approximately 3.8 per inch and it does not vary significantly over a wide range of densities. In an attic, 10 inches of cellulose insulation will have an R-Value of about R-38, regardless of the density of the material

Questions. HI or anyone on the thread.
It is proof that cellulose is a good all around insulating product. Why is the government not doing something about fiberglass insulation and phasing it out of the building environment.
I heard that under a micro scope fiberglass has a similar shape to asbestos. That the fiberglass partials enters your soft lung tissue in the same manor as asbestos only asbestos triggers a chain reaction that can trigger cancers. They are not certain about fiberglass.
The link was not what I was explaining but more on a note on fiberglass.

Safety and Health Concerns: Fiberglass

The American Conference of Governmental [FONT=Arial]Industrial Hygienist, the North American Insulation Manufacturers Association and other groups also underline the fact that conclusive research has not shown fiberglass to be a carcinogen in humans.[/FONT]

Thanks Marcel.
The American Conference of Governmental Industrial Hygienis

I like to see a third party with no ties to government or industry research to avoid any possible lobbing.
Carcinogen are one thing but overall oral orifice, breathing pathways and lung tissue and over all human health would be nice to see.JMO
Thanks again.

ACGIH is not an industrial nor government controlled organization. They are recognized around the world.


Thank You for putting that chart out there. It,s a nice tool to have in my arsenal!