# Does mineral wood insulation lose R-Value due to compression?



## Windows on Wash

How much are we talking about? 

http://www.bestofbuildingscience.com/pdf/Insulation%20inspections%20for%20home%20energy%20ratings%20HEM_22-1_p20-23.pdf


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## Bud9051

Hi TRK, you said "I know that with fiberglass batting insulation it loses a ton of R-Value if it is compressed." 
That's not exactly true. The primary loss is due to less thickness. The r-value per inch actually increases a bit with compression, this is for fiberglass insulation. With excessive compression you eventually eliminate a lot of the air spaces and it is those air spaces that provide the insulation.

As for mineral wool I would guess (no information) it would tolerate very little compression as it is very dense to start with.

Bud


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## Gary in WA

Compressing rock wool insulation increases the R-value to 4.1- if you can compress it that much... http://www.roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/Building%20Envelope/ROCKBOARD_BROCHURE_EN.pdf

FG also increases R-value, you just need to add more to fill a cavity, and you end up with a board- also. http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/

R-13 FG is just R-19 compressed from a low density to a high density insulation- without inherent convective loops as with all R-19 FG; pp 41/96, here; https://www.huduser.gov/Publications/pdf/walls.pdf

Gary


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## Bud9051

@gary "R-13 FG is just R-19 compressed from a low density to a high density insulation-"
I don't think that is what page 41 is saying, maybe I missed it. R-13 and r-19 are both basically low density insulations. Each is available as high density at R-15 and R-21 respectively. The nice part about using the high density products is they fit without fighting to compress them.

I haven't seen any discussion on compressing Mineral Wool but would think it more difficult and less beneficial than cutting it to fit.

Bud


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## TheRatKing

Well, the reason I am going to be compressing mineral wool is I only have access to batts and not comfortboard made by Roxul. I am going to be putting in mineral wool 2x6 24" OC batts into my attic. I was going to then lay a piece of comfortboard perpendicular to the joists and put a piece of 22/32 plywood on top. I know that the Roxul for 2x6 24" OC is R22, so I was going to buy some 2x4 24" OC batts, lay them perpendicular to the joists and put a piece of plywood on top. I know that the plywood will compress the mineral wool quite a bit, and I will be screwing everything down. I wanted to be able to get close to R30, and still be able to put down some plywood so I can walk up there to maintain the HVAC equipment. So I was hoping that would get me over the minimum R30 for attics.

Also I don't understand what you're saying when you claim..."That's not exactly true. The primary loss is due to less thickness.", that is exactly what compression is doing, reducing the thickness of the fiberglass batt. This also...

"The r-value per inch actually increases a bit with compression, this is for fiberglass insulation."

I don't believe this is true because looking at a chart of the R-Value of compressed fiberglass batting the R-Value per inch seems to stay relatively the same. Fiberglass batting is around R2.9 - 3.8, and when taking a 12" thick piece of fiberglass batt and compressing it down to 3 1/2" the R-Value per inch is R3.1, compressing the same batt down to 2 1/2" the R-Value per inch is R3.2. I just couldn't find any charts like this for mineral wool insulation.

http://www2.owenscorning.com/litera...ul Compressed R-Value Chart Tech Bulletin.pdf


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## TheRatKing

Argh, can't edit posts, so I was going to add a contingency to the above statement. I said...

"I don't believe this is true"

You are technically correct, but it is misleading to say the R-Value per inch will increase due to compression but gloss over the fact you can take a R38 12" fiberglass batt, compress it down to 2 1/2" which would be R8, losing 30 R-Value, but you did increase the R-Value per inch. See what I mean?


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## Gary in WA

In answer to post 5; R-13 is a* medium *density, I stand corrected, good catch.*Fig. 1* shows the compression*, *on pp. 41- the dotted lines. Lol, I miss a lot, or so I've been told.... comes with age- is my excuse. If you read the "compression chart" link- at bottom with R-19- 6-1/4" compressed to 3-1/2" above in chart = R-13. This way the manufacturer can make fewer material molds of the insulation, using the same amount of fiber just compressing it more before adding the glue= cost savings.

You may have missed this; "*if you can compress it that much...*" nothing like FG, compresses from 6-1/4" to 1-3/4" in floor over 20 years all by itself... should've added a smiley after it, I guess..  so you know not to try and compress rock wool. Just showing when you do, it increases the R-value/density per inch.

Gary


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## Gary in WA

Read "*15.3 Constructability and cost"* on pp.* 68/73;* http://apps1.eere.energy.gov/buildi...uilding_america/high-r_foundations_report.pdf

Gary
PS Table 1 shows the low, medium, high ratings in density; http://ws680.nist.gov/bees/ProductListFiles/Generic Fiberglass.pdf


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## Bud9051

TRK, we need to review how you are reading that chart.

The center of that chart gives you the resulting r-value when you take the r-value listed on the product label with its intended installation thickness (shown at the bottom) and install it into the thickness listed to the left.

At no point on that chart does it indicate: "and when taking a 12" thick piece of fiberglass batt and compressing it down to 3 1/2" the R-Value per inch is R3.1, compressing the same batt down to 2 1/2" the R-Value per inch is R3.2."
The only two options for 12" are the first two columns, R38 and R38C installed into a cavity depth of 11.25". The chart does not give values for compressing 12" down to other thicknesses.

As for the r-value increasing as you compress it, go to your chart for r-13 for 3.5" at the bottom. It gives you 3 options, 3.5", 2.5", and 1.5". If you divide the r-values shown for each by those depth options you get: 
13/3.5 = r-3.7 per inch 
10/2.5 = r-4 per inch
6.6/1.5 = 4-4.4 per inch
So, while the installed r-value is dropping from r-13 to r-6.6 the r-value per inch is increasing from 3.7 to 4.4

The primary controlling factor is the final depth. Multiply that times some slightly increased r-value per inch for your total value. With a 5.5" floor cavity, Roxul designed for that depth will give you r-22 or r-23. The bales I have are r-23, I checked.

Bud


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## Bud9051

@gary post #9:
First link: Interesting, but once the glue (or however they stick it together) is dry those batts have a memory, as we see when we open a highly compressed bundle, and each begins to fluff back to its intended thickness. 
Second link: I saw R-13 through r-38, but not low, medium, and high for various thicknesses. 3.5" is shown with 2 densities, but not identified specifically as low to high.

Bud


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## TheRatKing

Gary in WA said:


> Read "*15.3 Constructability and cost"* on pp.* 68/73;* http://apps1.eere.energy.gov/buildi...uilding_america/high-r_foundations_report.pdf
> 
> Gary
> PS Table 1 shows the low, medium, high ratings in density; http://ws680.nist.gov/bees/ProductListFiles/Generic Fiberglass.pdf


Lots of great information, and it will probably take me awhile to process everything, lol. Like a few months, hahaha.


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## TheRatKing

Bud9051 said:


> TRK, we need to review how you are reading that chart.
> 
> The center of that chart gives you the resulting r-value when you take the r-value listed on the product label with its intended installation thickness (shown at the bottom) and install it into the thickness listed to the left.
> 
> At no point on that chart does it indicate: "and when taking a 12" thick piece of fiberglass batt and compressing it down to 3 1/2" the R-Value per inch is R3.1, compressing the same batt down to 2 1/2" the R-Value per inch is R3.2."
> The only two options for 12" are the first two columns, R38 and R38C installed into a cavity depth of 11.25". The chart does not give values for compressing 12" down to other thicknesses.
> 
> As for the r-value increasing as you compress it, go to your chart for r-13 for 3.5" at the bottom. It gives you 3 options, 3.5", 2.5", and 1.5". If you divide the r-values shown for each by those depth options you get:
> 13/3.5 = r-3.7 per inch
> 10/2.5 = r-4 per inch
> 6.6/1.5 = 4-4.4 per inch
> So, while the installed r-value is dropping from r-13 to r-6.6 the r-value per inch is increasing from 3.7 to 4.4
> 
> The primary controlling factor is the final depth. Multiply that times some slightly increased r-value per inch for your total value. With a 5.5" floor cavity, Roxul designed for that depth will give you r-22 or r-23. The bales I have are r-23, I checked.
> 
> Bud


I definitely read that chart wrong! Thanks for pointing out the error, and it does seem the R-Value per inch increases by quite a bit more than I originally thought. I still feel though that you lose so much R-Value due to compression with fiberglass batts. I was just curious if this same type of loss happens with mineral wool batts since I was going to be laying plywood on top of the extra mineral wool I was going to place on top of the batts. Even though I tend to lose quite a bit of R-Value with the compression I think it will be worth it since I could still could hit R30 as a minimum. That was the decision I was trying to weigh out and ended up learning quite a bit along the way.


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## Bud9051

I currently have both 3.5 and 5.5 Roxul in the far end of my house and if I step on it, I just tested, the 5.5" goes to less than 1". If you screw it down to where the plywood is stable, my guess is you will have gone far beyond any increase in r-value per inch and well into the decrease in r-value headed rapidly towards zero. Mineral Wool is after all just a fluffy rock. Remove the fluffy and you are left with a rock. Probably not zero, but it would not be contributing much towards your r-30 goal.

If you added a 2x4 or 2x6 on top of the joists and then the Roxul and plywood you would retain much or all of that extra r-value.

Bud


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## Gary in WA

I figured you would have done more research yourself on low, medium and high densities, Bud, after my statements to see if you were correct or not, being an energy pro and all... . Many of my older links are gone, I feel the insulation manufacturers have gotten to them... some current ones for everyone; http://nutbetaxga.hexat.com/index/__xtblog_entry/572589-high-density-r-15-batt-insulation

You can group them in those categories due to weight per cubic foot; http://www.daviddarling.info/encyclopedia/F/AE_fiberglass_insulation.html But, many times you have to do the math to find the weight, especially blown-in FG. 

Two paragraphs under the box, here; https://books.google.com/books?id=7...e&q=fiberglass batt insulation weight&f=false

Another thread you mentioned having big air spaces for best performance, more little air spaces (hence- denser) is optimum; http://arc.aiaa.org/doi/abs/10.2514/6.1969-606

Compressing rock wool makes it a high R-value (dense) board (and good acoustical barrier) about R-4.1 or R-4.2; http://www.roxul.com/files/RX-NA_EN...ng Envelope/ENGLISH/ROCKBOARD_BROCHURE_EN.pdf

Gary


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