Can I compress insulation?

[StaggeringGoat]

I'm building a new work room inside my shop that's about 20x20 so I can actually heat and cool it. I'm hoping to get most of the materials from craigslist and the local building material recycling place. I've found some 6.5" thick fiberglass batts that I would like to use for insulation; however, my walls are only going to be 4" thick. Would it be OK to compress this insulation into my 4" wall cavity? What effect would that have on the insulation?

 

[Doug B]

The effect would be lowering the R value. Insulation is effective because of its loft- the airspace in between the glass fibers. When you compress it,that air space goes away.You may be able to peel away some layers of the fiberglass so it's closer to the 4" thickness you need.

 

[fury9]

X2 what he said

 

[green.bubbly]

It is pretty easy to strip away a layer so that it would fit in the cavity as Doug mentioned. You could then combine the stripped layers to fill other stud bays.

Insulation should not be compressed but I am not sure what R value you would yield if you put the full R19 batt in that wall.

 

[KinzeMech]

6.5" --> 4" is not severe compression in my opinion, but I am not a professional carpenter.

 

[rlitman]

My understanding is that you will be lowering the R value as rated on the batts.
However, it will still be higher than the R value of a 4" batt fully fluffed up (but not by much). I would suggest just compressing it, and not worrying about removing a layer of insulation. The higher density you would end up with is a waste of money if you're buying new, and but it should actually insulate a little better than the correct sized batt, because it restricts air flow a little better.

 

[csp]

My understanding is that you will be lowering the R value as rated on the batts.
However, it will still be higher than the R value of a 4" batt fully fluffed up (but not by much). I would suggest just compressing it, and not worrying about removing a layer of insulation. The higher density you would end up with is a waste of money if you're buying new, and but it should actually insulate a little better than the correct sized batt, because it restricts air flow a little better.

^^^^This^^^^

You already have this insulation. If you bought it take it back and save some cash, but if it was free or close to free, use it.

 

[djkeev]

Strip off two inches. Its easy to do, messy but easy. Insulation works by trapping pockets of dead air between the fibers, compress the fibers and you've squeezed out your dead air space thus no insulating properties exist any longer.
Now all you have is compressed fiberglass. Fiberglass sheets don't have much in the way of insulation value.

Dave

 

[djkeev]

........... but it should actually insulate a little better than the correct sized batt, because it restricts air flow a little better.

Bad info.... Ignore!

Dave

 

[DekeT]

If you can get the 6.5in batts for near the same price or lower as a 4.5in batts then get them. Compressed 6.5 batts will have a slightly higher R value within the thinner wall than 3.5 in material. This is because the R value per inch increases slightly with compression. Where that crossover point for compression is ???? The overall difference is so small that it is definately not worth compressing thicker batts unless you get a mind boggling bargain in relation to the cost of a 3.5in batt.

 

[DekeT]

Bad info.... Ignore!

Dave

Wrong, not bad info. The bad info would be to strip off 2 inches of insulation you already paid for.

6.5 inches of insulation stuffed in to a 3.5 inch cavity will have a higher overall R value than 3.5 in placed in the same volume.

The R value per inch increases when compressing fiberglass. A 6.5 in batt is rated at R-19 or 2.9 per inch. Put into a 5.5 inch wall it gets you only R-17 overall. That is an R value of 3.1 per inch. Nearly 7% per inch with compressed fibers. The point is compressing fibers is not cost effective because the increase in total R value is too small compared to price per foot.

 

[fflintstone]

Bad info.... Ignore!

Dave

Sorry but you are wrong Dave, but it is a common misconception.

You can strip off layers to make the insulation go further square footage wise. An R19 batt compressed into a 3.5” stud bay will yield a R14-R15 wall.
I worked in the fiberglass insulation manufacturing industry for a few years. the standard R13 and R19 batts are a compromise between cost of materials and R value.
Some places sell a “value” R 11 3.5 inch batt, it is nothing more than a less dense batt that is cheaper to manufacture.

 

[WheelsNT]

Air is what insulates, not the fiberglass. Fiberglass insulation works by having zillions of tiny air pockets within the batt and preventing the air from moving around. Air doesn't conduct heat well when it is still. Air conducts heat by moving "hot air" from one place to another. So, if you compress the batt, you have reduced the air in the batt, and reduced the R-value.

That said, if you want to spend less on heating and cooling (this is why we insulate, right?) Use 2x6's for the walls instead of the 2x4's you were planning on. You'll get full benefit from the insulation, not only in a heating and cooling sense, but also with sound transmission from the shop.

 

[Falcon67]

Can do - sure. Should do - no. Just peal off 2" and use the rest. Roll up and save the scraps and put them in the attic.

 

[rlitman]

Air is what insulates, not the fiberglass. Fiberglass insulation works by having zillions of tiny air pockets within the batt and preventing the air from moving around. Air doesn't conduct heat well when it is still. Air conducts heat by moving "hot air" from one place to another. So, if you compress the batt, you have reduced the air in the batt, and reduced the R-value.

This is dead wrong. For starters, the fiberglass does indeed insulate. Glass is a very poor conductor of heat, and the wandering and long path it would take to follow a glass strand between each side, contributes a lot to the insulating properties.

For another, reducing the size of an air gap can actually reduce heat transfer across it, if it reduces convective flow.

The point is, that for reasonable amounts of compression, while compressing fiberglass will reduce the R value from one that is printed on it, it will increase the R value per inch. This is why you can buy "high density" fiberglass bats (usually reserved for cathedral ceilings), which have a higher R value than standard batts. In that situation (against a roof, where you need to maintain an airgap), compressing a thicker batt would have other negative effects.

Now, if you are really cheap, you could strip 2" off each batt, and layer two of these together, to fill up wall cavities when you run out of the thicker insulation, but I'm of the opinion that you should cut fiberglass as little as possible, so as to avoid making more dust than necessary.

 

[thebear]

The biggest problem I see with compressing the batts is the drywall work. If you have to compress too much, the drywall can crack when installing due to uneven pressure.

 

[JimVonBaden]

I think the insulation properties are both air and fiberglass.

Use the table in the link and you will see: http://www.allwallsystem.com/design/RValueTable.html

If all the value was in the air pockets, why does foam insulation have more insulation per inch? It has less air trapped in it.

R-Value Table
Material
R/
Inch
R/
Thickness

Insulation Materials

Fiberglass Batt
3.14


Fiberglass Blown (attic)
2.20


Fiberglass Blown (wall)
3.20


Rock Wool Batt
3.14


Rock Wool Blown (attic)
3.10


Rock Wool Blown (wall)
3.03


Cellulose Blown (attic)
3.13


Cellulose Blown (wall)
3.70


Vermiculite
2.13


Air-entrained Concrete
3.90


Urea terpolymer foam
4.48


Rigid Fiberglass (> 4lb/ft3)
4.00


Expanded Polystyrene (beadboard)
4.00


Extruded Polystyrene
5.00


Polyurethane (foamed-in-place)
6.25


Polyisocyanurate (foil-faced)
7.20

Jim

 

[csp]

Air is what insulates, not the fiberglass.

So that explains why a thermos has a vacuum between layers.

 

[djkeev]

Well....... According to the California Energy Commision......

http://www.consumerenergycenter.org/home/construction/insulation.html

"You can greatly increase the energy efficiency and comfort of a home by installing insulation with an R-value higher than the minimum requirements. But to truly enjoy the benefits of insulation, it must be installed correctly. Compressing it or leaving gaps through which air can flow can cut insulation's effectiveness by as much as 50 percent."

I think this is going to be a highly debated topic. Apparently R19 wall insulation is rated at a 6 1/2" deep cavity...... Hmmmm we use 2x6's which are only 5 1/2" deep! That lowers your R19 to about R18 instantly!

What Is most important is that fiberglass must touch all 6 surfaces of the cavity it is applied into. You lose contact on only one side and the ability to insulates drops dramatically.

For something as minor as a garage, anything is better than nothing..... Stuff it in, the sun will still rise tomorrow, just stuff it in properly (within the parameters of using the improper material at the get go!)

Dave

 

[rsanter]

Strip off two inches. Its easy to do, messy but easy. Insulation works by trapping pockets of dead air between the fibers, compress the fibers and you've squeezed out your dead air space thus no insulating properties exist any longer.
Now all you have is compressed fiberglass. Fiberglass sheets don't have much in the way of insulation value.

Dave

Correct info here

Bob

 

[csp]

The point people are missing is that the R19 compressed is still going to be higher than the R13 that's made for 2x4 wall cavities. If the OP is getting a good enough deal on this R19 it's a no brainer!

 

[ironmutt]

what ever way you go compressed or not put a vapor barrior up on the inside and outside of wall tape all seams and use a little exspanda foam at doors and outlet openings less air flowing the better and put a foam runner under the bottom plate to make a good seal there too

 

[Angelfire]

Stuff it in there, but not the ceiling unless you have good ventilation in place.

http://www.buildingscience.com/documents/insights/bsi-043-dont-be-dense/

Dense packing works, and works quite well. The only reason I didn't go with this approach was because I wanted to use mineral wool batts.

 

[rsa]

Insulation works by trapping pockets of dead air between the fibers, compress the fibers and you've squeezed out your dead air space thus no insulating properties exist any longer. Now all you have is compressed fiberglass. Fiberglass sheets don't have much in the way of insulation value.

OC 700 series fiberglass boards have insulation values of R-4+ per inch and densities up to 6 pcf. Reference.

 

[rsa]

greenbuildingadvisor.com editor Martin Holladay addressed this exact subject in a finehomebuilding.com forum thread

Compressing a 5.5 inch batt to 3.5 inches will reduce the R-value compared to the same batt installed in a 5.5 inch cavity, but will increase the R-value per inch compared to a 3.5 inch batt installed in a 3.5 inch cavity.

In other words, fiberglass manufacturers have not optimized the density or the R-value per inch of their product. They sell a fluffier-than-optimum product in order to reduce the cost of manufacture. If you are willing to buy the 5.5 inch batt and compress it, your 3.5 inch cavity will have a higher R-value than if it were filled with a 3.5 inch thick batt.

 

[pattenp]

People on here that say compressing fiber glass insulation increases the R value don't know what they are talking about. Compressing R-19 in to a 2X4 wall is a waste. The glass in it self has virtually no insulating power, it's the trapped air in the fiber glass that provides the insulating power. You folks need to do a little research before providing information you think is correct.

 

[JimVonBaden]

People on here that say compressing fiber glass insulation increases the R value don't know what they are talking about. Compressing R-19 in to a 2X4 wall is a waste. The glass in it self has virtually no insulating power, it's the trapped air in the fiber glass that provides the insulating power. You folks need to do a little research before providing information you think is correct.

Look above your post. Several have shown proof contradicting what you say.

Jim

 

[pattenp]

Limited compression is okay such as putting 6" in a 5" cavity, but putting 6" R-19 in a 3.5 wall cavity is over compression and will reduce the overall R valve.

See this...
http://www.flashandfoil.com/pdf/Foam-with-compressed-batt.pdf

For those of you who want to compress your fiber glass insulation go right ahead.

Look above your post. Several have shown proof contradicting what you say.

Jim

 

[pattenp]

The quote in the post above mine is from a forum of a bunch of guys stating what they think is correct.That's far from proof. It's just like us here debating what's fact vs. fiction.

Look above your post. Several have shown proof contradicting what you say.

Jim

 

[djkeev]

Owens Corning information on compressing insulation,

http://www.owenscorning.com/around/insulation/CompressionChart.xls

R19 compressed to 3 1/2" results in an R13 value, why pay for R19 when purchasing R13 is cheaper and won't cause sheet rock bulge?

The R value drops are staggering! 3 1/2" R13 drops to R6.6 when compressed to 1 1/2"

Dave

 

[ddawg16]

Nothing worse than a bunch of girls arguing about something without any real facts to back it up.

This is from the Owens Corning site.....I 'think' they know what they are talking about....it took me longer to read the thread than find the blow facts.....

http://numsum.com/spreadsheet/show/21111

According to their chart, R19 insulation stuffed into a 4" wall will drop to R13

And Dave posted while I was typing....I type slow I guess....

 

[djkeev]

Nothing worse than a bunch of girls arguing about something without any real facts to back it up.

This is from the Owens Corning site.....I 'think' they know what they are talking about....it took me longer to read the thread than find the blow facts.....

http://numsum.com/spreadsheet/show/21111

According to their chart, R19 insulation stuffed into a 4" wall will drop to R13

And Dave posted while I was typing....I type slow I guess....

Great minds think alike? Your chart is better than mine!

Dave

 

[DekeT]

Limited compression is okay such as putting 6" in a 5" cavity, but putting 6" R-19 in a 3.5 wall cavity is over compression and will reduce the overall R valve.

See this...
http://www.flashandfoil.com/pdf/Foam-with-compressed-batt.pdf

For those of you who want to compress your fiber glass insulation go right ahead.

Well you really should do the math when you quote web sites. Because you have misinterpreted the results.

Examine the first R-11 graph at 1 in of foam. R-6 for the first inch of foam and R-9 for the next 2.5 inches of compressed fiberglass. Total equals r-15.
R-9/2.5 inches = R 3.6 per inch for the fiberglass.

Now read the same graph at foam = 2 inches. 2 inches of foam equal R-12. Remaining 1.5 inches oc compressed fiberglass is R-6. Total R is R-18. R-6 / 1.5 inches = R 4 per inch.

R-4/inch is better than R-3.6 per inch. More compressed equal better R value from your own example.

 

[DekeT]

Nothing worse than a bunch of girls arguing about something without any real facts to back it up.

This is from the Owens Corning site.....I 'think' they know what they are talking about....it took me longer to read the thread than find the blow facts.....

http://numsum.com/spreadsheet/show/21111

Well Nancy, I have been showing the math from the very beginning of this thread.

Your Owens Corning data. R-19 at 6.5 inches equals R 2.92 per inch. Compressed R-19 to 4 inches resulting in R-13 equals R value of 3.25 per inch.
19/6.3 = 2.92 13/4 = 3.25

Math don't lie.

 

[fflintstone]

Fiberglass batt insulative values per inch are directly related to the density (IE weight) of the fiberglass. More fiberglass = less air = higher R value.

It costs more to manufacture denser batts. It’s all about money.

You would not have new “high density” batts if it were not for new energy efficiency requirements.

This is a comparison of high density batts vs. regular.

http://www.owenscorning.com/insulationproguide/p10-cathedral.pdf

As stated before I worked in fiberglass insulation manufacture for a few years and know what the F I am talking about.
I studied green building science ad nauseum of my own volition.

R19 compressed into a 3.5 can produce R values of R14-R15.

By compressing insulation the novice installer may fill gaps and stand a better chance of reducing air infiltration. No batt works worth a **** if it is improperly installed.

 

[pattenp]

I haven't misinterpreted a thing. I'm talking overall R factor not per inch. You are arguing apples and I'm arguing oranges. The OP questioned the affect on the insulation if compressed. Bottom line is the over all R factor is reduced from what the product was manufactured to provide. I very well understand your math to get the R valve per inch if compressed. I don't care if the compressed R factor increased. The point is R19 in a 2x4 cavity is reduced to R13. Don't waste R19 where R13 should be used.

Well you really should do the math when you quote web sites. Because you have misinterpreted the results.

Examine the first R-11 graph at 1 in of foam. R-6 for the first inch of foam and R-9 for the next 2.5 inches of compressed fiberglass. Total equals r-15.
R-9/2.5 inches = R 3.6 per inch for the fiberglass.

Now read the same graph at foam = 2 inches. 2 inches of foam equal R-12. Remaining 1.5 inches oc compressed fiberglass is R-6. Total R is R-18. R-6 / 1.5 inches = R 4 per inch.

R-4/inch is better than R-3.6 per inch. More compressed equal better R value from your own example.

 

[jimp]

I like to fight, so.

Until the fiberglass density hits ~4 lbs/ft3. The R value per inch will increase. After that it will decrease at about the rate.

The R 19 batt properly installed will be about 2 lbs/ft3.

Just compress it, it's simpler and slightly better.

 

[RCStocker]

I had a cousion who just passed away. He was in his late 70's I am a contractor and had him do all my insulation work for 30 years. That is all he did. He had 5 trucks. He did blow in insulation, foam, bats and everthing else.
You can not compress thicker insulation . You loose R value. Yes you can peal off a layer. The foil backing makes a difference. Make sure it is facing the proper direction for the job. I would use foil to cover the spaces that do not have it. That is to say when you layer up what you pull off you can use foil on those. You can lay the extra up in the rafters of the regulary insulation. Make sure you get a perfect fit and cut. Gaps will kill you. Some wrap the walls with plastic depending on the siding and part of the country you live in.

I love all the I think's. If in doubt look it up. After all you are sitting with the largest book of knowledge at your finger tips.

Well....... According to the California Energy Commision......

http://www.consumerenergycenter.org/home/construction/insulation.html

"You can greatly increase the energy efficiency and comfort of a home by installing insulation with an R-value higher than the minimum requirements. But to truly enjoy the benefits of insulation, it must be installed correctly. Compressing it or leaving gaps through which air can flow can cut insulation's effectiveness by as much as 50 percent."

I think this is going to be a highly debated topic. Apparently R19 wall insulation is rated at a 6 1/2" deep cavity...... Hmmmm we use 2x6's which are only 5 1/2" deep! That lowers your R19 to about R18 instantly!

What Is most important is that fiberglass must touch all 6 surfaces of the cavity it is applied into. You lose contact on only one side and the ability to insulates drops dramatically.

For something as minor as a garage, anything is better than nothing..... Stuff it in, the sun will still rise tomorrow, just stuff it in properly (within the parameters of using the improper material at the get go!)

Dave

 

[Bob C]

Almost to page three. ******* contest soon to start.

 

[Kevin C]

My two cents.... There is not enough info about the OP material to say for sure. The chart shows mixed results on the performance of a given wall assembly VS what insulation it gets.

All of my comments are for overall R factor~ as in you want max performance and are limited by the stud size.

Based on the chart.....

In general the high density insulation is already optimized for maximum performance and compressing it does not provide a gain over the correct high density insulation.

Compressing standard density insulation does provide better performance for a given stud size compared to standard density design for that stud size.

Compressing standard density insulation often provides the same performance of high density that is not compressed ( R21 5 1/2" compressed to 3 1/2" is the same as R-15 3 1/2).

From what I can see... everyone is at least partially correct.

Odds are the Craigslist insulation is low density and this works out as a good solution.

BTW... I'm not in this to win a ******* contest. The next phase in my garage build is to add insulation. After seeing the charts that were posted, I have a lot better chance of making an informed decision on what is the right stuff to buy. I appreciate the discussion and the chance to learn.

http://www.owenscorning.com/insulationproguide/p10-cathedral.pdf

Good link in high density insulation.