Compressed Fiberglass R-Value

[Bud9051]

How many of you have heard the statement, "compressing fiberglass insulation reduces it's r-value"?  I'm going to assume everyone has, myself included.  Well, the statement is correct, but the interpretation or application of it often leads to a wrong conclusion, for example:  Stuffing extra fiberglass insulation into a wall cavity will reduce the walls r-value.  That statement is most often wrong, here's why.

I found it confusing that compressing fiberglass insulation would reduce it's r-value, yet we could buy high density fiberglass at a higher r-value.  So what is the difference between high density fiberglass and regular density that we compress into a tighter space?  It turns out to be, not as much as we thought.  The distinction is between the r-value of the fiberglass batt and its r-value per inch.  Take for example, r-19 batts.  They are nominally 6 1/4 " in thickness, thus an r-value of 3.04 per inch.  Now, that's not the r-value our books give us, but it is what the numbers say and what the pink panther says.  Now, install it in the normal 5 1/2" cavity and what do you get, 3.04 X 5 1/2" = 16.72, no.  The panther says we get r-18, so what happened to the r per inch.  It went up to 3.27.

So compressing the batts increased their r per inch, while reducing the total number of inches, resulting in a lower total r-value, and although the total went down, we can see that a limited amount of compression is not necessarily a bad thing.  Where the turning point is, I haven't determined and obviously we can't squash the fiberglass flat and expect it to continue to increase its r per inch.  But a little stuffing here and a little there can be a good thing.

Here is the panther's web page with the numbers I used and it does show that their high density for a  a 5.5 inch cavity is actually R-21.  Whether that's accomplished because they know how to increase the density better or because we need to add more insulation than the 6.25 in the example I started with, I don't know.
http://www.idimn.com/pdf/insulation/fiberglass/high-density/21149-D.pdf

I however do know I now understand the original statement better, I just hope my explanation helped you as well.  Corrections welcome if I am wrong.

Bud

[Glen Gallo]

Bud,

You have me thinking on this one.

 I thought that the ability of fiberglass to trap air was the key to its insulating property. Compressed fiberglass by virtue of its density and lack of air would than lose R value per square inch.

Furthermore the potential for incorrect installation would increase especially in batts. Compressing the fiberglass could potentially lead to air pockets for uncontrolled flow further decreasing the effective r value.

That is off the top of my head. I am off to work and will further investigate this when I return in the evening and can consult the oracle(internet search). I still think that compressing fiberglass is a bad thing. The argument you present requires me to re examine what I thought I knew to be valid.

[Robert Hronek]

If you read through it does show other fiberglass products have a lower R value when compressed. I dont think it means all fiberglass products would preform better when compressed.

Allthough is may preform better than other fiberglass products it still has many of fiberglass' weak points. It takes full contact on six side to perform near its stated R. For me I have a hard time with any product that takes near perfection for it to work right.  Yesterday I saw a good example of a poor install job. This was in a 2 story with an attached garage with the 2nd floor expending over the back half of the garage. It had fiberglass batts installed between the the garage cieling and the floor of the 2nd floor.  The attic over the front of the garage gave me a view of the end of the batts. What I saw was that the installers had pushed the batts up and middle was in contact with the floor. Each upper corner of the cavities had a sizeable gap that extended back as far as I could see. No wonder the homeowner compained that room was hard to heat and cool.

[Bud9051]

Hi Robert and welcome to the forum.  I re-read the link and not sure where it talks about other fiberglass products.  Direct me to what you read so I can comment.

But I agree, all fiberglass products have a lower r-value when compressed, but that is due to the reduced thickness and not a reduced r-value per inch.  That r-value per inch, actually increases for limited amounts of compression.  At what degree of compression the r-value per inch starts to drop, I do not know.  But a little bit of compression, like a 6.25" batt compressed into the standard 5.5" cavity is not necessarily a bad thing.

As Glen mentioned, air pockets, and as you stated poor installation and poor fit are extremely important.  What I'm trying to clarify is the assumption, which I have always had, that any compression was making the fiberglass less effective, when actually it is just making it thinner.

There is still the correct advice about not over stuffing the insulation around windows and doors as that can easily go beyond the optimal density of the fiberglass and besides foam works better.  I'll discuss foam around windows in another thread.

Bud

[Glen Gallo]

Bud,

After further investigation and thought on the subject I think it best to stick with the standard.

With the possibility of incorrect installation it outweighs any slight r value the might be achieved.

If one is looking for an increased r value I believe it wise to go with  another product such as foam.

Robert,

 I think if you examine it, most products require correct installation. A slight water or sewer leak is unacceptable. Most of the drywall meeting the corners is unacceptable. Shingles on most of the roof is unacceptable.

I think the issue with insulation as we can't see the mistake as easily as the above mentioned products.

 It is important in my opinion that we accept whatever material that has a proven cost effectiveness and performance and try to improve the process for which it is applied in our customers buildings.

Glen

[Bud9051]

Hi Glen,
I have to agree that trying to explain this to an insulation crew and then expect they would not mess it up would be difficult.  But for us, as energy auditors, it is important to understand the principles and properties of the materials we deal with, which I'm sure you do.  From what I'm reading, fiberglass has an optimal density where it exhibits its highest r value per inch.  Unfortunately they have never shared that information with us, and very likely for the exact reasons you state.  However, there is always a however.

When dealing with some of the older buildings with real 2x4's and 2x6's or some of the modern thicker walls and staggered stud configurations, we lack factory made materials to fill those spaces, at least in the fiberglass batt form, and for all of its problems, fiberglass is still a very popular insulation material.  So how do we fill a 4" cavity with fiberglass?  By understanding that some minor compression is better than under filling the cavity, we could actually be helping to provide a better solution.

I am in the process of re-reading the new Infrared standards and hope to open a discussion on how they will actually be handled in the real world.  I look forward to your input on that topic as well.

Thanks,
Bud

[Robert Hronek]

Glen

Fiberglass needs to be touching on all sides without bunches or gaps. I would invite you to inspect a house with fiberglass batts and see if there is 6 sided coverage. What about around electrical and plumbing? What about the closed side you can not inspect. Fiberglass will loose a considerable amount of its R value when there is even a 1/16 inch gap. It will take a very meticulas and detailed person to due such a job if at all possible.  Because of that I perfer a blown or sprayed product.

[Glen Gallo]

Robert,

I do not disagree with what you have said. I cannot dictate what materials are installed. Fiberglass is the most used product in my area. I would prefer to inspect only foam if I could, however I must work with what is out there.

When a installer is presented with these challenges they will always be restiveness.

There is no easy answer but we cannot accept inferior insulation installation when we are called on to verify.

I did not mean my comments negative in any way and I hope they were not received in that fashion. We have more common ground than not.


Glen