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Can Fiberglass Work if Done Properly?

BK24's picture

I know that most of the folks in this forum dislike the use of fiberglass because it does not maintain its R-value in leaky wall cavities due to air washing.  My question is: if a wall cavity was completely sealed - as in the sheathing was caulked or glued to the framing and the sheetrock was glued to the framing during installation - would fiberglass work ok?  If so, does it make sense to dense pack?  I know that R-19 (5.5 inch) does not keep it's full R-value if compressed into a 3.5 inch space, but does it do better than standard R-13 (3.5 inch)?


Edited 5/4/2008 2:23 am ET by bk24


Edited 5/4/2008 2:24 am ET by bk24

(post #115050, reply #1 of 94)

Bk24,


Q.  "I know that R-19 (5.5 inch) does not keep its full R-value if compressed into a 3.5 inch space, but does it do better than standard R-13 (3.5 inch)?"


A.  Yes.  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.


Edited 5/4/2008 6:45 am ET by MartinHolladay

(post #115050, reply #2 of 94)

But the wall R-value will be less than the rated R-19 of the 5.5"....right?


And what would the compressed 5.5" R-rating be?


Is there a quotable source for this discussion's kind of info? 


Sure would be nice to have to lay in front of a client during a discussions of energy efficiency versus cost effectiveness.


...............Iron Helix

.......Iron Helix

(post #115050, reply #81 of 94)

An R-19 compressed to 3.5" will likely be better than the HD R-13, but we really don't know what it might be. Two R-19 in a 6" wall will not be R-38, but it will be a lot more than an R-19.


Will a fiberglass batt give you an R-19? Installed properly and w/ proper construction, I think the answer is yes. Given a properly constructed wall cavity, I feel there is limited opportunity for air wash. Fiberglass should be installed w/out any cavities/voids (due to piping, wiring, over/under stuffing, obstructions, and other irregularities). Insulating, as such, is a skill like any other trade. Unfortunately it is often done by some of the lesser skilled tradesmen.


I had a guy tell me that wet blown cellulose doesn't pass air like fiberglass. I believe him. ... but his demo of actually forcing air through a fiberglass sample is not a reasonable reflection of what happens in real construction.


There is a time and place for everything and everything has its advanatages and disadvantages. Pay your $ and take your choice. I advocate informed decisions ... almost all involve some degree of compromise.

There ain't NO free lunch. Not no how, not no where!

(post #115050, reply #83 of 94)

In all means & methods....the devil is in the details!!!!!!


....Iron Helix

.......Iron Helix

(post #115050, reply #84 of 94)

"Two R-19 in a 6" wall will not be R-38, but it will be a lot more than an R-19."

Not quite. there is a point of diminishing returns where because you are substituting solid glass instead of containing dead air in place, you increase the conductive heat loss. I read an article based on a study of this back in the mid seventies. I forget ate wat point exactly, but recall that for practical purposes, you can only increase the batt density 30-50 percent. Your example goes to 100%, where the R-value might actually be R-18

 

 


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(post #115050, reply #91 of 94)

I'm not an expert, but I do have a good sense about this. High density fiberglass is frequently made for all kinds of applications with an R-value per inch that is higher than standard batts (3.1/inch). And those applications result in MUCH higher densities than 2 R-19 batts in a 5.5 inch wall.


My guess is that maybe the diminishing benefit isn't a linear function. I'm confident that 2 R-19 batts in a 6" wall will be significantly higher than an R-19, not worse. No offense intended, but I'd reread that article again to get the sailient points correct. If you do find it ... share it ... love to see what it does say. Some blind leading the blind hear ... and for that I apologize for maybe stepping into the conversation too far.

There ain't NO free lunch. Not no how, not no where!

(post #115050, reply #92 of 94)

I doubt that I'll ever see an article I read in the seventies again, LOL

And my instinct is in line with what you just said. I doubt very much that two crammed would be worth less, but there could be spots where that is so.

What is the R-value of the woven panel FG for ducting anyways? That would probably indicate the upper end.

BTW, ref previous post of yours, BIBs stands for Blown in Blanket, a trademark for chopped FG, not for DensPak Cellulose blown in. bIBs is a bit higher than Cells i cost.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #115050, reply #85 of 94)

What is the advantage (if any) of wet blown cellulose over denspak BIB cellulose? How do they compare in R-value and cost?

I have also been told that wet blown cellulose becomes part of the structure and gives greater structural rigidity. Hard for me to believe, but any truth to this?

Also, is it true that in wet humid areas that wet blown cellulose is at risk for mold?

(post #115050, reply #86 of 94)

"I have also been told that wet blown cellulose becomes part of the structure and gives greater structural rigidity. Hard for me to believe, but any truth to this?"

Maybe it's done differently in other areas, but I can assure you 100% that the wet blown cellulose in this area does not add ANY structural strength to the wall system.

I have heard that was the case with closed cell foam. In fact, I read a study (I think in "Coastal Contractor" by Hanley Wood) that the foam industry was pushing to get foam blown against the roof sheathing and rafters/trusses as being just as strong as doubling the nailing schedule on the roof sheathing.

 


Jon Blakemore

RappahannockINC.com

Fredericksburg, VA

 

Jon Blakemore

RappahannockINC.com

Fredericksburg, VA

(post #115050, reply #87 of 94)

Wet blown is a mold risk anyplace if they do not let it dry before covering it up.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
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We did the best we could...

(post #115050, reply #88 of 94)

So you have to wait for it to dry and lost site time. Why do it? Is there some advantage over denspack cellulose? I don't get it.

(post #115050, reply #89 of 94)

I never did either.. Maybe somebody else can take up for it with more info. There would be some labour savings in that you might not need to spend time stapling up the containment mesh

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #115050, reply #90 of 94)

Not sure of the differences in wet blown vs. BIB. Very similar results, though as far as R-value and filling voids. have no idea about cost comparisons. My guess is that the BIB may be a bit more, but I've really no feel for it.


I'd be skeptical re: increase in structural rigidity. I'm sure it does, but I'm sure it is negligible and unpredictable.


In humid areas ... may be tough ... you need to be able to dry it out. May have to A/C the space a lot to wring out the air to ensure that it does dry out before e.g. sheetrock is applied. An installer should be able to give you a better sense of what needs to be done in humid climates. Misapplied, it can definately be an issue!! Remember, drywall mud is much the same ... lots of moisture to get rid of (although not buried under 6" of insulation)!

There ain't NO free lunch. Not no how, not no where!

(post #115050, reply #93 of 94)

http://saveenergy.owenscorningblog.com/2006/11/

Question: What insulation should I use in my cathedral ceiling?
Clint writes from Catoosa, Oklahoma: "I have a 2'x8' cathedral ceiling (new construction) that I am trying to insulate. I have already put in nailers and screwed polyisocyanurate aluminum-faced foam board up inside the rafter cavity. With the foam board in place I only have a 5" space left to fill with fiberglass insulation. I was wondering what would be the best thing to do, I can put in R-19 6.25" thick or R-21 5.5" thick. I know either way I will be compressing the insulation which is undesirable. How much R-value loss could I expect if I compressed either of these insulation products into the 5" space?"

Answer: I suggest using the R-21 in order to get the most R-value in the cavity since you only get one chance at it before the cavity is enclosed. R-19 compressed to 5 inches is probably about 16.5 where R-21 compressed to 5 inches is around 19.6.

http://www.iccsafe.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=4;t=009647

R-19 (6 1/4") fiberglass bat insulation is rated as installed in an attic insulation situation. The R- value decreases to R-18 when compressed into a 5 1/2" stud cavity and to R-13 when compressed into a 3 1/2" stud cavity. R-13 (3 1/2") bats are available as are R-15 (3 1/2") high density bats. For purposes of prescriptive code compliance, the R-values are additive so you can install R-13 in the cavity with R-5 (foam) sheathing or R-15 with R-3 (foam) sheathing and meet code for zones 5 & 6.

.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
. William the Geezer, the sequel to Billy the Kid - Shoe

(post #115050, reply #94 of 94)

Thank you sir. What a Whiz kid!

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #115050, reply #3 of 94)

Really? I thought the value of fiberglass was really in the air. In other words, a 5.5" compressed fiberglass batt to 3.5" would have less r-value than an uncompressed 3.5" batt.

(post #115050, reply #82 of 94)

It is in the air ... but with more air pockets, you have fewer big, more inefficient pockets ... so you end up w/ a better insulation system. If if was ONLY in the air volume, leaving a cavity empty/void of any material would yield the most air space and therefore the best insulation value. It's the combination of materials and air space that provide efficiency.

There ain't NO free lunch. Not no how, not no where!

(post #115050, reply #5 of 94)

Martin Holladay.


    Not according to the publishers of this web site Fine Home Building  issue 160.  As I read the text over packing increases convextion or transfer of energy..


 Solid fiberglas would have extremely poor R value, loose fill will flow air too freely.. as you get away from perfect you spend more and gain less..

(post #115050, reply #6 of 94)

There was a thread on a JLC forum on this topic a while ago in which I participated.  See:


http://forums.jlconline.com/forums/showthread.php?t=35103


As I wrote then,


“In the recently published JLC book, "Tips & Techniques," on page 227, Mike Lacher answers a technical question on this topic. Mike Lacher formerly worked as a technical services manager for CertainTeed insulation. He … shares a CertainTeed chart displaying the R-values of compressed fiberglass insulation batts. The chart shows that when 12-inch thick R-38 batts are installed in a 9 1/4-inch rafter bay, the resulting R-value is R-33. This is less than the fluffed-up R-38, but it is more than the R-30 insulation that is usually sold for 2x10 rafters. Lacher writes, ‘as you compress fiberglass batt insulation, the R-value per inch goes up, but the overall R-value goes down.’”


 


 

(post #115050, reply #7 of 94)

Martin Holladay,


  Thank you for that tid bit, but given that the source works for a fiberglas company I wonder if there is a bias in there? 


  Please understand I'm not saying there is but you must admit the potential for bias would be strong.


   

(post #115050, reply #32 of 94)

A better choice is to purchase high-density FG batts instead of the standard ones. But, as you say, compressing standard batts will, within limits, improve the R value per inch of the product.

To answer the OP's original question, tightly air-sealing both sides of the stud cavity will greatly improve the effectiveness of fiberglass. Any sort of draft through the fiberglass significantly dimimishes its effectiveness.

It would be interesting to see FG mfgrs produce a product with some sort of imbedded draft stop, such as a Tyvek barrier halfway through, or plastic draft stops embedded at right angles to the batt somehow. Of course they wouldn't want to do this even if they could since it would be admitting that the standard product is seriously deficient.


What is wanted is not the will to believe, but the will to find out, which is the exact opposite. --Bertrand Russell


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115050, reply #42 of 94)

The movement of air through the fg batts is certainly a problem in terms of diminished insulating values, but movement of air within the stud space may well allow the wall to dry itself.


I can understand the importance of sealing the interior surface of the wall, and understand the logic involved with recommending sealing the exterior, but in wet climates such as the PNW, when you seal both sides of the wall, you are relying on the exterior allowing no water infiltration over the life of the house. Any water that does penetrate the wall cavity can not dry to the inside or out.


Sealing both sides may work well with foam, but with fg batts I am more comfortable with the approach our code dictates of gaps and holes in the exterior sheathing, coupled with a rain screen. It's just a more forgiving system.

(post #115050, reply #4 of 94)

bk24


 In Fine Home Building issue 160  (Jan 2004)  they debate this subject and do a comparison of the three systems. 


  In the end they said,, forget the product  pick the best contractor.. 


  However if you go through the whole text they mention convection, they mention leaks, and they mention the fact that when Fiberglas is wet it has virtually no R value.


    They speak of over compressing and how it hurts rather than helps the insulations R value.


 Bottom line, there is no replacement for doing the job perfectly.  If you cannot be absolutely sure of that, even the best system will fail you..

(post #115050, reply #8 of 94)

IMO,

Fiberglass can be an effective insulation. No it is not as good as cellulose or spray foams, but it can work.

The trick is to install it meticulously. Fill the cavities with no voids. It's as simple as that. It can be done. The problem is a lot of times the lowest guys on the totem pole install that batts.

Also, common sense suggests Martin is right: compressing a batt will result in a higher R value in a given space. Why else would a high density R21 batt have a higher R value than a low density R 19 batt?

(post #115050, reply #9 of 94)

Marson..


    Because R value isn't really correct!  You are aware that R value is tested at 70 degrees aren't you?   Not at zero or 40 below where the performance of fiberglas is severely compromised..  Quote,  "by up to 50% at 40 below"


 Fine Home Building # 160 Jan 2004 page 52


   In addition it's tested under controlled labrotory conditions where there is no humidity in the air or  wind blowing..


 All of which affect fiberglas most of all of the three comm on types of insulation.

(post #115050, reply #10 of 94)

Whoa, all I said is that fiberglass can be an effective insulation if installed properly. The way it is maligned on this board you'd think you might as well not use it--just go with uninsulated stud cavities. That's preposterous. No, it isn't as good as other systems. But it has it's place. Yes it does work. It actually reduces heat flow.

Every single decision in this business is a trade off between what is optimal and what is affordable. Sometimes those lower cost options make sense.


Edited 5/4/2008 8:30 pm ET by Marson

(post #115050, reply #14 of 94)

Marson, 


   Please let's not go to extremes.  I do agree some insulation is better than no insulation.


  However as has been pointed out the price differance is nil between between celuliose and fiberglas,   yes there is a small premium for spray foam but if we step back and look hard at alternatives SIP's and ICF's can become affordable..


  Oil is now 116 a barrel  in the next decade that can easily double if not more!


 If you want to build affordable homes you have to start calculating in the cost of heating. I'll grant that used to be chump change.. Not anymore! Heating will  quickly assume the importance of  payments!

(post #115050, reply #17 of 94)

Well Frenchy, I don't mean to start a war. I agree with you that money spent on insulation is money well spent, given where heating oil is headed.

But I have to disagree about your assertion that the price difference between fiberglass and cellulose is nil. Perhaps you are in a diffent market than I am, or maybe you are aware of some techniques that I don't know about. Cellulose may have a similar material cost, but the mesh used for blowing walls is not easily available. Nor do I have access to a blower capable of blowing it at the required density for walls. That leaves hiring an insulation contractor, and believe me, the difference in price between cellulose and fiberglass is not nil. Spray foam is more expensive yet.

(post #115050, reply #21 of 94)

Marson


 I certainly don't feel that we are at war, merely two people expressing differances over pricing.  I'll gladly shake your hand even if we wind up in total disagreement..


 As to the pricing between installed fiberglas and installed celluliose I've seen enough bids and heard it refered to by enough posters on this site to accept it as fact.


   If you are comparing DIY with any proffesional installation we're clearly talking apples and oranges..


   Again refering to professionally installed foam versis professionally installed fiberglas/celluliose In general I'm seeing a 10% to as high as 15% differnace in pricing..


  If you've watched the process,  all three take approximately the same time labor wise, all three typically are done with three man crews,  and material costs are not wildly differant at the wholesale level.. The only differance is that fiberglass batts have a slight advantage in that no expensive equipment is required to put them in..


  Thus clearly the potential for greater profit is with foam.. It's percieved as a superior product so a premium is commanded..


 The disadvantage of fiberglas is too well documented to be a serious question with Foam having a clear advantage over either fiberglas or celluliose.

(post #115050, reply #22 of 94)

"Again refering to professionally installed foam versis professionally installed fiberglas/celluliose In general I'm seeing a 10% to as high as 15% differnace in pricing.."

Frenchy,

If I'm understanding what you wrote above, that foam costs at most 15% more than fiberglass or cellulose, I will have to vehemently disagree.

In our area, you would pay about 2.5x more for closed cell spray foam vs. cellulose. FG would save you even more over cellulose. This is assuming you insulate to a comparable R value (FG and cellulose at roughly R3.5/inch, and SPF at R6.5/inch).

If spray foam was 10% more than FG, I think we would use foam exclusively.

 


Jon Blakemore

RappahannockINC.com

Fredericksburg, VA

 

Jon Blakemore

RappahannockINC.com

Fredericksburg, VA