First, lets assume fiberglass insulation is your only option. If you want to promote Cellulose, etc., please start a new thread.
Consider these two situations:
a) You use R-19 FG insulation in a 5.5″ space.
b) You force two batts of R-19 FG insulation into that 5.5″ space.
My questions:
1) Which 5.5″ space would have the higher R-value?
2) Which 5.5″ space would have the higher R-value/dollar?
I believe that the answer to (1) is (b), but the answer to (2) is (a).
I’ve seen conflicting information on this, but the sources I think are most reliable match the answers I’ve given. But I see many websites which state that compressing fiberglass lowers its insulating properties. I think that is a fallacy….in fact it insulates better but costs a little more. For the record, the “reliable” source I’m using is Charlie Wing’s book “The Visual Handbook of Building and Remodeling”.
Any thoughts on this?
Replies
My quess is that with over packing you will lose insulating value per inch, based on a fact and a supposition.
The fact is that it's the air bewteen the fiberglass strands which insulates, not the fiberglass itself. More fiberglass, less air.
The supposition is that the manufacturers have probably done testing and come up with the density that provides the best R value per inch to be as competitive as possible with competing products.
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Yes, but not all air is created equal. Packing the fiberglass more densely reduces convection within the batt and bulk air flow through the batt, so the remaining air insulates more effectively.
There's no doubt that a single R19 batt installed in a 5.5" space is a better insulator than a single R19 batt installed in a 2.75" space, but that wasn't the question.
Edited 12/31/2002 10:56:49 AM ET by Uncle Dunc
i agree with your conclusions completely,
which is why we use cellulose..... hah,hah, hah
Mike Smith Rhode Island : Design / Build / Repair / Restore
Me too.
R-19 fiberglass insulation was never intended for use in stud walls. It was originally produced for use in ceiling with addl. batts applied over the top or (for all you cellulose lovers) a topping of cellulose or blown fiberglass. You'd be better off forgetting your idea of two batts of R-19 in 5.5" and skip to what should really go in that stud cavity - R-22 batts specifically designed for use in 5.5" stud cavities.
>> You'd be better off forgetting your idea of two batts ...
Maybe. Maybe not. To decide that we'd have to know the costs and the R-value result of both approaches. Do you know of any studies that have evaluated the compressed fiberglass technique?
Joe,
I had no intention of cramming two batts in a wall cavity to increase R-value. My reference indicates that the total R-value would go up to R-24, but I'd be paying 100% more for a 26% gain in insulating value.
I disagree with your statement "R-19 fiberglass insulation was never intended for use in stud walls." It is commonly used around here, and Owens-Corning lists "exterior walls" as one of its applications. Using fiberglass insulation of the same thickness but higher R-value costs a little more per inch, which may be the reason R-19 is commonly used here. We don't have extremely cold winters here. However, I agree that it would be better to go for R-22 instead of using two R-19's.
My original question was intended to validate or debunk the notion that you should not cram fiberglass insulation into spaces behind electrical boxes, plumbing, etc. because it loses its R-value. I personally believe that it is better to cram it in. At this point, Mike Smith would point out that cellulose would fill around the boxes and plumbing, and I wouldn't try to dispute that.
I can't help debate the benefit or loss of cramming two batts into one cavity without benefit of the test results. I think it's odd that you rely on the opinion of the seller (Owens Corning) to substantiate the myth that R-19 batts are O.K. for wall cavities. In the climate I built in for 22 years R-19 was used commonly also. It was all that was available until the advent of the R-22 batt. The use of the R-19's until that time was the only choice - but not the right choice. I stand behind my original statement "R-19 batts were never intended for stud cavities". This information came from people outside the interest of selling their product to the uninformed consumer.
damn , ted... i wuz yust gonna say datMike Smith Rhode Island : Design / Build / Repair / Restore
Which has a higher R-value...fiberglass or rigid foam?
>> Which has a higher R-value ...
Do you mean higher R-value per inch, or higher R-value per dollar?
Whatever. If the rigid provides more insulation value per inch of thickness, then that tends to prove his point about compressing the FG, cause there's not much dead air in foam. Well, there's a lot of dead air in the little bubbles, but the foam material is denser than the FG strands.
Edited 1/3/2003 11:42:18 PM ET by ELCID72
But you also have to look at the heat transfer properties of the materials;
I don't know what they are, but strongly suspect that foam has a much lower transfer rate than glass.
With fiberglass insulation it is the air that provides the insulating value, but that isn't necessarily tru of all materials.
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Consider this situation. The R-value per inch for stone is listed at 0.08. In theory then, a 12" thick stone wall would have an R-value of only R-0.96. A sheet of 3/4" plywood has an R-value close to R-0.93 which is very close to R-0.96.
Now suppose that the temperature in this mythical land varies daily from 20 degrees at night to 120 degrees during the day, so that the average temperature is 70 degrees. Would you rather have your walls made of the 3/4" plywood, or the 12" of stone? Thermal mass can be important as well as thermal conductivity.
R-values give us a rough way of quantifying insulating value, but they aren't
Bob Walker has stated a couple of times that "it is the air that provides the insulating value". I agree that the air is important, but the fiberglass plays an important role too. Otherwise we'd just use the air space in the wall cavity (assuming we could make sure it wasn't moving). I also agree with him that the insulation manufacturers have optimized their product, but I think they're strongly considering cost/R-value in that optimization.
I agree that the air is important, but the fiberglass plays an important role too. Otherwise we'd just use the air space in the wall cavity (assuming we could make sure it wasn't moving).
Yeah, that's why the fiberglass is there, to keep the air from moving.
Of course, questions ahev been raised as to how good fg insulation is at doing that, buit that's another thread.
And I doubt if FG has much termal mass, since it's pretty light.
I have to admit, though, that I'm just speculating on this stuff and using what I think of as common sense, but I have found that often reality is counter-intuitive!_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
ted... back in the day.. when we used to use a lot of fiberglass.. we did quite a few 2x6 walls.. and the R-19 batts were junk, especially the kraft faced or foil faced ones.
Most of the time we used unfaced R-19 BATTS with a 6 mil poly vapor barrier.
If i were doing it today with F'gls.. i'd probably use (2) R-15 unfaced friction fit BATTS with a 6 mil poly vapor barrier . The first layer would go behind the elec. devices, and the 2d layer would get cut tight around the devices..
'Course, as you already guessed, i'd really just use cellulose....
here's Regal Wall... nylon mesh with cell blown in to about 4 lb/cf
Mike Smith Rhode Island : Design / Build / Repair / Restore
I tend to agree with the "cram it a bit" theory...at least there won't be drafts in the wall from insufficient fill. However, one spot never to do that cramming is between window frames. I have a pair of double hung windows (is that 4 hung?) I overstuffed that have slightly bowed casings that make the windows bind when raising and lowering.
There is a 'Limit' to how hard you cram. Say you eliminated all of the air (theoretical limit) I theorize it would it have the same thermal conductivity as solid fiberglass. That would be less efficient than the batts.
I'm sure if "price" options were available, the marketing gurus at the insulation companies would offer different r values for different size walls. Since they don't(not that I'm aware of) I think it's a safe bet that what they sell is optimized. If There was a useful r25(random number) for 2x6 walls in fiberglass, they would be selling it.
Hey, in my 1840 home that I'm gutting to replaster, the windows and top plates were insulated with corn silk and husks. You might want to try that. You know what they say about building them like they use to. ;-)
Don
the way i remeber it was " they don't build them like they used to. ..... Thank , God !"Mike Smith Rhode Island : Design / Build / Repair / Restore
If You a interested in compressed denisity of fiberglass the manufactures have run the test and have the charts available, do not know if they are on their web sites. As to r-22 it is a 7.5in material and should not be install in place of r-21 which is a high density product, high density products are conpressed batt insulation a r-21 has about the same amount of fiberglass as a low density r-30. In the past I have used compressed insulation charts to get insulation calculations on custom house to meet the energy codes for clients. It can often be less expensive to use low density insulation and conpress it then to locate and use high denisty
This gets to a question that I was idly thinking about. R15 is 3.5" thick. R21 is 5.5" thick. R30c is 8.125" thick.
If you stack R15 on top of R21, you might think it would be R36 and 9" thick. This would suggest though that the last .75" of the R21/R15 combo is worth R6, which can't be right.
What's wrong with the above calculations?
Pete