In a 2x6 wall is the any advantage to stuuffing 2 4in r12 batts instead of
the single 6in batts or does the compression just reduce the effectiveness?
I ask because I have several bales of the four inch stuff.
Not quite true. A fraction of the insulating value is lost as you
compress it, but you will still likely have more insulating value than
you would have with only the 6" batt. I looked this up on one of the
manufacturers web sites because I wanted to put 6" insulation into a
cavity made of 2x4s. As I recall the compression caused a loss of 20%
over what it would have been in a 6" cavity, but 6" insulation offers
almost twice the insulating value as 3.5", so it was still a large
improvement. You are correct that at some point if you keep compressing
the insulation you totally defeat the purpose, but he is only
compressing it about 25%.
Donald Gares wrote:
Two 4" R-12 layers of fiberglass compressed into a 2x6 stud space will
provide less than R-24 insulation value but still more than the R-19
of 6" batt because for fiberglass the insulation value per inch
increases with density, up to a point, so you'll probably end up with
R-20 or R21. None of this implies that you should compress fiberglass
if you don't have to because you'll get more insulation value by
leaving it as fluffy and thick as possible then.
The fiberglass batts I've used will pull apart into 2 layers. If yours
does, you could use one and a half 4" batts. Just make sure to remove the
vapor barrier from the half that it's attached to.
This is not necessarily true.
You are increasing the thickness substantially and not compressing the batts
to a level that the r value per inch is compromised. Insulation
manufacturers make thinner higher R-value batts for cathedral ceilings by
INCREASING the density of the insulation.
The density of typical fiberglass insulation is driven by the minimum amount
of fiberglass necessary to reach a certain rating (this allows it to be most
economical). It is not the optimum density. You can pay more and get denser
better performing insulation.
I would say that the thing to be careful about would be the presence of a
vapor barrier in the middle of the insulation, but then again, if it has a
vapor barrier on the outer or inner face, then it shouldn't make much
difference there either.
You are correct at least in part. It would be difficult to make an
absolute statement. There are a number of factors to consider, and we just
don't have enough information. Frankly I doubt if there is going to be a
serious increase or decrease under the conditions noted. What is certain is
that using batts designed for the space will give better insulation and less
problems (no concern about improper vapor barriers) than two smaller batts.
While there are some differences in R values from different materials,
the increase is not necessary result of increased density. In some cases
yes in others just the opposite. The shape, size and distribution of the
fibers is important. Changes in any of those factors would also change the
What a bunch of pretentious crap on your part. JackD is 100%
right, and you don't seem to have a clue because the R-value/inch
improves even when the density is doubled, as it almost is with
rigid ceiling tiles and was with rigid fiberglass insulation
used in old mid-priced water heaters (cheapest heaters had low
density soft fiberglass, premium-priced had polyurethane foam).
8" of regular fiberglass compressed between 2x6 studs will
give about R-21, compared to R-19 for 6" regular fiberglass.
Believe what you want. What I presented is based on the empirical
information I have seen. Note that actual results will depend on many
factors so in some cases compression may cause some increase in others some
decrease and it will also depend on the amount of compression.
More pretentious crap, and you actually didn't present any
empirical information - because you simply didn't have any.
The fact is that if you mash fiberglass to half its thickness,
which almost nobody is going to do, its insulation value per
inch is going to rise to about R-5 or R-6.
Journal of Light Construction says a slight compression of U.S. glass t fiber
insulation does increase R value. I was told this at a seminar on house
construction a couple of years ago and asked the magazine for proof. The did not
give what I would call proof, but quoted - as I remember - building researchers.
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