We were planning to use unfaced fiberglass insulation in new construction,
in a 9" wall consisting of a 2x4 and a 2x6 wall with the studs staggered. 
And a foil-poly vapor barrier over that.  Now the inspector says the
insulation will slump without facing, and we have to come up with a better
solution.  Has anyone any experience with unfaced insulation in walls?  We
were told you should not have two vapor barriers in the wall, but can you use
the kraft faced stuff, an R-11 and an R-19 layer, and slash the outer layer
or both layers?  Owens Corning said it was okay to have two vapor barriers
if they were in contact, but there will be a small gap between our foil-poly
and the insulation.  If you slash it, how much slashing is needed, and should
it be horizontal or vertical or diagonal.  Is there some other way to hold
up the insulation, such as wires inserted through it and attached to the
studs?  Fingerle's sells a non-vapor-barrier paper-faced R-11 but not in the
size we need (which is 22.5 inches wide).

14 responses total.

        If you anchor the strip at the top there would be no slumping
possible.

You don't want vapor barrier on a cold wall since you might get
condensation which will reduce the insulation value and could cause
rot to wood.  Two layers traps the moisture.  there are some types
of cellulose insulation that if installed wet will not sag over
time.  This may be worth looking into.  It may also do a better job
of filling gaps between your staggered studs.  If you wind up slitting
faced fiberglass, I would think that slitting it vertically would be
better since the weight of the remaining insulation/facing would have
more of a tendency to tear if slit horizontally.  I would also put in
as many slits as possible without compromising the vertical strength
too much.  There is no such thing as too many, I suspect.

I finally got through to the Owens Corning technical advisor, who agreed with
both of the above suggestions.  For unfaced insulation in walls which are
either over 8' or open on one side, they suggest putting 'tiger teeth'
(lightning rods, stiff wires) every 2' through the middle and jabbing them
through the studs.  Or use kraft-faced and slit it vertically, 3 or 4 times.
If the kraft facing is actually contacting the poly or foil vapor barrier put
over it, and has a lower perm rating, no need to slash, but if there is any
gap, and thus a temperature drop between the two barriers, moisture might
condense on the colder one.  We plan to use slashed insulation and on top of
that put horizontal 1x3s to hold the drywall, which will also help retain the
insulation (not that kraft-faced really needs it, but anything to keep the
inspector happy - he was telling us fora  while to use rigid insulation). 
We will also be using the more expensive R-21 because it is an actual 5 1/2
inches and we want to prevent bulging into the air gap so that we can have
extra R-value from reflective air space. ANd the slightly wider stuff made
for metal stud walls, which will be even less likely to slip since we can jam
it tighter (24 inch actual width instead of 23 inch).
        The reason for not just anchoring insulation once at the top, as we
had considered, is that the weight will stretch it out below and thin it a
bit near top and miiddle.
        We chose fiberglass because we can do our own, and don't trust
contrators.  There is a new roof on my block with two strips of melted snow
where someone obviously missed two rafter cavities.  Also it adds
sound-proofing, it is denser than cellulose.  There is also some new type of
foam out based on magnesium oxide, made from sea water, with talc added as
a filler, but it is quite expensive.  Are there other new products?

More information.
1.  Metal stud faced insulation does not have a nailing flange and is
therefore not suitable for wood studs.  When used in an open cavity with metal
studs, it will require those wire supports.
2.  There is a new product called 'quiet zone' insulation, which is faced with
a permeable paper (perm rating over 1.0, kraft paperm is just under 1.0, foil
faced about .5, our foil barrier approx. 0, polyethylene = ?).  It can be used
without slashing, but is only available in R-11 (15 or 23" wide). 
3.  We have 96" studs.  Wood-stud insulation is made for 93".  There are also
continuous rolls but only in R-11 15" and in R-19 (6 1/4", we need R-21 for
5 1/2 inch).  The people who first answer the telephone at Owens Corning had
never heard of continuous rolls.  If you ever call Owens Corning and cannot
get the answer you want, ask for Bob.  The subordinates tend to just read the
book at you, and their book is incomplete.

We just finished gluing together the attic part of the vent stack (thanks
Klaus for the PVC cement, do you want it back now) and hope to start
insulating tomorrow.  Any hints on the best ways to cut it?  I saw a picture
in a book or a special cutting tool that guards the hand but have never seen
one for sale, where do we get one?  Anything we can learn from someone else's
mistakes before we make our own?

I'm glad you found the cement.  I use a cheap, long-nosed pair of paper
cutting scissors to cut fiberglass insulation.  Wear gloves and some sort
of filter for the air you breath.  Otherwise those tiny bits of fiber-
glass will get into your pores and lungs.

Oh, keep the cement if you like.

How about some eye protection, too?

And nose and mouth protection, and hair and skin protection.  We bought a
disposable Tyvek painter's suit, slippery, will not catch the fibers, and a
helmet to match.  An old nylon running suit might also work.

Three bales full!  We have started on the ceiling.  To cut the insulation,
Jim is using a combination of butcher's knife and machete, both of which get
sharpened frequently using a belt sander, and compressing the stuff between
the wood subfloor and a large piece of old rigid clear plastic, as a straight
edge.  There does not seem to be any way unfaced high-density insulation can
slump out of a wall, or even slide down.

There are only a few hours to go on the ceiling insulation, but Jim has taken
off the day to recover from having his arms over his head all Sunday.  Any
heat we put into the house will now be slowed down on its way out by at least
the top two out of three layers.  (Low of 10-15 predicted, time to test.)

We are hopefully about to pass inspection on the ceiling insulation, but
before we do the vapor barrier, Jim worries about whether he left any metal
'lightning rods' in there.  ANyone have a metal detector?  Can they be dowsed?
Got to go, inspector due any moment now. 

Inspector came just before 5 p. m., and approved everything except the fact
that we used paper-faced insulation.  He would not believe that he had told
me to use it, in both layers on the wall, or that I had faxed him in great
detail just what we were planning to do, and what materials we would use, and
had phoned to make sure he understood before we started.  He said it was not
okay to use even the permeable paper, or to slash the other stuff. I told im
to call Owens-Corning, but by then it was too late.  He will call us Monday
to let us know if we can slash our ceiling inslation instead of redoing it
all.    Is this all possible?  He said to staple up the unfaced insulation
so it would not sag.  (OwensCorning insisted it would not sag, but if we
wanted to use paper just slash it three or four times....)  Last year he said
to use unfaced and make it an inch longer, earlier this year to use paper,
now to use unfaced and staple it.  I should have used a tape recorder.  So
we cannot continue with insulation or vapor barrier.  I will do taxes.

We passed the wall insulation inspection, he had his daughter along and was
in a hurry this time.  We are now working out how to put up foil-faced bubble
wrap over the fiberglass, and clamp it down with caulk and a board at the top
and bottom, where tape will not hold.  You have to screw through two layers
of bubbles without breaking them, and get the caulk into the little spaces
left between them.  Does anyone have any old scraps of fiberglass they would
like us to use up for them, going around windows and doors?

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