I assume you want to use batt insulation. If so then I suggest placing the batts between the joists, then adding a second layer perpendicular to the first to cover the joists. The second layer can be thinner (R-13) and should not have a moisture barrier.

If you are going to blow in insulation then the material should be high enough to cover the joists.

keep in mind OP joists for attic floor ceiling joists
rafters for roof. trusses can be either so specify.

ok...foam sheathing on undersides of roof rafters.
air at rest..trapped between underside of roof
decking and foam sheathing has a slight insulation
value so the benefit of not using any insulation
between rafters before attaching foam sheathing
is better than nothing.

first check your local code.
in my climate you can't use conventional insulation
in between roof rafters. if you planned on insulating
the roofline with batts and putting foam sheathing
in place attached to undersides of rafters it is
against code here. any insulation inbetween rafters
has to be foam only.
so check your local building codes.

keep in mind that if you make an air tight
foam sheathing install on the roofline that
when the roof leaks it will not show up
as easily as water will run down to lowest
exit point.

if all is a go, use foil faced ext polystrene
with foil facing into attic space. the foil
will act as a radiant barrier to reflect
radiant gain out of attic.
use button cap nails to attach sheathing
to rafters. tape all seams when finished.

having installed radiant barriers before..
there comes a point where you run out
of room to swing a hammer near the eaves
of the house. figure out how you will
secure the sheathing in these areas.
liquid nail on faces of rafters and
foam sheathing pieces to prop it in
place until liquid nail (or construction
adhesive) dries..

you are correct that in putting foam
sheathing boards on top of insulation
& ceiling joists will create condensation.

best of luck. its going to be a LOT of work.

I'd keep an eye on it. moisture/condensation
and the paper backing of the sheetrock/food
source can cause mold.
even if your attic floor has wood to the
attic side and sheetrock to interior,
water can cause problems.

what did you hope to achieve?

best of luck.

What is wrong with my suggestion of putting in batt insulation with a vapor barrier between the joists then a second layer over this without a vapor barrier perpendicular to the joists? You don't need a layer of polystyrene and eliminate a potential a problem.

If you only want to use one layer of batts, and are concerned about the insulation value of the joists, then you could cut the batt into strips and place them over the joists. It is a lot of work, but it is a solution which should not give you a problem. The batt strips could be thinner than the other batts between the joists. This would save money and make it easier to cut.

no question about it.

how much, over what length of time..
who knows? depends on how much air leakage from
ceiling below, temps of attic, time of year, tightness
of foam sheathing install. lots of factors.

but if the end result causes more problems...why go
there?

I understand that you are trying to 'trap' the
warm air from the living space below.
putting foam board on top of
joists is just the wrong solution.

Ideally you want to 'trap' the air inside the
living space so that it doesn't leak into the
attic at all.

so you create an air barrier. usually the ceiling
material is the air barrier. sheetrock, plaster,
etc are pretty good air barriers, less so with
individual boards comprising the ceiling.

on top of this air barrier between living
space and attic.. you create a thermal barrier to
keep attic temps in attic.
the air barrier is the ceiling of the living
space. the thermal barrier is the insulation.

if you feel that the polysterene is what you want
to use, then use it to create the air barrier.
cut it to fit tightly between ceiling joists,
then caulk it in place.
seal all holes & gaps.
then insulate on top of it.
Lot of work, but like a diy foam spray of attic floor. probably better if you take the time and effort to
seal seams etc.

insulation doesn't stop air leakage.
as air moves through the insulation, it robs
it of its R-value.

if you are worried about thermal transfer through
the joists themselves..do this.

after you've created your air barrier put
batts in the joists. make sure that you fill the joist
bays entirely. then put another layer across the joists.

say the joists run front of house to back.
fill joist bays, then come from side to side with
the next layer. you want to make sure that there
is no air gap between layers of insulation or this
too will de-value insulation.

personally, I'd air seal the ceiling/attic floor
then insulate with double layer of batts.
use foam sheathing somewhere else.

do you have knee walls in the attic where ceilings
change heights?
best of luck.

sealing with rockwool isn't an air seal,
so that may work in your favor.

could you not take the foam sheathing up?

to take the wrong install and try to make
it right isn't something I advocate.

best of luck. truly.

Energy rater, an expert, has called what you did a "wrong install" and has said that your follow-up isn't a good idea.

You've made an honest mistake, no harm no foul. If you want to make the best of the situation, as you say, I'd try following the expert's advice.

Being stubborn in continuing what you're doing isn't very smart. Trapped moisture leads to mold - then what you gonna do?

You may want to check and see if the polysturene is fire rated. Please check with the building department of your city. Fires are not good.... and the material may give off a gas.

If it were me.... I would use standard materials, no makeshift stuff.

Good Luck,

Scott

you do plan to remove foam board to put
plastic barrier? then just do as above.

are you in a heating climate?
I've seen visqueen used behind wall
sheathing over insulation in heating
climates...

best of luck.

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/54110 /You-Don-t-Need-a-Vapor-Barrier-Probably

after reading the article, read the comments.
then on the right side of the screen...read some
of the articles listed concerning insulation,
air barriers and air infiltration.

putting a vapor barrier UNDER the foam sheathing
is creating another vapor barrier under the foam
sheathing vapor barrier.
two where there shouldn't be any.

did you ever check code in your area as I suggested
in my first post?

I think ZERO suggestions have been accepted by the OP. :-)

I think ZERO suggestions have been accepted by the OP.

unless the foam boards come off you are
setting yourself up for problems.

seal the air leaks. insulate with conventional
insulation. use foam sheathing somewhere else.

best of luck.

If you really want poly boards, then what you should probably do is use Isoboard, the one I used has a mylar facing which adds a couple of R values, and is rated (in Canada at least) as a vapour/air barrier. It's very efficient, something like R12-14 per inch or so, if I remember right. There's also a fibreglass 'paper' faced one that's NOT a vapour barrier if you wanted to double up. I paid CDN$ 25 for that, $27 for the mylar faced, per 4x8' sheet. Going between joists it'd go further.

You could also probably go for a sprayfoam direct onto the top of the ceiling material, between joists. It's expensive though.

I'm puzzled by some of the comments re condensation here. In new construction, poly vapour is stapled to the underside (room side) of the ceiling joists. Drywall is put over that. Unless you're using bathroom grade paint (and I don't know what happens there, I assume they feel it won't get breached, and moisture will not get trapped) any moisture/condensation will only go from the house side to the poly, remain in the drywall which will dry out again. Drywall, AFAIK, is considered a kind of permeable barrier.

If you build a house with an unfinished basement, the outside walls still need to be drywalled so you don't get condensation directly on the poly on the outside walls - the drywall's kind of a buffer.

Put it between the ceiling joists, caulk with Acoustic/Acoustical (sic) sealant - the black, gooey stuff. Then put blown-in or batts above that.

poly vapor install is climate dependent.
here in La. it would cause mold growth
behind walls or ceiling under ceiling joists.
by the same token we don't use enamel or
oil based paints on interior as it creates
an interior vapor barrier.

best of luck.

I lived for many years in Australia and have often wished they were more aggressive about insulating (winters can be damp and chilly in some parts) but whether a vapour barrier would be required is good question. Mould is an issue there, and dehumification is probably the #1 requirement.

Paradoxically, as far as I know clothes dryers don't have to be vented outside. Go figure.

I'm a southern climate, are you up north?

best of luck.

I'm even guessing they're in the Antipodes.

I am thinking a vapour/air barrier IS apropos, and, whilst I defer to your knowledge Energyrater, you should be more upfront with the fact you're in a unique climate compared to the rest of the country and most of North America.

If a vapour barrier is mandated, I see no issue with a barrier-rated polystyrene vs poly film. I can't see why one would be a bigger issue than another, neither breathe. If the polystyrene enhances the insulation greatly, then it should be less of an issue, with regards to a dew point.

you and I both recommended putting foam sheathing
in between joists (and caulking and sealing for air
seal ...from my pov)

I think I did ask if OP was in a heating climate.

it has several times been suggested that OP
check with his local codes.

putting vapor barrier on top of ceiling joists
doesn't work for heating or cooling climates.

best of luck.