Extruded polysterene as attic insulation- between joists??.

elbowAugust 23, 2012

I was wondering which of these plans is best: to cut and place rigid extruded polystyrene boards between attic joists, or to form a "floor" of polystyrene boards on top of the joists - in this case, I assume I would have warm air between the wood ceiling boards and the polystyrene boards. So some of the heat from the house would accumulate there between the joists. Would that mean "losing" heat from the house? On the other hand, placing the insulation between the joists, do you think the joists themselves would act as reasonably good insulators? (I think they are 10 cm wide x 15 cm high). Thank you.

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Joists and studs are a significant heat bridge when insulation is installed between them. Losses are pretty high.

    Bookmark   August 24, 2012 at 2:55PM
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attic joists? usually a joist is a floor supporting thing? a house might have roof trusses above the ceiling. or rafters. are you insulating the attic space above a room?

    Bookmark   August 24, 2012 at 5:58PM
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Thanks for the replies.. My original idea was to install the polystyrene boards on TOP of the joists, thinking that it would form a more complete, leak-proof insulation, and the 8cm boards seem rigid enough to walk on (space between joists around 60 cm), in the case that I would need to go into the attic for maintainance etc. *But, someone mentioned about the possibility of condensation forming between the joist spaces (if the polystyrene is laid on top). I was also wondering about the possible accumulation of hot air in that space between ceiling boards and the insulation boards - would that represent heat "lost" form the house? Also, for light bulb cables. I assume the correct procedure would be to drill a hole through the polystyrene board, with a small plastic "sleeve" to pass the cable through? I mean a sleeve that would have a generous amount of space (maybe 3 or 4 times bigger than the cable?) for the cable to pass through - of course I also want to avoid too many "gaps" in the insulation.

    Bookmark   August 27, 2012 at 7:02AM
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The joists have an R value of about 6, so it is not terrible but not as good as the insulation you are adding.

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.

    Bookmark   August 27, 2012 at 9:40AM
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I'm not understand if the foam boards are
going on the roof rafters or the ceiling joists.
must just be me??

    Bookmark   August 27, 2012 at 7:45PM
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Circus Peanut

I believe placing the boards on top of the 'joists' (rafters) in the attic is the best practice to mitigate heat loss. The wood is too much of a bridge otherwise. One way to proceed would be batts between the rafters and then poly board over the top. Although more expensive, the benefit of the boards is that you don't lose your attic space to storage.

    Bookmark   August 28, 2012 at 8:31AM
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wood has an insulation value of R-1 per inch.
not much.

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.

    Bookmark   August 28, 2012 at 11:06AM
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Thanks, I appreciate the replies. Finally I just lay the 8 cm extruded polystrene boards on top of the joists (joists meaning the wood beams that hold up the ceiling underneath). I didn't put anything between the joists. Hopefully it won't lead to condensation issues. How would I know if it did? Thanks

    Bookmark   September 6, 2012 at 1:22PM
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back of sheetrock gets wet, over time it
comes through to interior as water stain.
depending on how much condensation, time for
this to happen will vary.

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.

    Bookmark   September 6, 2012 at 2:45PM
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Sheetrock? Around 80% of the upstair ceilings are parquet boards 14 mm thick x approx 9cm wide. The other part of the ceiling is plaster. Then in the attic I have laid the rigid polystyrene boards on top of the joists, so I understand that there will now be an "empty" space between the ceiling boards and the polystyrene, where I assume heat from the house could get "trapped" - of course I would prefer this heat to stay *under the ceiling boards. But if condensation is now likely, do you think a solution would be to lift the polystyrene boards and lay rockwool batts between the joists, and then replace the polystyrene on top of the joists? Thanks

    Bookmark   September 7, 2012 at 4:27AM
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My concern is the polystyrene will trap moisture. I have never seen this done so I can't tell you from experience.

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.

    Bookmark   September 7, 2012 at 12:00PM
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polystyrene on top of ceiling joist will trap condensation.

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

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.

    Bookmark   September 7, 2012 at 1:00PM
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Well, the thing is I have already laid the polystyrene boards on top of the joists. Of course I would prefer to "trap" all the heat inside the house, not between ceiling and insulation boards. I even tried to seal along the walls (of the attic floor) with rockwool, where the polystyrene boards have little gaps, which I now think may be worse for condensation. *But, would there be any point in, say, making several fairly big holes in the polystyrene, and filling those holes with Rockwool, to counteract the condensation? Those holes could be possibly 6-inch square cut-outs? So maybe in every square meter, about 1 or 2 holes?

    Bookmark   September 7, 2012 at 2:52PM
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I wouldn't venture to say that making holes in the
foam board would work.

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.

    Bookmark   September 8, 2012 at 1:04PM
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well, the new polystyrene boards don't come that cheap, so I want to try to make the best of the situation. If trapping moisture is the probable downside, I assume making those holes (perhaps two or three 3-inch holes in every square meter of polystyrene, and replacing with rockwool to try to avoid heat loss) would allow the moisture to escape into the above attic space which *is well ventilated. Or maybe even 1-inch holes drilled through the boards - maybe 6 or 8 every square meter and filled up with rockwool, may be better? My idea is to get ventilation in there.... thanks.

    Bookmark   September 8, 2012 at 2:46PM
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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?

    Bookmark   September 8, 2012 at 7:55PM
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Can I put my question another way? I think my proposed solution of drilling 6 to 8 one-inch holes filled by rockwool, per square meter of polystyrene board, would solve the vapour issue, especially bearing in mind that otherwise the attic in general is very well ventilated. Could you give me any rational, scientific / physics, or common sense reason why you think it *wouldn't work? Thanks.

    Bookmark   September 9, 2012 at 6:53AM
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Ok, meanwhile I have been researching and now I'm inclined to go with the idea installing a vapour barrier just on top of my ceiling, under the polystyrene boards. Hopefully I will be able to get that space properly sealed with a good plastic barrier. Thanks

    Bookmark   September 9, 2012 at 3:47PM
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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,


    Bookmark   September 9, 2012 at 5:14PM
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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.

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

best of luck.

    Bookmark   September 9, 2012 at 5:16PM
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But, would it also be ok to just lay a good sealed vapour barrier on top of the joists - under the polystyrene boards? And maybe a bit of rockwool between the joists, below the vapour barrier? (to keep more heat under the ceiling). Also, if I have the vapour barrier under the insulation, I assume it would then not be necessary to seal or tape the polystyrene boards? Thanks

    Bookmark   September 9, 2012 at 5:52PM
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Maybe I should have started with this??


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?

    Bookmark   September 9, 2012 at 7:12PM
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"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. :-)

    Bookmark   September 9, 2012 at 8:11PM
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"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.

    Bookmark   September 9, 2012 at 8:12PM
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Thank you for that informative link about vapour barriers. This is for a relatively isolated mountain house, where winter average night-time temperatures would be around 0 celsius, but can drop to -15 or so, but not common. I like the idea of sealing all the air leaks, and I take your point that 2 vapour barriers isn't a good idea. However, would it then be a good solution just to seal all the air leaks with good quality caulk, I mean by leaving the polystyrene boards in place and sealing every possible gap I find, including along walls, where the biggest gaps are? Obviously in this case I would be basically sealing leaks at the top of the boards. So, I wouldn't install any separate vapour barrier underneath, or add any other insulation on top of the polystyrene? Thanks.

    Bookmark   September 10, 2012 at 7:09AM
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no. you seal the air leaks at the point where they
leak. otherwise you are allowing the air to
enter into the joist bay with the vapor barrier
on top.
keep reading energyvanguars's articles,
try the air infiltration articles next.

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.

    Bookmark   September 10, 2012 at 10:23AM
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Assuming your poly boards are rated as an air/moisture barrier, then you don't want TWO barriers which is what you'd have with added poly.

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.

    Bookmark   September 10, 2012 at 2:27PM
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alan..did you read this article?

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.

    Bookmark   September 10, 2012 at 5:00PM
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Energyrater, I will read it presently. I am indeed in a colder area. I assumed OP was too.

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.

    Bookmark   September 10, 2012 at 6:19PM
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elbow..where do you live?

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

best of luck.

    Bookmark   September 11, 2012 at 12:13PM
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Yeah, scrolling up a bit, the OP is in a cold-ish climate - and alpine so I'm thinking dry, and maybe not even North America, so they should research their local regs.

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.

    Bookmark   September 11, 2012 at 3:07PM
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I've tried to change my user name to energy rater La
but would have to re-create my account & change
my email. I'm not willing to use another email.

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.

    Bookmark   September 11, 2012 at 4:09PM
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I appreciate all the information, and I have learned a lot! My general plan now is to properly seal any air gaps I can find, and (probably) install a rockwool layer on top of that, as suggested earlier by Energy rater, I think. (now that I understand better) So, basically I would be using the polystyrene as a vapour barrier, and the rockwool above that would mean that the area around my sealed gaps would stay warm, preventing condensation. Thank you all for your help. I hadn't realised that insulated an attic was that involved...but I'm very happy to have learned more about it!

    Bookmark   September 13, 2012 at 5:00AM
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FWIW - ERLA - I instantly knew where your were from. A quick thought about Los Angeles but that passed quickly. Either way, certainly not a cold climate.

    Bookmark   September 13, 2012 at 5:25AM
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