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Old 08-23-2012, 06:32 AM   #1
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Polystyrene between attic joists or on top of joists?


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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Old 08-23-2012, 07:46 AM   #2
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Polystyrene between attic joists or on top of joists?


Why not just loose fill?

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Old 08-23-2012, 08:48 AM   #3
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Polystyrene between attic joists or on top of joists?


I don't know anything about loose fill. Isn't a special blower machine needed? What kind of product is used in loose fill? Rock wool? I'd rather not work with fibre glass. But I have since seen that it is sometimes recommended to insulate BETWEEN joists and then on top as well, perpendicular to joists? That sounds like a lot of insulation!
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Old 08-23-2012, 10:33 AM   #4
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Polystyrene between attic joists or on top of joists?


Loose fill is much cheaper.

Most big home improvement stores will lend you the blower if you buy the insulation from them.
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Old 08-23-2012, 10:42 AM   #5
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Polystyrene between attic joists or on top of joists?


Where do you live?
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Old 08-27-2012, 05:20 AM   #6
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Polystyrene between attic joists or on top of joists?


The house is in the north of Spain. 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.
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Old 08-27-2012, 05:29 AM   #7
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Polystyrene between attic joists or on top of joists?


The insulation boards are not walkable at all.

You would need to put down another subfloor over top of them and that could create a bit of a moisture trap.

Why is the access to the attic for foot traffic?
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Old 08-27-2012, 05:55 AM   #8
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Polystyrene between attic joists or on top of joists?


ok, the reason I would walk on the insulation boards is just in case of having to go into the attic for any routine maintainance. What about if I used a couple of "walkways" - I mean something like plywood sheeting of 2 meters x 1 meter?
Apart from the walking on the boards, would there be any problems with heat building up under the insulation, on top of the ceiling timber? And the electric cables? Drill holes in insulation that are maybe 2 or 3 times "fatter" than the cables? In this case the cables are mainly for low power use, like light bulbs and bedroom plugs etc. But, summarising, do you think it woukld be a safer bet to install the polystyrene BETWEEN the joists? And maybe at some future time lay other insulation on top of the joists? Thanks


Last edited by seanog; 08-27-2012 at 05:58 AM.
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