polysterene ok for attic insulation??

I've just bought a house and plan to put more insulation in the attic and walls. However when I was in the attic I found about 6 large pcks of inch thick polystyerene.

I was thinking since it's there and I'm pretty broke after you know buying a house!! That I'd just cut these to size to fit in between the joists and lay them over the fleece insulation that is in place already.

Is this safe? I've read on the internet that polysterene is supposed to always be behind plasterboard.

Also because the polystyerene isn't foil backed will it cause water vapor problems?

Thanks for your help.

brogan said:

Also because the polystyerene isn't foil backed will it cause water vapor problems?

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I have been told that vapour build-up is a problem, I have also been told it makes no difference as any moisture gets reabsorbed back into the room below.

If the boards are polystyerene just try to insure they dont touch any wiring as the two react.

brogan said:

I've just bought a house and plan to put more insulation in the attic and walls. However when I was in the attic I found about 6 large pcks of inch thick polystyerene.

1. I was thinking since it's there and I'm pretty broke after you know buying a house!! That I'd just cut these to size to fit in between the joists and lay them over the fleece insulation that is in place already.

2. Is this safe? I've read on the internet that polysterene is supposed to always be behind plasterboard.

3. Also because the polystyerene isn't foil backed will it cause water vapor problems?

Thanks for your help.

Click to expand...

1. Its a good idea IF you first take up the fibreglass and put the polystyrene under it. Then relay the fibreglass over.

2. It is safe. However, you must ensure to carry all electrical cables up and over the fibreglass insulation that you are going to lay over the polystyrene. Electrics react with polystyene unless contained within conduits.

3. It should be ok.. once you lay the fibreglass over it. this will create the dew (condensation) point somewhere within the fibreglass. once the attic is properly ventilated, this condensed moisture will be drawn to the top of the fibreglass and allowed to exhaust away...

is your ceiling plasterboard foil-backed? If so, no need to worry about condensation.

Wow, They were quick answers thanks!

Galwaytt I will have to check if the ceiling plasterboard is foilbacked when I go home. But by the sounds of your advice I think I'll put the polystyerene to some good use.

One more question, is there such a think as a maximum thickness of insulation you should put in your house?

are you talking about in the attic again? Then there is a threshold where by you are getting not enough benefit to justify the costs of the insulation. I think the max that seems to be put in is one roll between the joists and then another roll on top and running perpendicular to the first layer. Not sure exaclty where the cutoff of cost vs benefit is but I'm sure someone might have calculated this.
If its walls, then its more a case of the amount of insulation you can actually apply. For cavity wall, you can only put in enough to fill the cavity. For internal wall insulation you're restricted by the amount of space in the room you will be taking away from.