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Extra Attic Insulation. Can I add polystyrene panels over existing fibreglass batts? Answered

I have access to 6in thick polystyrene panels which I would prefer to re-use rather than take to the garbage dump. Can I lay them over existing fibreglass batts to increase the insulation in the attic ? The house is 4 yrs old so is up to code and is located in Ontario, Canada where the winters are cold. I'm worried about causing a moisture problem.


Code put aside, I would put the insulation on the underside of the roof sheathing, between the studs.

Put simply. No!
The problem is, that you and your family produce water vapor, this rises and passes through the ceilings and any gaps, it will build up under the polystyrene and make everything wet, leading to mold and wood rot.

However, if you remove all the existing insulation and place it directly on top of drywall, face to face contact, then the drywall will stay damp (its always damp-being so provides a 30 minute fire brake) make sure you cut it to a push tight fit.
You can then place your existing insulation on top.....it won't do anything but, its one way to loose it.

If you allow air circulation between the drywall and polystyrene, you may as well throw it away.

What is the fire rating on the polystyrene you have? Is it standard or flame retardant? If it's standard you will probably be going against the building code any you may be voiding your fire insurance.

Oh, I forgot about that - ignore my answer.

As long as you allow air circulation, you're OK.