The top half of my basement walls are covered with the original fibreglass and drywall. If I'm planning to use rigid foamboard insulation can I just use this on the lower half and leave whats currently there - and then drywall overtop? Or would I need to remove everything and use the foamboard on the entire wall?
You can use rigid at the bottom as long as you achieve the R value of R 20 as per table 18.104.22.168. of the SB-12 in the ontario building code. Vapour barrier is still required to be continued down up to 200 mm above the slab.
I myself would remove the existing insulation. The reason the boards are so great, is the fact that you have zero water damage to it if ever you have a leak of any kind. Remember to install an R factor of 20.
Enjoy finishing the basement.
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