Basement insulation is a common feature on newer homes. If you're finishing your basement you're probably wondering "what should I do with the insulation that is already on my basement foundation walls?" The big silver padding that you see nailed to the wall of your basement is the insulation.
Should you remove the existing basement insulation before framing your basement walls? My opinion on this topic is purely that, an opinion. This really is an "it's your choice" decision. I left my basement insulation up and framed the walls outside of it. Today I'll walk you through my thought process so you can make the best choice for your basement project. It really was a decision making process, this question bugged me for about 4 or 5 days.
Will I save some space?
If I take my basement insulation down and then frame the walls right up against the foundation will I save some space? The answer, is yes. But not much.
Going purely by the numbers you will save about 3 inches on each side of the room as the insulation more or less puffs out at about 3 inches depending on the brand of insulation. That's 6 inches in total. However...
There's always a however isnt' there? Why can't it just be that's it. That's the answer. It's not. The space where your basement foundation wall meets the basement floor is likely not perfectly clean and square.
In my brand new basement floor there are all kinds of concrete bits and mortar overrun that creeps out of the seam. This is a good sign because it tells me that it's well sealed from critters and bugs but it also means that I can't exactly build my framed wall straight up against the foundation wall. I'm going to have to come out an inch or so to ensure my wall will be flat against the floor.
Aha! You say. I will just clean up all that mortar goop and then frame my wall. Very well and good luck to you, but make sure that the lumber you are buying is perfectly straight. Because if that framing lumber has the slightest bow in it then you'll have to face that side out towards the room. This will make your drywall guys cry in their pillows. Which in turn will cost you more money or worse, leave you with wavy walls.
You probably still want that extra inch of room so that you can turn the bowed side of a stud towards the foundation wall, thus keeping your drywall side of the wall even.
You will save about 4 inches by removing your basement insulation
So now you know that you are likely going to have to give back and inch on each side, that's 2 inches in total for two sides of a room. So let's see, now we're down to a total of 4 inches of extra space. I mean 4 inches is 4 inches. In a heated ping-pong match or game of pool that can make a difference. To me though, that's not enough of a difference to go through all the hassle of tearing down the insulation and putting it back up. But, my basement was pretty big to begin with, in a smaller basement that could be a bigger deal.
Do I need this insulation?
If you decide to tear it down do you need to put it back up? Yes, you should insulate. Granted, the walls you are going to frame will also act as an insulator but have you ever touched drywall that isn't insulated on a cold winter morning…. It's cold baby, ice cold. I would either cut up your old insulation into strips and then put it back into the stud cavities or trash it and buy some new insulation that's already pre-cut to the right width.
If your basement doesn't have any insulation to begin with there are several options that you can consider. This website does a good job of explaining how to insulate your basement with extruded polystyrene foam.
What if I frame over it ?
I know what you're thinking, I thought the same thing. When I frame my basement walls I will just squish the wall in over top of the insulation to save an inch or two. Bad idea. If you compress basement insulation (or any insulation) then it loses it's "R" value. It is no longer an effective barrier of cold and heat. The framed wall studs can touch the insulation but don’t let them crush it.
Oh, and if you see any holes in your insulation go ahead and cover them with some duct tape before you close in everything. My cat "Badger" used our basement insulation as a ladder to get to the basement window, so that was kind of a problem for me.
Hopefully that sheds some light on this issue, it's a very common question. I'll have a different post about insulation for sound control - plus a very long post about sound abatement, so look for that in the coming weeks.