Can you add new insulation to old wall insulation if there are no holes in the old insulation?

I'm not sure I understand your question. Are the walls open to allow access? What is the existing insulation? How old is old?

You don't want to compress the insulation more than the manufacturer recommends. So putting 6" insulation in a 3.5" wall cavity won't get you more R-value, it will actually get you less. Doubling up vapor barrier is a bad idea, so if the existing insulation has a craft paper face or foil face you need to use unfaced insulation if you're putting the new behind the old or remove the craft paper facing if you're putting it on top of the old. But again, you don't want to compress the insulation more than the specs say.

The bottom part of the wall is open so I could cut the old insulation and add newer insulation for that space. I have no idea of the R value as the house was built in 1979 and I just bought. I am trying to make a crawle space better insulated and have R19 faced insulation for the walls that currently dont have any insulation.

I was needing to know if I can just put the R19 over the old (no vapor barrier)or did I need to cut the old and insert the new?

R-19 can only be used in walls made with 2x6, the material is 6-1/4 inch thick per the Depot website. you would have R-13 or R-15 for a 2x4 wall. jamming R-19 in there would kill the insulating value down to something laughable like R-5 because the air pockets in insulation are what does the job.

if you have open gaps, you certainly can trim things up and insert new insulation. but you will still have gaps from old batt to new batt. you need to install insulation in an unbroken batt to have it perform to standards and keep out drafting around the material.

as I understand your circumstance, you want to put a batt over an existing one, perhaps overlapping it. this is not efficient. the best way to do this is pull down the old stuff, and pull up a continuous new strip of insulation. this reinstallation could be done by cutting a narrow strip of wallboard off at the top, then used by some means like a strip of plastic duct-taped to the sleeve of the new insulation to pull it up the wall cavity, and pushed tight to the top of it. then yank off the helper plastic and use it in the next cavity.

This message has been edited. Last edited by: swschrad,