To remove, or not to remove (80 year old mineral wool)?
1940 farmhouse in Zone 3B with 2×4 ceiling joists in a vented attic. New metal roof going on soon is bringing the opportunity to upgrade the existing four inches of pre-war mineral wool insulation. This upgrade is all about keeping the house cool in extreme hot/dry summers which costs us far more than keeping it warm during short mild winters.
Is it worth removing the mineral wool “batts” in order to provide decent access to performing air sealing before blowing in loose fill? Admittedly the idea of also removing nearly a century of rodent droppings and a fair amount of cedar shingle shards is fairly attractive.
The only practical alternative seems to leave the mineral wool (which I don’t care for sending to a landfill) and just peel it back to perform air sealing less successfully.
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Replies
Depends on how bad it is. If you do have a lot of rodent damage, removing is the only option.
For century homes with similar amount of insulation, I pull back the areas that need air sealing, clean and spray foam those, then blow in cellulose to required depth.
Also don't forget to install baffles at the eaves.
Before you filling up the attic with insulation, make sure to check and replacing any old wiring, much easier now than digging through a foot of insulation.
I am dreading the loss of easy access to existing wiring (and joists to crawl on). There are a multitude of prior owners' wiring modifications to button up.
Another vote here for removing the old insulation. If you have a lot of critter damage and mouse poo, cleaning that up is a good idea. You’ll have a much easier time both finding the things that need to be air sealed and sealing those things if the old insulation is out of the way. Once you’ve done your air sealing work, blow in how ever much new insulation you need to get to the R value you want.
I would also do as Akos’ suggested and do any electrical work now while you have easy access. That would would include replacing any old knob and tube wiring (which can’t be buried in insulation), and I’d also replace any old “type R” wire (cloth over natural rubber) that is risky to leave in place.
Bill
Fortunately, there is no knob and tube. In fact the entire house was wired with flexible steel conduit to the center of each room's ceiling and then down the walls from there. It's pretty amazing work. It's only where subsequent work has caused concern that I am addressing.
The other big issue is attempting to seal up the chimney passing through the attic. There is about an 8" air gap (that was air sealed in 1940 with cardboard !) and drop ceilings on either side of it creating a fairly challenging number of intersections to deal with. To make matters more interesting, the dropped ceilings are 1/4" plywood "panel" as are the walls that surround the chimney. There is no button board or plaster parallel to the chimney, only on the true ceiling two feet higher than the dropped ones.