Any reason to keep 55 year-old fiber wall sheathing during siding job?

Question

Our 1965 DC-area home has original aluminum siding that is dented, scuffed, with drain-holes painted shut, and is so full of other holes and caulk-bodged window / door flashing, etc. that we are getting bids on having new vinyl siding put on. In the attic it's easy to see the original exterior sheathing, something like a 1/4" thick fiber board. I can also feel it inside a defunct vent fan duct on the outside of the house that is plugged with insulation but otherwise allows access to the wall cavity. Phone camera even managed to capture it:

I assumed the right move here, given the age of the house and the best-case condition of this stuff (brittle and crumbly) is replacing with 7/16" OSB and new house wrap. This process would also give us a chance to address any bays empty of insulation, clean out mouse nested insulation and replace, etc. And the OSB will take fasteners for the few spots where it's necessary to put them in. (We've got hide-a-lines for mini-splits that run up the walls, and a standing pipe for the radon mitigation fan, for instance.)

Is there any reason we would want to consider leaving that existing sheathing in place and just wrap it (other than trying to save the $3k=$4k extra cost that adds)?

To be clear, I am not trying to avoid that cost. Just wondering if there aren't other issues / consequences that I haven't thought through that would argue for leaving it if it's in OK condition for the most part. (The stuff I have seen is in surprisingly good condition. It's brittle and old, but not rotted. But I'm sure there are spots that are rotted.)

Answer 1

That is asphalt-impregnated fiber board. It is a structural component of the house and it should not be removed unless it gets replaced with either OSB or RMAX The area will also need to be wrapped with a house wrap as the fiber board also acts as a water repellent.

You may or may not have any luck with keeping the fiber board in place if removing the old siding. It breaks apart pretty easily. My suggestion is to check with your local building inspector and see if RMax can be used to sheath the house. If it is allowed then find out any specifics to how it needs to be installed. RMAX is a rigid foam board that is backed in aluminum. It has an r value of 3 per 1/2" of thickness and it also acts as a radiant barrier. The real benefit comes from the reduction in the thermal bridging that occurs from studs. (r-value of 4ish). If they will not allow it then you will have to sheath the house if you are not able to save the fiberboard.

have at least 50% more money than what you are paying to have the siding done on the ready. you might end up finding some things that you do not want to find in the walls. Like rotted framing. You will also have the opportunity to improve the insulation in the house if you so choose. It might be worth replacing the fiberboard considering how expensive it is getting to heat and cool houses these days. putting on the new sheathing and a house wrap is a fantastic way to reduce drafts as well.