Air sealing exterior Celotex fiberboard sheathing
We are desiging the residing of our house located in central New York State. The house was built in 1973 with some later additions done by prior owners. We are going to take the opportunity to add air sealing, water management, rigid insulation, a rainscreen, some new windows, then fiber cement lap siding. Planning to DIY it.
My particular question concerns the air sealing. I want to get the house as tight as possible while we have access.
Originally I was intending to seal the seams of the sheathing using tape and/or caulk. However I’ve discovered that a lot of our sheathing is a fiber board material (Celotex), not plywood or OSB. The pattern seems to be that plywood us used in specific places like corners or under windows, and fiberboard everywhere else. (For whatever reason ).
Anyway, I don’t know how to air seal this fiberboard. I guess my options are:
1. Seal the seams as intended
2. Replace all the fiberboard with OSB and seal as intended, or even better as I can seal to the studs somehow (glue / caulk / gaskets?)
3. Cover the fiberboard with 1/8″ plywood and seal as intended ($$$?)
4. Use a single membrane to provide both air and water management
5. Tape the rigid insulation making it the air / water barrier.
Regarding #1, would the fiberboard allow air to permeate? I don’t know what would bond to the material well. I think the surface is some kind of asphalt coating. It also is friable and I fear would disintegrate under adhesive stress.
Regarding #4, my impression was that housewraps eg Tyvek are not really good as air barriers.
Regarding #5, our plans have been to do the air management, water management, and windows all in the plane of the sheathing with insulation on top of that; this would void that idea.
Thanks for any advice!
From what I can tell, the way the house was sided has a lot of flaws. No tar paper or similar material at all; part of the siding is dense cardboard-ish fake stucco boards which are rotting where reverse shingled “Tudor” details have trapped water, etc.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part