Rather large 2 story home in Western WA. I currently have cedar singles for siding that are EOL and I want to replace them along with the rotting fascia board with T1-11. It turns out the sheathing is a rather thin foil faced paperboard(although it seems to be rated as structural). Some of the sheathing is also shot and must be replaced. Furthermore, it appears it no longer meets building codes due to the thinness of it.

If I were to replace the old sheathing with OSB, what benefits would I gain, if any? I have the time and money to replace the sheathing(and understand the additional complexity) - that's not a concern.

  • I am not sure I understand the type of sheathing you now have, but the t1.11 may be all that is required, if you want to strengthen the home the osb not on the same orientation as the t1.11 would add sheer strength and is used in earthquake areas for this (west coast). The added layer would help to seal the home but the thicker material is going to require additional work around doors and windows. Left as a comment since I haven't used the foil/ foam type of sheathing.
    – Ed Beal
    Mar 25 '19 at 20:59
  • Why would you want to use T1-11? It's just exterior plywood, and will require routine maintenance. A problem I had with a T1-11 sheathed vacation home is that rain water would wick up into the bottom edges of the plywood and rot the bottom inch or two of the sheathing.
    – SteveSh
    Dec 21 '19 at 13:47

Sounds as though your old system has what was / is referred to as "Thermo-Ply (which is a brand name I think, but I don't know of a generic term for it), a cellulosic press board with a weatherproof layer on it. The "good" thing about it is that the foil backing meant it was both the shear wall system AND the vapor barrier in one. It was also, despite being cellulose based, fairly water resistant in case your siding leaked. OSB would NOT be, and would require that you add a vapor barrier, such as Tyvek. In addition if you are concerned about such things, OSB will have formaldehyde in it (part of the "glue" holding the flakes together). Then again, so will the T1-11...

But as mentioned by Ed Beal, the OSB UNDER the T1-11 would be kind of redundant anyway. If you want to put anything under it, consider foam board insulation, especially if you don't already have insulation inside of the walls.


I would not recommend T1-11.

I just had a major renovation done that included re-doing a lot of the exterior of the house. On the outside, the buildup up is: OSB (1/2" I think), 1" polyisocyanurate board, and finished with cement board siding.

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