Replies

1. GBA Editor
   Martin Holladay | Dec 30, 2010 11:58am | #1

   Jason,
   Obviously, homes built in 1898 didn't have OSB sheathing. So do the walls have extensive rot?

   You don't want to sandwich OSB sheathing between two layers of impermeable foam. One of these two foam layers has to go. I would vote to keep the exterior rigid foam, and to install dense-packed cellulose insulation between the studs.

2. 
   user-916173 | Dec 30, 2010 12:12pm | #2

   To clarify, regarding the OSB we plan to remove the old plank sheathing and replace with OSB. Purpose to establish an air barrier on the plane of the OSB. I don't know the condition of the existing sheathing. Though, I assume it has some rotten sections. Thanks for your suggestion...

3. GBA Editor
   Martin Holladay | Dec 30, 2010 12:18pm | #3

   Jason,
   I'm glad to hear that you are thinking about air tightness, but I'm not sure that your plan makes sense.

   First of all, it can be dangerous to remove existing board sheathing unless you are sure that the walls are well braced to prevent racking.

   Secondly, board sheathing is in most respects superior to OSB, in almost all characteristics except air tightness.

   Airtightness is certainly important. If you feel that the OSB is essential, why not install it on top of the existing board sheathing?

4. 
   user-916173 | Dec 30, 2010 12:40pm | #4

   Ok, that makes good sense. What then is the best strategy for improving the airtightness of old plank sheathing? Do you recommend installing thin OSB on top, then rigid foam? I'm concerned that the rigid foam / plank sheathing interface will nullify the insulating benefits of the rigid foam.

5. GBA Editor
   Martin Holladay | Dec 30, 2010 12:51pm | #5

   Jason,
   Unless your board sheathing is unusual, you shouldn't have any problems installing rigid foam on it. I would probably recommend installing a layer of housewrap between the boards and the foam, although the housewrap could be omitted.

   Before installing each piece of rigid foam, use a bead of caulk around the perimeter of the foam, so that the caulk is squeezed between the foam and the housewrap (or sheathing). Then tape the foam seams with an appropriate tape.

6. 
   user-916173 | Dec 30, 2010 01:31pm | #6

   This solution makes a lot of sense. Thank you for your time!

7. 
   jklingel | Dec 30, 2010 01:37pm | #7

   If I may add, replace the OSB w/ plywood in case it ever gets wet.

8. 
   Riversong | Dec 30, 2010 02:22pm | #8

   Jason,

   I will ditto John about using CDX plywood rather than OSB - it's superior in every way (though a bit more costly). But, if you don't need the additional sheathing for shear bracing, then I would simply install and tape housewrap over the existing sheathing (making sure to integrate it with the flashings over doors and windows for proper drainage), hang and tape exterior XPS (not foil-faced foam) to allow minimal breathing to the outside, a furred drainscreen and then your siding. And use dense-pack cellulose in the stud cavities for all its many health, ecological and moisture-management benefits.

   You don't need to fuss over caulking the foam to the housewrap. With dense-pack in the walls, housewrap and taped foam board, you will have a reliable air barrier (as long as you tighten up the basement and attic where most air leakage occurs).

9. 
   Jim Merrithew | Dec 30, 2010 02:23pm | #9

   Martin, If the exterior rigid foam is unfaced PolyIso, would this give enough permeability to allow the OSB to be installed between the layers of foam? How does the thickness of the rigid foam affect the perms?

10. GBA Editor
    Martin Holladay | Dec 30, 2010 02:34pm | #10

    Jim,
    Any foam, including unfaced polyiso, will significantly slow drying. I think you should choose to put your foam on just one side of your sheathing, not both. As I said before, it makes more sense to install rigid foam sheathing on the exterior of your wall, and to use dense-packed cellulose instead of spray foam between your studs.

    Reducing the thickness of exterior rigid foam sheathing to increase vapor permeance is a fool's errand. When it comes to foam sheathing, thick foam is better than thin foam, because it reduces the chance of condensation or moisture accumulation within your stud cavities.