Subscribe or Renew Membership Subscribe Renew

1/2 - 3/4 XPS between studs and exterior OSB sheeting

strawmyers's picture

I've seen plenty of examples of people adding thin (<1") sheets of XPS on the outside of the exterior OSB sheeting for additional insulation (outsulation?)  Any issues with adding 1/2" XPS to the outside of the studded wall, then applying the OSB sheeting over over that?  Issues with the OSB rotting?  Seems like it would be similar to "flashing" the sheeting from the inside with spray foam before adding FG/cellulose, only maybe even a little better because it would provide a break between the OSB and the studs.  Thoughts/input?

straw (post #207445, reply #1 of 6)

With the cushion of foambd. you'll be losing your shear wall connection.  At the minimum behind it you'll have to use let in bracing either wood or metal in compliance with your inspection dept.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


Thanks for the info... that's (post #207445, reply #2 of 6)

Thanks for the info... that's exactly why I look into things before just doing it.  It's a garage/shop space; so I am actually putting 5/8" OSB on walls on the inside.  Only doing the lower 8', though; and it's going to have a 14.5' ceiling. 

straw (post #207445, reply #3 of 6)

Talk with your inspector.  Is this an outbuilding or an addition?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


1/2" insulation SPX (post #207445, reply #4 of 6)

Do you have air conditioning?

Water vapor created within a warm wet home is programed by nature to head for the nearest cold surface, where it condenses. Usually a cold window.


Keeping the walls warm by having an inch or more of insulation on the outside of the the OSB helps to stop condensation and wood rot in all parts of the wall, by keeping it above dew point.

If you have air conditioning, then things work differently.

The XPS fitted on the outside of the frame, will keep the warm wet and cold dry air inside the frame, leaving the OSB on the outside totally subject to the outside weather.

XPS at 1/2 inch may leave the OSB subject to condensation on humid days when the inside of the home is cold unless it is completely protected from outside humidity. This may be achieved by painting all sides of the OSB panels with yacht varnish.

The condensation will then land on the varnish and dry later with no ill effect.

So how would it be different (post #207445, reply #5 of 6)

So how would it be different if you left out the foam and just used fiberglass?

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

This will be an addition (post #207445, reply #6 of 6)

This will be an addition (attached garage / workshop).  Will be heated (prob 55 degrees just to take winter edge off when working on vehicles); so I want to insulate it best I can.  Will not be air conditioned, though.  Right now planning on R-10 XPS under the slab and R-5 between the slab and foundation block.  Wet-blown cellulose in the 2x6 walls; and ~14" of loose blown-in cellulose in the attic space.  I appreciate the info from everyone; and will just be leaving the XPS on the outside of the sheeting if I decide to use it at all.  Don't know if it's worth the extra cost for a mildly-heated non-living space.