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OSB before Rigid Foam?

I'm planning the renovation of a small (920sf) home in Indiana (Zone 5). The original home is 112 years old, but at some point it there was an addition (yes, it is still only 920 sf). So the original exterior walls have no sub-siding (sheathing) but instead has a very hearty lathe on the interior of the stud wall. The exterior siding and the hearty lathe both have to go. The addition has 1x6 sheathing boards on the exterior of the studs, hung horizontally. We'll be insulating the wall cavity and want to hang rigid foam on the exterior before siding and I want to know if there should be sheathing hung first, such as 4x8 sheats of OSB.

I have these questions:
Is it needed structurally? Many GBA videos show rigid foam going right up on the new construction studs. With such a small house, I'm thinking it might not be required for this reason.

Would the OSB need a water-resistant barrier, such as housewrap, before the rigid foam? Or, is that redundant since the foam will be taped and sealed tightly?

Our wall cavity insulation is currently planned as spray-cellulose, and so I very much want to keep this stuff away from any exterior moisture.

Finally, if I sandwich sheathing between an insulated wall cavity and rigid foam, am I creating wall rot potential?

My contractor and I are game to figure it out, so any advice is appreciated. Thanks!

Asked by William Wagnon
Posted Dec 19, 2012 9:28 PM ET


5 Answers

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If the 1x6 board sheathing is installed structurally sound then you are good, if not, you should install Plwd/OSB sheathing on top. The APA has not finished the testing about insulation between framing members and board sheathing, so installing 1-2” rigid foam on top of the wood sheathing would be good; make sure you seal and tape all seams (2-1’’ foam layers with staggered seams is even better). Rigid foam can be a moisture barrier if it is taped and sealed, so you won't need house wrap, unless you want to have additional security. House wrap is best used as a drainage plain; but can be used as an air-barrier if it is taped and sealed, and no tears.
Install 1x4 rain screens and siding on top; plus blown dense-packed cellulose in the cavities for a great wall assembly.

Answered by Armando Cobo
Posted Dec 19, 2012 10:28 PM ET
Edited Dec 19, 2012 10:32 PM ET.


To clarify: the 1x6 boards are solid but only cover about 1/3 of the perimeter of the house. On the front 2/3 of the house there is currently only siding | tar paper | studs | lathe. All but the studs will be removed, and I want to be sure the lateral strength being lost by taking out these 3/4" tongue-groove lathe boards does not need replacing, or that the foam sheathing will be enough.

Answered by William Wagnon
Posted Dec 19, 2012 11:09 PM ET


I should have phrased it better; it's not about being sound, it’s about meeting the structural code. If you only have a third of a small house covered with 1x6 sheathing boards, you maybe better off to strip it all down and build a good wall assembly thought out.

Answered by Armando Cobo
Posted Dec 20, 2012 12:08 AM ET
Edited Dec 20, 2012 12:09 AM ET.


Every exterior wall needs bracing. And every exterior wall needs a WRB.

Right now, your lath and plaster are providing bracing. Once those are removed, your walls are at risk of racking. That's why you should install a layer of plywood or OSB on the exterior of your studs.

The addition with 1x6 board sheathing also needs to be braced. (If the builders who built the addition had installed the sheathing boards diagonally, the board sheathing would provide excellent bracing. But they didn't.) You can install plywood or OSB directly on top of the existing sheathing boards.

Once the plywood or OSB has been installed and securely nailed, you can install your rigid foam.

If, for some reason, you don't want to install plywood or OSB, you can install diagonal steel strapping to brace your walls. It's easier to install this type of strapping on a new construction job than a retrofit, however, and I don't recommend that you go this route.

Every wall needs a water-resistant barrier (WRB). That can be asphalt felt, housewrap, or (in some cases) rigid foam. Most builders find it easier to use housewrap as a WRB rather than rigid foam, because it's easier to integrate the WRB with your window flashing and penetration flashing if you use housewrap.

All this, and more, is explained in the following articles:

How to Install Rigid Foam Sheathing

Where Does the Housewrap Go?

Using Rigid Foam As a Water-Resistive Barrier

Calculating the Minimum Thickness of Rigid Foam Sheathing

Answered by Martin Holladay
Posted Dec 20, 2012 6:09 AM ET


Thanks, guys. Was leaning that direction and appreciate the confirmation. Martin, a thoughtful and detailed answer with points to more detailed information--as usual. It's just why I'm a GBA Pro member.

Answered by William Wagnon
Posted Dec 20, 2012 9:13 AM ET

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