OSB versus Styrofoam
daviddsims
| Posted in General Questions on
I live in climate 3 in warm humid Louisiana and will be building in the next six months. Most of the builders in the area use OSB in the corners and 1/2 styrofoam over the rest of the home for sheathing. I understand the styrofoam has much better insulating value than OSB but I also don’t want a home that is structurally weak in a wind storm. Any advice on this?
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Replies
Shane,
In most areas of the country, it's possible to build a strong frame using rigid foam sheathing -- a frame that is properly braced and able to resist racking. Here is a link to an article that discusses the different method used to brace foam-sheathed walls: Four Options for Shear Bracing Foam-Sheathed Walls.
That said, homes in high-wind regions of the country (for examples, homes near the ocean) need to be built to different engineering standards than homes in sheltered locations. You should consult an engineer to make sure that your planned frame meets local requirements before proceeding.
Shane,
As Martin rightly points out you can in some areas build a house without sheathing which can withstand shear forces, and substantially improves the insulating value of the walls. However I'm not sure substituting 1/2" foam for the OSB really fits that description.
It's also good to remember that in terms of flashing, attaching trim, the windows and doors, and WRBs, building this way is just like building with no sheathing at all. As you say, it can and is done, but it is a lot more difficult to perform those tasks well without a solid substrate like plywood or OSB.
The code in my area apparently must allow for just OSB sheathing in the corners for strength and styrofoam can be used in straight runs. It must be very popular as 75% of builders here do it that way. I still want the benefit of stopping thermal bridging but want the structure to have strength. I am wondering if it is more cost effective to just use OSB for sheathing and spray foam all the exterior walls?
Actually 1/2 styrofoam cost almost double OSB from what I can tell so it can't be a cost cutting effort by the builder.
Malcolm,
If you're talking about rigid foam that is 1/2 inch thick, the rigid foam has an R-value of R-2 or R-2.5. That's not much. It doesn't provide much of a thermal break.
If you really want to install rigid foam in a way that makes sense, you'll need to spec thicker rigid foam. If the house uses OSB at the corners for bracing, you could (for example) use 1 1/2 inch thick rigid foam for the sections of wall with no OSB, and 1 inch thick rigid foam at the corners (over the OSB), so that the sheathing is co-planar.
Whether or not you want to do this depends on your R-value goals. It may or may not make sense, depending on a variety of factors, to invest in a high-R wall in your climate zone.
I'm not a fan of using only foam sheathing in certain wall areas. Even if it makes a code approved assembly it doesn't pass my common sense assembly.
For starters I don't think the walls will remain square and plumb for the long term. Take a good look at most 100 year old houses, check with a level, and you'll see what I mean.
Secondly, with fiber cement or vinyl siding one good kick from the exterior and I'll be inside your house. This is not good for burglary protection or dealing with flying projectiles from a storm. Third, varmints have a very easy time getting through foam.