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I don't like insulating sheathing...

Posted by jb on March 7th, 2002 02:35 PM
In reply to PLYWOOD OR MS STYROFOAM SHEATHING? by JOE on March 7th, 2002 01:22 PM [Go to top of thread]

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I don't have experience with this particular product but generally I am against the use of insuating panels instead of real plywood or OSB sheathing, unless they can be shown to have equivalent shear strength.

When I worked out in Syracuse NY we were building houses with no ply or osb wall sheathing but using 2", double foil faced Celotex and "letting in" metal "T" wind-bracing for wind bracing. These houses were a JOKE! A stiff breeze or a slammed front door made the ENTIRE house shake. I was not impressed. The wall sheathing keeps the house from blowing over from lateral (usually wind) loads. As important as the sheathing's shear strength is how well it is fastened. The nailing schedule must me followed to the letter to achieve design strength for shear forces. For example, frequently hack carpenters use an air nailer to fasten plywood wall sheathing without carefully adjusting the air pressure. The nails blast through 1/2 (or more) of the thickness of the plywood before the heads stop. So, the heads are commonly countersunk 1/4" or more. This is roughly equivalent to using 1/4" plywood instead of 1/2" on your sidewalls. So, the nailing can be as important as the material, and this goes double for nailing insulation board. Also, insulation sheating can compress where it is nailed, at each stud or at the windows. This causes ripples afteer the siding is nailed up and can cause nightmares for extension jambs at the windows if the windows aren't installed correctly.

Maybe the stuff you are talking about is different, but I doubt it. I only use PLYWOOD, not even OSB on sidewalls. If you really need that much insulation then make 2x6 walls.


Foam Sheathing Trade-Offs

Q. Here in Wisconsin, I have always built my homes with 2x6 studs, R-19 fiberglass batts, and 1/2-inch OSB sheathing. I am considering switching to 2x4 studs, R-13 batts, and 1-inch R-7 foam sheathing, using metal T-braces for racking resistance. The cost of the two systems appears to be about the same. I like the fact that the foam sheathing stops thermal bridging at the studs. My question is: Will the metal T-braces provide a frame that is rigid enough?

A. Paul Fisette replies: You are correct that wrapping the entire house in foam reduces the problem of thermal bridging at the studs. By elevating the temperature of the stud cavities, the foam also reduces the likelihood of condensation on the interior face of the sheathing.

However, there are three drawbacks to using foam sheathing. First, the foam complicates the exterior trim details at doors and windows. Second, there is the real risk of ants attacking and nesting in the foam. Third, substituting foam sheathing for OSB will seriously compromise a home’s structural rigidity. Metal strap braces alone will not provide the racking resistance that you need. In fact, the technical literature by Simpson Strong-Tie clearly indicates that the metal braces are for temporary use during construction. They are not intended for long-term bracing of the framing. For more information, see "Bracing Foam-Sheathed Walls," JLC, 4/93.

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