I am supervising the construction of several Habit for Humanity homes in the Northwest where temperatures are mild. We have been using OSB as sheathing followed by 1-inch sheets of Dow Extruded Foam Insulation panels. The latter are generously donated by Dow free of charge. This combination makes for a strong, well-insulated building (R-5 on top of the regular insulation and eliminates the need for a vapor barrier); however, one of the construction committee members wishes to omit the OSB other than on shear corners to save on cost. Most of us are opposed to the idea, primarily for personal reasons, in other words, we do not have hard evidence that it makes for a lower quality construction. What say you all? I should mention that we use either Hardiplank or vinyl siding.
We live in a potentially seismic region; hence, my concern with strength, others feel you can almost punch a hole in the wall when using a vinyl exterior, etc. I would appreciate any ideas, comments, whatever, on this aspect. Thank you in anticipation.
Replies
another aside is the Dow is 1''..osb corners is 7/16''..now what?
skip the corners and use rack bracing (if allowed by local codes) and you are ok..
me? I'd not change what you are doing.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
>> Most of us are opposed to the idea, primarily for personal reasons, in other words, we do
>> not have hard evidence that it makes for a lower quality construction.
Well, let's see if we can define what full OSB sheathing adds to quality.
Resistance to penetration is one facet. It would slow down a guy with a knife or machete, wouldn't help much against a guy with an axe or a chain saw. Better than nothing against airborne debris, but not a lot better in the case of high wind and big chunks. I saw a report of a test that shot an 8' stud at a variety of different walls at 100 mph. I was researching brick at the time, so the one I remember was 4" brick veneer, 7/16" OSB, fiberglass insulation, and 1/2" sheetrock. The stud penetrated the brick, the OSB, and the sheetrock.
A fully sheathed wall will certainly resist more racking force than one that just meets the bracing code, but is that extra strength useful? Would that much force have already destroyed some other component and led to structural failure?
Wind resistance. I think full sheathing will exclude wind better. I don't have any experience with house wrap, but I think I'd rather staple it to OSB than to XPS. If you're not using house wrap, I know for sure I'd rather try to caulk all the seams in an OSB shell than in an XPS shell.
Thermal mass. Not very much, and not very useful since it's outside most of the insulation.
Fire rating. Most fires don't burn from the outside in, so any added fire rating due to the OSB would only be important very late in the game.
Acoustical insulation?
So maybe the cheapskates are right. Maybe the increased quality of full sheathing isn't worth the extra money.
PS - I'd still use it on the corners anyway, instead of let in bracing. You can make up the thickness with some 1/2" XPS.
jack
I used to do a little work for HFH a few years back, the local track home builder in the area wanted to build a couple houses for Habitat, they built theirs w/o osb, just angle braces and 1/2" foil board(don't remember what its real name is).
I know that HFH turned them down, said that they were building to some National code(don't know much about it) and that they had to use osb, besides they wanted it done that way.
I've seen these track homes built with just the foil board, vinyl hung on that! Makes for a really flimsy house.
I would stick with the osb.
Doug