Exterior Storm Room Wall, Zone 4A/3A?
I am working on an exterior wall detail for a guest bedroom and guest bathroom in an addition on our house that will double as storm rooms. The FEMA plans for a storm room show a double layer of 3/4″ plywood fastened to wood studs.
Right now I’m thinking of 2×6 studs at 16″ on center. From the interior to exterior:
GWB (mold proof);
2×6 studs with unfaced fiberglass insulation;
3/4″ Advantek (subflooring) screwed to studs and lapped onto floor and ceiling band joists;
1-1/2″ extruded polystyrene;
3/4″ Advantek (subflooring) screwed through foam into 1st layer of Advantek;
Breathable housewrap;
175 mph vinyl siding applied with truss head screws and washers.
I might run the exterior layer of sheathing horizontally with the tongues up, with a horizontal 2×4 at the bottom, at horizontal joints, and at the top.
No vapor barrier on the interior side.
Location is in zone 4A, but very close to zone 3A.
Any advice and critiques would be much appreciated.
Bill
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Replies
Bill,
I call this a site-built SIP.
You should consult an engineer (perhaps one is reading GBA and will comment here) to determine whether your site-built SIP will be as strong as the FEMA recommendation (two layers of 3/4" plywood). The idea of a storm room is that it needs to resist a horizontal 2x4 (or an oak tree) been flung at the wall by hurricane-force winds. Or something like that.
Maybe ICF's? Those are some nasty winds.
Since this is my own house, I'm not that worried about whether or not the 2 separated layers of Advantek will be equal to the double layer of 3/4" plywood. When I price out the addition, I am going to see what it would cost to add a layer of 1/8" steel plate on the exterior of the 1st layer of Advantek.
I guess I'm concerned primarily about any moisture / vapor issues. And, I guess the best place to add the layer of steel plate as far as moisture control goes. Although, I would think that the layer of steel plate would be OK as long as it was in contact with either side of the foam, which is a vapor barrier, and in this zone, the foam should be thick enough that the dew point would occur within the thickness of the foam.