Help with wall assembly for hurricane repair (hot-humid)
elenan2
| Posted in GBA Pro Help on
We live in Houston Texas in a two-story house that has a pier and beam foundation with a ventilated crawl space. The house according to members of our neighborhood association that investigated it was built around 1914. It has been modified over time. We are trying to make the best decisions for a repair, but we haven’t had any luck thus far in finding a building science consultant in Houston to help with the decision making.
During a hurricane this summer, a water leak developed on the east wall upstairs and water came into the house from a couple of window openings downstairs buckling the wood floor. We had to rip out the wet drywall and insulation and discovered that there isn’t any exterior sheathing. The rear segment of the east wall (the wall is split by an exterior chimney) is where the leak occurred. This is part of an extension to the building that was done by the prior owner in 2000. It consists of a mix of old and new elements:
– Portion of wall with older materials in it (contains an original window that we are required to keep): Fiber cement plank siding | tar paper | faced fiberglass batt insulation | interior shiplap | woven fabric/wallpaper covered by drywall
– Remainder of the wall (contains 4 flangeless windows downstairs and 2 upstairs; circa 2000): Fiber cement plank siding | tar paper | faced fiberglass batt insulation | interior 3/4-inch plywood (not continuous; consists of a large corner piece where the east wall meets the south wall—I’m assuming this is structural and small pieces of 1×4 to hang the drywall) | drywall
Our plan for the repair is to add exterior sheathing and replace the 2000 windows with 6 new flanged windows. For the portion of the wall with older materials, the plan is to rehabilitate the old window (we are required to keep it) and leave the original interior shiplap and drywall in place. The following are the components of the planned wall assembly.
– Portion of wall with older materials in it: Fiber cement plank siding | 30# tar paper ASTM D226-97 Type II | 15/32 3-ply CDX plywood | Rockwool Comfortbatt R15 insulation | interior shiplap | woven fabric/wallpaper covered by drywall
– Remainder of the wall: Fiber cement plank siding | 30# tar paper ASTM D226-97 Type II | 15/32 3-ply CDX plywood | Rockwool Comfortbatt R15 insulation | Georgia Pacific – Toughrock Fiberguard X Mold-Guard Gypsum Board
My husband would also like to cover all the exposed internal walls in the newer segment with interior 3/4-inch plywood prior to putting up the gypsum board to further strengthen the wall. We would like to include a drained and ventilated rainscreen furred with 1/2-inch-thick plywood strips. We also plan on air sealing the exterior sheathing.
Will we be able to get vapor flow in both directions (drying in both directions) with the assemblies outlined? I’m wondering about the 3/4-inch-interior plywood my husband would like to add (I’m thinking it is overkill) and the shiplap interfering with this not to mention the type of gypsum board chosen.
Any advice you can provide is much appreciated.
Elena
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Elena,
The two aims - a better enclosure, and a stronger structure - require different approaches. You can get good advice as to what your walls and roof should look like from an efficiency and moisture management perspective here on GBA. For the structure you need a very targeted approach using either the Fortified Standard, or to engage a structural engineer. You should resist the temptation to add elements like the interior plywood, without knowing what their exact purpose is.
https://www.greenbuildingadvisor.com/article/building-to-the-fortified-home-standard
https://www.greenbuildingadvisor.com/article/build-a-hurricane-resistant-home-without-breaking-the-bank
https://www.greenbuildingadvisor.com/article/the-fortified-roof-system
Thank you Malcolm. We have a good structural engineer that we will call to get his 2 cents about the interior plywood and the soundness of the structure in general. My assumption (perhaps a naive assumption) is that it is structurally sound as is since the city had to approve and inspect the addition. We will check with him to be sure.
Assuming no 3/4 interior plywood is added, are the wall assemblies acceptable for moisture management? Again, the interior shiplap on the portion of the wall with older materials is of concern as well as the gypsum board on the newer portion.
Elena,
The walls sound good to me. Neither the shiplap, gypsum, or the interior plywood are a problem as they are all fairly vapour-open
Good luck with your reno!
Only thing that stood out to me was the wallpaper. A lot of wallpaper is not very vapor-open. Not sure what kind you have but at the least your assembly can dry to the outside which is usually good enough. Drying to both sides is even safer but not necessary.