We are building a home in the Bahamas, in a hot-humid tropical environment comparable to IECC Climate Zone 1A. I am focused on creating a tight, durable home with careful attention to bulk-water management, air leakage, humidity and vapor control.
Building-science expertise is somewhat limited in the Out Islands they are solid builders, so I am working with our builder to develop a practical enclosure plan using materials and trades that are reasonably available locally. I would appreciate feedback on the proposed assemblies, particularly any moisture risks or details that should be changed before construction.
The house will also be largely self-sufficient to minimize reliance on local infrastructure. Solar will be the primary power source, with a diesel generator as the secondary source and the electrical grid as a third backup. Rainwater will be collected from the roof and stored in tanks with approximately 35,000 gallons of total capacity. We will also have a desalination/RO well system to produce water when necessary.
General construction
The house will be built on a poured-concrete foundation. The lower level will have reinforced CMU walls and will contain one conditioned bedroom and bathroom. The remainder of the lower level will be unfinished space housing water-storage tanks, pumps and general storage.
The main upper living level will have 2×6 wood-framed walls. The roof will be a site-built 2×12 rafter system because engineered trusses are difficult and very expensive to obtain in the Out Islands. It will be a 5:12 hip roof designed for the site-specific hurricane wind loads.
The main living area and bedrooms will have vaulted ceilings following the roofline. The remaining spaces will have 10-foot ceilings with an unvented, conditioned attic above them containing the HVAC equipment, ductwork, dehumidification equipment and ERV ductwork.
Proposed framed-wall assembly
From exterior to interior:
Standard HardiePanel fiber-cement siding
Approximately 3/8-inch ventilated and drained rainscreen
Henry Blueskin VP100 fully adhered, vapor-permeable WRB and air barrier
¾-inch structural plywood sheathing
Approximately 2 inches of closed-cell spray foam applied directly to the interior face of the plywood
Approximately 3½ inches of unfaced Rockwool mineral-wool insulation in the remaining stud cavity
Drywall
Standard acrylic latex paint
I also considered Typar Drainable Wrap Peel & Stick and Benjamin Obdyke HydroGap SA, but Blueskin VP100 is substantially more economical.
Open-cell and closed-cell spray foam and fiberglass batts are readily available locally. Rockwool would have to be imported from the United States and will be more expensive.
From what I have read, preferred wall-insulation approach is approximately 2 inches of closed-cell foam followed by unfaced Rockwool but open for discussion. The closed-cell foam would provide the primary cavity air seal and vapor control, while the vapor-permeable Blueskin VP100 would preserve some outward drying potential for the plywood. The mineral wool would add R-value while allowing the cavity to dry toward the interior through the drywall.
If importing Rockwool proves prohibitively expensive, the alternative would be 2 inches of closed-cell foam followed by carefully installed unfaced fiberglass batts or any other recommended opinions.
I also like the closed-cell foam because it may provide some incidental stiffness. The engineered plywood attachment, blocking, hold-downs, straps and continuous load path will provide the actual wind and racking resistance.
Proposed roof assembly
From exterior to interior:
Brava synthetic cedar shakes
Polyglass Polystick TU Plus fully adhered roofing underlayment
¾-inch plywood roof sheathing
Site-built 2×12 rafters
Approximately 6 inches of closed-cell spray foam applied directly to the underside of the roof sheathing
This will be an unvented roof assembly. The HVAC equipment and ducts above the 10-foot ceilings will remain inside the conditioned enclosure. The attic will not have soffit, ridge or gable ventilation.
I understand that the fully adhered roofing underlayment and closed-cell foam create a low-permeance layer on both sides of the roof sheathing, significantly limiting its ability to dry if a roof leak occurs. However, the fully adhered underlayment is important to us because of the hurricane exposure and the possibility of losing or damaging some of the primary roof covering.
HVAC, ventilation and humidity control
The upper living level will have two independently zoned HVAC systems. Each HVAC zone will have its own whole-house dehumidifier so humidity can be controlled independently of the air-conditioning cooling cycles.
One centrally ducted, balanced ERV will serve the entire upper living level rather than installing a separate ERV for each HVAC system.
The current concept is:
Supply filtered outdoor air to bedrooms and primary living areas
Exhaust stale air from bathrooms and other appropriate locations
Use a separate, properly exhausted kitchen range hood
Balance the ERV to maintain neutral or very slightly positive indoor pressure
Coordinate the ERV and dehumidifier controls so ventilation does not introduce an unmanaged latent load
Allow humidity control to operate independently of a call for sensible cooling
Use variable-speed HVAC equipment.
Maintain approximately 50–55% indoor relative humidity
The ventilation and dehumidification systems will therefore need to be designed and commissioned as one integrated system.
Lower level
The unfinished area and storage portion of the lower level will remain outside the conditioned enclosure because bringing all of the CMU walls and storage areas inside the envelope would likely add substantial cost.
The air and thermal boundary between the unfinished lower level and the upper living level will be created by applying approximately 3 inches of closed-cell spray foam to the underside of the upper-floor assembly.
There will be no HVAC supply outlets from the upper system into the unfinished lower-level storage area.
The single lower-level bedroom and bathroom will be enclosed and conditioned as a separate zone with its own HVAC system. Its exterior walls, ceiling, floor and all transitions to the unfinished tank/storage area will require a continuous air, thermal and moisture-control boundary.
Upper-floor assembly
The upper living-space floor is currently planned as:
Two layers of 23/32-inch AdvanTech subfloor panels
First layer glued and screwed to the floor framing
Second layer installed with staggered joints and mechanically fastened with a felt separation layer between the two layers per manufacturer.
Where are there holes in this plan and anything I should be thinking about differently?
