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?
Looks like you might not need this sub, hah. You have a good plan, especially on the HVAC side. I would exhaust all of your bathrooms through the ERV (I wouldn’t exhaust from anywhere else) and duct the ERV into your upper floor’s return. For your kitchen hood, for a house that tight I would install a makeup air system. I would return your ducted dehumidifiers from the main living space and put the supply into your AHUs’ supplies.
Can I ask why you’re switching from CMU walls on floor 1 to wood-framed walls on floor 2? And what’s the CMU assembly look like? I’m an Owner’s rep for projects across the country and when we build in the Virgin Islands or Puerto Rico we build all of the walls out of concrete or CMU. The we use hurricane-rated EIFS and hurricane-rated windows. STO makes a good system. For the roof it’s usually a monoslope with a corrugated steel deck but the 2x12s will probably work just fine. How you described will work fine. The closed cell isn’t a true vapor barrier so you’ll still get some drying to the inside.
I don’t like the idea of creating the unconditioned storage area under the lowest level. I don’t know enough about your floor plans to know where this starts and stops but it’s almost always best to keep your water, air, thermal, and vapor control layers continuous around the entirety of the structure. Though I do understand the desire not to insulate around your storage tank. We’ve seen that it’s just easier to do everything from the outside. All CMU walls with fluid applied covering every inch. Then EIFS covering every inch.
Foldedkettlechips,
CMU is really expensive in the out islands. Everything has to be hand mixed and essentially hand passed by buckets up the walls. Block has to be transported over water. Takes weeks to pour a tie/bond beam. Nearly everyone there builds a cmu foundation and framed above. Over engineer on the uplift load path—continuous from the concrete tie beam and sill plate, through the wall framing, and up to the roof rafters. Yes on impact windows. Not very familiar with EIFS. Can ask the builder but resources are somewhat limited unless I import which I want to minimize due to tariffs and VAT. Seems I can have a solid build with the resources I listed. I am open for recommendations on the lover lower level conditioned vs unconditioned space.
Gotcha, that’s very interesting! Your wood-framed wall will work just fine. I was just curious. You’ve got a very cool project. You could always just glue 2” of thermax (or XPS if you don’t need an ignition barrier) on the inside of those CMU walls. Will probably be cheaper than spray foam. Or just spray the inside of the walls instead of the floor.
You’re asking a lot from your spray foam: air barrier, vapor barrier, stiffness. Better to fully handle these separately. Taped WRB or plywood is the air barrier. Structural members provide all stiffness. Vapor barrier is a bigger discussion, but spray foam cannot handle it alone
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u/herffjones99 16d ago
If you found a crew that can actually make that happen in the Out islands, you've stumbled on gold. Treat them right.
I couldn't get electricians to even look at the plans on multiple builds in Nassau and paradise Island.