Air Conditioning an Island Home in The Bahamas
Apart from being beautiful, it is HOT and really HUMID in The Bahamas. The locals are used to it, and visitors put up with it just to be in The Bahamas. We certainly put up with it during our visit this past July, and it was so worth it.
I mean, really. So worth it…
To get a break from it, we either plunged in to clearest water we’ve ever seen…
…took refuge in the shade of some beach-side bar…
…or, found shelter inside with the comfort of nice, cool and dry, conditioned air of our rented beach cottage. As much as we prefer 100% fresh air, we also appreciated the break from the hot and sticky.
This brings me to the whole reason we were even in The Bahamas to begin with. I was hired to design the Air Conditioning, Heating (yes, heating too), and Ventilation (HVAC) systems for a home on a private island near Great Exuma. Below is the view from my temporary “office”, which was 1 of 2 RVs that the builder set up for consultants and construction supervisors. Not bad, eh?
The “Bali” House
The design of the HVAC systems for this home was somewhat complicated (which I enjoy very much). Not just because of the weather conditions, but also because the architecture of the house didn’t exactly “favor” conventional, or even non-conventional, ducted systems. It wasn’t the curves, multiple buildings, or 18′-0″ sculptural roofs that made this house particularly challenging. It was the fact that the design of the home was not designed to fully accommodate any type of air conditioning systems, other than, perhaps, a system without duct work. In the case of this home, the owners “do not want to see or hear” any part of the mechanical systems. Not the equipment, and especially not the air. So, a very quiet, low-profile, concealed ducted systems was about the only option. Selecting that system and getting it to fit within the architecture of the home is where I fit in, and in a few weeks (finally) we’ll be headed back down to supervise the installation by a Nassau-based HVAC contractor.
Before I talk particulars and geeky stuff, here’s a quick photo tour of the exterior of the home, referred to as the “Bali House” for it’s Balinese-them architecture and interior design.
Image 1: Looking East at sunrise. Main Living Wing on the left, Outdoor Dining Room on the right.
Image 2: View of home looking north. Main Living (left), Outdoor Dining (Right)
Image 3: View looking South in to the Master Bedroom
Image 4: Looking South in to the Main Living wing of the home
Image 5: Looking South at the Guest Wing.
Image 6: From the South looking in to the Guest Wing.
THE DESIGN, PART I – Weather Conditions, Great Exuma, Bahamas
As I mentioned above, no matter where you are in The Bahamas, or what time of year it is, it’s typically hot, humid and breezy. When it’s not breezy, it feels even hotter. Here is the Average Weather for Georgetown, Great Exuma, The Bahamas, which is just south of the island where this home is. Below are a just a few of the weather factors that are important when designing any home and its mechanical system. I made slight adjustments when design the systems for the “Bali House” based on its precise location design.
Image 7: Average Temperatures for Great Exuma, Bahamas (photo credit: weatherspark.com)
The average high temperature, in early August, is around 90 degrees Fahrenheit, and the average low, in early January, stays around 67 degrees Fahrenheit. According to the locals, the lowest temperature they’ve ever experienced is 60 degrees Fahrenheit.
Image 8: Average Relative Humidity (RH) for Great Exuma, Bahamas (photo credit: weatherspark.com)
The average high RH is pretty steady around 86%, and the average low has a slightly bigger swing between 55% and 66%.
Image 9: Average Dew Point Temperatures for Great Exuma, Bahamas (photo credit: weatherspark.com)
The dew point temperature (i.e. when condensation occurs) is relatively close to the actual air temperatures (shown above), which tells us that the specific humidity (the ratio of the water vapor to total air) is fairly high. This is why it feels sticky most of the time.
Image 10: Average Wind Speeds for Great Exuma, Bahamas (photo credit: weatherspark.com)
Finally, a look at average wind speeds gives us an idea of the wind’s contribution to air leakage (faster wind = greater pressure on the house = more leakage), which affects the heating and cooling loads, and ultimately the equipment size and type for the home.
Mind the Salt
On the next island over from the “Bali House”, the same owner built the “Mexican House”. For one reason or another, it has been left vacant and incomplete for approximately 3 years. They’ve recently taken steps to finish it (our next project with them), which is why you see a fresh coat of paint. In Image 12 and 13, you can see what happened to the steel reinforcement bar that has been exposed to the “Bahama Breeze” while the house sat unfinished.
Image 11: The “Mexican House”
The salt-laden moisture being pushed around by the ocean breezes has corroded the exposed steel reinforcement bars (re-bar). For this reason, a lot of outdoor mechanical equipment, made of metal, is given a protective coating to extend its useful life. Although it does reduce a system’s efficiency and performance, there are some coatings that minimize this loss and do not void the manufacturer’s warranty. BlyGold is one of these, and the one we specified for all exposed equipment in the “Bali House”.
Image 12: Exposed steel reinforcement bars of The “Mexican House”
Image 13: Re-bar when left exposed to salt-laden moisture
Image 14: Propane tank corroded by salt-laden moisture
If we were to install a dedicated fresh air ventilation system in the “Bali House”, we would also apply the protective coating to the insides, and use stainless steel ductwork for the intake and supply. The reason for this is that mechanical fresh air systems (e.g. ERV, HRV, in-line fan) use a motor to drive a fan that pulls the outside air in. That air flows through these systems, made mostly of metal, and it would not take long for the insides of these systems to corrode just like this re-bar of the “Mexican House”. Replacing corroded equipment is much more costly than this one-time coating.
THE DESIGN, PART II – The opportunities in the Architecture
Here are a couple examples of what I typically see when designing mechanical systems for historic homes, and the owner has requested a ducted systems. The two photos below are from a home on near St. Michaels, MD (eastern shore), where the owner was reluctantly going to use duct-less mini-splits everywhere, until he discovered that there are ducted mini-splits that are smaller, concealed, versatile, and usually have smaller ductwork than normally seen with conventional systems. After a week at this house, we came up with a design that required only one dropped soffit (12″ wide x 8″ high) for one run of duct. Everything else fit within these small areas, which I asked to be brought in to the building enclosure by installing rigid foam on the outside, and employing other air sealing and moisture management methods.
Image 15: Knee-wall area of Maryland home.
Image 16: Limited attic area in Maryland home. From top of joist to peak was just over 18″
The “Bali House” is mostly an all concrete and block house, with the exception of the Main Living area and Guest House. They are both covered with a sculptural roof feature (made in Bali) that has a continuous ventilated attic (see in Image 19) that is no more than 6″ clear. The rest of the house with concrete roofs that are 6″ – 10″ thick, with a tile or 12″ – 60″ of earth on top of it, and the entire house is on a slab-on-grade foundation, so there is no crawlspace or basement. The design calls for all of the ceilings to be “exposed” concrete. To maximize floor area, the design also calls for all the walls to have ether tile attached directly to the wall, or 2×2 or 2×4 furring strips to mount drywall to. At first glance, one might call this impossible to install a ducted system.
Image 17: Early stages of construction showing the block and concrete walls. 2×2 wood furring is installed on a few of the interior of the block walls to attach drywall to, where there isn’t tile, which is mostly what covers the interior walls.
Image 18: The very cool roof structure for over the Main Living area and Guest House, being assembled in Bali before being shipped to the island.
Image 19: Detailed section through the eave of the roof structure.
Note the continuous ventilation and “insulation”. That’s the only insulation specified for the house that we discovered has no radiant barrier, which makes it almost useless. No insulation will be put in the walls or ceilings, because they are all mostly below grade (thermal mass). The only I asked the builder to add insulation is on the concrete roof (with tile finish) over the Master Suite. Three inches of polyisocyanurate (approx. R-21) reduces the cooling load for the Master by 40%.
If the cavity of the wood roof structure was sealed tight and filled with high density (higher than R-4.2) insulation, the cooling loads for the Main Living Room area and Guest House would decrease by 20%. This wasn’t enough to justify insulating and air sealing, so it was left uninsulated.
Image 20: A view of the interior finish side of the roof over the Main Living area.
Image 21: Another view of the interior finish side of the roof over Main Living area.
Image 22: Looking north along the eave of the wood roof feature. The 5′-0″ (or more) overhangs of this roof were its greatest asset, next to the beauty and drama it added to the home.
Image 23: Continuous concrete slab on grade, looking toward the Dining Room. The generous overhang over the west-facing floor-to-ceiling window in the Dining is going to be a huge help in the afternoons, but there is still a significant heat gain.
Image 24: A Service Wing room with 2×2 or 2×4 furring on block walls and ceilings.
Image 25: 10″ concrete slab roof over the “Service” wing. Eventually, three (3) to five (5) feet of earth will cover this roof.
Image 26: View from Master Bedroom, looking north. Roof above this area is 10″ concrete with tile on top. This is the area we’re adding R-21 insulation to help reduce heat gain.
FINALLY – The Design
Given the climate, the architecture, the enclosure performance, and the owner’s desire to not see or hear the systems, we determined that a ducted variable refrigerant flow (VRF) system, known as CITY MULTI, from Mitsubishi, was the most appropriate. Indoor and outdoor equipment is very quiet, and the air handlers are approximately 8″ – 10″ tall. This provided the flexibility with placement in the home, which we needed. The owner also wants the option to simultaneous heat and cool, if necessary, which this equipment can do. In fact, the efficiency of the equipment improves when in this mode. Although these systems will handle a portion of the humidity, we’re installing standalone permanent dehumidifiers throughout to help handle the anticipated latent (moisture) loads from people, showers, cooking, infiltration, etc.
Here are the final plans:
Image 27: Main House HVAC Plan. Air handler AHU1, AHU2 and AHU3 are on top of a concrete ledge. Two of them serve the 2-story Living Room, Library and TV Room, and the third will serve the Dining Room. All diffusers are linear slot diffusers, and you can see one of the line set (refrigerant lines) had to be buried beneath the slab. AHU4, serving the Master Suite, will be installed on the roof and will be encapsulated in a built-up air tight, well-insulated “box” on top of the roof, and will require cutting a hole in the concrete roof to route the ductwork in to the space.
Image 28: Service Wing HVAC Plan. Air handler AHU7, AHU8 and AHU9 are on suspended from the Storage Room ceiling, and all the ductwork is being routed through a 16″ wide cavity that surrounds the building. All refrigerant lines are running in the 2×2 furring of the ceiling.
Image 29: Guest House HVAC Plan. AHU10 is also on a ledge, like the Main Living area, and will be encapsulated in a drywall and rigid insulation enclosure to reduce the sight and sound.
Image 30: This is the “ledge” where one of the three air handlers in the Main Living area and its ductwork will be installed to serve the Living Room, TV Room and Library.
Image 31: This is the other “ledge” in the Main Living area where the other two air handlers will be installed to serve the Living Room, TV Room and Library and Dining Room.
That’s it for now. Thanks for taking the time to visit and read what’s happening at LG Squared. Lots of exciting things to come. In the meantime, here are a few “FUN” photos of The Bahamas that we took in between calculations 😉 It is one of the most amazing places on earth.
I would also like to take a second to thank my friend and mentor, David Butler, whom I consulted with on aspects of this and many other mechanical system design projects. He is one of the most patient, thorough, and knowledgeable people I know. I couldn’t be where I am today without him.