AlbertoArriaga33 is wrestling with plans for a new home in a tropical location. He doesn’t say exactly where, but conditions are certainly challenging: high heat and high humidity, with nighttime temperatures dipping only into the upper 70s and outdoor relative humidity hovering at about 70%.
Exterior walls will be grouted concrete block, what Alberto says is standard construction in the tropics. Outside walls will be finished with a cementitious plaster and then painted. On the inside, light steel framing and stud cavities filled with R-15 mineral wool batts.
“However,” Alberto writes in this Q&A post, “I have many doubts and still not sure if I quite understand how thermal mass works. I’ve read that typically you would want to keep the thermal mass inside in a hot climate in order to keep it from absorbing solar energy, and subsequently heating up your house at night.”
Alberto wonders how the mass of the concrete-block walls will affect interior conditions, and whether the interior insulation will be enough.
“My guess is it would slow down the transfer of thermal energy from the walls to the interior space as it radiates throughout the night. Is the thermal performance of the assembly affected substantially by whether the insulation is on the exterior or interior?”
Those are the questions for this Q&A Spotlight.
Let’s get our terms right
The term “thermal mass” pops up frequently, as it did in Alberto’s post. But, DCContrarian says, it’s not technically correct.
“‘Thermal mass’ is not a term used in science or engineering,” he writes.
Maybe not, says Malcolm Taylor, but in building design the term is commonly understood to mean the “capacity of a high mass material to absorb and store and release energy as heat.”
“And this is why I find the term so grating,” DCContrarian replies. “It’s not the mass that gives it that capacity, it’s the heat capacity. People act like simply being heavy makes something a good storage medium. And often the substances being suggested have rather low heat capacities.”
Tom May suggests that if you built a wall out of wood and the same sized wall out of brick, the brick wall would have more mass and thus be able to hold more heat.
“This sort of thinking is why I rail against the use of ‘thermal mass,’ ” DCContrarian responds. “An object’s heat capacity is not determined by its weight, or even its mass! It’s the product of the mass and the specific heat. Most of the substances that are proposed as good candidates for ‘thermal mass’ actually have low specific heats, they just happen to be dense.”
“That makes no sense,” May says.
‘Thermal mass,’ ” says Tyler Keniston, “is little more than synonym for ‘heat capacity.’ If you’re suggesting to use that term instead, I’m not necessarily arguing with you (perhaps even agreeing).”
Mass is relevant, Keniston continues, because the values that can be altered in design are the specific heat of the material and the amount of material (its mass). “Since we are often restricted in tweaking volume to significant degree, we are inevitably left tweaking density in order to increase mass (though we can include or exclude existing mass depending on where we put the insulation.)”
Heat retention in exterior walls won’t help
DCContrarian says that unless the desired interior setpoint is about the average of the daily high and low, the heat retention properties of the exterior walls won’t much much.
Alberto replies that the ideal setpoint would be 69°F or 70°F, but the average of high and low will be about 86°F with peak temperatures in the 90s.
“Heat retention is not going to benefit you,” DCContrarian says. “You’re going to be cooling all the time. Your best strategy is to make the building as tight and as shaded as you can stand it.”
And, says Dana Dorsett, framing interior walls with light-gauge steel and insulating the bays with mineral wool would be a waste of time.
“Steel studs are highly thermally conductive, and will reduce the thermal performance of the insulation by about half,” Dorsett writes. “Using shallow steel furring to hold a continuous layer of stacked batts in place (no steel passing through the mineral wool layer) would allow the batts to provide full function.
“Whether R15 would make an appreciable difference over R7-ish in peak or average cooling loads in that assembly & climate is questionable though,” he adds.
Insulation and air sealing will be key
The original question, Alberto says, was not so much how to use heat retention characteristics of the concrete block to his benefit but rather whether it would be detrimental to have the block on the exterior of the wall.
The key, says Roger Berry, will be to insulate and air seal very carefully. Judging from a dew point chart, it seems likely that outside air will condense on cool surfaces.
“Removing the latent load will be tricky and likely to be done in stages by dropping your set point slowly over days,” Berry says. “At that, you will need to have a very tight structure to keep exterior air intrusion from being so high you need constant dehumidification, which, unfortunately, would be dumping warm (but drier) air back into the living space.”
Alberto’s goal should be to isolate interior mass from as much exterior heat gain as possible. A ground-coupled slab with tile and insulation at the perimeter to separate the block walls from the interior would be a good start,” he says.
“Interior walls of half thick CMU [concrete masonry units] and plaster would be a next step,” Berry continues. “All this mass, once cooled to your set point would act like the food in a freezer or refrigerator. Opening and closing the door does allow warm air to come in, but the relative specific heats and masses means the air cools fast and the food mass warms negligibly. The risk for you is the very high humidity you face. Think of how frost build up occurs in freezers. Cooled thermal mass will act in the reverse manner of a giant stone fireplace up north. Oddly, you would benefit from an airlock style front door as much as an Alaskan.”
Low-gain, double-pane windows, and possibly exterior shade trellises, will help minimize direct heat gain, Berry says. High ceilings will stratify air, so the lower part of the room where people sit and relax will be the coolest
“The exterior walls of CMU and plaster could be insulated and clad, but I suspect that method of construction would baffle the locals,” Berry says. “Besides, the choice of CMU for walls may also reflect durability decisions learned over time. I would think termites and ants would be extra large in such a warm moist environment.”
‘Tried and true’ regional design
Take a tip from conventional house designs of South America, suggests Armando Cobo, a Texas-based designer of net-zero houses.
“Tried and true designs for over centuries in South America have wide overhang tile roofs over purlins to keep attic cool and shaded walls, since the sun is mostly straight up,” he says. “White walls, ample hallways, courtyards in the interior, thick tile or brick floors to keep floor cooler, and lots of cross ventilation. Growing up in a super hot humid climate, and I don’t recall any of our houses having AC.”
Alberto is aware of these design details, but he says that no air conditioning is not really an option where he lives.
“On a cloudy day we may get by with fans, but not on a sunny day,” he writes. “Cross ventilation is also a pain because not only does it bring pollutants in (I live in a fairly industrialized city) but it also brings in copious amounts of dust. We will however, use white colored exterior paint and exterior shades for windows on the east and west.”
A word from Peter Yost
This topic was covered to some extent in this Q&A, “Thermal Mass in Hot Humid Climates,” from 2010. I’ll use that as a starting point.
There are two requirements for thermal mass to work as a flywheel for energy-efficiency and thermal comfort:
- The mass must be well connected to the interior
- Temperatures swings allowing the mass to charge and discharge its heat content
Any interior insulation to the thermal mass dampens the ability of the mass to take on or give off its heat content. Any insulation needs to be on the outside of the mass walls.
If the climate is hot but dry and temperatures at night drop enough, you can connect the interior to the exterior and use night-time cooling to discharge the heat content of the thermal mass.
If you don’t have temperature swings that align with thermal comfort demands, you will get little benefit from the thermal mass. And it’s tricky to get the mass effect in balance with different temperature swings and steady thermal comfort needs of occupants. The thermal mass effect depends on heat capacities and rates of heat loss and gain. Even in locations with ideal diurnal temperature swings, there are times when occupants get a bit too warm late in the day and a bit too cool in the early mornings.
All of this gets more difficult when you introduce humid conditions. If humidity is high day and night, night-time cooling means little to no moisture management of the interior.
If you can use building mass to take on and give off sensible heat content, can’t you use interior finishes to manage latent load? See this video, “Plasters and Humidity,” from the Living Plasters website for a demonstration along those lines.
But again, the question is going to be just how much and at what rate moisture moves in and out of the moisture-open interior finish. Just as you need swings in temperature to moderate sensible interior temperature, you need swings in moisture content to manage interior relative humidity.
There is an interesting “Wingnut”-like test video on the Living Plasters website but keep in mind that while this shows how well the plaster takes up the given amount of moisture, it does not show the moisture discharge of the plaster.
Also, you might check on some of the research that’s been done on thermal mass in hot-humid climates:
- “The Role of Thermal Mass in Humid Sub-Tropical Climate: Thermal Comfort and Energy Demand of CSET Building, Ningbo.”
- “Strategy for energy efficient buildings in tropical climate” (2013 International Conference on Alternative Energy in Developing Countries and Emerging Economies).
There are three important takeaways from this research:
- People who are used to tropical climates have more forgiving thermal comfort demands and so night-time ventilation may be more of an option for thermal discharge of building masses.
- Using thermal mass for both sensible and latent load management is really complex, and it’s easy to end up with either moisture-related problems (condensation and mold/mildew) or thermal comfort issues.
- Solutions for residences—where people are in the building at all hours of the day—may be quite difference than solutions for office buildings.
Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. Peter Yost, our expert, is GBA’s technical director and founder of a consulting company in Brattleboro, Vt. called Building-Wright.