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Radiant cooling in humid climates

Pros and Cons of Radiant Cooling in Humid Climates

The building sector is one of the largest consumers of energy globally. In South East Asia it accounts for around 30% of the primary energy demand and 41% – 51% of the operational energy used in a building is attributed to air conditioning and ventilation. 

According to the United Nations Environmental Programme, “Buildings use about 40% of global energy, 25% of global water, 40% of global resources, and they emit approximately 1/3 of GHG emissions.”

The race to reduce cost and increase efficiency and effectiveness is on. Traditional air conditioning systems, have been more widely adopted, but with the rise of climate change and new consumer demands architects and engineers are looking towards radiant cooling as a possible solution.

Concerns over performance in humid climates and the initial cost of these systems have been a major deterrent to widespread adoption. There is, however, significant evidence that proves these concerns are a thing of the past and that a well-designed radiant cooling system is effective in most climates.

This blog takes a look at the pros and cons of a radiant cooling in humid climates and tries to answer a few of the questions surrounding this cooling system.

Pros of Radiant Cooling

Architectural Freedom

Radiant cooling systems allow for substantial reductions in ductwork required to cool a building.

Depending on the particular climate and system design, airflow can be reduced by up to 80% which in turn results in less ducting required to move air around the building.

The space gained from the reduction in required ductwork can be very beneficial. An architect may choose to utilize the additional space by reducing the size of the overhead plenum. This expansion of floor-to-ceiling height enables the inclusion of much larger windows in a given space. The net effect of larger windows is that more natural light is allowed to enter deeper into the building.

Alternatively, the downsizing of ductwork and in some instances the elimination of ducting altogether can lead to significant cost savings but reducing floor-to-floor heights. Lower floor-to-floor heights mean lower material and construction costs. These savings are particularly accentuated in large multi-story constructions.
Architectural freedom with radiant cooling


Radiant cooling is particularly adaptable and lends itself well to mixed-mode ventilation design. “Mixed-mode” refers to a hybrid approach to space conditioning that utilizes a combination of natural ventilation and HVAC (Heating Ventilation Air Conditioning) system.

A well designed mixed-mode system provides maximum comfort while maintaining minimal energy use and lowering the cost of year-round air conditioning.

The two main form of mixed-mode design are zoned and concurrent. While a concurrent design allows natural ventilation and an HVAC system to work in the same space at the same time, the zoned design uses a mixed-use design. This means that in a zoned mixed-mode design, natural ventilation can be used in one space while an HVAC system is used in another but both are used at the same time.

Radiant cooling is adaptable to both mixed-mode design. The use of radiant panels is more appropriate in a zoned mixed-mode design. The panels have rapid cooling times and thus allow for a more compartmentalized approach which is extremely effective in humid climates.

Hydronic slabs, on the other hand, are more suited to concurrent space conditioning. Slabs operate at more moderate surface temperatures and thus have slower cooling times than a panel system. They can, however, be paired well with natural ventilation in a broader range of climates without the need to worry about condensation.

Thermal Comfort

Radiant cooling systems inherently provides a healthier indoor air quality as there is no recirculation of air in the system. The use of a Dedicated Outdoor Air System (DOAS) provides treated fresh air to the occupants in order to maintain healthy conditions as well as to remove moisture from the space.

Thermal Comfort is largely influenced by two main factors, air temperature and mean radiant temperature (MRT). While an all-air-system only controls the air temperature, a radiant system when combined with a ventilation system can control both.

Thermal comfort
The radiant system offers direct thermal control of active surfaces (panels and slabs) and indirect control of temperatures of non-active surfaces. The influence of an active surface over a non-active surface occurs through radiant exchange. Heat will flow from objects, occupants, equipment and lights in a space to a cooled surface as long as their temperatures are warmer than that of the cooled surface and they are within the line of sight of the cooled surface. The process of radiant exchange enhances thermal comfort as the cooling within a space is steadily and evenly distributed.

Due to the manner in which our bodies respond thermally, this type of cooling allows occupants to reach the same level of thermal comfort but at higher temperatures. The higher operating temperature means less energy is required to meet building occupants’ thermal needs.

Energy Saving Solution

Perhaps the largest driving force behind the adoption of radiant cooling is the potential energy and operating cost savings. There has been significant progress in the last few decades in the development of radiant cooling systems. The technology has highlighted itself as a comfortable and energy and material efficient alternative to traditional HVAC solutions.

A radiant cooling system uses approximately half the horsepower and materials required to effectively cool a space when compared to other traditional cooling methods. Low-flow injection-pumping systems can help make radiant-cooling systems even more efficient. Injection-pumping systems deliver heating and cooling energy to a variety of terminal units, including chilled ceiling panels, chilled beams, fan coils, and heat pumps, in the same piping-distribution system, even if each unit requires a different temperature.
An energy simulation study was conducted by the Pacific Northwest National Laboratory (PNNL). The aim was to achieve 50% energy savings, in new medium-sized office buildings, relative to a building that just meets the ASHRAE Standard 90.1-2004. The PNNL concluded that meeting this 50% target would not be feasible even when using a high efficiency forced VAV system. The report went on to recommend the use of a radiant cooling system in conjunction with a DOAS for ventilation to achieve this savings target.

The Bullitt Foundation in Seattle used radiant heating and cooling slabs with a DOAS system and natural ventilation to help target ZNE in its new headquarters building. The final design has a modelled Energy Use Intensity (EUI) which is 3 times lower than a similar Seattle Energy Code compliant office building.

Cons of Radiant cooling


The fear of condensation forming on cold radiant surfaces has stopped the widespread adoption of radiant cooling. This perceived limiting factor is accentuated in high humidity areas such as South East Asia. Condensation occurs when the surface temperature of a cooling panel or slab is equal to or below the dew point. The dew point is the temperature at which dew forms.

Some building standards suggest a limit for relative humidity to around 60% or 70%. That means that at a room temperature of 26 °C the dew point would be between 17 °C and 20 °C.

Times, however, have changed. The simple inclusion of a dehumidifier or DOAS will eliminate the threat of condensation altogether and increase cooling capacity. With proper design, radiant cooling does not create condensation. The largest radiant cooling installation in the world is at the Bangkok (Thailand) Airport. The concourses and main terminals contain 1.6 million square feet of radiant cooling.

The ASHRAE Journal reports that the Bangkok airport supply water temperature is 12 °C, and the return temperature is 18 °C. The floor surface temperature is 21 °C and with a dew point of 10 °C, there is no condensation.

Ventilation in Buildings

Radiant cooling systems do not on their own circulate air through a building. The lack of adequate ventilation means that stagnant air pockets form and no clean fresh air is introduced. Ventilation is mainly used to control indoor air quality by diluting and displacing indoor pollutants. It can also be used for purposes of thermal comfort or dehumidification when the introduction of outside air will help to achieve desired indoor psychrometric conditions.

Design firms and owners are striving to meet heating, ventilation and air-conditioning (HVAC) loads with optimum comfort and minimal energy. Radiant systems provide heating and cooling through pipes, most commonly filled with water, while ventilation and any humidity control requirements are efficiently met by a DOAS.

With space conditioning and ventilation responsible for 41-51% of the total energy load in conventional offices, leading design teams are turning to radiant systems that can cut this energy use in half. These savings come in large part because using water to move and remove heat in a building is inherently more efficient than using air. Water transfers thermal energy about 7 times more effectively than air.

What’s the catch with radiant cooling?

To be frank, there isn’t one. Straying away from the norm is a daunting task for an architect or engineer. Traditional forced air systems are widely adopted and well known by designers, investors, contractors and building occupants. HVAC systems have little-perceived risk and because there are so many professionals that recommend all-air systems, they seem to be the obvious choice.

The questions, however, is do all-air these systems meet the brief? Reduced cost, energy efficient and optimal thermal comfort are all extremely pertinent factors to look at when weighing up a cooling system. Hydronic radiant heating and cooling systems have been plagued by halve truths for a long time, specifically in humid climates with the ever-present fear of condensation and constant issue.

The simple fact is that radiant cooling is effectiveIts effectiveness is enhanced when properly designed with a DOAS for ventilation and humidity control. The Bangkok Airport in Thailand and the Infosys Software Development Building in India, prove that radiant cooling can perform exceptionally well in hot humid climates reducing energy consumption and increasing internal thermal comfort.

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