The Issue
IEN Consultants was approached by a contractor to investigate the risk of thermal bridging and hence the risk of condensation on the ceiling separating a room with 24-hour air conditioning and one with 8-hour air-conditioning.
Our Approach
For this study, the steady-state finite element software, Therm, was used to investigate the risk of condensation for the ceiling below the 24-hour air-conditioned room in the hospital building.
Figure 1: Cut-off area used for simulation
Figure 2: Construction details of the cut-off area built on Therm
The study methodology and approach follow the following steps:
A cross-section area was built on Therm.
Stated below are the inputs for the simulation:
- indoor conditions of 24hr air-conditioned room (21 degC & 50% relative humidity)
- indoor conditions of neighbouring room when not air-conditioned (28 degC, 83% relative humidity and 24degC dew point)
- indoor surface film coefficient (8.3 W/m2K)
- thermal conductivity of building materials (refer to Figure 2)
Using steady-state finite element simulation, the Therm software computes the heat flow and temperature profiles through the construction and the resulting surface temperature.
The surface temperatures are then compared against the dew point temperature to see the potential risk of condensation.
Shown below is the detailed sectional drawing of a 24-hour air-conditioned space surrounded by the neighbouring room that is not air-conditioned for 16 hours per day:
Figure 3: Detailed sectional drawing of the spaces used for simulation
The simulations were performed under 3 scenarios:
Worst case scenario to understand the heat flux across the ceiling with no insulation
Proposed design by the contractor; and
Recommended design by IEN Consultants
Our findings
In order to understand whether condensation will occur in the non-air-conditioned room, it is important to understand what the dew point temperature is of the space. Using the worst case assumption that the space is fully naturally ventilated, i.e. not benefitting from any moisture removal of the intermittent air-conditioning (approx. 8 hours of air-conditioning per day), the annual frequency of dew points of the outdoor air is plotted below:
Figure 4: The dew point temperature frequency plot for Kuala Lumpur
The most commonly occurring dew point is in the range of 22-24 degC, but for 1.4% of the year (122 hours of the year), the dew point goes as high as 27-28 degC.
(A) Scenario 1: Worst Case Scenario
We started the simulation with the worst-case scenario, that is, to have no insulation on the ceiling in the non-air-conditioned neighbouring room.
Figure 5: Simulation results for the worst-case scenario
As a result, it can be seen in Figure 4(a) that the ceiling/floor separating the 21 degC space with the 28 degC space has a high heat flux magnitude (13 W/m2). Thermal bridging is also observed at the edges as the heat travels from the 28 degC space to the cooler 21 degC space.
However, the condensation risk is not shown in the simulation, hence, the thermal bridging is not a big problem as it pre-cools the room, hence, reducing the air-conditioning consumption of the room below, albeit, still causing a slightly overall air-conditioning consumption.
(B) Proposed Design - 1500mm extended insulation
The next simulation performed was the proposed design. This was the design proposed by the contractor, namely to extend the insulation 1500mm beyond the 24-hour air-conditioned room.
Figure 6: Simulation results for the proposed-case scenario
We could see from Fig.6(a) that, upon insulating the ceiling in the 28 degC space, the heat flux through the ceiling/floor slab separating the two spaces is reduced significantly from 13 to 4 W/m2.
Even so, in Fig.6(b) thermal bridging is still observed at the edge of 21 degC air-conditioned space as the heat travels mostly horizontally from the adjacent 28 degC spaces.
It can also be observed from Fig.6(b) that, beyond a certain point, the heat flux is insignificant. Hence, there is a chance of reducing the length of insulation.
(C) Our Recommendations
Since there was no condensation observed on the ceiling below the 24-hour air-conditioned room, we figured there is a potential of reducing the length of insulation. We then simulated various cases with different lengths (shorter than 1500mm) of insulation. These simulations were done for a representative section of the 24-hour air-conditioned room; see purple box:
Figure 7: The U-values were calculated for a representative section of the room, namely half the wall height and half the room width.
Figure 8: The simulated U-values for the representative section
The Therm U-value simulations show that extending the simulation along the ceiling beyond the 21 degC air-conditioned room has virtually no effect on the U-value. In other words, the additional insulation cannot be justified on basis of reducing heat gains.
Next, we will examine how the ceiling insulation extension will affect the condensation risk. This is done by identifying the lowest ceiling surface temperature. As can be seen from the below simulations, the lowest ceiling temperature occurs at the end point of the ceiling insulation:
Figure 9: Heat flux and temperature distributions of the case with 500mm extended insulation
Additional Therm simulations were undertaken to determine the minimum surface temperature for different lengths of insulation; refer to the table below:
Insulation Extension Length (mm) | Minimum Surface Temperature (degC) |
0 | 28.0 |
275 | 27.9 |
525 | 27.9 |
1000 | 27.9 |
1500 | 28.0 |
The simulated values show very little difference in the ceiling surface temperature, all of which at about 28 degC, which is well above the average dew point temperature of 22-24 degC, and just below or matching the infrequent maximum dew point temperature of 27-28 degC, which only occurs 1.4% of the time across the year.
On this basis, we are recommending a safe insulation extension length of about 500mm. Reasons being:
Even though a minimal insulation extension likely will suffice, as a safe-guard against sub-cooling of the 24-hour air-conditioned space (which potentially goes as low as 18 degC), extending the ceiling insulation will prevent surface condensation from happening in the space below.
Extending the insulation will slightly reduce the thermal bridging on the left corner between the ceiling and column.
The U-values between 1500mm and 500mm are more or less the same. Therefore, going for the latter would be more economical.
However, it is also important to take note of the following:
The floor of the adjoining room to the 21 degC air-conditioned has a bigger condensation risk than the ceiling of the room below, and no easy insulation fix can solve this potential floor condensation problem.
The bottom of the insulated RC slab is now below the dew point of 24 degC, so if we puncture or remove the ceiling insulation, for example by drilling suspended ceiling hangers into the RC slab, there could be a risk of condensation. One way to overcome this is by applying insulation foam to any installations drilled into the ceiling. Moreover, the ceiling insulation itself must be impervious to water vapour (for example foam insulation, and not mineral wool) in order to prevent condensation taking place between the RC slab and the insulation.
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