As mentioned earlier, the main criteria for construction materials selection should always be in tune with the locally appropriate bioclimatic design strategies. As concluded in Section 3.5, the most effective bioclimatic design strategies for SLP up to year 2080 are:
• High thermal mass (3)
• High thermal mass with night flushing for summer season (4) • Internal heat gains for winter season (7)
• Passive solar gain high mass for winter season (8)
From the bioclimatic design strategies above high thermal mass with night flushing (4) will not be considered for this research. However, high thermal mass (3) is and will be (for future weather scenarios) the most effective bioclimatic design strategy in providing natural thermal comfort for San Luis Potosi City. Therefore, it will be considered as a main design criterion. As for passive solar gain high mass (8), this relies on high thermal mass (3) with an
appropriate orientation and sun exposure due to windows sizing. In consequence, construction materials selection will be based on this criterion.
3.8.1 Thermal mass as a bioclimatic design strategy
Generally speaking, thermal mass is considered as the ability of a material to absorb and store heat (CCAA, 2010). However, for a construction material to be useful in the build
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environment (thermal comfort), it is required to have a combination of three basic properties. Firstly, a high specific heat capacity; secondly, high density since the heavier the material, the more heat it can store; thirdly, a moderate thermal conductivity, so that the rate of heat flow in and out of the material is slow enough to take advantage of the diurnal temperature shift (mpa The Concrete Centre, 2012, p. 4).
Heavyweight construction materials such as brick, stone and concrete share the aforementioned properties; all of them combine a high thermal storage capacity with moderate thermal conductivity (heat will be absorbed and released slowly).
When designing with thermal mass (TM) it is important to understand concepts such as diurnal temperature variation, which stands for the daily temperature shift that occurs from daytime to night-time (24hours cycle). In order for TM to properly work, a minimum of 6°C daily temperature variation is needed. As shown in Section 2.7, in SLP the average daily temperature variation is of at least 12°C, condition that makes it very suitable for the implementation of TM as a bioclimatic design strategy.
This daily temperature shift is important since it is closely related to the heat flow to and from a building over the course of 24 hours also, the temperature shift occurs slower inside the building than outside. This is due to time lag (measured in hours) which is the time for heat to pass through a material or construction element. It is also the delay between the peak outer and inner surface temperature of a wall or roof (mpa The Concrete Centre, 2012, p. 2).
During summer time buildings with high-medium TM slowly absorb excessive heat during the day, generating cooler inner temperatures (it is necessary to keep windows closed to take advantage of the difference in temperature from the exterior). At night, when external temperatures drop down, the construction materials slowly release the stored heat generating warmer internal temperatures. This is particularly advantageous during winter time.
In summer, during night time, it is necessary to get rid of the stored heat through ventilation; by doing so, the construction materials discharge the absorbed heat and get ready to be charged again. This discharged heat process that occurs during night time will leave the TM ready to begin a new 24hr cycle. One of the main advantages of this process, is that it provides stable inner temperatures due to the material’s lag time and this is translated in naturally more comfortable conditions inside the building.
During winter time, the ventilation strategy consists in leaving windows closed and allowing sun light in; this way, the buildings TM will be heated (slowly) through the day. At night, when exterior temperatures drop sharply, the stored heat will be slowly released, providing
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naturally comfortable temperatures. During the cold season some discomfort is expected through the early morning hours when the building’s TM is depleted until it is charged again, chiefly by the sun and/or by the residual heat of human activities/appliances.
For TM to work properly, the main energy source is the sun, therefore, orientation plays a key role. The biggest windows need to face south with a ±30° East-West tolerance to make the most out of winter sun’s low angles while avoiding over heating during summer time.
According to The Concrete Centre (2012), as a general rule, windows should be at least 15% of a room’s floor area to provide adequate daylight, and no more than 40% of the façade area in order to prevent excessive heat gains/losses. Proportions can change depending on the type of crystal and if using single, double or triple glazing. However, according to research conducted in Mexico by Mora Juarez (2014), thermal mass was shown to be more effective in combination with single glass when daily temperature variations are large (such as is the case for SLP), note that the aforementioned research was done for Mexico City climate on a heavy weight and thermally insulated prototype.
3.8.2 Construction materials selection by concept
In Section 3.8 it was concluded that high thermal mass is the most appropriate
bioclimatic design strategy for San Luis Potosi, while in Section 3.8.2 it was concluded that the most relevant construction elements regarding their impact in thermal comfort are:
• Walls • Slabs, and
• Inner walls coating
Further steps consist in matching each construction concept with high thermal mass construction materials that are locally available and accordingly with common construction practice in the social housing sector in San Luis Potosi.