Slabs thickness analysis

After performing window size optimization, the next step consisted of examining the impact that different slab thicknesses would have on the prototype’s thermal comfort

performance. This is particularly interesting given that the main bioclimatic design strategy is ‘high thermal mass’ and changes to slab thicknesses would modify the amount of available mass. The impact of those changes in terms of thermal comfort are explored in this sub chapter.

Simulations in Chapter 7 Thermal Comfort Simulations began by employing poured concrete with 200mm thickness for the internal floors and roof slabs as this is the regular practice in the social housing sector in San Luis Potosi City, Mexico. Tables 8.11, 8.12 and 8.13 show a direct comparison between the baseline extruded brick prototype (best performing) with 200mm concrete slabs and with ±50mm variations in slab thickness. Simulations were performed in order to find out the impact of such variations in terms thermal comfort and their climate change resilience.

The decision to only simulate a ±50mm slab thickness variation over the 200mm thickness baseline was because a thinner than 150mm slab thickness would generate additional acoustic and structural challenges that would make it impracticable in real life. In opposition, going beyond a 250mm slab thickness would risk being impracticable due to the economic burden imposed by the additional concrete volume given that social housing is rather cost sensitive. Overall, the changes in slab thickness were an attempt to represent feasible and probable changes that could be implemented in order to improve thermal comfort without

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compromising structural stability nor adding an economic burden on a social housing construction project.

Table 8.11 shows that for the extruded brick prototype a 50mm reduction in slab thickness (150mm) would increase discomfort time by 179 hours across the studied time period when compared with the 200mm baseline. On the other hand, a 50mm increase in thickness (250mm) would decrease discomfort time by 162 hours. Overall it can be concluded that a slab thickness between 200-250mm would provide a good thermal comfort at the present time and for future weather scenarios (climate change resilience).

It is important to note that in all the studied scenarios the extruded brick prototype discomfort time was always less than 10% of the year (<876 hours). This means that bioclimatic design (high thermal mass) and a natural ventilation regime might be enough for thermally

comfortable conditions in the prototype almost year-round. Results are relevant for the residential sector and when employing the adaptive model of thermal comfort (AMTC) given that those provide flexibility in terms of the adaptation measures that house inhabitants can take to make themselves thermally comfortable. This is also possible due to the relatively mild weather conditions prevailing in San Luis Potosi City and the large diurnal temperature shifts that allow for the effective use of high thermal mass.

Table 8.11Extruded red brick prototype slab thickness & discomfort time comparison by year

Extruded brick prototype slab thickness & discomfort time comparison by year

Year 150mm Baseline 200mm 250mm High temp. Low temp. Total High temp. Low temp. Total High temp. Low temp. Total 2010 0 314 314 0 285 285 0 257 257 2020 59 419 478 46 380 426 31 338 369 2050 270 129 399 241 103 344 223 82 305 2080 565 75 640 531 66 597 506 53 559 Total 894 937 1831 818 834 1652 760 730 1490

Note that, although quantitative changes in discomfort time (number of hours) do not seem to be particularly large, they are still relevant as any reduction in discomfort time would benefit the building occupants especially since the studied prototype is a free running building with no mechanical heating/cooling systems.

Table 8.12 shows the impact of a ±50mm variation on slab thicknesses for the hollow concrete block (HCB) prototype which is the second best performing. In this case a 50mm reduction on slab thickness would increase discomfort by 185 hours whilst an increase of 50mm would reduce discomfort time by 166 hours.

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For all the studied scenarios discomfort time occurred during less than 10% of the year (<876 hours), which makes it feasible not to have mechanical heating nor cooling since the building would be naturally comfortable year-round just by the implementation of high thermal mass as the main bioclimatic strategy coupled with a careful south orientation design and natural ventilation. Also, an adequate window size plays a major role on achieving this, as seen on Section 8.1.

Table 8.12Hollow concrete block prototype slab thickness & discomfort time comparison by year

Hollow concrete block prototype slab thickness & discomfort time comparison by year

Year 150mm Baseline 200mm 250mm High temp. Low temp. Total High temp. Low temp. Total High temp. Low temp. Total 2010 0 296 296 0 267 267 0 236 236 2020 60 401 461 46 367 413 31 326 357 2050 270 120 390 243 97 340 225 79 304 2080 577 73 650 528 64 592 500 49 549 Total 907 890 1797 817 795 1612 756 690 1446

Table 8.13 shows the impact of a ±50mm variation in slab thicknesses for the red brick prototype, which is the worst performing. In this case a 50mm reduction in slab thickness would increase discomfort by 198 hours whilst an increase of 50mm would reduce discomfort time by 156 hours.

Table 8.13Red brick prototype slab thickness & discomfort time comparison by year

Red brick prototype slab thickness & discomfort time comparison by year

Year 150mm Baseline 200mm 250mm High temp. Low temp. Tota l High temp. Low temp. Tota l High temp. Low temp. Tota l 2010 0 347 347 0 310 310 0 285 285 2020 73 444 517 56 412 468 49 378 427 2050 308 149 457 274 122 396 252 99 351 2080 621 89 710 585 74 659 547 67 614 Total 1002 1029 2031 915 918 1833 848 829 1677

Even though the red brick prototype is the worst performing of the three prototypes, discomfort time for all the simulated scenarios occurs for less than 10% of the year (<876 hours) which means that the prototype will still manage to be comfortable almost year-round without having to rely on mechanical heating nor cooling when employing the adaptive model of thermal comfort.

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Overall, with the employed simulation parameters, every studied wall construction material, regardless of slab/roof thickness, managed to generate discomfort time for less than 10% of the year (<876 hours) which, in turn, suggest the possibility of not to rely on mechanical heating nor cooling. Results seem to point out that it might be possible to build year-round comfortable climate change resilient social housing in San Luis Potosi City, Mexico by implementing bioclimatic design with locally available construction materials and natural ventilation. This would be particularly true for the residential sector for which any increase beyond 200mm thickness in the roof and internal slabs thickness seems to improve thermal comfort conditions by the added thermal mass, hence, it would be advisable for the concrete slabs to be at least 200mm thick or more.

As mentioned in previous chapters, the quantitate results of the building energy simulations should be taken with due care. In this particular case, although results seem to point at the possibility of not needing mechanical cooling/heating, a more in-depth analysis is necessary to validate such statement. Especially because extreme weather conditions seem to become more common due to climate change, such as heat waves. Therefore, the results shown are not meant to be considered as design prescriptive as they only depict the analysed prototype scenario with its simulation pitfalls.

Final steps consisted of a floor by floor temperature and energy balance analysis to better understand how thermal mass behaves during summer and winter design days in providing thermal comfort. This will be studied in the next chapter.

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