Accounting for exposure duration in overheating risk assessment--a Chicago retrofit case study.
In recent years there have been increasing occurrences of record high temperature, hot spells, and heat waves. In their November State of the Climate report, the National Climatic Data Center stated that 2012 is likely to be the warmest year ever on record for the US (NCDA 2012). This announcement is not entirely surprising especially in light of IPCC's prediction of worldwide increase in the frequency and magnitude of warm daily temperature extremes (IPCC 2012). The report is nevertheless alarming particularly for the US where on average more people died from heat every year than from all other weather-related disasters combined (NOAAWatch 2012). In fact, over the quarter-century period from 1979 to 2003, more than 8000 excess heat-related mortalities were reported in the US (CDC 2009), almost 3500 deaths were from just the last 5 years of this period (Luber et al 2006).
Although overheating in buildings is often defined with respect to thermal comfort, given the escalating warming trends it stands to reason that overheating risk assessments must include means to quantify the state of being overheated in ways relevant not only to comfort but also to health, especially considering the growing elderly population who are among the most vulnerable to heat stress. Specifically, this entails accounting for not only the occurrence and the severity of overheating, but also the length of time one remains in the state of being exposed to an overheated environment, henceforth referred to as exposure duration in this paper.
Because the adaptive model accounts for behavioral and psychological adjustments that can influence occupants' comfort (de Dear and Brager 2002), short hot periods may be more likely to cause thermal discomfort than do extended warm spells because people have not had the opportunities to adapt. Longer hot periods, however, are more prone to cause heat stress (Semenza et al. 1996; Tan et al. 2007; Gosling et al. 2009). Specifically, sustained periods of high temperature have higher mortality risk than do individual hot days (Kinney et al. 2008; Anderson and Bell 2011). Rocklov et al (2011) have also shown that mortality risk increases proportionally with increasing duration of high temperature exposure.
While these epidemiological studies have demonstrated the importance of overheating exposure duration to thermal health, existing overheating risk assessment methods for buildings have yet to account for the length of consecutive time for which indoor temperature remains continuously high. Currently the most utilized overheating assessment method in Europe is CIBSE's summer peak time temperature and overheating criteria as laid out in CIBSE TM36 and CIBSE Guide A, which specifies that overheating has occurred when more than 1% of the occupied hours have exceeded the benchmark operative temperature of 28[degrees]C/82.4[degrees]F for living areas or 26[degrees]C/78.8[degrees]F for bedrooms (CIBSE 2005, 2006). This approach and similar variations based on exceedance of certain benchmarks and certain percentage of occupied hours have been criticized for not considering the severity of the overheated state (Nicol et al. 2009; Spires 2011). The binary judgment of whether overheating has occurred is also sensitive to the way occupied hours and thresholds are defined as well as the weather files used for thermal performance simulations.
The British/European Standard EN15251 (2007) addresses the issue of severity by using degree hours and defining three categories of adaptive thermal comfort limits based on the running mean of outside temperature. This method allows a more detailed understanding of overheating and serves as the basis of a new overheating risk assessment approach proposed by the CIBSE Overheating Task Force (OTF) (Spires 2011). The new approach uses not just one but three different criteria of overheating, specifying separate thresholds for 1) the percentage of occupied hours exceeding the adaptive comfort limit, 2) the daily sum of degree hours over the adaptive comfort limit, and 3) the maximum operative temperature at any time. If any two of the three criteria have been violated then the building is considered to have unacceptable levels of overheating (Spires 2011).
In the US, ASHRAE Standard 55 (2010) also provides several different methods for determining acceptable thermal conditions, namely the graphical method using defined comfort zones on a psychrometric chart, the computer model method that is based on the relationship of predicted percentage of dissatisfied (PPD) and predicted mean vote (PMV), and a third method specifically for naturally conditioned spaces that defines adaptive thresholds in a manner very similar to the adaptive comfort limits defined by BSEN 15251.
While these criteria cover both overheating occurrence and severity, they do not account for the length of time for which the hourly temperature continuously exceeds comfort or overheating thresholds. We suggest two measurements that address the issue of exposure duration in overheating and may be used to characterize the indoor thermal environment with respect to both thermal comfort and thermal health. This study is a continuation of a paper (currently under review) aimed to demonstrate how the proposed measurements may be used to assess overheating exposure duration between thermally distinctive building types. In the same vein but with a different focus, the present paper illustrates how the measurements may be used to evaluate retrofitting options, specifically those aimed to reduce winter energy consumption but may either exacerbate or ameliorate any existing overheating problems.
Despite its relative short cooling period, Chicago has experienced one of the worst heat disasters in the history of US when at least 700 excess heat-related deaths were recorded over a span of only a few days in 1995 (Semenza et al 1996). Unfortunately, a recent study by Hayhoe et al (2010) projected that before the end of this century, heat waves similar to the one in 1995 are likely to occur every other year under the low emission (B1) scenario and may happen as frequently as three times a year under high (A1F1) scenario. More disconcerting is that their study further suggested that heat events of magnitudes akin to the 2003 European heat wave could also happen in Chicago as many as 5 to 25 times by mid-century. In addition to more frequent and more intense heat waves of longer durations, the time period within which heat waves are expected to occur will also lengthen (Vavrus and Van Dorn 2010). In the absence of adaptation, a study by Peng et al (2011) projected that Chicago could experience up to more than 2200 excess heat-related mortality per year between 2081-2100. Clearly overheating is and will continue to be an urgent public health concern for the foreseeable future in Chicago. It follows that any thermally consequential retrofit will need to be examined for potential overheating implications.
A typical Chicago uninsulated timber-frame single-family bungalow (Figure 1) occupied by two adults and two school-age children was modeled and dynamically simulated in IES-VE (version 22.214.171.124). Airflows and solar shadings were modeled by MacroFlo and SunCast, respectively. These are two modules within IES that work in tandem with the main thermal simulation program, ApacheSim. A typical meteorological year (TMY3) weather file representing the current climate (1991-2005) for Chicago Midway was downloaded from the US Department of Energy's EnergyPlus website (Wilcox and Marion 2008) and used in the simulations. Hourly indoor operative temperatures for the entire year for the Living Room and Bedroom 3 (occupied by two adults) were then collected and analyzed.
[FIGURE 1 OMITTED]
Physical building attributes (number of stories, constructions, ventilation systems, etc.) for the case study house were based on the information and assumptions for a single-story pre-1942 timber frame home in the Chicago metropolitan area as described in a recent study (Spanier et al. 2012) conducted by the Partnership for Advanced Residential Retrofit (PARR), a research branch of the US Department of Energy's Building America program. Additional details including the layout, glazing area, occupancy profile, and internal heat gains were determined at the authors' discretion. Wherever possible, values and assumptions were taken from or calculated according to ASHRAE Fundamentals (2009), Building America Research Benchmark Definition (2009) and House Simulation Protocol (2010), International Energy Conservation Code (2009), and CIBSE Guide A (2006). The key building characteristics include: natural ventilation (no mechanical ventilation or cooling system) in summer, windows open (during occupancy) whenever outside is cooler than inside and the inside temperature is greater than the CIBSE summer comfort temperature (25[degrees]C/77[degrees]F for living areas, 23[degrees]C/73.4[degrees]F for bedroom), gas furnace operating during occupancy (18:00-8:00 on weekdays) from Sept. 16 to May 31 with heating set point maintained at 20[degrees]C/68[degrees]F for living area and 19[degrees]C/66[degrees]F for bedroom (based on City of Chicago Heat Inspections ordinance), total glazing area approximates 17% of total conditioned floor space, and no local or tree shading (except 0.61m/2ft roof overhang) for windows.
The objective of the PARR study was to identify the top housing groups with the largest cost-effective energy-saving potential from four retrofitting options: infiltration reduction, roof insulation upgrade, space heating equipment upgrade, and water heater upgrade. Wall insulation and window upgrades were not considered by PARR due to high cost. Interestingly, cooling equipment upgrade was also omitted due to short cooling season. Housing group 14, on which the case study house in this paper is based, represents 11.2% of the total building stock in the region and is ranked first in the amount of energy saved after the retrofits.
In the present study, nine retrofitting "packages" consisting of different combinations of upgrades were simulated in addition to the current condition. The individual building elements and their upgraded counterparts are described in Table 1. All retrofitting packages include infiltration reduction (from 0.55 to 0.25 ACH). In addition, each package includes one or more upgrades (referred to as underlined letters in Table 1) as followed: roof insulation (upgrade D), wall insulation (A), roof and wall insulations (A and D), white roof paint (E), roof and wall insulations and white roof paint (A, D, and E), floor insulation (C), roof and floor insulations (C and D), low-e glazing (B), roof insulation and low-e glazing (B and D). It should be noted that these nine packages were selected for illustrative purpose only. More comprehensive set of options with different upgrade combinations will be explored in a separate investigation.
All simulations were done first with Bedroom 3 facing south, then with the Living Room facing south. The resultant hourly operative temperature data then were analyzed for the south-facing Bedroom 3 and the south-facing Living Room using the two proposed measurements to quantify the duration of overheating exposure: consecutive hours of exceedance (CHE) and cumulative degree exceedance of continuous exposure (CDECE).
Rather than counting the total number of hours (or degree hours) above a certain threshold in a given period of time, CHE only counts the number of hours (and CDECE only counts the number of hours (and CDECE only counts the degree hours) for a stretch of time where the operative temperature ([T.sub.o]) is continuously over the threshold. It follows that the given period may have several stretches of time where [T.sub.o] remains high; these stretches are named continuously overheated intervals (COI). For example, if a given period has five such intervals, there would be five pairs of CHE and CDECE measurements associated with each interval. Specifically, each interval would have a CHE that represents its length (number of hours) and a CDECE that sums the exceeded degree hours over that interval. CHE and CDECE can be described mathematically as followed: for each Continuously Overheated Interval, COI:
Consecutive Hours of Exceedance (CHE, in hours) = b - a; a = the time the COI begins, b = the time the COI ends
Cumulative Degree Exceedance of Continuous Exposure (CDECE, in degree [degrees]C hours)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (1)
where [T.sub.o] is the hourly indoor operative temperature and [T.sub.th] is the threshold temperature, which can be either an absolute overheating benchmark or an adaptive comfort limit that changes with the outside temperature. For illustration purposes, the CIBSE overheating benchmarks (28[degrees]C/82.4[degrees]F for living areas or 26[degrees]C/78.8[degrees]F for bedrooms) were used as the threshold temperature in this present study. It should be noted that intuition suggests that CHE and CDECE are related; the more hours (CHE) the continuously overheated interval has, the greater the sum of degree exceeded (CDECE). This is preliminarily explored by the authors in a related paper (under review).
The indoor temperature data were also analyzed via the CIBSE and BSEN overheating risk assessment approaches. Specifically, the indoor operative temperatures were computed to show 1) percentage of annual occupied hours exceeding CIBSE overheating benchmarks; 2) cumulative annual occupied degree hours exceeding these benchmarks; 3) percentage of summer occupied hours exceeding BSEN 15251 adaptive comfort upper limits by at least 1K for three levels of expectations (categories); and 4) cumulative summer occupied degree hours exceeding these limits. The same analyses were repeated for all hours to illustrate how the definition of occupancy profile can dramatically alter the "overheated status" of a building.
RESULTS AND DISCUSSIONS
Before the indoor temperature data were computed according to the existing overheating assessment approaches and the two proposed measurements, the raw time-series data (not shown) were examined. This preliminary inspection revealed that retrofitting packages with floor insulation (upgrade C) incurred substantial increase in daytime temperatures in the summer without the benefit of such increase in the winter. This result is not entirely surprising as the basement, which is assumed to be completely underground in this study, serves as a thermal mass that attenuates the indoor temperature swing caused by outside conditions. Insulating the floor essentially eliminates this beneficial effect and the indoor condition is thus rendered undesirable. It should be noted that this particular retrofit, which is conceivably cost prohibitive, was not considered by the PARR study and the simulation results confirm that such "upgrade" would in fact be a downgrade.
Retrofitting packages with low-e glazing (upgrade B) showed almost no difference from the today scenario. Since a wide variety of treatments can be applied to improve glazing U-values and only one particular scenario was considered here, window retrofit warrants further investigation in order to determine whether it can undermine or contribute to the reduction of overheating either in isolation or in tandem with other upgrades. While retrofitting packages with higher albedo (lower solar absorptivity) roofs (upgrade E) showed some potential in reducing summertime indoor temperature, the improvement is most pronounced when this upgrade was applied in conjunction with other upgrades, most notably wall insulations (upgrades A). Painting the roof white in addition to roof insulation produced almost no difference, as the reduction of roof surface temperature from higher albedo can be rendered superfluous when given enough interior roof insulation. Simply painting the roof white without the insulation produced similar result as applying roof insulation alone in the summer. In fact, the white roof paint alone rendered even lower minimum nighttime temperatures. Unfortunately, this reduction is negligible in the summer and the most pronounced in the winter when it is undesirable (winter heat penalty effect).
In contrast to the ambivalent or even counterproductive retrofitting packages involving low-e glazing, floor insulation, and white roof paint (upgrade B, C, and E respectively), packages involving roof and wall insulations (upgrade D and A, respectively) warrant further overheating risk analysis as both provide substantial thermal improvements in the winter. Inspection of the time-series data showed that wall insulation alone fared better than roof insulation by itself in the winter. Appling both upgrades provided the best thermal performance in the winter. The effects of these upgrades and their combination in reducing overheating in the summer are less obvious by time-series inspection alone and are further examined as discussed below.
Figure 2 shows the simulation results computed as both percentages of hours exceeded (1 and 2) and as cumulative degree hours (3 and 4) according to CIBSE's benchmarks for four scenarios: the current condition, upgrade D, upgrade A, and combined upgrades A and D. Simulation results were also computed according to BS EN 15251's adaptive comfort limits, but not shown due to space limitation. Several interesting observations are noted. First, overheating (at least as defined by CIBSE's benchmarks) can occur outside the typical summer months even in such a high-latitude city, as evidenced by the October segment in each monthly breakdown of all scenarios, most notably in the living room. Second, how occupied hours are defined can dramatically change the apparent overheating risk of a space. When only occupied hours are considered, the living room is substantially more at risk than the bedroom in terms of both overheating occurrences and severity. However, when all hours are considered (which is conceivable if the occupants were a work-at-home parent with young children or elderly adults), the bedroom is more overheated. The disparity is the most pronounced when measuring overheating severity (degree hours, Figure 2-3 and -4). Nevertheless, both rooms in all scenarios are deemed overheated by the CIBSE overheating criteria (more than 1% annual occupied hours exceeding benchmarks). Third, how thresholds are defined can also alter the relative overheating risk, as observed by comparing the results calculated using the CIBSE benchmarks -- which differ based on space utility, with the results calculated using the BS EN adaptive comfort limits -- which vary with the outside temperature but do not make distinctions between rooms. Although the two are not directly comparable as CIBSE considers an entire year and the BS EN limits are only applicable to summer, results computed using the CIBSE benchmarks showed appreciable contrasts between the living room and the bedroom regardless of occupancy or months, whereas results evaluated using the adaptive comfort limits showed little difference between the two rooms when accounting for all summer hours (not just occupied). With respect to the retrofits, all upgrade scenarios fared better than the current condition except for the bedroom when evaluated using the CIBSE benchmarks and only occupied hours are considered. In general, combining upgrades A and D seems to yield the most overheating reductions; upgrades A and D alone are rather comparable. To confirm this and to understand how these retrofits affect the overheating exposure duration, the indoor temperature data were investigated using the two proposed measurements: consecutive hours of exceedance (CHE) and cumulative degree exceedance of continuous exposure (CDECE). All hours within a year were included to compute CDECE and CHE, so that no stretches of overheating--the continuously overheated intervals (COI)--would be "interrupted" by the occupancy profile.
[FIGURE 2 OMITTED]
The overlaid histogram "areas" of CHE and CDECE for the living room (Figure 3-1 and 2, respectively) confirm readily what the existing approaches have already suggested, that an retrofitting package including both wall and roof insulations (upgrade A and D) is better able to reduce overheating than do either insulation alone. This is illustrated by the smaller/lower blue areas. What CHE is able to further impart is that while insulating the roof (upgrade D) reduces the occurrences of continuously overheated intervals (COI) of longer duration (higher CHE)--as shown by the lower yellow peaks compared to the red peaks, insulating the wall (upgrade A, green peaks) not only moderately reduces the number of occurrences but also shifts the peaks leftwards, indicating shorter COIs. These effects are most prominently seen when both the roof and wall are insulated: the blue peaks are both shifted and reduced, implying less COI occurrences and shorter lengths for those that do occur. The overlaid histogram "areas" of CDECE for the living room (Figure 3-2) show similar pattern. Even though all four scenarios still incur a few COIs of greater severity (higher CDECE), the COIs for the retrofitting package including both roof and wall insulations lie consistently to the left of all other scenarios (implying lower severity), even though it has one COI that lasts slightly longer (far right blue blip in Figure 3-1). Worth noting is that while at lower severity (lower CDECE) applying wall insulation alone (upgrade A, green peaks) seems to incur similar number of COIs as applying roof insulation alone (upgrade D, yellow peaks), at greater severity its COIs are consistently more intense (green peaks to the right of yellow peaks at higher CDECE).
[FIGURE 3 OMITTED]
The results for the bedroom (Figure 3-3 and 4) are similar to that of the living room but less pronounced, especially for the CHE. It is unclear whether this is simply a "side-effect" of using a lower threshold (26[degrees]C/78.8[degrees]F instead of 28[degrees]C/82.4[degrees]F), or a true response due to different physical attributes (internal heat gains, floor areas, etc.). Further examinations and sensitivity analysis will be conducted in subsequent studies to investigate this issue. For the time being, the results suggested by the CHE/CDECE analyses and by the existing assessment approaches seem to support each other; that insulating both the wall and the roof (upgrades A and D) appear to be the most effective retrofitting package in reducing summertime overheating risks without compromising wintertime thermal performance. More importantly, CHE and CDECE unpack the state of overheating into individual continuously overheated intervals (COI) to reveal stretches of overheating that may be critical to health, such as COIs that are particularly long or severe (the blips towards the right hand side of each graph in Figure 3), which are otherwise "hidden" in the aggregated assessment results shown in Figure 2. In this way, CHE and CDECE contribute to the understanding of the manner with which retrofits affect overheating characteristics (such as whether certain upgrades can bring the CHE and CDECE of COIs down to a certain range). This information cannot be gleaned from aggregated reduction, which also may not accurately reflect the overheating situation as experienced by the occupants in real time.
This paper demonstrated how two proposed measurements designed to quantify overheating exposure duration can be employed to impart additional information to assist the selection of retrofitting options, particularly in places with continental climates such as Chicago where both the cooling and heating seasons can prove challenging. Furthermore, the two measurements--consecutive hours of exceedance (CHE) and cumulative degree exceedance of continuous exposure (CDECE)--allows an assessment based on continuously overheated intervals (COI) that more appropriately capture the state of overheating as relevant to the occupants' thermal health than does aggregated assessment which is easily influenced by how thresholds and occupancy schedule are defined.
In the context of the pre-1942 single-story uninsulated timber-frame bungalow in Chicago, while wall insulation is more effective than roof insulation in the winter, there is little difference in the overheating risk reduction potential of the two in the summer. However, when wall insulation is applied in tandem with roof insulation, thermal performance is improved in both seasons. Further analysis is needed to investigate the energy and cost savings of these upgrades. Follow-up studies will investigate more building types of different construction in different climates and different retrofitting scenarios in order to generalize the usefulness of CHE and CDECE. Specifically, heat stress will be investigated in the context of both temperature and humidity, and how this addition may influence the calculation and usefulness of CHE and CDECE.
More broadly, should future epidemiological studies be able to relate distinct exposure duration thresholds to specific physiological consequences, then the measurements of CHE and CDECE could be used to produce clearer and more exact comparisons. For example, if future health studies determine that only COIs over 28[degrees]C/82.4[degrees]F lasting longer than 12 hours should be a cause for concern, then neither roof, wall, nor combined insulation would reduce the overheating risk for the living room as all three retrofit scenarios in this study incur similar numbers of continuously overheated intervals that stretch over 12 hours as the current condition (Figure 4-1). In the same way, if COIs over 26[degrees]C/78.8[degrees]F lasting longer than 12 hours were deemed unacceptable, then upgrading both the roof and wall insulation could potentially reduce the number of COI occurrences in the bedroom from 26 to 17 -- a 35% reduction (Figure 4-2)! Information like this then can aid in the identification of potentially health-threatening building stocks within a city or region, and in the selection of appropriate retrofitting options to reduce heat--related health risks in buildings.
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W. Victoria Lee
Koen Steemers, PhD RIBA
W. Victoria Lee is a PhD researcher in the Department of Architecture, University of Cambridge, Cambridge, United Kingdom. Koen Steemers is a professor and the Head of the Department of Architecture, University of Cambridge, Cambridge, United Kingdom.
Table 1. Case study house construction and retrofit upgrade descriptions and U-values Current Condition Construction U-Value [SI: W/[m.sup.2]k]; [I-P: Btu/h[ft.sup.2] x [degrees]F] External Wall Outside to inside: wood siding, plywood 1.493 (SI); 0.274 (I-P) sheathing (1/2"), wood stud 2 x 4 frame (23% framing factor), gypsum board drywall (5/8") Internal Wall Uninsulated light wood frame (2 x 4) 1.539 (SI); 0.269 (I-P) with gypsum board drywall on both sides (5/8") Doors and Windows Weatherstripped double-glazed (1/2" 2.691 (SI); 0.512 (I-P) gap air filled) sash windows w/ PVC-U frame Weatherstripped solid wooden door to 1.134 (SI); 0.200 (I-P) outside Solid wooden internal doors 3.200 (SI); 0.618 (I-P) Floor/Ceiling Plasterboard ceiling with carpeted 1.232 (SI); 0.222 (I-P) uninsulated wood frame floor (2 x 8) (11% framing factor) Basement Wall: uninsulated medium weight cast 1.775 (SI); 0.313 (I-P) concrete; Floor: slab on ground, screed over insulation Roof Hipped ([degrees]25[degrees]) roof with 1.462 (SI); 0.265 (I-P) R3 insulation and dark red shingles (([rho] = 0.75) Retrofit Upgrades Construction U-Value [SI: W/[m.sup.2]k]; [I-P: Btu/h[ft.sup.2] x [degrees]F] Upgrade A: upgrade to polyurethane 0.320(SI); 0.057 (I-P) foam insulating sheathing (1") and add batt insulation to the frame cavity No upgrade Upgrade B: upgrade to low-e double 1.950 (SI); 0.363 (I-P) glazing No upgrade No upgrade Upgrade C: same as A but applied to 0.166 (SI); 0.030 (I-P) floor/ceiling frame cavity No upgrade Upgrade D: upgrade to R38 insulation 0.146 (SI); 0.026 (I-P) Upgrade E: reduce solar absorptivity--white roof paint ([rho] = 0.13) Figure 4 Hypothetical categorization of continuously overheated interval (COI) based on consecutive hours of exceedance (CHE) bands (1) Living room Consecutive Hours Frequency of Exceedance (Categorized) Current Upgraded D Upgraded A Upgrades A+D 1-3 hrs 15 25 22 20 4-6 hrs 27 15 15 13 7-12 hrs 34 27 28 18 13-24 hrs 6 5 6 5 >24 hrs 0 0 0 0 (2) Bedroom Consecutive Hours Frequency of Exceedance (Categorized) Current Upgraded D Upgraded A Upgrades A+D 1-3 hrs 16 19 23 24 4-6 hrs 27 20 24 23 7-12 hrs 32 34 34 30 13-24 hrs 26 22 20 17 >24 hrs 3 3 3 3 Note: Table made from bar graph.
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|Author:||Lee, W. Victoria; Steemers, Koen|
|Date:||Jul 1, 2013|
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