ENERGY PERFORMANCE R-VALUE: PART 2, EXAMPLES OF INTEGRATED METHODOLOGY FOR EVALUATION OF ENERGY EQUIVALENT R-VALUE FOR BUILDING ENCLOSURES;

Transcription

1 ENERGY PERFORMANCE R-VALUE: PART 2, EXAMPLES OF INTEGRATED METHODOLOGY FOR EVALUATION OF ENERGY EQUIVALENT R-VALUE FOR BUILDING ENCLOSURES; Part 2: Examples of application to residential walls Thomas Thorsell and Mark Bomberg 1 The paper is submitted to the Journal of Building Physics; here is an extended abstract: It is often forgotten that the building sector consumes more energy than the transportation sector. To meet expectations and needs of our society we must seek significant improvements in the efficient use of energy for this purpose. In many instances our normal approach that is based on conventional testing methods, is not comprehensive enough. For instance, the thermal performance of a wall is defined by tests performed on walls with dry materials, without consideration of air and moisture movements even though we know that the energy performance of materials and building assemblies is affected by both moisture and air flows. The authors believe that a more precise means of evaluation of the thermal performance of assemblies must be used to guide us in developing construction practices that lead to better performance. That should include consideration of air and moisture transfer under field conditions. The previous part of this paper described the limitations of conventional thermal resistance testing using calibrated hot boxes and explained that the effect of climate on thermal performance must also involve use of computer models that are capable of simultaneous calculations of heat, air and moisture (HAM) transfer. In this paper, the integrated testing and modeling methodology proposed is applied to a few selected residential walls to highlight the magnitude of air flow effects compared with steady-state thermal resistance without air flows. Effectively, to characterize energy performance of the building enclosure one must use an integrated methodology that uses both testing and modeling. The paper represents a first Step in this direction. 2. Construction of tested residential wood-frame walls The material selection and construction technique used was identical for 6 residential test walls in pilot study and for the two walls (1 and 4) reported in this paper. 2.1 The reference wall (wall 1) The following describes the construction of 1 1 Syracuse University, 149 Link Hall, Building Energy and Environmental Systems Laboratory, PhD student and Research Professor respectively 1

2 Glass fibre batt insulation (R13) with dimensions 3 ½ x15 x93 (manufactured by a large US manufacturer) with asphalt backed Kraft paper purchased from a building materials supplier. The dimensions of the wall were about 6'x8' and the tolerance of the frame dimensions was ¼ inch. A double sill plate was used at the top, and a single plate was used at the bottom. To represent typical workmanship there was no sealing between the sub-flooring and the wall assembly. The facer on the insulation batts was stapled to the inside of the studs on both sides of the cavities at a 12-inch spacing. The recommended procedure for installation was acquired from the manufacturer To ensure that the installation of batt insulation represented typical workmanship, several sources, such as Energy Gauge programmers and Building Professional Institute etc. were contacted for advice. The final choice of workmanship level was to represent the middle of category 1 (good workmanship) as defined in Energy Gauge (5 6 percent of deficiency). Because the facer was stapled to the studs, the area of unfilled corners was reduced by approximately 3% (i.e.the area of insulation was 97%). Corners next to the stud, on the back side of each cavity were unfilled. To ensure uniformity special triangular templates were prepared, see Figure 4. A similar approach to insulation installation was used by Brown et al, (1993). The 7/16" thick oriented strand board (OSB) sheathing was installed to the framing with galvanized roofing nails 1¼, at about 1 foot spacing (30 cm). The OSB sheathing was installed with 1/8 inch (3 mm) gaps in the joints to allow for hygrothermal movement. The same layout was used for construction of all wood-frame test walls In all cavities using batt insulation, they were joined at a height of 6 feet. The drywall was mounted carefully on the interior surface to not affect or damage the sensors on its surface. Drywall screws were used for the installation and the joints where taped and finished. At the bottom edge of the walls, the bottom edge of the gypsum board was installed 1/4 up from the sub floor. The polymeric WRB product selected was stapled to the OSB according to the manufacturer s instructions with a stapling distance of 1 ft (30 cm) throughout the whole surface. Overlap between two rolls was about 6. The WRB was also taped with a compatible tape according the manufacturer s instructions. (a) For full sized cavity 2 x 2 inch triangle 2

3 (b) For narrow cavities 1.5 x 1.5 inch triangle Figure 2: Typical spacer used to maintain 3% of unfilled corners on the back side of batt insulation. 2.2 The hygroscopic fibrous insulation (wall 4) The same layout was used for construction of all wood-frame test walls. The other wall tested ( 4) was identical except for the thermal insulation material that will not be described except that it was blown in material based on cellulose fiber. When the construction of each test wall was completed, a layer of 1 inch thick expanded polystyrene with a nominal density of 1 lb/ ft3 was mounted and adhered to the interior surface of each wall. It was used as a calibrated boundary layer (CBL) for measuring heat flux. 3. Thermal resistance measurements of walls 1 and 4 These tests were performed at exterior temperature of 3 o F (-16 ). The heat flux transducer (HFT) placed on thermal insulation at mid-height of wall 1 malfunctioned and is not reported in this paper. 3.1 Step 1 measurement of the reference on the section through insulation Figure 3 shows results of measurements performed during the Step 1 on wall 1 under standard conditions namely room 24 and 50 %RH; weather -16, uncontrolled RH and no air pressure difference. 3

4 Figure 3: Results of heat flux and temperatures measured on wall 1 Tables 1 and 2 presents temperatures measured on both wall surfaces and heat fluxes measured at the interior surface of each wall. Since a 25 mm layer of EPS was placed against the inner surface of each wall, its effect was included in the measured Rsi-value 2, and when its thermal resistance is subtracted, one obtains a local, apparent Rsi-value of the wall itself as indicated by the HFT. To continue the analysis of the primary measured data (temperatures and heat fluxes) with a view to establishing thermal resistance of the wall we need to define a few terms that will be used in this paper: 1) in this paper represents the air to air resistance to heat flow. It is an inverse of the U-value. Note that this definition is different from that used in ASHRAE. This definition permits us to avoid several measurements on the wall surface to establish an average surface temperature when multi-dimensional heat transfer causes large local differences. 2) Apparent = a ratio between temperature difference between indoor and outdoor air to the local heat flux measured at the local point without subtracting the resistance of calibrated boundary layer (CBL). 3) Local = a ratio between temperature difference between indoor and outdoor air to the local heat flux measured at the local point when the resistance of calibrated boundary layer (CBL) is subtracted. 2 Rsi-value is thermal resistance in SI units, is thermal resistance in Imperial (IP) units 4

5 4) = thermal resistance under steady-state, unidirectional heat flow through the center section of the insulation i.e. in the symmetry point between thermal bridges. s measured under standard conditions (Step 1) are considered as the reference. 5) Clear-wall = the ratio between average temperature difference between indoor and outdoor air to the mean value of the heat flux across the whole wall. This would be calculated for the central area of the wall (without consideration to increased heat flow on the wall perimeter, corners etc). Clear-wall is used for calculating effects of air and moisture either in the laboratory testing protocol or for the field performance evaluation. 6) Energy performance = the clear-wall that accounts for effects of air and moisture flows measured under specified conditions. Using these terms we can proceed from the measured local s to the clear-wall s. Table 2 through the insulation section measured on 1 during Step 1 Cold side, air Warm side, air Temp. difference Heat flux W/m 2 Apparent Local Local Top bottom Average () Table 3 through the insulation section measured on 4 during Step 1 Cold side, air Warm side, air Temp. difference Heat flux W/m 2 Apparent Local Local top middle bottom Average () Adding up the s for the individual component materials, namely R13 fiberglass batt, OSB, drywall and surface air film resistances, one obtains the nominal of R14.9 for 1 and 5

6 R15.8 for 4. This agrees with the value measured on 1 but disagree for 4. These tests were, therefore, performed once more and results are presented in Tables 4 and 5. Table 4 through the insulation section of 1 during repeated Step 1 test Cold side, air Warm side, air Temp. difference Heat flux W/m 2 Apparent Local Local Top bottom Average () Table 5 through the insulation section of 4 during repeated Step 1 test Cold side, air Warm side, air Temp. difference Heat flux W/m 2 Apparent Local Local top middle bottom Average () Tables 4 and 5 show repeated measurements of nominal in the section through the thermal insulation. The agreement remains good for 1 but the change in test results for 4, namely from 18.5 (ft 2.h. o F)/Btu to 17 5 (ft 2.h. o F)/Btu warrants a review of stability for that wall. 3.2 Analysis of the stability of on 4 4 showed significant changes in thermal resistance during the extended period of testing. One hypothesis that comes to mind for this change is that this could be an effect related to moisture gain as this wall includes a non-traditional insulation system containing a hygroscopic material. The following section will provide theoretical background and elucidate the effect of moisture on the apparent. The hygrothermal properties of the insulation material were established in another project. Figure 4 shows measurements of heat flux on the surfaces of a sealed specimen having a constant total moisture content of 0.94 percent by volume (9.4 kg/m 3 ). 6

7 Figure 4: Heat flow flux measurements performed in an apparatus built for testing in accordance to ASTM C518 but performed on a sealed specimen having a moisture content of 9.4 kg/m 3. This method is used for verification of the HAM model 3. Calculations (solid line with triangles) show better agreement with the heat flux recorded on both surfaces (continuous lines) than calculations with moisture content of 6.8 kg/m 3 (dashed line with triangles). Under the influence of a thermal gradient the moisture moves towards the cold side causing changes in the heat flux with time. The agreement shown in Figure 4 is believed to be sufficient, and we will use this set of hygrothermal input data in the current project. Figure 5 shows the results of HAM model calculations based on this data. g Heat flux (conduction) in [W/m2] HFT_stud HF_whole HFT Time in [h] Other results from MBES project led by Syracuse U in and sponsored by BASF, Fortifiber and Greenfiber Corporations were published in several papers. 7

8 Figure 5: Calculated heat flux through 4 using hygrothermal data as shown in the verification tests (Figure 4) Figure 5 shows the results of 2-D calculations performed with CHAMPS-BES 4 software developed under joint project between Technical University of Dresden, Germany and Syracuse University. Figure 5 presents three curves. The top curve represents values of local heat flux expected to be measured by a HFT placed in the center of the insulation section. The second curve (HF whole) represents the mean value of the heat flux for the whole wall The third curve represents the maximum heat flux that would be measured at the stud. Measurements of his value were attempted with small commercial HFT sensors but they were found to be unreliable. We must note that we do not know the degree of connectivity between the cavities of 4 and the indoor environment. We have, therefore, made calculations for the worst case scenario i.e., a full connectivity with indoor environment. This is the case presented in Figure 5. Heat flux calculated at the 2 nd day of simulation was 10.1 W/m 2 while on the 3 rd day it was 10.7 W/m 2-6 % higher. The measured change in the of 4 was about 5 % which is in the same order of magnitude difference. Now, we pose the question as to what resistance to water vapor diffusion is required to eliminate the effect of moisture on of this wall providing that the effect of air flow was also eliminated by use of an air barrier. To provide an answer to this question, the same HAM model was used but assuming that the CBL layer was air impermeable and that its water vapor permeability was as recommended by ASHRAE for thermal zone 5 of the US. The results of that simulation are provided in Figure 6. Heat flux (conduction) in [W/m2] HF_whole HFT1 HFT_stud Time in [h] Figure 6: Calculated performance of 4 with the same hygrothermal input data as for Figure 5 but with a perfect air barrier and with a required water vapor retarder placed on the inner face of the wall. 4 This software was developed under a joint project between Technical University of Dresden, Germany and Syracuse University, NY. 8

9 Two conclusions can be drawn from comparison between Figures 5 and 6. The first relates to the effect of moisture ingress. The wall provided with the water vapor retarder on the inner face showed practically constant heat flux (HFT1) with time. It only varied from 11 to 11.2 W/m 2 over the 350 hours simulated. On the other hand, the wall with an air bypass carrying moisture into the wall cavity (Figure 5) showed that over that same period of time the heat flux at HFT1 increased from 9.6 W/m 2 to 11.2 W/m 2 (a 14.3% change). The second conclusion is far more surprising a short term thermal resistance measurement using a calibrated hot box of hygroscopic systems may give an overrated result. While we are familiar with the situation when movement of moisture towards the cold side of a material increased the apparent conductance of heat and reduced the thermal resistance, we are not familiar with the opposite situation. We have not seen any report on the situation when an air bypass brings moisture to the hygroscopic material and therby reduces heat flux entering the wall. Let us now compare the measurements and calculations. If the apparent R18.5 (h. o F.ft 2 )/Btu shown in Table 2 is reduced by 14 % one obtains R15.9 (h. o F.ft 2 )/Btu. The nominal for this wall based on adding the s of the individual components was 15.8 (h. o F.ft 2 )/Btu and this value was also derived from the HAM calculations when the air bypass was eliminated. Therefore, the nominal for 4 in our integrated testing and modeling program is 15.8(h. o F.ft 2 )/Btu. 3.3 Clear-wall s as measured in this test program Figures 4 and 5 show a difference of approximately 14% between heat fluxes on the inside face of the thermal insulation and the average value of the wall as calculated by 2-D software. This agrees with values measured for this type of the wall by Thorsell and Bomberg (2008). Attributing the 14% difference as the effect of thermal bridges to the nominal one obtains clear-wall s (Table 6). Table 6 and clear-wall s, measured and calculated for s 1 and 4 number measured R value (though insulation) Clear-wall R value from 2-D calculations Difference, percent < <1 3.4 Effect of standard air infiltration on the apparent s 50 Pa air pressure was applied in the weather chamber and the bottom orifice plate on the weather side and the top orifice plate on the room side were opened to create a long-path for air infiltration inside the wall cavities. The results are presented in Tables 7 and 8 for s 1 and 4 respectively. Table 7 Apparent measured on 1 during a standard air infiltration test Temp. Heat Apparent Reference Reduction 9

10 differ. flux W/ m 2 (IP units) of % Top bottom Avg % The above measurements demonstrate that cold air infiltration has a very significant effect (up to 21% reduction) at the bottom level close to where the air entered and a much smaller decline at the top of the wall where the air exited. Table 8 Apparent measured on 4 during a standard air infiltration test. Temp. differ, C Heat flux W/ m 2 Apparent measured in Step 1 Reduction, percent top middle bottom Avg We observe that heat flux near the bottom of 4 increased about 22% while the value in the middle was reduced. Note that measurements performed under all steps consistently showed that heat flux in the middle of this wall were higher than at the other two s. Under air infiltration all three values of heat flux are closer to each other but our information is insufficient to comment on these observations. 3.5 Effect of doubling the area of air infiltration orifices on the apparent Table 9 measured on 1 during the high air leakage test Temp. differ., Heat flux W/ m 2 Apparent Reference (Step 1) Reduction of, percent Top bottom average Table 10: measured on 4 during the high air leakage test 10

11 Temp. differ., Heat flux W/ m 2 Apparent Reference (Step 1) Reduction. of, percent top middle bottom average Reduction of thermal performance caused by the increased level of air leakage appears excessive but one must bear in mind that this is a calibration test for estimating the air flow pressure relationships for the tested wall and the measured influence of that flow on thermal performance. At least two levels of air flow (hence airtightness) are necessary for a linear approximation of s as a function of air flow. Knowing or assuming the overall airtightness of the wall (construction quality test) one can select the reduction in caused by air flow through the wall. 3.6 Effect of wetting on the apparent s Step 3 required a change in the weather chamber from cold to hot and humid conditions (40 and 85 % RH). The standard opening of the orifices was used again and moist air entry lasted 5 days. Table 11: for wall 1 during the Step 3 of the test sequence Temp. differ., Heat flux W/ m 2 Apparent Reference (IP), Step1 Reduction, percent Top bottom avg Table 12: for wall 4 during the Step 3 of the test sequence Temp. differ, Heat flux W/ m 2 Apparent Reference (IP), Step1 Reduction, percent 11

12 top middle bottom avg Evidently the combination of reverse thermal gradient and moisture entry will cause a much higher cooling load than it was in case for dry cold air entry. 3.7 after a period of standard exposure Step 4 Tables 13 and 14 show the measured after a transition period of 1 day plus 5 days of drying. This selection is arbitrary because different walls will dry to a different extent depending on many factors. But, for comparative purposes it is sufficient to examine the drying ability of the wall assembly. Note that this test is not included in the energy performance concept but serves as primary means of model verification. Table 13: for 1 during the Step 4 of the test sequence Temp. difference, Heat flux W/ m 2 Apparent Reference (IP), Step1 Top bottom average Table 14: for 4 during the Step 4 of the test sequence Temp. difference Heat flux W/ m 2 Apparent Reference- * (IP), Step1 top middle bottom average */ Note that the actual reference value is 15.8 (h. o F.ft 2 )/Btu but the one measured during Step 1 was 17.5(h. o F.ft 2 )/Btu. 12

13 4. Discussion on Energy Performance (R EP ) of s These two papers (Part 1 and Part 2) are aimed at two distinctly different, yet complementary issues: 1) There is a need for an index to appropriately rate the thermal performance of wall systems. We gave that index the name Energy Performance (Rep). Such an index, as explained in the introduction to Part 1, should include the effect of air and moisture infiltration on thermal performance. 2) There is a need to develop a methodology for verification of HAM models so that they could be used for prediction of the actual field performance of materials and construction systems The approaches taken in these two papers constitute a first step in providing information addressing both issues, yet before we propose any solution we need to step back and make a few observations. Firstly, currently in the Building Physics domain there is a sufficient capability for characterizing air flow paths and for measuring / calculating the effect of dry air flow on thermal performance of construction systems. Secondly, currently in the Building Physics domain there is insufficient capability to characterize the ingress of moisture carried by air or the capability of calculating moisture removal under simultaneous heat and air flows. To build confidence in the use models we need to verify them by comparing with the experimental that are related to the rate of those processes versus time More research in this area is urgently required. In the meantime it is proposed that a Rep indicator be used that only includes two effects: 1) the effect of thermal bridges (framing correction) 2) the effect of standard air flow conditions on apparent Using this concept we obtain results shown in Table 15. Table 15: Summary results of measurements on s 1 and 4 wall R- value, h o Fft 2 /Btu Measured nominal R- value ) Effect of airflow, percent R energy performance; h o Fft 2 /Btu We also propose to use a concept of thermal insulation efficiency. Using the nominal as 100 percent we find the efficiency of 1 is 73.3 percent and that for 4 is 82.3%. Thorsell and Bomberg (2008) showed that using continuous exterior insulation modifies the thermal 13

14 bridging reduction correction and improves the thermal efficiency of walls. For example, if 4 had been covered with a 1.5 inch layer of continuous spray polyurethane foam the multiplier against the nominal increases from to and the total becomes R =(R15.9x0.918x0.955+R9.0) i.e., R22.94 increasing the efficiency factor to (22.94/24.9) = 92%. 5. Concluding Remarks The methodology proposed here develops an interim indicator that includes two measured effects: effect of thermal bridges (framing correction) effect of standard air flow conditions on apparent Furthermore, extended information obtained from this test protocol will be used for the verification of HAM models. It has been concluded that use of these models for calculating effects of moisture is necessary and only when heat, air and moisture transfer models are adequately verified can they be used for this purpose. The verification must performed at both the material level and at the assembly level. The latter is one of the objectives of the proposed integrated methodology. While it may be premature to include moisture effects in the test energy efficiency indicator, it was shown that addressing effects of moisture on thermal performance is necessary when testing hygroscopic insulation materials. Finally, the extended information derived from Steps 3 and 4 will be used for verification of HAM models. Only two walls are discussed in this paper. However, this study dealt with 8 different walls to examine differences between several foamed and fibrous insulations. The foremost important conclusion was that air ingress into wall cavity has significant effects on thermal performance of walls. Changes caused by air intrusion to typical frame-wall constructions varied from 1 or 2 % when airtight foam filled the whole cavity to 20% reductions when insulation was permeable for air and moisture, or a large unfilled air space existed in the cavity. Obviously these numbers were lower (e.g. 4.5 % and 14.5 % shown in this paper) when the wall construction were more airtight. 14

15 Syracuse University - Building Energy & Environmental Systems Laboratory Integrated methodology for evaluation of energy equivalent for BE, 2: Application to residential walls Thomas Thorsell PhD student Mark Bomberg Research Professor 1 1 (reference wall) with MFI, R13 batt ) 2

16 Reference wall (left) and test wall in series 1 before drywall placement 3 3% unfilled corners spacer for MFI batt, facer stapled on side of studs a) For full sized cavity 2 x 2 inch triangle (b) For narrow cavity 1.5 x 1.5 inch triangle 4

17 Effect of air flow is different depending on the path Series 1 no gaskets /sealing to the subfloor or floor joist 5 Reference wall initial tests First wall with window and siding results were unreliable because the test method was under development Second wall window and siding removed, el-outlet remains and no gaskets on the perimeter of the wall Calibrated inlet / outlet introduced for measurement of air flow effects on the energy equivalent R 6

18 Details of test wall preparation RH, T and air pressure sensors at different s WRB has an overlap made in accordance with mfg instructions 7 Air pressure taps on the inner side of the batt 8

19 Airtight connection between tested wall and the chamber 9 Climatic chambers at SU are used for H, A, M, and Pollutant tests 10

20 8 different residential walls were tested but only 2 are reported Results obtained during test method development are not reported Published results represent typical and best achievable on market place energy equivalent s R with a small modifications 4 has continuous, permeable air barrier on inner side and dense pack CFI. There is a small air gap between drywall and AB material. 11 Definitions in this paper in this paper represents the air to air resistance to heat flow. It is an inverse of the U-value. U This definition is different from one used in ASHRAE to avoid several measurements on the wall surface to establish the mean surface temperature when multi- dimensional heat transfer causes large temperature differences. 12

21 Definitions = and Local s R = thermal resistance under steady-state, state, unidirectional heat flow through the center section of the insulation. Local R = a ratio between temperature difference between indoor and outdoor air and heat flux measured at this point (The R of calibrated boundary layer is already subtracted). 13 Energy equivalent that includes effect of multidirectional heat flow and air flow measured under specified conditions. Effect of moisture is not included as it requires performing a HAM model calculations. 14

22 Reference wall, step 1, nominal Cold Warm Temp. Heat App. Local Local side, side, differe flux Rsivaluevalue, (IP Rsi- air air nce W/m 2 units) Top bottom Avg This agrees well with R15.0 h o Fft 2 /Btu obtained as the sum of R13 batt, OSB, drywall and surface film resistance values. 15 Calculation of multidimensional heat flow effect This calculation is performed with the heat flow model (we used two 2-D 2 models to eliminate calculation errors) With 14.3% effect for this geometry (see introductory paper to session 4) one obtains R12.9 h o Fft 2 /Btu 16

23 Measurements in stage 2 (50 Pa) locat. Temp differ. o C Heat flux W/ m 2 App. Rsi, Rsi, Refer. Reduced by % Top Bott Avg % From nominal multi-dimensional flow 14.3% and air flow 14.5 % total 28.8%. Energy equivalent is 15.0 ( ) = 10.7 h o Fft 2 /Btu 17 4, step 1, measured nominal Cold side, air Warm side, air Temp. difference Heat flux W/m 2 Apparent Local Local R- value (IP units) top middle bottom Average Repeated test top middle bottom Average

24 Heat flux measured in C518 apparatus on sealed, wet specimen vs calculated one For moisture characteristics used as input consult other papers 19 Calculated heat flux during the test in climatic chamber caused by moisture Heat flux (conduction) in [W/m2] g 50 HFT_stud HF_whole HFT Time in [h] Maximum, minimum and mean values of heat flux vs testing time 20

25 The test as above but AB material on inner side is also a water vapor retarder Heat flux (conduction) in [W/m2] HF_whole HFT1 HFT_stud Time in [h] With the mean heat flux calculated here, the nominal is 15.9 and the one expected from material testing is 15.8 h o Fft 2 /Btu 21 Effect of air flow and energy equivalent Temp. differ, C Heat flux W/ m 2 Apparent Rsivalue, measured in Step 1 Reduction, percent top mid bott Avg For wall 4 combined effects of thermal bridges and air flows is 18.8 i.e.10% less than for wall 1. Energy equivalent is 15.9 ( ) = 12.8 h o Fft 2 /Btu 22

26 Conclusions Demonstration of the proposed test method show a reduction from the nominal R15 to energy equivalent R10.7 h o Fft 2 /Btu Furthermore, different air flow effect causes a difference about 10% in the energy equivalent R value. The proposed test method identified transient effect of moisture, an important issue in verification of HAM models. 23