iibec - a field study of thermal and hygrothermal performance … · 2019-10-21 · one bethel...

A Field Study of Thermal and HygrothermalPerformance of Attics With Various

Retrofitting Strategies

Ming L Shiao PhD

Sudhir Railkar PhDGAF

1 Campus Drive Parsippany NJ 07054 Phone 973-531-2648 bull E-mail mshiaogafcom

and

William A Miller PhD PEOak Ridge National Laboratory

One Bethel Valley Road Bldg 3147 MS 6070 Oak Ridge TN 37831Phone 865-574-2013 bull E-mail wmlornlgov

B u i l d i n g E n v E l o p E T E c h n o l o g y S y m p o S i u m bull n o v E m B E r 1 3 - 1 4 2 0 1 7 S h i a o a n d m i l l E r bull 1

Abstract

The field performance of energy and ventilation retrofitting strategies in attics was studied in a hot humid climate to reveal their hygrothermal performance and potential net energy savings Attics with building-integrated photovoltaics (BIPV) were included in this study The speakers will discuss observations sheathing temperature and partial pressure measurements to illustrate moisture and heat flow in seven attics with various retrofitting roofing assemblies below or above sheathing insulation unvented or 1300 to 1150 attic ventilation with increasing solar reflectivity with radiant barriers with BIPV or underlay-ment permeability The introduction of BIPV over roof decks was studied by increasing roof reflectivity for their potential thermal and moisture benefits A cyclic moving of moisture in and out of the depth of the roof was observed when permeable spray foam was applied to the underside of the roof deck The moisture transfers back to the attic air as solar irradiance bears down on the roof

Speakers

Ming L Shiao PhD mdash GAF

MING SHIAO PHD currently holds the position of principal scien-tist with GAF Corporation He has been conducting roofing research and product development for the past 23 years Shiao has authored or coauthored over 20 publications and holds more than 50 US patents He received the 100 RampD Innovation Award in 2012 and the Presentation Excellence Award from ASTM Committee D08 in 2003 Shiao received his PhD in mechanical engineering from the University of Massachusetts Amherst in 1993

William A Miller PhD PE mdash Oak Ridge National Laboratory

WIllIAM MIllERrsquos work experience includes 37 years of research in building science vapor compression refrigeration systems and heat absorption and mass transfer He holds a joint faculty position with the University of Tennessee at Knoxvillersquos College of Engineering and also teaches in its mechanical aeronautical and biomedical department He has worked with the Tile Roof Institute PolyFoam Corporation the Metal Construction Association the Copper Development Association louisiana Pacific Billy Ellis Roofing SPRI and GAF Current work at ORNl focuses on developing and demonstrating high-performance roof and attic systems with the support of Owens Corning

Nonpresenting Coauthor

Sudhir Railkar PhD mdash GAF

2 bull S h i a o a n d m i l l E r B u i l d i n g E n v E l o p E T E c h n o l o g y S y m p o S i u m bull n o v E m B E r 1 3 - 1 4 2 0 1 7



aBSTRaCT The field performances of attics with

various insulation and ventilation retrofit-ting strategies were studied in a hot humid climate to reveal their hygrothermal perfor-mance and potential net energy savings In addition attics having roof-integrated photovoltaics (RIPV) were also included in this study A field testing facility consisting of seven attics configured with various roof-ing assemblies including below or above sheathing insulation unvented or 1300 to 1150 attic ventilation increasing solar reflectivity RIPV or metal shingles were constructed in ASHRAE Zone 3 These attics were instrumented and monitored over a two-year period to collect data on the thermal moisture and heat transfer across roof decks and through attic floors These results were then compared and contrasted with a control attic having a code-compliant attic configuration to reveal the best- and worst-performing attic retrofitting strate-gies

The introduction of RIPV over roof decks was also studied by increasing roof reflec-tivity and adding an air cavity underneath the PV for potential thermal and moisture benefits The result revealed that the lowest heat flux into HVAC conditioned space can be achieved by combining above-sheathing insulation and ventilation whereas the application of spray foam underneath the roof deck does not appear to be the most energy-efficient case A cyclic moving of moisture in and out of the depth of the roof was observed when permeable spray foam was applied to the underside of the roof deck The moisture transfers back to the attic air as solar irradiance bears down on the roof These findings were further ana-lyzed by hygrothermal simulations using WUFIreg to provide practical recommenda-tions for attic retrofitting strategies across various ASHRAE climates

INTRODUC TION For low-rise residential dwellings in

North America attics are commonly used

to manage heat and moisture buildups between the roof assembly and the condi-tioned space1-2 Typically attic ventilations and attic insulations are the major strate-gies to manage the atticrsquos thermal and hygrothermal performance as part of the overall building envelope design They have been proven effective and their performance has been extensively studied3-6 As the building envelope design advances towards higher energy and airtight performance challenges regarding these common prac-tices have been raised and new methods for managing attic thermal and moisture problems have been proposed For example sealed attics via spray foam applied directly to the underside of the roof sheathing have been gaining significant traction as an energy-saving alternative over the more traditional vented attics7-8 Although they may provide potential energy savings some performance concernsmdashparticularly the moisture buildup in roof decksmdashhave been discovered9-13 As a result a new solution to address this issue by venting moisture at roof deck level has also been proposed1014-16

Although this may help to prevent moisture buildup the heat flux through attic ceiling and into conditioned space for an open-cell and a closed-cell spray foam insulation has been found to be higher than typical venti-lated attics having a code-compliant level of insulation placed on the attic floor13 As a result the question of potential energy sav-ings on sealed attics still remains Also for sealed attics using closed-cell foam insula-tion below the roof deck their thermal and hygrothermal performances are yet to be fully understood

As we strive to achieve the net-zero energy goal the deployment of rooftop solar panels has been steadily climbing and new types of solar panels the so-called RIPVs are becoming a popular choice due to their aesthetics and integration as part of the roofing system In this case the roof-ing materials or shingles are replaced by PV arrays that are either directly applied to the roof deck or have some air gaps

that are designed mostly for managing power electronics Although there have been studies on their effects over rooftop tem-peratures17-18 their impact on overall attic performance is not yet fully understood For instance their effect on attic moisture behavior and their impact on additional lev-els of attic ventilation have yet to be studied In addition the use of roofing materials with higher solar reflectance to reduce the HVAC loading has been proven effective and has been adopted into building code in some parts of the country When combined with RIPV however it is unclear how they may impact the overall energy and moisture per-formance of the roofing assembly As RIPV installation increasesmdashespecially in areas such as California where a cool roof is also popularmdashthis remains an area of interest to homeowners or roofing contractors who may seek a sound deep energy retrofitting strategy

Besides these new considerations other means of combating thermal challenges in the built environment such as radiant bar-riers above-sheathing ventilation or the combination of above-sheathing insulation and ventilation have been proposed19-22

Consequently a wholistic study of the over-all attic performance regarding these newer roofing and attic conponents would be needed especially one that monitors their real field performance and collects data to build meaningful simulations so that their combined effects can be modeled and performance can be anticipated in different climate zones

In this study we examined the thermal and hygrothermal aspects of some of these attic strategies by constructing full-scale test attics and monitoring their performance over a two-year period This paper is part of a larger study Results of the effects of sealed attics using open-cell spray foams13

the use of cool shingles13 strategies for underlayment breathability23 and the ben-efits of an insulated vented nail base (IVNB) system22 have already been published In this paper we expand the scope of sealed


Figure 1 ndash The southern exposure of the NET Facility in Charleston SC

attic strategies to include both open- and closed-cell spray foams such that their potential energy and moisture performance can be compared to other attic construc-tions including the IVNB system We also investigated the effects of RIPV with and without air cavities over roofing assemblies that have different levels of solar reflectivity as well as the effects of metal shingles Each construction was compared to the perfor-mance of a typical 1300 vented insulated attic WUFI simulations were then conduct-ed to measure against these field data and to predict moisture movements within attic assemblies

ATTIC CONFIGURATION AND CONSTRUCTION

This field study was conducted in South Carolina using the existing test facility known as Natural Exposure Test Facility (see Figure 1) located in ASHRAE Climate Zone 3 The facility consists of a building approximately 25 x 80 ft (76 x 244 m) having attic space and a pitched roof at 312 slope in a northsouth orientation

The attic and the roof deck were subse-quently subdivided into seven bays for vari-ous atticroof configuration studies Each bay is approximately 11 ft (34 m) in width and is separated by barrier walls thermally insulated to about R-15 The barrier walls were air-sealed using caulk and spray-foam sealants

These bays were then configured with combinations of attic ventilation permeable and impermeable underlayment above-sheathing ventilation metal shingles and asphalt shingles with varying solar reflec-tance to study their effects on attic per-formance In addition two types of RIPV were added on the south-facing roofs of two adjacent bays to study their effects The space below the attic floors was conditioned by an air-to-air heat pump that maintained a constant temperature of 70ordmF (21ordmC) A schematic of the attic assemblies is shown in Figure 2 Configurations for each attic roof system are listed in Table 1 In this study no additional moisture sources were introduced in the attic to simulate moisture accumulation in attics due to human activi-

Figure 2A ndash Test attic assembly layout at NET

ties it was done in a previous study13 where humidifiers were used in the attic to match up with the moisture-loading measured from occupied houses in the same region

Attic 1 was the control case having standard attic construction with 1300 soffitridge ventilation and R-38 batt insu-lation over the attic floor The roof deck had a permeable underlayment (16 perms) and was covered in dark-colored asphalt shingles having a solar reflectance of 003 The soffit and ridge openings were of equal area to match up to 1300 attic ventilations

The attics in Attics 2 and 3 were sealed with spray foam insulation applied to the underside of the roof decks to a thickness covering the roof rafters in order to achieve similar R-value between the two sealed attics Attic 2 was constructed with 2 x 6 rafters About 6 in of open-cell spray foam was applied from the underside of the roof deck to about frac12 in past the underside of the rafter to achieve an R-value of approxi-mately R-20 to R-23i Attic 3 had closed-cell spray foam applied to a thickness ~4

in from the underside of the roof deck and covering roof rafters of 2 x 4 construction with about frac12 in of closed-cell foam the resulting R-value for a total of 40 in of foam rang-es from R-28 to R-20 (dependent on foam aging)ii and is close to the R-value applied to the open cell case

Attic 4 had a small nonactive RIPV array representing photovol-taic panels that were


Attic Attic Attic Roof Deck Shingle Solar No Insulation Ventilation Underlayment Configuration Reflectance

1 R-38 on attic floor 1300 soffit-ridge Synthetic breathable Asphalt shingles 003 ventilation film (16 perm)

2 60rdquo open-cell spray foam on roof sheathing

None 15-lb felt (8 perm) Asphalt shingles 003

3 40rdquo closed-cell spray foam on roof sheathing

None Synthetic non-breathable (004 perm)

Asphalt shingles 003

4 R-38 on attic floor 1300 soffit-ridge Synthetic non-breathable Asphalt shingles 028 ventilation (004 perm) RIPV on south deck

5 R-38 on attic floor 1300 soffit-ridge Peel-and-stick Asphalt shingles 003 ventilation non-breathable RIPV on south deck

6 R-38 on attic floor 1300 soffit-ridge ventilation

15-lb felt (8 perm) IVNB (1100) amp asphalt shingles

003

7 R-38 on attic floor 1150 intake-ridge ventilation

30-lb felt (5 perm) Metal shingles ~010

Table 1 ndash Attic configuration summary

directly applied to the roof deck with little or no air gap This was to compare to another type of RIPV in Attic 5 where the PV panels were attached to the roof deck with a ~2-in air gap between the PV cell and the roof deck Again both RIPVs were nonactive so no extra heat was generated by the PV cells Furthermore asphalt shingles with increased solar reflectance were used to study the effects of cool roofs in combination with the RIPV array

In Attic 6 the roof deck con-struction was an IVNB system deck where extra above-sheathing venti-lation (1100) and an R-5 insula-tion were introduced This type of system was the subject of earlier studies22 23 In this paper we will only cover the relative comparisons to other attics that were not part of

Figure 2B ndash Instrumentation layout for each attic in the ORNL NET

the results from previous study In Attic 7 granulated metal shingles

were used as the roofing material and also the attic ventilation was increased to 1150 using a fascia intake vent For the underlayment different levels of perme-abilitymdashfrom completely non-permeable to breathable underlaymentmdashwere used and their results were studied and reported by Railkar et al23 which showed that under-

Specimen Thickness

(in)

Density

(lbfft3)

Apparent Thermal Conductivity

(Btu-in)(ft2 middothrmiddotdegF)

Thermal Resistance

(hrmiddotft2middotdegF)Btu

13-3411 12000 053 03056 393

13-3422 12001 053 03417 351

13-3423 12000 054 03448 348

13-3423 12000 054 03448 348

layment with higher permeability can help 1 in = 00254 m 1 lbfft3 = 1602 kgm3 1 (Btu-in)(ft2middothrmiddotdegF) = 1442 W(mmiddotK)to reduce condensation potential during 1 (hrmiddotft2middotdegF)Btu = 67 (m2middotK)Watt

winter Table 2 ndash Calibration of fiberglass batt insulation used as guard for the ceiling heat flow of Attic 1


Figure 3 ndash The summer temperature profile across roofing assembly in Attic 1

ControlReference Attic

bull frac12-in gypsum board

bull R-38 batt insulation

bull Pitched attic vented air space (1300 ratio at soffit and ridge)

bull Plywood roof deck

bull Breathable underlayment (16 perm)

bull Dark asphalt shingles


Attic 5





bull Peel-and-stick nonbreathable underlayment

bull Nonactive RIPV



Attic 4





bull Synthetic non-breathable underlay-ment

bull Nonactive RIPV


THE INSTRUMENTATION Temperature sensors relative humidity

(RH) sensors and heat flux transducers (HFT) were attached at pertinent locations to analyze the in-situ performance of the individual attic bays The sensors were placed in an identical pattern for each attic cavity as shown in Figure 2B to standardize the comparison of results For Attic 6 with the IVNB system the temperature sensor locations were the same except additional temperature and RH sensors were added in the air gap within the IVNB system

Thermistor and humidity sensors were calibrated by ORNlrsquos Metrology lab The response of the thermistors was measured at the following nominal conditions 158 185 212 239 and 266degC All probes met the manufacturerrsquos specification for the temperature response of +-02degC Humidity sensors were checked at 25 50 75 and 90 RH The error in RH ranged from 2 of the reading at 25 RH and 15degC to 65 of the reading at 90 RH and 26degC

All heat flux transducers were calibrated in accordance with ASTM C518-10 (ASTM 2010) by RampD Services from Cookeville TN They used a Fox Heat Flow (FHF) meter having a 0305- by 0305-m footprint to measure the thermal resistance of batt insulation insulating the floor of Attic 1 as shown in Table 2 Each HFT was placed in a 0305- by 0305-m (30- by 30-in) guard made of gypsum board and insulation sam-ples from the same lot of batt insulation and were calibrated in the FHF meter

RESULTS AND DISCUSSIONS Case 1 Attics With RIPV

The temperature profiles across the roof deck for attics having RIPV on the south-facing side during the summer season are shown in Figure 4 for Attic 5 and in Figure 5 for Attic 4 This is to compare to the control case of Attic 1 shown in Figure 3 As can be seen Attic 5 (Figure 4) which has inactive RIPV and traditional dark-colored shin-gles showed higher rooftop temperatures at about 5~8ordmC higher than the control case of Attic 1 However we noticed that the temperatures at underlayment and at the sheathing side of Attic 5 actually showed slightly lower temperatures which resulted in lowered heat flux through the roof deck for Attic 5 (see Figure 6A) as compared to the control case of Attic 1 This is due to the fact that this particular RIPV system has an air gap of ~2 in between the solar panel

and the underlayment which may provide some level of above-sheathing ventilation thereby reducing the heat flux into the attic space Please note that the RIPV system in this study was not active during the study and hence no extra heat was generated by the PV as part of the power generation Therefore it is not clear yet if this additional air gap can provide enough heat reduction that would be comparable to the standard

Figure 6A ndash The roof temperatures and roof deck heat flux comparisons for Attics 1 4 and 5 during summer

Figure 6B ndash The roof temperatures and roof deck heat flux comparison for Attics 1 4 and 5 during winter

attic without the PV array Nevertheless the use of an effective air gap between the PV panel and the deck in reducing the heat passing through the roof deck could be a good strategy to consider for reducing heat loading when introducing RIPV to the exist-ing roof deck

On the other hand Attic 4 with direct-to-deck RIPV having no air gap and coupled with shingles having higher solar reflec-


Figure 7 ndash Percent RH comparison for Attics 1 4 and 5 during the winter season

Figure 8 ndash The specific humidity comparisons during the winter period for Attics 1-5

tance was found to have slightly lowered roof surface tem-peratures by about 3 to 5degC as well as the lowest sheathing and attic air temperatures (Figure 5) This combination also resulted in the lowest heat flux through the roof deck (Figure 6A) which is about 25 to 30 drop in peak heat flux as compared to the control Attic 1 This reduction in the heat flux is similar in magnitude to the case of attics with cool shingles that have about 25 solar reflectance Hence the use of shingles having higher reflectance may provide a solution in helping reduce the heat buildup when there is limited or no ventilation between the RIPV and roof deck

The roof sheathing temperatures and heat flux across the roof decks during the winter period are shown in Figure 6B for the comparison of Attics 4 5 and the control case of Attic 1 Again we noticed a similar trend in lowering peak sheath-ing temperatures in Attics 4 and 5 whereas the control Attic 1 was found to have higher solar heat gain during the daytime hours but higher heat loss during the nighttime radiative cooling In this case the nighttime heating penalty due to the heat loss may be cancelled out by the solar heat gain in the daytime hours of winter and therefore the overall thermal performance of Attic 1 is similar to Attics 4 and 5 having less heat flux through the roof deck On the other hand for their moisture performance (shown in Figure 7) during the cold months the data indicated that the attics with RIPV systems showed a slightly lowered percent RH (RH) in the attic air as compared to the control attic By taking out the temperature influence and calculating the specific humidity (Figure 8) for these three attics the actual moisture contents were found to be about the same This is expected since all of them have the same attic ventilation strategy In previous studies we have found that underlayment breathability could be a useful strategy for moisture management in attics having insufficient ventilation especially during the winter months23 Similarly previous results showed that attic moisture accumulation may be reduced by increasing attic ventilation beyond 1300 when the impermeable underlayment was used

Attic 7



bull Pitched attic vented air space (1150 ratio at soffit fascia intake and ridge)


bull 30 felt

bull Metal shingles

Figure 9 ndash Temperature profile across the roof deck in Attic 7 during summer


Case 2 Attic With Metal Shingles The temperature profile across the roof deck for Attic

7 which was roofed with metal shingles having a solar reflectance of 010 is shown in Figure 9 As can be seen the shingle surface temperatures showed a significantly lower peak temperature as compared to the control of Attic 1 during the summer period (Figure 3) and was even lower than the shingle surface temperatures of Attic 4 (with shingles having higher solar reflectance) However as we move from the top shingle surface and into the attic the temperature profile in Attic 7 showed only 4 to 5degC drop from the shingle surface to the underlayment whereas the control Attic 1 showed a higher drop of ~8degC in peak temperatures This was somewhat unexpected since it has been suggested that metal shingles typically have some level of air gap and may provide some venti-lation benefits due to their raised shape One potential explanation may be the fact that the metal shinglesrsquo underside has lower thermal emittance and hence lower capacity to re-emit the heat back into the air Therefore the heat may be retained The attic air temperatures in Attic 7 were found to be slightly higher than those of Attic 4 with cool shingles The amount of heat influx via the roof deck was also slightly higher in Attic 7 (Figure 10A) but when compared to the control case the attic with metal shingles appeared to have significantly better thermal performance with a potential of ~20 reduction in heat flux into the attic during the summer season This may be due in part to the fact that Attic 7 also has increased ventilation from 1300 to 1150

During the winter period the data shown in Figure 10B indicated that the thermal behavior of Attic 7 was similar to that of the attic with a cool roof Attic 4 As a result the solar heat gain during the winter periods was also found to be reduced for metal shingles This may raise the con-cern for potential moisture performance since the solar drying power was reduced However the moisture data collected during the winter and summer showed similar percents of RH in the attic air and moisture contents in the roof decks were also found to be comparable to that of the control case (Figure 11) Again this may be due in part to the fact that Attic 7 has increased ventilation (1150) compared to the control attic (1300) More study may be needed in order to understand its hygrothermal perfor-mancemdashparticularly in colder regions in order to prevent potential moisture problems during winter

Case 3 Unvented Attics With Open- and Closed-Cell Spray Foams

In a previous study13 we have shown that sealing an attic with open spray foam applied to the underside of roof sheathing results in significant moisture buildup in the attic air or roof deck as well as higher heat flux through the attic floor when compared to the control attic having 1300 ventilation and attic floor insulation In this study we conducted further investigations on the sealed attic using spray foams in both open (Attic 2) and closed cell (Attic 3) Their thermal performance is summarized

Figure 10A ndash Roof deck heat flux comparison between Attics 1 4 and 7 during the summer

Figure 10B ndash Heat flux through roof decks for Attics 1 4 and 7 during winter

Figure 11 ndash Percent RH for the roofing assembly in Attic 7


Figure 12A ndash Roof deck thermal performance for Attics 1 2 3 and 6 during summer

Figure 12B ndash Thermal performance of the roof deck in Attics 1 2 3 and 6 during winter

ed in the highest heat reduction among all attics studied Furthermore when we exam-ined the heat flux through the ceiling and into the living space underneath the data in Figure 13A clearly showed that both open-cell and closed-cell spray foams have the

highest heat gain dur-ing summer and worst heat loss during win-ter periods (see Figure 13B) Again this result was consistent with previous studies where only the open-cell spray foam was tested In this study the open-cell foam insulation was found to have slightly worse thermal perfor-mance than the closed-cell foam but the dif-ference was relatively minor One should note that in the IVNB sys-tem where the above-sheathing ventilation and insulation were added to the roof deck (in addition to standard 1300 attic ventilation) the heat flux data in Figure 12A and Figure 12B clearly demon-strated the exceptional thermal performance in blocking heat gain during the summer and the heat loss during the winter It is there-fore suggested as one of the best approaches for deep energy retro-fitting which can be easily adapted during a reroofing project where the roof deck is readily accessible

For the moisture performance of the sealed attics the RH in the attic air was plot-ted Figure 14A shows the summer num-bers and Figure 14B the winter As can be seen in Figure 14A for Attic 2 with open-cell spray foam the data

in Figure 12A for the summer period and in Figure 12B for the winter As one can see both the open-cell and closed-cell spray foams showed similar heat flux via the roof deck with the open-cell foam being slightly lower in peak heat flux during the daytime

Comparing to the control the spray-foam insulation did show a significant heat reduction of about 80 during the peak hours of the day However we noticed that Attic 6 with the IVNB system had even lower heat flux through the roof deck and result-

1 0 bull S h i a o a n d m i l l E r B u i l d i n g E n v E l o p E T E c h n o l o g y S y m p o S i u m bull n o v E m B E r 1 3 - 1 4 2 0 1 7

indicated that a significantly higher RH is building up in the attic during sum-mer particularly during the late afternoon hours This may raise concern for potential mold growth which is more sensitive to the RH This may be further analyzed using the WUFI Pro Mold Index add-on For the closed-cell foam case the RH level was found to be more moder-ate compared to the control attic but again the buildup of moisture during evening hours was observed

By calculating the specif-ic humidity we noticed that the specific humidity level in both Attics 2 and 3 was similar to those of the control attic (see Figure 15A) with the slightly elevated moisture level noted in Attic 2 for the open-cell foam Since we did not introduce moisture load-ing into these sealed atticsmdash unlike our previous study where moisture was added to simulate the attic moisture accumulation for occupied housesmdashwe have not seen very high levels of moisture accumulation in the roof deck (Figure 15B) except that the data revealed a much higher degree of daily cyclic swing between moisture absorption and desorption during day and night times

By plotting the RH data collected at the spray-foam level for Attic 2 as shown in Figures 14A and 14B we observed that moisture in these sealed attics moved from the attic into the roof assembly during the night-time and then back into the attic during daytime hours particularly during the sum-mer period which resulted in the observed lag on peak RH behind the control attic This can be confirmed by examining the vapor pressure differentials between the roof deck the insulations and the attic air (see Figure 16) Here we can clearly see that

the moisture in the sealed attics was driven cell spray foam than the closed-cell foam out of the roof deck during the daytime because of the difference in permeance of hours and accumulated in the attic space open-cell foam as compared to closed-cell during the late afternoonevening hours foam The magnitude of this moisture transport In the winter months results for the was found to be much higher for the open- sealed attic shown in Figure 14B indicated

Figure 13A ndash Comparison of heat flux via attic floors in all attic assemblies during summer

Figure 13B ndash Heat flux through ceiling and into living space for all attics in the study during winter

B u i l d i n g E n v E l o p E T E c h n o l o g y S y m p o S i u m bull n o v E m B E r 1 3 - 1 4 2 0 1 7 S h i a o a n d m i l l E r bull 1 1

that the RH was relatively moderate com-pared to the control attic and that there was no sign of cyclic moisture movement possibly due to the lack of solar heat drive during the winter By examining the RH at the spray-foam location we did observe higher RH and the trend of lagging behind the control attic which may suggest that

moisture could still be moving from the roof assembly and into the attic air but with a much smaller magnitude On the other hand the specific humidity shown in Figure 8 revealed that the moisture content in the sealed attics was actually higher than those of the control Attic 1 whereas the open-cell foam appeared to be running slightly higher

Figure 14A ndash RH comparison between Attics 1 2 and 3 during summer

Figure 14B ndash RH comparison in attic air between Attics 1 2 and 3 during winter

than the closed-cell foam Due to relatively moderate attic air and joist temperatures in the sealed attics the potential for conden-sation may not be an issue however the potential accumulation of moisture content over longer periods of time may need to be studied further This will be discussed in the next section via the WUFI simulation

ANNUAL MOISTURE FLOWS AND WUFI SIMULATIONS

Moisture pin (MP) mea-surements are again shown in Figure 17A to better assess and understand the flow of moisture to and from the roof deck over the full two years of the field study The MP data is corrected for temperature effects (see Miller24) The MP data for the two sealed attics is the same data as shown in Figure 17A

As stated previously the movement of moisture in the roof decks of the two sealed attics is very similar in val-ues and trends During the heat of the day the mois-ture content of the sheathing drops which is likely due to the movement of the moisture from the sheathing into the foam and attic air As the sun sets the roof cools and mois-ture is reabsorbed from the attic air and the foam back into the roof deck During the course of study the mois-ture content of all roof decks dropped slightly during the hot summer months and rose slightly during the colder win-ter months The movement is cyclical and shows that water accumulation in the roof deck is highest during the winter while during the summer the decks all dry somewhat Also the moisture content for all roof systems in this study was well below 20 Viitanen25

reported that fungus require a moisture content (MC) of 25-28 for growth Therefore this study shows there is only a small potential for mold


growth for the attic configurations that were tested However it should be men-tioned that we did not introduce additional moisture loading into the attic to simulate occupied home conditions during the two- year field study

Previously Miller24 had reported and shown evidence of moisture damage to the plywood sheath-ing of an attic sealed with open-cell spray foam After the study was completed the roof shingles and underlay-ment were removed to verify workmanship of the roof A Delmhorst meter was used to measure the moisture content on the topside of the bare ply-wood deck All meter readings showed both the south- and north-facing decks were dry Wood resistance was too high for the Delmhorst to yield a measurement However the Delmhorst showed 10 to 14 moisture content at the abut-ting of two 4 x 8 plywood sheets where the plywood is nailed to the roof rafters Miller24 did observe water damage to a small section of the north-facing roof deck and observed water marks on the underside of the roof sheath-ing after foam was removed for inspection of the sheathing The inspection was conducted during early summer when moisture would be driven from the sheathing into the foam The data and observations are somewhat confounding The MP measurements are made into a 05-in depth of the wood and represent the aver-age MC over this depth of wood Also it is very feasible that the plywood had an initial defect when installed on Attic 1 WUFI 1D simulations were made using the measured roof deck and attic air tempera-tures to check the field mea-surements

Simulations were con-ducted using WUFI PRO v61 to estimate the water content of the OSB roof sheathing

Spray foam has a moisture capacity of only 033 kg per cubic meter of foam while ply-wood at about 60 RH has a hygroscopic storage capacity of 48 kg per cubic meter of wood The wood is far more absorbent than the foam and as the sun drives moisture from the wood into the foam a moisture

layer forms at the wood-foam interface Therefore absolute humidity probes were installed on the topside of the plywood the underside of the plywood and midway through the depth of a new application of open-cell spray foam (ocSPF) and at the foamrsquos surface adjacent to the attic air The

Figure 15A ndash Specific humidity in attic air for Attics 1 2 and 3 during summer

Figure 15B ndash Measured moisture content in north-facing roof deck for Attics 1 2 and 3 during winter


Figure 16 ndash Vapor pressure differential between the roof deck and the attic during summer

topside measurements and the measure-ments made at the foamrsquos surface were used as input boundary conditions for the WUFI tool to estimate the change in water content of the sheathing Results in Figure 17B show the sheathing sealed with ocSPF to have the higher water accumulation from January to March 2015 However during the summer months there is little differ-ence between the sheathing of the ventilated attic and that of the sheathing with ocSPF The permeance of the closed-cell spray foam (ccSPF) insulation clearly shows it retarded the release of moisture during the winter of 2014 but also allowed less moisture absorption and desorption during the 2015 portion of the study Hence the ccSPF can better protect the sheathing from moisture accumulation

SUMMARY AND CONCLUSIONS The field performance of attics with dif-

ferent ventilation strategies sealed attics with open- or closed-cell foam an attic with a metal roof or attics having RIPV with and without air gaps and coupled with cool shingles were studied These attics were constructed and instrumented in a hot humid climate to collect data over a two-year period to understand their perfor-mance The results showed that the insu-lated ventilated nail-base system had the best thermal performance during summer or winter seasons by effectively blocking heat flux through the roof deck and the

1 4 bull S h i a o a n d m i l l E r

attic floor On the other hand although the sealed attics having spray-foam insula-tion underneath the roof deck showed sig-nificantly reduced heat flux into the attics they were found to have the highest heat flux through the ceiling and into the con-ditioned space The data also showed that the closed-cell foam insulation performed slightly better in reducing heat flux into the attic For the attic with RIPV the introduc-tion of an air gap between PV and the roof deck was found to reduce the heat gain into the attic space whereas similar thermal performance can be achieved by combining RIPV shingles with higher solar reflectance when no air gap is available

The effect of underlayment breathability to the hygrothermal performance of attics was studied by WUFI simulations Results showed that the low permeance of closed-cell spray foam insulation limits the accu-mulation of water as compared to sheathing sealed by open-cell spray foam insulation or sheathing exposed to attic ventilation The open-cell spray foam insulation showed a wintertime accumulation that exceeds that of sheathing exposed to attic ventilation but during summer months there is no distinguishable difference between the two applications

REFERENCES 1 FB Rowley CE Algren and CE

lund 1939 ldquoCondensation of Moisture and its Relation to Building

B u i l d i n g E n v E l o p E T E c h n o l o g y

Construction and Operationrdquo ASHVE Transactions 44 1115

2 J lstiburek 2006 ldquoUnderstanding Attic Ventilationrdquo Building Science Digest Westford MA

3 ASHRAE 2017 ASHRAE Handbook ndash Fundamentals American Society of Heating Refrigerating and Air-Conditioning Engineers Inc Atlanta GA

4 A TenWolde and C Carll 1992 ldquoEffect of Cavity Ventilation on Moisture in Walls and Roofsrdquo Thermal Performance of Exterior Envelopes of Building V ASHRAE

5 DS Parker 2005 ldquoliterature Review of the Impact and Need for Attic Ventilation in Florida Homesrdquo Florida Solar Energy Center

6 KE Wilkes and Jl Rucker 1983 ldquoThermal Performance of Residential Attic Insulationrdquo Energy and Buildings 5

7 A Grin J Smegal and J lstiburek 2013 ldquoApplication of Spray Foam Insulation Under Plywood and Oriented Strand Board Roof Sheathingrdquo Building Science Corporation

8 D Chasar VV VonSchramn J Sherwin and S Chandra 2010 ldquoMeasured Performance of Side-by-Side South Texas Homesrdquo ASHRAE Thermal Performance of the Exterior Envelopes of Whole Building XI International Conference Clearwater Fl

9 J Straube R Smith and G Finch 2009 ldquoSpray Polyurethane Foam The Need for Vapor Retarders in Above-Grade Wallsrdquo Canadian Urethane Foam Contractors Association

10 J lstiburek 2015 ldquoVenting Vaporrdquo ASHRAE Journal August pp 46-51

11 D Derome January 2005 ldquoMoisture Accumulation in Cellulose Insulation Caused by Air leakage in Flat Wood Frame Roofsrdquo Journal of Building Physics Vol 28 No 3 pp 269-287

12 D Prahl D and M Shaffer 2014 ldquoMoisture Risk in Unvented Attics Due to Air leakage Pathsrdquo Oak Ridge TN Department of Energy

13 A Desjarlais W Miller S Railkar and A Chich 2012 ldquoEnergy and Moisture Performance of Attic Assembliesrdquo Proceedings of the RCI Symposium on Building Envelope

S y m p o S i u m bull n o v E m B E r 1 3 - 1 4 2 0 1 7

Technology Phoenix AZ 14 P Boudreaux R Jackson and S

Pallin 2013 ldquoMoisture Performance of Sealed Attics in the Mixed-Humid Climaterdquo Oak Ridge TN Oak Ridge National lab ORNlTM2013-525

15 P Roppel N Norris and M lawton 2013 ldquoHighly Insulated Ventilated Wood-Framed Attics in Cool Marine Climatesrdquo Thermal Performance of the Exterior Envelopes of Whole Buildings XII International Conference Clearwater Fl

16 Kohta Ueno and Joseph lstiburek 2015 ldquoField Testing Unvented Roofs with Asphalt Shingles in Cold and Hot-Humid Climatesrdquo Westford MA Building America Report - 1409 Building Science Corporation

17 MD Bazilian H Kamalanathan and DK Prasad 2002 ldquoThermographic Analysis of a Building Integrated Photovoltaic Systemrdquo Renewable Energy V 26 pp 449-461

18 S Pantic l Candanedo and AK Athienitis 2010 ldquoModeling of Energy Performance of a House With Three Configurations of Building-integrated PhotovoltaicThermal Systemsrdquo Energy and Buildings V 42 pp 1779-1789

19 N McNabb 2013 ldquoStrategies to Achieve Net-Zero Energy Homes A Framework for Future Guidelinesrdquo Washington DC National Institute of Standards and Technology Special Publication 1140

20 W Miller M Keyhani T Stovall and A Youngquist 2007 ldquoNatural Convection Heat Transfer in Roofs with Above-Sheathing Ventilationrdquo ASHRAE Building X

21 William A Miller and Ethan Herman 2012 ldquoThe Tradeoff Between Solar Reflectance and Above-Sheathing Ventilation for Metal Roofs on Residential and Commercial Buildingsrdquo Oak Ridge TN ORNl

22 Ml Shiao S Railkar WA Miller and A Desjarlais 2016 ldquoField Study on the Thermal and Hygrothermal Performance of Insulated Ventilated Nail Base Systemsrdquo Thermal Performance of the Exterior Envelopes of Whole Buildings XIII ASHRAE

23 S Railkar Ml Shiao A Desjarlais and WA Miller 2015 ldquoThermal and Hygrothermal Performance of

Ventilated Attics With and Without FooTNoTES Breathable Underlaymentsrdquo BEST-4 i 2013 ASHRAE Fundamentals Table Conference Kansas City April 2015 1 pg 268 Open-cell spray foam

24 WA Miller S Railkar Ml Shiao has thermal conductivity range of and A Desjarlais 2016 ldquoSealed 026 to 029 Btu-in(hrmiddotft2middotdegF) Attics Exposed to Two Years of ii 2013 ASHRAE Fundamentals Table Weathering in a Hot and Humid 1 pg 268 Closed-cell spray foam Climaterdquo Thermal Performance of the aged for 180 days has thermal con-Exterior Envelopes of Whole Buildings ductivity range of 014 to 020 Btu-XIII ASHRAE in(hrmiddotft2middotdegF)

Figure 17A ndash Moisture content of the north-facing roof sheathing for Attic 1 (control) and Attic 2 and Attic 3 sealed with spray foam insulation

Figure 17B ndash Simulations using WUFI Pro v61 to estimate the moisture content of the roof sheathing over the full two-year study for Attic 1 (control) and Attic 2 and Attic 3 sealed with spray-foam insulation


Abstract

The field performance of energy and ventilation retrofitting strategies in attics was studied in a hot humid climate to reveal their hygrothermal performance and potential net energy savings Attics with building-integrated photovoltaics (BIPV) were included in this study The speakers will discuss observations sheathing temperature and partial pressure measurements to illustrate moisture and heat flow in seven attics with various retrofitting roofing assemblies below or above sheathing insulation unvented or 1300 to 1150 attic ventilation with increasing solar reflectivity with radiant barriers with BIPV or underlay-ment permeability The introduction of BIPV over roof decks was studied by increasing roof reflectivity for their potential thermal and moisture benefits A cyclic moving of moisture in and out of the depth of the roof was observed when permeable spray foam was applied to the underside of the roof deck The moisture transfers back to the attic air as solar irradiance bears down on the roof

Speakers

Ming L Shiao PhD mdash GAF

MING SHIAO PHD currently holds the position of principal scien-tist with GAF Corporation He has been conducting roofing research and product development for the past 23 years Shiao has authored or coauthored over 20 publications and holds more than 50 US patents He received the 100 RampD Innovation Award in 2012 and the Presentation Excellence Award from ASTM Committee D08 in 2003 Shiao received his PhD in mechanical engineering from the University of Massachusetts Amherst in 1993

William A Miller PhD PE mdash Oak Ridge National Laboratory

WIllIAM MIllERrsquos work experience includes 37 years of research in building science vapor compression refrigeration systems and heat absorption and mass transfer He holds a joint faculty position with the University of Tennessee at Knoxvillersquos College of Engineering and also teaches in its mechanical aeronautical and biomedical department He has worked with the Tile Roof Institute PolyFoam Corporation the Metal Construction Association the Copper Development Association louisiana Pacific Billy Ellis Roofing SPRI and GAF Current work at ORNl focuses on developing and demonstrating high-performance roof and attic systems with the support of Owens Corning

Nonpresenting Coauthor

Sudhir Railkar PhD mdash GAF










































003









Specimen Thickness

(in)

Density

(lbfft3)



Thermal Resistance


13-3411 12000 053 03056 393

13-3422 12001 053 03417 351

13-3423 12000 054 03448 348

13-3423 12000 054 03448 348













Attic 5






bull Nonactive RIPV



Attic 4






bull Nonactive RIPV


















Attic 7





bull 30 felt

bull Metal shingles





























































































































003









Specimen Thickness

(in)

Density

(lbfft3)



Thermal Resistance


13-3411 12000 053 03056 393

13-3422 12001 053 03417 351

13-3423 12000 054 03448 348

13-3423 12000 054 03448 348













Attic 5






bull Nonactive RIPV



Attic 4






bull Nonactive RIPV


















Attic 7





bull 30 felt

bull Metal shingles






























































































Attic 5






bull Nonactive RIPV



Attic 4






bull Nonactive RIPV


















Attic 7





bull 30 felt

bull Metal shingles





































































































Attic 7





bull 30 felt

bull Metal shingles





















































































iibec - a field study of thermal and hygrothermal performance … · 2019-10-21 · one bethel...

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