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A Tale of Two Rating Systems - Edited 2014
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A Tale of Two Rating Systems - Edited 2014

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A comparison study between the NFRC and Passive House Institute rating of windows, using one specific profile as a case study.

A comparison study between the NFRC and Passive House Institute rating of windows, using one specific profile as a case study.

Published in Education , Technology , Business
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  • This was very helpful. Those hidden NFRC values make conversion between systems difficult for consumers like my housing cooperative. So much for just converting units. Much of the time U-values are given without units in US market, leaving us to guess if OPTIWIN just converted units or got NRFC values. If NFRC just provided those 2 hidden model values, then an online converter could be made available to estimate U-value for particular window dimensions and estimate the conversion between systems.

    Why is the National Institute of Standards and Technology, or ASTM, in on this? This deals with fundamentals of materials and testing that are not unique to fenestration.
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  • 1. A Tale of Two Rating Systems: NFRC & PHI window testing protocols Apples to apples? Bronwyn Barry, Assoc. AIA., CPHC First draft presented at: Passive House California Mtg. October 23, 2011 Published: Nov.22, 2011 Updated: Mar.2, 2014
  • 2. Passive House needs good windows Triple glazing: the new normal? They can be part of your heating system - or not! Image 1: Enersign.de, Image 2: passivehausfenster.at , Image 3: optiwin-usa.com
  • 3. Windows have pluses and minuses And are complicated! Essence of the PHPP: Heat Demand = Heat Losses – Heat Gains For windows: ‘Loss’ items • Frames • Glazing spacers • Installation edge ‘Gain’ item • SHGC of glass http://passipedia.passiv.de/passipedia_en/planning/calculating_energy_efficiency/phpp_-_the_passive_house_planning_package
  • 4. How PHPP calculates windows Lf Uf ψspacer ψinstall Af Ug Lg Ag (Ug*Ag) + (Uf*Af) + (ψspacer*Lg) + (ψinstall*Lf) Uw-installed = Aw
  • 5. Two rating systems now on US market: National Fenestration Rating Council: • US based • No minimum standards • Simulation and product test Passive House Institute: • German based • Set performance standards • Simulation only
  • 6. PHI’s certification criteria: 1. Uw </= 0.8 W/m2K (0.14 BTU/hrft2F or 6.7 hrft2F/BTU) 2. Uw installed </= 0.85 W/m2K 3. fRsi (temperature factor at edge of glass) (Varies for other transparent components, including curtain walls, roof domes and skylights. See document: Certification criteria and calculation regulations Passive House Suitable Transparent Components Version 1.0 E, 16. May 2011) Image from ENERsign brochure: http://www.qplus-llc.com/index.php?id=1191&L=
  • 7. NFRC’s certification criteria: Simulation: 1. Uwindow (required) SHGC (required) 3. Air Leakage (optional) 4. Visible Light Transmittance (optional) 2. Verification: 1. Destructive test of window sample 2. Factory inspection Image from NFRC website: http://nfrc.org/fenestrationfacts.aspx
  • 8. My comparison sample: Pazen ENERsign Tilt and Turn Jamb profile This study is based on the simulation results for the ENERsign profile, calculated by others, using the two testing protocols being compared here. Images from ENERsign brochure: http://www.qplus-llc.com/index.php?id=1191&L=
  • 9. How PHI calcs window U-value Uf Ug = U-value glass ψspacer Ug Lg Ag = Area glass Uf = U-value frame Af = Area frame Af Ag Ψspacer = psi L g = Length spacer Aw = Area Window Uw = (Ug*Ag) + (Uf*Af) + (ψspacer*Lg) Aw
  • 10. PHI Therm file protocols: Boundary Condition: Temp: 14 degF (-10degC) Film Coeff: (Rs) 4.35 BTU/hr.ft2F or 0.23 hr.ft2F/BTU (24.69 W/m2K) U-factor Surface: none Ug Boundary Condition: Temp: 68 degF (20 degC) Film Coeff: (Rs) 1.35 BTU/hr.ft2F or 0.74 hr.ft2F/BTU (7.67 W/m2K) U-factor Surface: Interior Uf Boundary Condition: Temp: 68 degF (20 degC) Film Coeff: (Rs) 1.35 BTU/hr.ft2F (7.67 W/m2K) U-factor Surface: Frame * This page verified by Passive House Academy
  • 11. PHI also calculates FRsi: Exterior: Boundary Condition: Temp: 23 degF (-5degC) Film Coeff: (Rs) 4.35 BTU/hr.ft2F (24.7 W/m2K) (Cursor placed at inside edge of glass and frame) Interior: Boundary Condition: Temp: 68 degF (20 degC) Film Coeff: (Rs) 1.42 BTU/hr.ft2F (4 W/m2K) * This page awaiting verification from PHI – TO BE CONFIRMED!
  • 12. And Uw – Installed: Lf Uf ψspacer ψinstall Af Ug Lg Ag (Ug*Ag) + (Uf*Af) + (ψspacer*Lg) + (ψinstall*Lf) Uw-installed = Aw
  • 13. PHI Uw testing values: Ug = 0.7 W/m2K Ag = 1.32m2 Uf = 0.675 W/m2K Af = 0.50m2 Ψspacer = 0.0325 W/mK L spacer = 4.62m Aw = 1.82m2 Uf total = (3*Uf jmb+hdr + Uf sill) = (3*0.64 + 0.78) = 0.675 W/m2K 4 Image from: http://passivehouse.com/07_eng/01_dph/HerstLi_e/01Bauw_e/Fenst_e/Daten/D_Pazen_ENERsign.pdf 4
  • 14. PHI calculation: Uw = (Ug*Ag) + (Uf*Af) + (ψspacer*Lg) Aw (0.7*1.32) + (0.675*0.5) + (0.0325*4.62) = 1.82 (0.924) + (0.337) + (0.15) = 1.82 = 0.77 W/m2K (or 0.135 BTU/hft F) 2
  • 15. PHI certification for ENERsign: Uw = 0.77 W/m2K Image from ENERsign brochure: http://www.qplus-llc.com/index.php?id=1191&L=
  • 16. How NFRC calcs U-window: Uf Ucog = U-value glass Uedge Ucog Acog = Area glass Uf = U-value frame Af = Area frame Af Uw = Aedge Acog Uedge = U-value edge of glass Aedge = Area edge of glass Aw = Area Window (Ucog*Acog) + (Uf*Af) + (Uedge*Aedge) Aw Thanks to Christian Kohler at LBNL for formula verification.
  • 17. NFRC Therm file protocols: Ucog Boundary Condition: Boundary Condition: NFRC 100-2001 Exterior Temp: -0.4 degF (-18degC) Film Coeff: (Rs) 4.58 BTU/hr.ft2F (26 W/m2K) Emmissivity: 0.84 Temp: 69.8 degF (21 degC) Film Coeff: (Rs) 0.39 BTU/hr.ft2F (2.19 W/m2K) (Defined using WINDOW) Uedge Boundary Condition: Temp: 69.8 degF (21 degC) Film Coeff: 0.39 BTU/hr.ft2F (2.1 W/m2K) U-factor Surface: Edge Uf Boundary Condition: U-factor Surface: SHGC Exterior (frame exterior only) Interior Wood/Vinyl Frame Temp: 69.8 degF (21 degC) Film Coeff: (Rs) 0.43 BTU/hr.ft2F (2.44 W/m2K) U-factor Surface: Frame
  • 18. NFRC simulation results: Ucog = 0.13 BTU/hrft2F Uw = 0.14 BTU/hrft2F
  • 19. Hidden NFRC Therm outputs: Uf = 0.146 BTU/hrft2F Uedge = 0.161 BTU/hrft2F Ag = 11.6 ft2 Acog = 8.96 ft2 Aedge = 2.65 ft2 Af = 7.7 ft2 Aw = 19.37 ft2
  • 20. NFRC calculation: Uw = (Ucog*Acog) + (Uf*Af) + (Uedge*Aedge) Aw (0.13*8.96) + (0.146*7.75) + (0.161*2.65) = 19.37 (1.16) + (1.13) + (0.42) = 19.37 = 0.14 BTU/hrft2F (or 0.789 W/m2K)
  • 21. Agrees with NFRC report: Uw = 0.14 BTU/hrft2F
  • 22. Where are the differences? Component NFRC PHI Window size 1.8 m2 1.82 m2 Width of frame * 0.15 m 0.1 m 39 deg C 30 deg C 0.73 W/m2K 0.7 W/m2K 0.22 W/K 0.15 W/K 0.79 W/m2K 0.77 W/m2K Final U-window value (IP) 0.139 BTU/hr.ft2.°F 0.135 BTU/hr.ft2.°F Final R-value 7.18 hr.ft2.°F/BTU 7.37 hr.ft2.°F/BTU Delta T in Therm Boundary condition U-glass Spacer vs edge of glass Final U-window value (metric) * Frame size variation is due to different profile options submitted by manufacturer’s representative to the two testing agencies and is not a protocol difference. Superficially that looks comparable, BUT…
  • 23. What if we adjust for the differences? (Metric units:) PHI NFRC Uw = = = Ug Ag Uf Af Psi spacer 0.7 1.318 0.675 0.5 0.0325 0.9226 0.73 0.3375 1.08 0.7884 0.675 Aw 4.62 1.82 0.15015 0.72 0.486 Lg Uwindow 0.775 1.8 0.22 0.830 (Ug*Ag) + (Uf*Af) + (ψspacer*Lg) A (0.7884)w+ (0.486) + (0.22) 0.830 W/m K 2 1.8 NFRC results become much more conservative! NFRC test numbers inserted into PHI formula.
  • 24. Preliminary Conclusions? • Enough differences exist between these two testing protocols that conclusive results are difficult. • The study was based on one window profile only and does not provide sufficient sample size. • More investigation is needed. However, qualified simulators could easily provide data for both protocols, should the market demand this data become commonly distributed.
  • 25. Possible solutions? Passive House Institute: • Modify Window Tab in US version of PHPP to account for NFRC simulation protocols • Coordinate simulation protocols with NFRC and allow for ‘translation’ by NFRC for PHPP values NFRC: • Work with PHI to add PH protocols as another cert. option Other: • Independent Testing Labs coordinate with PHI to provide PHPP performance numbers to US market?
  • 26. Update: (March, 2014) Since this study was first presented, a more comprehensive investigative report was conducted by RDH Engineering and can be requested directly from them here: http://www.rdhbe.com/about_us/news/articles/313.php Their study confirms the preliminary findings of this presentation and concludes: “… it is important for designers, specifiers, and manufacturers to recognize that the different certification systems will yield different Uvalues and solar heat gain values. In other words, comparing an NFRC U-value to a Passive House U-value is not an apples-to-apples comparison.”
  • 27. There is still much to do… Let’s figure this out together! Special thanks to: QPlus Llc for use of the ENERsign info Marles McDonald for NFRC review Christian Kohler for NFRC Uw formula Bob Ryan and Passive House Academy for Therm training & review Thank You! Questions? Contact: Bronwyn Barry bronwynbarry67 at gmail.com bronwyn at oneskyhomes.com