A Tale of Two Rating Systems:

NFRC & PHI window testing protocolsApples 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

Passive House needs good windowsTriple 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

Windows have pluses and minusesAnd are complicated!

Essence of the PHPP:

http://passipedia.passiv.de/passipedia_en/planning/calculating_energy_efficiency/phpp_-_the_passive_house_planning_package

For windows:

‘Loss’ items• Frames• Glazing spacers• Installation edge

‘Gain’ item• SHGC of glass

Heat Demand = Heat Losses – Heat Gains

How PHPP calculates windows

(Ug*Ag) + (Uf*Af) + (ψspacer*Lg) + (ψinstall*Lf) Aw

Uw-installed =

Ug

Ag

Uf

Af

Lg

Lf

ψinstall

ψspacer

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

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=

NFRC’s certification criteria:Simulation:

1. Uwindow (required)

2. SHGC (required)

3. Air Leakage (optional)

4. Visible Light Transmittance (optional)

Verification:

1. Destructive test of window sample

2. Factory inspection

Image from NFRC website: http://nfrc.org/fenestrationfacts.aspx

My comparison sample:Pazen ENERsign Tilt and Turn Jamb profile

Images from ENERsign brochure: http://www.qplus-llc.com/index.php?id=1191&L=

This study is based on the simulation results for the ENERsign profile, calculated by others, using the two testing protocols being compared here.

How PHI calcs window U-value

(Ug*Ag) + (Uf*Af) + (ψspacer*Lg) Aw

Uw =

Ug

Ag

Uf

Af

Lg

Ug = U-value glass

Ag = Area glass

Uf = U-value frame

Af = Area frame

Ψspacer = psi

L g = Length spacer

Aw = Area Window

ψspacer

Uf Boundary Condition:

Temp: 68 degF (20 degC)

Film Coeff: (Rs)

1.35 BTU/hr.ft2F (7.67 W/m2K)

PHI Therm file protocols:

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)

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: Interior

U-factor Surface: Frame

U-factor Surface: none

* This page verified by Passive House Academy

Exterior:

Boundary Condition:

Temp: 23 degF (-5degC)

Film Coeff: (Rs)

4.35 BTU/hr.ft2F

(24.7 W/m2K)

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!

PHI also calculates FRsi:

(Cursor placed at inside edge of glass and frame)

And Uw – Installed:

(Ug*Ag) + (Uf*Af) + (ψspacer*Lg) + (ψinstall*Lf) Aw

Uw-installed =

Ug

Ag

Uf

Af

Lg

Lf

ψinstall

ψspacer

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) 4= (3*0.64 + 0.78)

4= 0.675 W/m2K Image from: http://passivehouse.com/07_eng/01_dph/HerstLi_e/01Bauw_e/Fenst_e/Daten/D_Pazen_ENERsign.pdf

PHI calculation:

(Ug*Ag) + (Uf*Af) + (ψspacer*Lg) Aw

Uw =

(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/hft2F)=

PHI certification for ENERsign:

Uw = 0.77 W/m2K

Image from ENERsign brochure: http://www.qplus-llc.com/index.php?id=1191&L=

How NFRC calcs U-window:

(Ucog*Acog) + (Uf*Af) + (Uedge*Aedge)

Aw

Uw =

Ucog

Acog

Uf

Af

Ucog = U-value glass

Acog = Area glass

Uf = U-value frame

Af = Area frame

Uedge = U-value edge of glass

Aedge = Area edge of glass

Aw = Area Window

Uedge

Aedge

Thanks to Christian Kohler at LBNL for formula verification.

NFRC Therm file protocols:Ucog Boundary Condition:

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)

Uf Boundary Condition:

Interior Wood/Vinyl Frame

Temp: 69.8 degF (21 degC)

Film Coeff: (Rs)

0.43 BTU/hr.ft2F (2.44 W/m2K)

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

U-factor Surface: Edge

U-factor Surface: Frame

U-factor Surface:

SHGC Exterior (frame exterior only)

NFRC simulation results:

Ucog = 0.13 BTU/hrft2F

Uw = 0.14 BTU/hrft2F

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

NFRC calculation:

Uw =

(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)=

(Ucog*Acog) + (Uf*Af) + (Uedge*Aedge)

Aw

Agrees with NFRC report:

Uw = 0.14 BTU/hrft2F

Where are the differences?

Component NFRC PHI

Window size 1.8 m2 1.82 m2

Width of frame * 0.15 m 0.1 m

Delta T in Therm Boundary condition 39 deg C 30 deg C

U-glass 0.73 W/m2K 0.7 W/m2K

Spacer vs edge of glass 0.22 W/K 0.15 W/K

Final U-window value (metric) 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

Superficially that looks comparable, BUT…

* 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.

What if we adjust for the differences?

NFRC results become much more conservative!

(Ug*Ag) + (Uf*Af) + (ψspacer*Lg) Aw

Uw =

(0.7884) + (0.486) + (0.22) 1.8

=

(Metric units:) Ug Ag Uf Af

Psi spacer Lg Aw

U-window

PHI0.7 1.318 0.675 0.5 0.0325 4.62 1.82  

0.9226 0.3375 0.15015   0.775

NFRC0.73 1.08 0.675 0.72     1.8  

0.7884 0.486 0.22   0.830

= 0.830 W/m2K

NFRC test numbers inserted into PHI formula.

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.

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?

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 U-values 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.”

Update: (March, 2014)

There is still much to do…Let’s figure this out together!

Contact:

Bronwyn Barry

bronwynbarry67 at gmail.com

bronwyn at oneskyhomes.com

Questions?

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!