| Date post: | 15-Jul-2015 |
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Energy Performance of Windows: Navigating North American and European Window Standards
ROBERT LEPAGE, MASC, P.ENG.,
RDH BUILDING ENGINEERING LTD. VICTORIA, BC
ON BEHALF OF BRITTANY HANAM, MASC, P.ENG., AL JAUGELIS, AND GRAHAM FINCH, MASC, P.ENG.
Outline
à Project Origins à Importance of high performance windows à Applicable Standards à Difference between standards
à Boundary Conditions à Topology à U-Values à Solar Heat Gain Coefficient (SHGC)
à Impacts on Performance à Summary
Understanding Window Rating Systems
à Recently completed a large industry research project to look at the validity of the Canadian ER Rating and to evaluate/rank windows in terms of U-values SHGC while also assessing thermal comfort
à Differences between North American & European ( and Passive House) window rating systems being studied as part of a follow-up task -Today: What we have uncovered so far…
Window Selection for High Performance Homes
à High performance windows form integral part of strategy to achieve whole building energy target à Provide necessary solar heat gains
à Reduce heat loss to a point where window becomes a gain
à High performance windows provide high interior surface
temperatures for thermal comfort & prevent
condensation/surface mold growth à Selection of window properties is climate & building
dependant – though general guidelines exist
à Windows from Europe are rated differently than in North
America
≤ 0.80%
0.8 to 1.01% 1.0 to 1.4
10%
1.4 to 2.089%
North American Window Products
à What are some of the best performing windows available from North American manufacturers?
à From the ENERGY STAR Canada product database: à 326 of 583,120 listings have U ≤ 0.8; triples have
surface 6 low-e coating and/or Krypton gas fill, or quad glazing
à Of these listings, highest SHGC is 0.33 à How are European manufacturers
achieving low U-values and high SHGC with only triple glazing and argon gas fill?
Window Rating Standards
à North America – NFRC 100 (U-value) and NFRC
200 (SHGC/VT) à Computer simulation (THERM) using
laboratory validated test for calibration/confirmation of model
à NFRC 100& 200 are ISO 15099 compliant methods
à Europe – ISO 10077-1 (Whole Window U-value),
ISO 10077-2 (Frame U-value), EN-673 (Glazing U-
value), EN-410 (Glazing g-value/SHGC)
à Passive House Institute Darmstadt (PHI-D) –
references ISO 10077, EN 673, EN 410 à Plus minimum surface temperature criteria
Key Differences Between Window Rating Standards
à Boundary conditions (temperatures &
air film resistances)
à Window geometry
à Calculation methodologies
(algorithms) for IGU and frame U-
values
à SHGC (g-factor) for the windows and,
à Treatment of sloped glazing
Heat Flow Basics for Windows
à Conduction à Heat is lost or gained through window when there is a
temperature difference between inside and outside à Measured in terms of U-value, Btu/hr-ft2-F or W/m2-K
à Solar Gain à Heat gained through direct or indirect solar radiation à Measured in terms of the Solar Heat Gain Coefficient
(SHGC) à Infiltration
à Air leakage through cracks in fenestration
Key Difference – Boundary Conditions
Window Rating Standard
Exterior Temperature
Interior Temperature
Exterior Boundary
Condition – W/m2·K
Interior Boundary
Condition – W/m2·K
NFRC 100 & 200
-18 oC (0oF) 21 oC (70oF)
26.0 2.44 -3.29* convection
ISO 10077-1 and 10077-2 and EN 673
0 oC (32oF) 20 oC (68oF) 25.0 7.7 combined
ISO 15099 0 oC (32oF) 20 oC (68oF) 20.0 3.6 * convection
Passive House Cert. Criteria
-10 oC (14oF) 20 oC (68oF) 25.0 7.7 combined
For U-value Calculations (Insulated Frames)
This matters because temperature affects gas thermal resistance (NFRC/CEN account differently) and interior/exterior air films add thermal resistance directly
Key Difference – Boundary Conditions
Window Rating Standard
Exterior Temperature
Interior Temperature
Solar Insolation W/
m2
NFRC 100 & 200
32 oC (90oF) 24 oC (75oF)
783
ISO 10077-1 and 10077-2 and EN 673
30 oC (86oF) 25 oC (77oF) 500
Passive House Cert. Criteria
30 oC (86oF) 25 oC (77oF) 500
For SHGC Calculations
Different exterior temperatures create different temperature profiles, and different solar insolation affects solar heat gain calculations. SHGC includes both long and shortwave radiosity of the system.
Key Differences: Standard Sizes
à NFRC sizes depend on operator type
à For example:
Fixed: 1.2 m x 1.5 m
Casement – Single: 0.6 m x 1.5 m
Passive House has one standard size for fixed and operable punched windows – 1.23 m x 1.48 m German operable windows typically Tilt & Turn – larger sizes
Tilt & Turn: 1.2 m x 1.5 m
Key Difference: Window Geometry – Design
European (EU) Style Window North American (NA) Style Window
Operable Hardware Preference – EU (Inswing) vs NA (Outswing)
EU Frames tend to be deeper (avg. ~4.75”) than NA frames (avg. 2.75”)
EU glazing spacer buried within frame vs inline with NA frame sightline
SAME Argon & SAME low-e emissivity coatings But Different Results!
IGU gap, 1/2” optimum under NA NFRC vs 5/8” optimum under EU CEN/ISO
More standard EU 4mm vs NA 3mm glass panes
Key Difference: Rating Procedures for U-Values
ISO 10077 – European Style Window NFRC 100 – North American Style Window
Uframe x Aframe
Standard Window Size 1.23m wide x 1.48m high (48” x 58 ¼”)
Standard Window Size 1.2m wide x 1.5m high (47 ¼” x 59”)
Uglazing x Aglazing
ψspacer x L glazed perimeter
ψinstall x L window perimeter
Uframe x Aframe
Uglazing x Aglazing
Uedge glz x Aedge
glz 2.5”
Uedge glz (NFRC) can be converted into a ψedge glz EN/ISO relatively easily (but not vice versa)
Key Difference: Rating Procedure for SHGC
ISO 10077 – European Style Window NFRC 100 – North American Style Window
g-value in Europe / SHGC in North America,
g-value provided for center of glass only (neglects frames) Convert to whole window by multiplying by glass/window ratio (becomes lower by 20-40%+)
SHGC provided for whole window (includes frame effect) Convert to just glazing by dividing by glass/window ratio (becomes higher by 15-25%+)
Many European glazing manufacturers also use low-iron glass to get the SHGC a few percent higher
Key Differences: Algorithms
à The NFRC algorithm for centre of glass U-value are more
accurate
à NFRC follows ISO 15099, Passive House follows ISO
10077-2 and EN 673 à Footnote in ISO 10077-2, section 6.2 (reference to EN
673):
“NOTE The correlations for high aspect ratio cavities [in glazing] used in EN 673 and ISO 10292 tend to give low
values for the equivalent thermal conductivity. More accurate correlations are given in ISO 15099.”
How do these differences affect energy performance?
à Study evaluated U-value, solar heat gain of three
windows using NFRC and ISO/PHI methods à North American Vinyl Frame
à North American Fibreglass Frame
à European Vinyl Frame
à Each window had same
glass, gas fill and spacer
à Showed how same product
performs under different
rating systems
Centre of Glass U-Value
à Triple glazing, argon gas fill, two low-e coatings à Big difference between U-values for NFRC and ISO methods
and standard temperatures
0.5
0.6
0.7
0.8
0.9
10 12 14 16 18 20
Centre of G
lass U-‐Value
, W/m
2 -‐K
Gap Size, mm
NFRC, -‐18°C
ISO, 0°C
Centre of Glass U-Value
à Triple glazing, argon gas fill, two low-e coatings à Differences when only changing exterior temperature of
methodology
0.5
0.6
0.7
0.8
0.9
10 12 14 16 18 20
Centre of G
lass U-‐Value
, W/m
2 -‐K
Gap Size, mm
NFRC, -‐18°C
NFRC, 0°C
ISO, -‐18°C
ISO, 0°C
0.5
0.6
0.7
0.8
0.9
10 12 14 16 18 20
Centre of G
lass U-‐Value
, W/m
2 -‐K
Gap Size, mm
NFRC, -‐18°C
NFRC, -‐7°C
NFRC, 0°C
NFRC, 5°C
ISO, -‐18°C
ISO, -‐7°C
ISO, 0°C
ISO, 5°C
Centre of Glass U-Value
à Triple glazing, argon gas fill, two low-e coatings à Add in climate-specific temperatures for Passive House
certification…
0.5
0.6
0.7
0.8
0.9
10 12 14 16 18 20
Centre of G
lass U-‐Value
, W/m
2 -‐K
Gap Size, mm
NFRC, -‐18°C
NFRC, -‐7°C
NFRC, 0°C
NFRC, 5°C
ISO, -‐18°C
ISO, -‐7°C
ISO, 0°C
ISO, 5°C
Centre of Glass U-Values
à Examples à 12.7 mm gap: NFRC U-0.72, ISO U-0.70
à 18 mm gap: NFRC U-0.73, ISO U-0.57
0.5
0.6
0.7
0.8
0.9
10 12 14 16 18 20
Centre of G
lass U-‐Value
, W/m
2 -‐K
Gap Size, mm
NFRC, -‐18°C
NFRC, -‐7°C
NFRC, 0°C
NFRC, 5°C
ISO, -‐18°C
ISO, -‐7°C
ISO, 0°C
ISO, 5°C
Centre of Glass U-Values
à Optimal gap size different for NFRC and ISO
NFRC optimal gap size is approx. 13 mm
ISO optimal gap sizes are larger, approx.18 mm
Centre of Glass U-Values
à Six other IGU configurations were simulated
à Biggest difference in U-values for larger gap sizes à Double glazing 15.875 mm gaps
à Triple glazing 12.7 mm gaps
0.00.20.40.60.81.01.21.41.61.8
Double -‐High Solar Gain
Double -‐Low Solar Gain
Triple -‐High Solar Gain
Triple -‐Low Solar Gain
Centre of G
lass U-‐Value
, W/m
2-‐K
NFRC
ISO
19% 23%
0% 2%
0.0
0.5
1.0
1.5
2.0
Fixed -‐ Head Fixed -‐ Sill Fixed -‐ Jamb
Triple -‐ 180/180
Fram
e U-‐Value
, W/m
2 -‐K
Triple Glazed North American Vinyl Frame Window
NFRC
ISO
Frame U-Values
à NFRC frame U-values determined with actual IGU and
spacer; ISO values determined with ‘calibration panel’ of
specified conductivity – lower ISO frame U-values
à Also different standard material properties, e.g. fibreglass
11% to 16% difference
0.00.20.40.60.81.01.21.4
Fixed -‐ Head Fixed -‐ Sill Fixed -‐ Jamb
Triple
Fram
e U-‐Value
, W/m
2 -‐K
Triple Glazed Fibreglass Frame Window
NFRC
ISO
2% to 4% difference
0.00.20.40.60.81.01.2
Fixed -‐ Head Fixed -‐ Sill Fixed -‐ Jamb
Passive House Triple
Fram
e U-‐Value
, W/m
2 -‐K
Triple Passive House European uPVC Window
NFRCISO
13% to 16% difference
1.2
1.3
1.4
1.5
1.6
1.2 1.3 1.4 1.5 1.6
ISO Frame U-‐Value
NFRC Frame U-‐Value
No Correlation!
-‐15%
-‐10%
-‐5%
0%
5%
10%
15%
Fixed Operable Fixed Operable
Triple -‐ 180/180 Triple -‐ 366/180
Percent Difference in NFRC & ISO U-‐Values for Triple Glazed Windows
North American Vinyl
North American Fibreglass
European Vinyl
Whole Product U-Values
à Highest percent difference in window U-values was 18%
ISO Lower U-Values
NFRC Lower U-Values
Triple - High Solar
Triple - Low Solar
Solar Heat Gain Values
à Centre of glass NFRC values were 1% to 8% lower than ISO
à Greater difference for low solar gain glazing
à Big difference between centre of glass and whole product
values!
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Double -‐ 180 Double -‐ 366 Triple -‐ 180/180 Triple -‐ 366/180
Solar H
eat G
ain Co
efficient
NFRC Centre of GlassISO Centre of GlassNFRC Fixed SHGCNFRC Operable SHGC
Fixed: 18% - 19% reduction Operable: 46% - 48% reduction
Double High Solar
Double Low Solar
Triple Low Solar Triple High Solar
Summary – Biggest Difference?
à Many differences, but a significant one is centre of glass
U-value calculations
0.00.20.40.60.81.01.21.41.61.8
Double High SolarNA Vinyl
Double High SolarNA Fibreglass
Double High SolarEU uPVC
Triple High SolarNA Vinyl
Triple High SolarNA Fibreglass
Triple High SolarEU uPVC
Centre of G
lass U-‐Value
, W/m
2-‐K
NFRC
ISO
0.00.20.40.60.81.01.21.41.61.8
Double High SolarNA Vinyl
Double High SolarNA Fibreglass
Double High SolarEU uPVC
Triple High SolarNA Vinyl
Triple High SolarNA Fibreglass
Triple High SolarEU uPVC
Windo
w U-‐Value
, W/m
2-‐K NFRC
ISO
Centre of Glass
U-Values
Whole Window U-Values
Lessons Learned
à Neither NFRC nor ISO system is “better” à NFRC uses more accurate algorithms, compares all
products using the same conditions
à PHI uses more realistic climate design conditions,
components allow for better energy modeling
à Today products are optimized to perform best under
the rating regimes in effect in Europe, North America à Rating regimes drive product design
à North American simulation tools have the capability to
model products for Passive House standards
Summary and Conclusions
à NFRC and EN/ISO calculate and report window U-values and SHGC differently and under different conditions (apples vs oranges) à Neither is necessarily better, both have limitations
à Careful what values you input into energy models (PHPP is
EN/ISO calibrated, most other NA software uses NFRC) – “NFRC values appear conservative, EN/ISO values appear optimistic”
à Design for your climate/site/building – guidelines exist à U-value specification to meet energy target & comfort/surface
temperature criteria à SHGC to meet energy target & thermal comfort (but watch
overheating without shading)
