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 Project Origins Importance of high performance windows Applicable Standards Difference between standards Boundary Conditions Topology U-Values Solar Heat Gain Coefficient (SHGC) Impacts on Performance Summary
Understanding Window Rating Systems 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 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 High performance windows form integral part of strategy to achieve whole building energy target Provide necessary solar heat gains Reduce heat loss to a point where window becomes a gain High performance windows provide high interior surface temperatures for thermal comfort & prevent condensation/surface mold growth Selection of window properties is climate & building dependant though general guidelines exist Windows from Europe are rated differently than in North America
North American Window Products What are some of the best performing windows available from North American manufacturers? From the ENERGY STAR Canada product database: 326 of 583,120 listings have U 0.8; triples have surface 6 low-e coating and/or Krypton gas fill, or quad glazing Of these listings, highest SHGC is 0.33 How are European manufacturers 0.8 to 1.0 0.8 achieving low U-values and high 1% 0% 1.0 to 1.4 SHGC with only triple glazing and 10% argon gas fill? 1.4 to 2.0 89%
Window Rating Standards North America NFRC 100 (U-value) and NFRC 200 (SHGC/VT) Computer simulation (THERM) using laboratory validated test for calibration/ confirmation of model NFRC 100& 200 are ISO 15099 compliant methods Europe ISO 10077-1 (Whole Window U-value), ISO 10077-2 (Frame U-value), EN-673 (Glazing Uvalue), EN-410 (Glazing g-value/shgc) Passive House Institute Darmstadt (PHI-D) references ISO 10077, EN 673, EN 410 Plus minimum surface temperature criteria
Key Differences Between Window Rating Standards Boundary conditions (temperatures & air film resistances) Window geometry Calculation methodologies (algorithms) for IGU and frame Uvalues SHGC (g-factor) for the windows and, Treatment of sloped glazing
Heat Flow Basics for Windows Conduction Solar Gain Heat is lost or gained through window when there is a temperature difference between inside and outside Measured in terms of U-value, Btu/hr-ft2-F or W/m2-K Heat gained through direct or indirect solar radiation Measured in terms of the Solar Heat Gain Coefficient (SHGC) Infiltration Air leakage through cracks in fenestration
Key Difference Boundary Conditions For U-value Calculations (Insulated Frames) 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 -10 oc (14oF) 20 oc (68oF) 25.0 7.7 combined Passive House Cert. Criteria 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 For SHGC Calculations 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 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 NFRC sizes depend on operator type For example: Fixed: 1.2 m x 1.5 m Tilt & Turn: 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
Key Difference: Window Geometry Design European (EU) Style Window North American (NA) Style Window 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 Operable Hardware Preference EU (Inswing) vs NA (Outswing) 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 SAME Argon & SAME low-e emissivity coatings But Different Results!
Key Difference: Rating Procedures for U-Values ISO 10077 European Style Window NFRC 100 North American Style Window Uframe x Aframe Uframe x Aframe Uedge glz x Aedge 2.5 glz ψspacer x L glazed perimeter Uglazing x Aglazing ψinstall x L Uglazing x Aglazing Uedge glz (NFRC) can be converted into a ψedge glz EN/ISO relatively easily (but not vice versa) window perimeter 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 )
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 The NFRC algorithm for centre of glass U-value are more accurate NFRC follows ISO 15099, Passive House follows ISO 10077-2 and EN 673 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? Study evaluated U-value, solar heat gain of three windows using NFRC and ISO/PHI methods North American Vinyl Frame North American Fibreglass Frame European Vinyl Frame Each window had same glass, gas fill and spacer Showed how same product performs under different rating systems
Centre of Glass U-Value Triple glazing, argon gas fill, two low-e coatings Big difference between U-values for NFRC and ISO methods and standard temperatures Centre of Glass U- Value, W/m2- K 0.9 0.8 NFRC, - 18 C 0.7 ISO, 0 C 0.6 0.5 10 12 14 16 Gap Size, mm 18 20
Centre of Glass U-Value Triple glazing, argon gas fill, two low-e coatings Differences when only changing exterior temperature of methodology Centre of Glass U- Value, W/m2- K 0.9 NFRC, - 18 C 0.8 NFRC, 0 C 0.7 ISO, - 18 C 0.6 ISO, 0 C 0.5 10 12 14 16 Gap Size, mm 18 20
Centre of Glass U-Value Triple glazing, argon gas fill, two low-e coatings Add in climate-specific temperatures for Passive House certification Centre of Glass U- Value, W/m2- K 0.9 NFRC, - 18 C 0.8 NFRC, - 7 C NFRC, 0 C NFRC, 5 C 0.7 ISO, - 18 C ISO, - 7 C 0.6 ISO, 0 C ISO, 5 C 0.5 10 12 14 16 Gap Size, mm 18 20
Centre of Glass U-Values Examples 12.7 mm gap: NFRC U-0.72, ISO U-0.70 18 mm gap: NFRC U-0.73, ISO U-0.57 Centre of Glass U- Value, W/m2- K 0.9 NFRC, - 18 C 0.8 NFRC, - 7 C NFRC, 0 C NFRC, 5 C 0.7 ISO, - 18 C ISO, - 7 C 0.6 ISO, 0 C ISO, 5 C 0.5 10 12 14 16 Gap Size, mm 18 20
Centre of Glass U-Values Optimal gap size different for NFRC and ISO NFRC optimal gap size is approx. 13 mm Centre of Glass U- Value, W/m2- K 0.9 ISO optimal gap sizes are larger, approx.18 mm NFRC, - 18 C 0.8 NFRC, - 7 C NFRC, 0 C NFRC, 5 C 0.7 ISO, - 18 C ISO, - 7 C 0.6 ISO, 0 C ISO, 5 C 0.5 10 12 14 16 Gap Size, mm 18 20
Centre of Glass U-Values Six other IGU configurations were simulated Biggest difference in U-values for larger gap sizes Centre of Glass U- Value, W/m2- K Double glazing 15.875 mm gaps Triple glazing 12.7 mm gaps 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 19% 23% 0% 2% Double - Double - Triple - Triple - High Solar Gain Low Solar Gain High Solar Gain Low Solar Gain NFRC ISO
Frame U-Values NFRC 1.6 frame U-values determined with actual IGU and spacer; ISO valuesno determined with calibration panel of Frame - Value, W/m /m22- K - K Frame UU U- Value, - Value, W W /m Frame ISO Frame U- Value specified lower ISO frame U-values 1.5 conductivity Correlation! Also different standard material properties, e.g. fibreglass 1.2 2.0 1.4 1.0 1.2 1.5 1.0 0.8 0.8 0.6 1.0 0.6 0.4 0.4 0.5 0.2 0.2 0.0 0.0 0.0 Triple PGassive Horth ouse uropean PVC Window Triple lazed N AEmerican Vuinyl Frame Window Triple Glazed Fibreglass Frame W indow 1.4 NFRC ISO NFRC 1.3 ISO NFRC ISO 1.2 Fixed - Head Fixed - 1.2 - H Head ead Fixed Fixed - Sill Fixed - J amb Jamb - Jamb 1.4 1.5 Fixed - Fixed Passive House TFriple NFRC rame U- Value Triple - 180/180 Triple Fixed 1.3 Fixed - Sill- Sill 11% 16% difference 2% 13%toto to4% 16% difference difference 1.6
Whole Product U-Values Highest percent difference in window U-values was 18% 15% Percent Difference in NFRC & ISO U- Values for Triple Glazed Windows ISO Lower U-Values 10% 5% North American Vinyl 0% North American Fibreglass - 5% European Vinyl - 10% NFRC Lower U-Values - 15% Fixed Operable Triple - - 1High 80/180 Triple Solar Fixed Operable Triple - 3-66/180 Triple Low Solar
Solar Heat Gain Values Centre of glass NFRC values were 1% to 8% lower than ISO Greater difference for low solar gain glazing Big difference between centre of glass and whole product values! Fixed: 18% - 19% reduction Operable: 46% - 48% reduction Solar Heat Gain Coefficient 0.7 0.6 NFRC Centre of Glass ISO Centre of Glass NFRC Fixed SHGC NFRC Operable SHGC 0.5 0.4 0.3 0.2 0.1 0.0 Double High Double - 180 Solar Double Low Double - 366 Solar Triple- High Triple- 3 Low Solar Triple 180/180 Triple 66/180 Solar
Summary Biggest Difference? Many differences, but a significant one is centre of glass Centre of Glass U-Values Centre of Glass U- Value, W/m2- K U-value calculations 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 NFRC ISO Whole Window U-Values Window U- Value, W/m2- K Double High Solar Double High Solar Double High Solar Triple High Solar Triple High Solar Triple High Solar NA Vinyl NA Fibreglass EU upvc NA Vinyl NA Fibreglass EU upvc 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 NFRC ISO Double High Solar Double High Solar Double High Solar Triple High Solar Triple High Solar Triple High Solar NA Vinyl NA Fibreglass EU upvc NA Vinyl NA Fibreglass EU upvc
Lessons Learned Neither NFRC nor ISO system is better NFRC uses more accurate algorithms, compares all products using the same conditions PHI uses more realistic climate design conditions, components allow for better energy modeling Today products are optimized to perform best under the rating regimes in effect in Europe, North America Rating regimes drive product design North American simulation tools have the capability to model products for Passive House standards
Summary and Conclusions NFRC and EN/ISO calculate and report window U-values and SHGC differently and under different conditions (apples vs oranges) Neither is necessarily better, both have limitations 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 Design for your climate/site/building guidelines exist U-value specification to meet energy target & comfort/surface temperature criteria SHGC to meet energy target & thermal comfort (but watch overheating without shading)
Questions Robert Lepage rlepage@rdh.com 250.479.1110